Marine Boilers

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MARINE BOILERS & STEAM ENGINEERING.

MARINE BOILERS. By Prof. K. Venkataraman. CEng; FIMarE; MIE.

For BE (Marine Engineering) Cadets.

Introduction to Marine Boilers: Boilers of varied design and working conditions are installed in both steam and motor vessels. The most modern steamships have boiler plant of a sophisticated nature, and even on motor ships the steam plant can be quite extensive, providing useful services and enhancing the overall efficiency of the vessel. The demand for steam propulsion is currently very low, being confined to specialised ships such as liquid natural gas (LNG) carriers. However, a number of steamships may still be found in service having boiler plant resulting from many years of development. Design modifications have been made to eliminate problem areas and to adjust to changing operational constraints in much the same way that the diesel engine has progressed to its present advanced state. Water tube marine boilers have been dominant, as far as steam propulsion is concerned, since the period between the two world wars. Even the generation of steam for auxiliary purposes aboard ship has come into the province of the water tube boiler, a practice which grew to prominence with increasing demand for large quantities of auxiliary steam and which persists today in ships such as the large motor tanker. Nevertheless in the field of auxiliary steam production many non-water tube boilers can still be found, especially where steam output and pressure are not high. Water tube boilers can be made for steam duties as low as 1.5 ton/ h and as high as 2.5 X 103 ton /h; at the lower end of the range, the water tube boiler is found to be uneconomical and would only be considered for very special applications where very high steam pressure was involved. Boilers having duties in the upper end of the output range would be found in central power stations ashore. Steam pressure in water tube boilers can vary between 7 bar and supercritical values such as 225 bar, although natural circulation would only be applicable to pressures below about 175 bar. Steam temperature could range from saturation to 600~650*C, depending upon the fuel and method of firing. With this vast range of duties it is not surprising that the shape and detail of water tube boilers should vary considerably. Although the marine sphere is only a particular section of the whole range, the number of different boiler designs available is large. For merchant ships building to classification requirements and adopting the low forcing rates that experience has shown will enable good levels of the other factors to be obtained achieve a good compromise. Even so, there are distinctions to be observed, such as between main propulsion and auxiliary boilers. Auxiliary boilers, receiving possibly much less use than main propulsion boilers, may usefully employ higher forcing rates. There are two main types of boilers in Marine use:

1. TANK BOILERS OR FIRE TUBE BOILER OR SMOKE TUBE BOILER. 2. WATER TUBE BOILER.

TANK BOILERS OR FIRE TUBE BOILERS: These boilers were used in olden days with steam reciprocating engines. Now used in some ships for auxiliary steam requirements. These boilers have poor efficiency and low power/weight ratio. Names of some of the Tank boilers: 1. Scotch Boiler. 2. Clarkson Boiler. 3. Cochran Boiler 4. Spanner Boiler.

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WATER TUBE BOILERS: Water tube boilers have to a large extent superseded the Scotch boiler for the supply, of steam to main and auxiliary machinery. Even donkey (auxiliary) boilers are frequently found to he water tube boilers and certainly all-modern turbine plants use them for main steam supply. Names of some of the Water Tube Boilers: 1. Babcock & Wilcox. 2.Foster Wheeler. 3. Yarrow. Use of Steam on Steam Turbine or Motor vessel:

a. To drive the main propulsion steam turbine. b. To run Cargo pump turbines in Tankers c. Heating duties: ME Fuel oil heater, Purifier heater, Oil tank heating, Cargo heating, Air

conditioning & heating plant, Calorifier, Galley supply, sea-chests, tracer lines for pipeline heating etc.

d. Run Steam Turbo Generators. e. Run cargo pump turbines in tanks. f. Drive steam driven deck machineries like winches etc. g. Operate bilge, stripping and other steam driven pumps. h. Drive boiler feed pump turbine. i. Evaporator/Fresh water generator heating media. j. Tank washing in tanker ships and general cleaning. k. For boiler Soot blowing and for the steam atomised burners. l. Fire fighting as used in steam smothering system. m. Main engine Jacket F. W. pre-heater and lubricating oil sump and drain tanks n. Use in the waste oil, incinerator and slop tanks. o. Use as a steam ejector media for ejector pumps and vacuum devices. Classification of Boilers:

Classification Criteria.

For Steam Ships. For Motor Ships. Add. Information.

Capacity. High capacity. 100,000kg/hr and over.

Low capacity. 1,000 – 5,000kg/hr.

For Motor Tankers. 20,000-50,000 kg/hr.

Pressure. High pressure. 60 bars and above.

Low pre: 6-to15 bar. Medium pressure: 17 –30 bar.

Mid pressure used In Tanker vessels.

Shape. Drum type. ‘D’ – Type.

Cylindrical, Vertical, Tank type.

Package, Tubular, Coil type.

Usage.

Main Propulsion Boiler.

Auxiliary boiler. Donkey boiler.

Assist propulsion, Hotel purpose.

Type of fuel

used.

Heavy fuel oil, Gas.

Light diesel oil, Heavy fuel oil, Gas.

Coal, Electricity, Exhaust gas.

Working principle.

Water tube. Fire tube, Water tube.

Steam raising method.

Circulation type.

Natural. Natural, Forced. Tank, Drum. Coil Exh. Economizer.

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KEY CONCEPTS, FEATURES & BOILER TYPES: Main Boiler: Used for the main propulsion of the vessel. Auxiliary Boiler: Aids the propulsion in some way; e.g., heating of heavy fuel oil using a steam heater, necessary for propulsion would quality the supplying boiler to be referred to as an auxiliary boiler. Donkey Boiler: A boiler which is used only for the ‘hotel’ needs of the ship; e.g., supplying hot water to the galley. Tank Boiler: A boiler with large water carrying capacity where the shell is being used as the pressure vessel. Most low-pressure auxiliary boilers will come into this category. Vertical Boiler: Any boiler where the shell is upright and the furnace is usually contained within the shell at the lower half. Horizontal Boiler: This is also referred to as cylindrical boiler; here, the boiler cylindrical shell is lying across its length parallel to the structure of the ship or the ground level. Exhaust Gas Boiler: Boiler operated by hot gas from engine or other exhaust gas sources. Drum Type Boiler: Water tube boilers employing steam and water drums. They are also known as bent tube type boilers. Package Boilers: Fully automatic, low capacity boilers packed inside a box type casing. Capable of quick steam production and flexible in being positioned anywhere. This could be coil type or fire tube type. Some Popular Boiler Makes:

Medium Pressure: Water Tube, Drum Type.

Low Pressure: Package, Coil type,

Fire Tube Type.

Low Pressure: Tank Type:

Oil fired/ Composite.

Low Pressure: Exhaust Gas. Forced circulation tubular

Tanker Vessels: All type of Vessels: All type of Vessels: All type of Vessels: Babcock & Wilcox.

M11. M11M. Cochran Chieftain. Alborg.

AQ3,AQ5, AQ9, AQ12 Alborg AV.

Foster Wheeler D4. Steambloc. Sunrod. CPH, CPDB*. Sunrod PL,PT*. Combustion Engineering. Stone-Vapour. Osaka Howden Johnson

Kawasaki. Clyton. Hitachi Zosen HV. Hitachi. Miura VWS. Spanner Swirlflo.

Mitsubishi MAC*. Cochran. Mitsubishi.

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Comparison of Smoke tube and Water tube boilers:

Smoke Tube Boilers: Water Tube Boilers: Water Circulation: 1. There is no definite circulation in a cylindrical boiler, the direction in which the water moves being irregular. 2. This indefinite circulation causes severe strains, due to unequal expansion, which 3. After a time, owing to the rigidity of the boiler structure and the use of stays, entail heavy repairs from the straining or possible rupture of its weakest parts. Repairs: 4. When repairs become necessary they are usually of a very serious nature, and entail the employment of skilled boilermakers at considerable expense. 5. Repairs, when completed, have frequently to be followed by a reduction in the working pressure, because, by their very nature, they are usually not of a sufficiently good character to admit of the boiler being considered as strong as previously. 6. It follows then, particularly as regards high pressure boilers, that the cost, of repairs, when taken over a series of years, is higher with the smoke tube type, than with water tube boilers Safety: 7. An accident in connection with a cylindrical boiler may easily lead to serious disasters owing to the large diameter of the boiler, and the great amount of energy stored in the large volume of water. The collapse of furnace crowns, which is of comparatively frequent occurrence, is often attended with danger. Cleaning: 8.In a cylindrical boiler men working inside the shell in a hot atmosphere must do the whole of the internal cleaning. A cylindrical boiler cannot be conveniently entered until it has cooled for 24 to 36 hours, and much of the cleaning has to be carried out in very confined and awkwardly situated spaces, indeed, some places cannot be efficiently, cleaned at all.

1.The water circulation is definite and continuous in one direction. 2.The temperature of the water throughout the boiler is uniform, and thus there can be no strains in the structure due to inequality of temperature. 3. Provision is made for expansion of all the parts separately; there being no stays whatever.

4.Should repairs become necessary owing to incrustation due to neglect in cleaning, the worst that could happen would be the overheating of tubes, and these can be readily replaced by a mechanic in a short time, and at slight cost. 5.When tubes are replaced by new ones, it follows that the initial strength of the boiler is not affected in any way. There is thus no necessity for a reduction in the working pressure, which may be indefinitely maintained. 6. Over a long period of operation the cost of repair will be lower. 7. The safety of water tube boilers is undoubted far less than cylindrical boilers. . 8. It can be very easily inspected and cleaned, provision being made for ready access to every part. The external cleaning of the whole of the section tubes is affected from the stokehold, where the men work in a natural and pure atmosphere. Cleaning can be commenced almost immediately the boiler is blown down.

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Time required to raise steam: 9. A period of from five to six hours is necessary for properly raising steam in this construction of boiler, and it is advisable to allow even a longer period in the case of large units. Space occupied: 10. A cylindrical boiler has proportionately small grate area for a given floor space. 11. Water content very high.

9. Under ordinary, conditions, steam can be easily raised within an hour in a water tube boiler. 10. A material saying of space is effected by the use of this boiler, which has a large grate area in proportion to the floor space occupied, hence a given evaporation can be obtained with less space. 11. Water content very low.

THE ADVANTAGES OF WATER TUBE BOILERS:

1. High efficiency (generally greater than 85%) hence reduced fuel consumption. 2. Flexibility of design -- important space consideration. 3. Capable of high out put (i.e. high evaporation rate). 4. High pressures and temperatures improve turbine plant efficiency. 5. Flexible in operation to meet the fluctuating demands of the plant, - superheat control rapidly

responsive to changing demands. 6. Generally all surfaces are circular hence no supporting stays are required. 7. Steam can he raised rapidly from cold if the occasion demands, (3 to 4 hours compared to 24

hours for a Scotch boiler) because of the positive water circulation. 8. Compact and relatively light (water content up to 7.5 tons compared with 30 tons for a Scotch

boiler). 9. With double easing radiation loss can he cut to 1% or less. ******************kv********************

Construction of Water Tube Boilers: Materials: Drums: Good quality low carbon steel, the main constituents are. 0-28% carbon maximum, 0.5% Manganese approximately, 0.1% Silicon approximately, remainder mainly Ferrite. Ultimate tensile strength 430 to 490 MN/m2 with about 20% elongation. Steels with chrome, molybdenum, manganese and vanadium are increasingly being used. The increased strength and creep resistance enable less material to be used. Reduced weight, cost, machining and assembly time being advantages. Superheater tubes: Plain low carbon (0. 15% Carbon approximately) steel up to 400*C steam temperature. 0.5% Molybdenum low carbon steel up to 480*C steam temperature. Austenitic stainless steel: 18% Nickel, 8% Chrome, stabilised against weld decay with niobium, for steam temperatures up to 590*C. Weld decay: When stainless steel superheater tubes, in some earlier boilers, were welded to header stubs the microstructure of the metal adjacent to the weld changed. Corrosion protection by the chrome in the alloy steel was lost due to precipitation of the element as chromium carbide. A band of corrosion around the tube was named “weld decay". Creep considerations predominate in the case of superheater tubes since they are subjected to the highest temperature (especially the last pass) and to boiler pressure.

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The actual metal temperature will depend upon:

a) Steam flow rate and temperature. b) Gas flow rate and temperature. c) Tube thickness and material. d) Condition of tube surfaces externally and internally

For normal conditions the temperature difference between inside and outside of the tube may be of the order, or less than, 38*C. Creep tests are usually carried out over a period of 20,000 hours for superheater tubes in order to ascertain the creep rate and maximum strain. Creep rate would be approx. 10-6m/mh and maximum strain 0.02. For other boiler tubes, i.e. water tubes, the material used is generally plain low carbon steel since their operating temperature will be the saturation temperature corresponding to the boiler pressure. Uncooled superheater element supports and baffles must have resistance to creep and corrosion. Alloys of nickel and chrome or steels containing high proportions of these elements are suitable. For steam and water drum, welding of preformed plating is the most usual method. Low-pressure boilers have the steam drum plating uniform in thickness, with a single longitudinal welded seam. High-pressure boilers may use two plates, tube plate and wrapper plate, with two longitudinal welded seams. The tube plate is thicker than the wrapper plate and it is machined to the thickness of the wrapper plate in the region of the weld. Refer to figure on page 7: Test pieces made of the same material as the drum would be clamped to the drum and, using a machine welding process of the protected are type, the weld metal would be continuously deposited on to drum and test pieces. When the longitudinal seam or seams are completed, the test piece is then removed and the preformed drum ends would be welded into position. At this point, the welded seams, longitudinal and circumferential, are radio graphed. The shadow pictures obtained will show up any defects such as porosity, slag inclusions and cracks, etc, these defects would then be made good by chiseling or grinding out and then welding. Openings for boiler mountings, etc, would then be made and all the necessary fittings would be welded into position, i.e. branches, casing flanges, feet, etc. When all welding to the drum has been completed and radio graphed the drum and test piece would then be annealed by heating slowly in a furnace up to about 600*C and then allowing it to cool down slowly. The test piece is then cut up as shown in Fig. 1 and tested according to Class 1 welding regulations. These regulations apply only to boilers whose working pressure is in excess of 4.5bar and consist of:

a) 1: Tensile test of the weld metal to cheek upon its strength and ductility. b) 2: Tensile test of parent and weld metal to check upon joint strength. c) 3 & 4: lzod tests to determine the materials notch brittleness and ability to withstand impact. d) 5 & 6: Bend tests to cheek ductility and soundness of material.

Tests 1 to 6 are well known and understood by engineers hence detailed descriptions are not warranted, however, macro and micro-examination require elucidation. Electro slag welding of uniform thickness preformed plate to produce drums for high-pressure boilers is being used. The drum is arranged vertically and the welding machine moves up a beam parallel to the seam. Figure on page 7 shows simply a cross section through the seam and water-cooled copper guide shoes. Main advantages of this welding process are:

1. One weld run, this reduces possibility of inclusions. 2. Up to 200mm. plate thickness can be welded in one run. 3. Sound, reliable weld is produced.

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Boiler Drum Construction:

Longitudinal Seam Of A Water Tube Boiler Drum:

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Macro-Examination: Preparation of test piece: This would consist of grinding and polishing until scratch free when viewed with the naked eye, then washing it in alcohol and then water to remove grit and grease, etc. Next the test piece would be etched with an acid solution to remove the thin layer of amorphous (i.e. structure less) metal, which will have been burnished over its surface. Examination of the prepared test piece with the aid of a hand-magnifying lens (X10) may reveal cracks, porosity, weld structure and heating effects. Micro-Examination: Preparation of the test piece would be similar to that described above but the polishing process would be continued until the surface was scratch free when viewed under a microscope. After etching, the test piece would be examined under the microscope for defects. The pearlitic structure will be seen and so will any martensitic and troostitic structures, the latter two giving indication of hardening of the metal. Different etching agents can be used, a. typical one being NITAL which consists of 2 ml of Nitric acid and 98 ml of alcohol (methylated spirits). Tube holes would now be drilled into the drum and the tubes fitted. Tubes: These are arranged to form the furnace walls, etc, and to give positive circulation. Circulation is created by a force set up by the gravity head caused by differences in water density between tubes. This is affected by heat input, friction and head losses due to sudden contraction and enlargement and inertia loss. Steam bubbles generated in tubes have lower density than the water and this gives natural circulation. However, difference in density between the steam and water decreases as pressure increases and there is no difference at critical pressure (220 bar). This causes problems for the boiler designer. Water wall tubes frequently form the rear and side walls of the furnace and they may be fed with water from floor tubes which are supplied from the water drum, alternatively unheated large bore down-comer tubes external to the furnace may be used to supply the water wall tubes via their lower headers, no floor tubes being required. Often, water wall tubes have studs resistance welded to them in order to serve as retainers for plastic refractory. The advantages of water wall tubes are:

1. Cooler furnace walls. 2. Reduced boiler size, since more heat is extracted per unit furnace volume. 3. Saving in refractory, initially, and because of the cooling effect, there will be less maintenance

required. Generating tubes are situated in the path of the furnace gases and are arranged to obtain as much of the radiant heat as possible in addition to baffling gas flow. Return tubes, for water wall feeding and water drum feeding may be situated in the gas path in a lower temperature region or external to the furnace. Superheater tubes may be situated in between generating and return tubes, in this way they create the necessary temperature difference to produce positive circulation in generating and return tubes, or they may be arranged in the gas uptakes after the generating section. The headers to which the superheater tubes are connected would be supply and return headers. Steam from the drum passes to the supply header the steam then passes through the superheater tubes to the return header and thence to the turbo generator or main engine (steam propulsion). Superheating methods are discussed later. Finally we have economiser tubes and air preheater tubes which are arranged in the flue gas uptake. In the case of the economiser tubes they may or may not be fitted with shrunk on cast iron gilled sleeves, the purpose of which, in addition to giving extra heating surface, is to protect the steel economiser tube from corrosion. The choice depends upon operating metal surface temperature. Operating metal surface temperature depends upon, tube thickness; feed temperature, feed flow rate, gas flow rate and temperature.

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Generally if feed water inlet temperature, to the economiser is 140*C or below, the sleeves will be fitted (Sulphuric acid vapour dew point is approximately 150*C or above, it depends upon various factors. the main one being fuel sulphur content). If the feed inlet temperature is above 140*C the economiser tubes may be of the extruded fin type. Air preheater tubes made of plain low carbon steel generally have air passing through them and the gases around them, although reverse arrangement has been used. These tubes are usually situated in the last gas-path hence they operate at the lowest temperature and are more likely to be attacked by corrosion products from the gases. Various methods have been used to reduce the effect of corrosion of these tubes, inserts have been provided in order to reduce heat transfer and corrosion over the first 0.3m or so of the tubes, by-passes for the air used during maneuvering to keep tube surface temperature high. Recently it has been suggested that glass tubes be used in place of the steel ones. Vitreous enamel has also been used as a coating for these tubes unfortunately where it cracked off severe corrosion resulted. Superheater Header and Tubes:

The above figure shows various methods of securing superheater tubes:

a) Used if access to both sides of the weld is possible. b) Welding from one side only. c) This shows a five tube element welded to two 90* stubs on each heater. d) Tubes expanded into header. Access doors must be provided; this method is not used at

temperatures above 470*C.

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Headers: For superheaters and water walls these are usually solid forged square or round section tubes with nozzles and ends welded on. Doors or plugs, opposite tube holes, may be provided to allow access for inspection, expanding or plugging of tubes. The main methods used for securing tubes to headers are: 1. Expanding. 2. Expanding followed by seal welding. 3. Welding to nozzles or stubs. Since welding on site can prove difficult (superheater elements may have to be renewed during the boiler's working life) it is usual to arrange for the number of site welds to be kept to a minimum with best possible access. Casings: Once the boiler is tubed, it becomes a self-supporting unit and around it the double casing, of welded construction, is fitted. The double casing, which conducts the combustion air or forced draught to the boiler fronts in addition to reducing radiation losses, also prevents gas and soot leakages into the boiler or engine room. The casings are provided with detachable panels at strategic points for access to water-wall headers, superheaters, etc., and are often required to support the weight of the economizers and airheaters above the boiler. Provisions are made in the casings for essential fittings such as soot blowers, inspection windows and smoke-detecting gear. Water drums are normally provided with sliding feet, which while taking care of drum expansion also transfer most of the weight of the boiler to the tank top. In some designs, however, the boiler proper is cradled into the casing, which in its turn again transfers the weight to the tank top. In both cases, flexible connections are embodied between the boiler and the easing to avoid high stresses and gas leakages occurring through differential expansion. Once the boiler is tubed and work is proceeding on the casings, attention can be concentrated on the superheater. The procedure for superheater construction varies considerably as there are many different designs and they are placed indifferent positions in the boiler. Superheater: In the straightforward D-type boiler, the superheater usually consists of a pair of forged-steel rectangular-section header with welded-in ends placed parallel to one another in a more or less vertical position either at the front or back of the boiler and supported from the boiler casing, The actual superheater tubes are normally of hairpin or multi-loop type which project into the gas path and terminate at their ends in the aforementioned headers. Present-day practice is to avoid expanded joints in superheaters, the tube ends being butt welded to stubs embodied in the headers during their welded fabrication. It is possible in some designs to install and withdraw the complete superheater. This makes for easier assembly as the superheater can be built up as a separate unit, which can be tested prior to installation in the boiler. Once installed, and with the headers secured, the question of supporting the weight of the longitudinal tube nest of the superheater has to be attended to. Superheater supports vary in type with different boiler designs, but in the normal ‘D’-type boiler they usually consist of heat-resisting-steel spectacle plates through which the superheater tubes pass, the plates being clamped on to special support tubes running between the steam and water drums. The support tubes act as heat sinks for the spectacle plates, thus prolonging their useful 1ife. It is important that the correct superheater-tube material is used for the steam temperatures involved.

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Whereas mild steel is used up to 400*C, once this figure is exceeded, and dependent on the actual temperatures involved, special steels are required which have a high resistance to creep; 0.5 % molybdenum and 1.0 % chrome steels are commonly used. Trouble is often experienced in service with superheater supports and as they operate at very high metal temperatures, it is extremely important that the materials used are suitable. A typical spectacle-plate arrangement with its water-cooled support tubes is shown in the figure below.

Water-cooled Superheater Tube-Support:

Economizers: A typical economizer consists of a bank of mild-steel tubes with cast-iron extended surface or gills shrunk-on, over which the flue gases pass and through which the feed-water passes.

Extended Surface Economiser:

Above figure shows a part element of a Foster Wheeler Green economizer; in which117.5 O.D. cast-iron gills are shrunk on to 50.8 O.D. mild-steel tubes.

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These Tubes are supplied as U-bend elements, which are welded to stub tubes on the inlet and outlet headers and are interconnected by welded-on bends to form the required number of feed-water paths. The stub tubes are an integral part of each header and hand holes are located opposite each tube end to allow internal inspection and emergency plugging of a water path, should an element fail in service. As will be seen from Fig. 4, the elements are carried in mild-steel support plates, being secured by locking rings at one end and free to expand through sealing rigs at the other. The economizer casings are fully insulated and doors are fitted at each end for access to the headers and return bends. It is usual to buildup the economizer separately from the boilers and to install it-in one piece after satisfactory hydraulic test. After the economizer and superheater have been assembled into the unit and all integral piping and mountings fitted, the completed boiler is hydraulically tested to 1.5 times the design pressure, which it has to withstand without any signs of weakness or defect. A satisfactory hydraulic test having been witnessed any casings left off for access, are completed and the boiler prepared for lifting into the vessel, where brickwork and insulating materials are then fitted. Particular care being taken to ensure that any parts of the steam and water drums which could be exposed to the radiant heat of the furnace, are efficiently protected by refractory material.

Construction of Smoke Tube Boiler: In ships using diesel engines as main propulsion, the auxiliary boilers fitted onboard are normally used for air conditioning and heating purposes such as fuel oil heating, fuel tanks heating, lubricating oil sump tank heating, purifiers heater, domestic hot water system etc. Steam turbo generators are commonly found on large container ships and tankers. Steam turbine driven cargo pumps and boiler feed pumps are also commonly used on board tankers. Fire-tube Boiler or Smoke-tube Boiler: Scotch Boiler: A number of Scotch boilers are still in use today as main and auxiliary units and few if any are being manufactured. Only a brief description will be given. See figure given on page 13. Construction: Three types of constructions are being used:

1. All riveted. 2. Riveted and Welded. 3. All welded.

Majority of modern Scotch boilers are mainly the all welded. Riveted type of construction is obsolete. Material: Plain low carbon open hearth steel of good quality having an ultimate tensile strength between 430MN/m2 to 54OMN/rn2 is used. Steel produced by the "Kaldo"or"L.D.” or any other oxygen process would be acceptable. Most steel producing plants are phasing out there ‘Bessemer’ converters and open hearth furnaces, but some open hearth furnaces have been converted to oxygen blast. All flanged plating and stays, etc., which have hail their ends upset must be heated to 600*C and then allowed to cool slowly in order to stress relieve.

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Details of Scotch Boiler: Combustion Chamber: The figure on page 14 shows an all welded construction of a Scotch boiler combustion chamber. As the name implies, these are chambers in which combustion, apart from that which has already occurred in the furnaces, takes place. These chambers, surrounded by the water content of the boiler, are in addition heating surfaces, and it is from them that the products of combustion return to the uptakes via the plain and stay tubes. The combustion chamber, being at all times under compression, lends itself admirably to that modern method of fabrication, electric welding.

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Welding Details of Chamber Tube & Back-plate Edges:

The combustion chambers are made up mainly of flat thin plating and hence have to be given support by means of stays, girders and tubes. Boiler tubes, in addition to carrying gases from combustion chamber to boiler uptake, support the boiler front tube plate and the combustion chamber front plate. Stays and, stay tubes give support to the boiler back and front plating between the combustion chambers in addition to giving support to the combustion chamber plating.

All-welded chamber with square corners and no flanged plates:

Combustion chamber girders, which support the top of the combustion chamber, may be built up or welded types as shown in the above figures. Combustion chamber bottom plating requires no support by means of stays, etc., since it is curved and is hence the ideal shape for withstanding pressure. See figure below.

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Scotch Boiler Section:

Furnaces:

Various Types of Furnace Corrugation:

Corrugated for additional strength and longitudinal flexibility (See figure above) this also gives increased heating surface area. Not to be thicker than 22 mm. Excess material thickness would give poor heat transfer and could possibly result in the excess material being burnt off.

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Most Scotch boiler furnaces are suspension bulb type corrugation, since, for a given working pressure and furnace diameter, their thickness would be less than for other corrugation types. This gives better heat transfer and efficiency. The number of furnaces in each boiler is usually dependent upon the boiler diameter. For those up to 2.66 meters diameter, two furnaces are usual; from 2.66 to 3.55 meters diameter, three are used, while in boilers over 3.55 meters in diameter there are usually four furnaces. Furnaces must be arranged and fitted so as to ensure that furnace renewal can be carried out with rninimum possible inconvenience (furnaces are always made withdraw able). In the case of furnaces riveted to the combustion chamber, this is accomplished by terminating the inner end of the furnace in a neck and flange (gooseneck), the flange being of such a shape as to be withdraw-able through the front end-plate opening. In welded construction the furnace is butt welded direct to the flange opening in the combustion chamber front or tube plate. The two methods are illustrated in the figure below.

Riveted Type: Welded Type:

Methods of Making Furnace Withdrawable: Boiler End Plating: The boiler end plating, i.e. frost and back, is supported by means of the combustion chamber stays and tubes. In addition, main stays about 67 mm diameter are provided. These are in two groups of three between the furnaces and in two or three rows with about 400 mm pitch at the top of the boiler. The front and back end plates may be built up from several plates of varying thickness with lap-riveted or welded cross-seams. Both front and back end plates are flanged to fit inside the shell in the case of riveted boilers, and to butt against the shell for welding in the case of welded boilers. It should be observed here that the plate thickness required depend on the working pressure, amount of support given by stays and flanging, etc., and so it is quite usual to find end-plates built up from two or three different thickness of plate, each plate thickness meeting the requirements of its individual loading. In riveted boilers, the end plates are usually riveted into the shell with the flanging outside of the circumferential seam, although in some cases when hydraulic riveting was utilized for the closing seam, the end plate was flanged inwards.

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The boiler end plates are usually in two parts, the top or steam space portion being somewhat thicker than the lower portion. It is present-day practice, after the plate edges have been prepared by planing, to butt-weld this seam and on completion of welding to flange the complete end. The flanging is effected in the hydraulic flanging press, working around the plate at about 1.83 meters per heat, each portion of the flange being completed before proceeding to the next. On completion of the flanging the end plates are annealed and levelled. The flange edges are then dressed and in the case of the front end plates, locating holes for tubes and furnace openings drilled, after which furnace openings are machined out and manhole openings are plunged. The end plates are then marked off, and tube holes and combustion chamber back-stay holes drilled in the front and back plates respectively. Boiler Shell Plating: Having concentrated on the production of the combustion chambers and the two end plates, attention is then transferred to the shell. When several similar boilers are being constructed at one time it is advantageous that all the chambers and end plates proceed simultaneously, so that on completion of these components a certain amount of “pairing up” can be resorted to (a difference in circumference of more than 12.7 mm between front and back end-plates is undesirable). The shell plates are then marked off to suit the end-plate circumference their plate edges planed to give the required weld preparation, combustion chamber side-stay holes drilled and manhole opening machined out. The plates are then rolled, and as the rolls do not impart correct curvature to the longitudinal edges, these have to be separately faired. The shell plates are then assembled and with their longitudinal edges suitably strapped together and weld test plates tacked in position, the longitudinal seams, together with test plates, are butt welded, either by hand or machine, using a balanced welding technique (part of the seam welded from outside and part inside to avoid distortion from weld contraction). The main shell plating is usually in two parts, which are joined together by means of double strap butt joints of special design. Since the shell plating is circular it requires no stays for support against pressure. Manhole openings are usually made in the bottom of the boiler back plate and in the top of the shell plating. These openings have to be compensated to restore the plate strength and the opening in the shell plating must have its minor axis parallel to the boiler axis, the reason for this is that the circumferential (or hoop) stress is twice as large as the longitudinal stress. Boiler Tubes: The front tube plate and combustion-chamber tube plate are tied together by means of stay tubes. The stay tubes screwed through both tube plates vary in thickness according to the area of plate which they have to support, and there may be several thickness of tube in one tube-nest, the minimum thickness allowed for such tubes, measured at the base of the threads, being 6.4mm for marginal tubes and 4.8 mm for others. The normal pitch for the threads of these tubes is 2.82 mm, the smoke-box end of the tubes being enlarged and the thread being continuous, so that when inserted they can be screwed simultaneously through both tube plates. In some cases the ends of the stay tubes are attached to the tube plates by welding, the weld execution being as shown in figures below.

Welded Stay Tubes: Plain Tube Seal Weld:

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This method ensures pressure tightness under working conditions, but presents difficulties when renewals are called for. The material used for stay tubes is normally wrought iron or steel. Plain tubes form the major part of the heating surface of the Scotch boiler, and are generally made of either lap-welded wrought-iron or steel, seamless steel or electric-resistance-welded steel. The inner tubes in a nest are very inaccessible on the waterside, and unless considerable care and attention is paid to their cleanliness, on both this and the fireside, their heating-surface value rapidly decreases. The normal size of plain tubes vary from about 63.5 mm to 88.9 mm outside diameter with thickness of 3 mm to 4.5 mm, and they are made tight in the tube plates by means of expanding, supplemented in some cases by seal welding.

Attachment of Combustion Chamber Stays: Attachment of Longitudinal Stays: Showing Two Methods ‘A’ & ‘B’: Stays: In general, Scotch boilers are fitted with stays of the screwed type, although for the past twenty years plain bar stays, welded at both ends, have gradually been taking their place. Details of the method of attachments of welded combustion chamber stays and longitudinal stays are shown in above figure. Smoke Tube Boilers used in Steam Vessels: The earliest marine boilers used for main propulsion had a working pressure of approximately 0.7 bar and were chiefly have the rectangular or haystack type. As technology advanced, the compound, triple and quadruple expansion steam engines were introduced which called for gradually increasing scantlings and pressures until the practical limit of pressure was reached for the scotch type of boiler i.e. approximately 21bar. To exceed this pressure in a normal sized scotch boiler would require a construction so heavy that it would not be a commercial proposition. The ordinary scotch boiler with return tubes was introduced in the 1880's and has basically remained altered since it is robust, easy to operate and requires the minimum of maintenance. ‘Howden-Johnson’ and also ‘Capus’ boilers have introduced modifications to the basic design, each with its own claimed advantages. By the late 1930's many crack turbine passenger liners were steamed by Scotch boilers of the double-ended type. However the quest for increased overall efficiency and higher speeds has led to the adoption of water tube boilers where propulsion is by turbine, and the majority of such turbines operate around 60 bar and 500*C, working pressure increased from 5 bar (1880) to 15 bar (modern). Modifications mentioned included water drum, steam drum and circulating tubes also superheaters.

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Assembly of Scotch Boiler During Construction:

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Scotch Boiler: (See figure on page 13): The Shell: A mild steel plate forming a ring, usually constructed from two plates having two treble riveted double butt strap joints for the older designs, and welded in modern designs. The tensile strength of the plating is normally 430 to 490 MN/m2 although lighter scantlings are permitted when using higher tensile steel. The End Plates: These may be manufactured from a single plate or fabricated from a number of plates of varying thickness. The plate thickness required is determined from the working pressure and the amount of support given by the stays and flanging, etc. The Combustion Chambers: These are surrounded by water in the boiler and are suitably stayed (always under compression). They are situated after the furnace and the products of combustion pass through these chambers before entering the tubes, thereby giving an additional heating surface. The Furnace: The modern scotch boiler furnace is always made of corrugated steel with welded seams, the corrugations giving for a given thickness additional strength and longitudinal flexibility. The number of furnaces in each boiler usually depends upon the boiler diameter i.e. two furnaces for boilers up to 3660 mm diameter, three furnaces for diameters between 366Omm and 4890 mm, and above this it is usual to have four furnaces. In each case the furnaces are made withdrable, this being accomplished by terminating the inner end of the furnace flange in a 'goose neck' when riveting is used. Welded construction boilers have a butt joint. The more common types of corrugation are Leeds suspension, Deighton, Morrison, Fox and Purves. Boiler Tubes: The front tube plate and combustion chamber plate are tied together by means of stay tubes, which are normally made from wrought iron or steel. Plain tubes, which form the greater part of the heating surface, are made from the same material as stay tubes. The tubes vary from about 63 to 89 mm outside diameter and approximately 3 to 4.5 mm thickness of wall, are expanded into the tube plates, and often welded to give a good seal. Stays: Screwed type stays form the majority of stays used for the chamber tops, sides and backs. Materials used are iron or steel for combustion-chamber stays and steel for longitudinal stays. The more modern boilers however are fitted with welded stays. SCOTCH BOILER MOUNTINGS: On page numbers 21 and 22 Scotch Boiler mounting figures are shown.

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Scotch Boiler mountings:

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Waste steam outlet.

Line Diagram of Scotch Boiler Mountings:

The points of importance relating to boiler mountings are:

1. Water level gauges: The water cock orifice for water gauge glass fitting is at least 5 inches above the top of the combustion chambers. Two gauge glasses are fitted to determine the water level.

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2. Feed cheek valve: This valve internal feed pipe is closed at the end and perforated throughout its

length in order to distribute the relatively cool feed water over a large space, thus avoiding local over cooling of hot boilerplates.

3. Blow down valve: The blow down valve at the bottom of the boiler is for empting the boiler. This is also used to remove sediments at the bottom of the boiler. This valve is either a non- return valve or a cock, usually the former to prevent back flow in to the boiler of cold water if the boiler is blown down at sea.

4. Scum valve: This valve (cock) with the pipe is provided to remove any scum floating on the water level.

5. Stop valve: Main steam out let is usually fitted with a dry pipe to reduce moisture carry over in the main steam supply.

6. Whistle steam valve: Steam from this valve goes direct to the whistle from the boiler. If steam Steering gear is fitted in the ship, then one steam line has to be provided from boiler to steering gear direct.

7. Safety valves: These are generally of the improved high lift type. These are provided to relieve excess boiler pressure. They are fitted with easing gear, which could be used in emergency. Drain lines are provided with out any cock or valve to drain all condensed water, or accumulated water, in the safety valve chest. This drain should be checked at all times and made sure that they are clear.

MANHOLE AND DOOR: Manholes are provided for access in to the boiler for cleaning and inspection. One manhole in the shell near the top of the boiler and two in the front end near the bottom between and below the furnace are provided. Manholes are elliptical in shape (Usual size is 400 mm. major axis and 300mm. minor axis).

Howden-Johnson Boiler:

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Capus Boiler:

Howden Johnson and Capus Boilers: (See figures above): These boilers are basically of the tank type and in each the furnaces pass from end plate to end plate, the combustion chambers containing circulating tubes bounded by brickwork. This arrangement gives a 'dry-back' combustion chamber, which simplifies construction and also allows higher pressures to be obtained when compared with Scotch Boilers. Generally both types may be said to have certain advantages, which are common to water tube and scotch boiler, particularly where the water tubes assist to produce rapid and effective circulation, but since the tubes in the combustion chamber are of small bore and of bent forms then boiler feed water treatment is more critical. The combustion chamber also houses the superheaters. Cochran Boiler: Cross sectional sketch of Cochran boiler is shown in figures on pages 25 and 26. It is a vertical cylinder fire tube type with hemispherical top, constructed of steel plates welded together the furnace being one-piece seamless hemispherical dome. A composite boiler is used in motor ships, so that when the diesel engine is working, the heat from the exhaust gas could be passed through this boiler and there by steam could be raised. Some times this boiler is also called waste beat boiler. The amount of heat extracted from the exhaust gas boiler will depend on the quantity and temperature of the exhaust gas. These boilers are fitted with burners to generate steam, when the exhaust gas temperature falls due to slow running of diesel or when the diesel is stopped.

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Exploded View of a Cochran Vertical type Boiler:

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Cochran Boiler:

Cochran Composite Boiler:

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An auxiliary boiler is some times called a DONKEY BOILER: An auxiliary boiler is used in Port to supply steam to auxiliaries. At this time the main boiler may be shut down. In motor ships this boiler is used to supply steam both in port and at sea. The working pressure is usually in the region of 7 to 10bar; and may be a small size scotch boiler or of special design of which there are many types. Cross sectional sketch of Cochran boiler in shown above. It is a vertical cylinder fire tube type with hemispherical top constructed of steel plates riveted or welded together, the furnace being a one-piece seamless hemispherical dome. A Composite boiler is used in motor ships, so that when the diesel engine is working, the heat from the exhaust gas could be passed through this boiler and there by steam could be raised. Some times this boiler is called waste heat boiler. The amount of heat extracted from the exhaust gas boiler will depend on the quantity and temperature of the exhaust gas. It is some times possible to extract 40% of the heat in the exhaust gas. Those boilers are fitted with burners to generate steam when the exhaust gas temperature falls due to slow running of the diesel or when the diesel is stopped. COCHRAN BOILER MOUNTINGS:

1. Pressure Gauge and cocks: Shows the steam pressure, normally has two red lines marked on the dial. Lower line shows normal working steam pressure, the upper the maximum permissible pressure of the boiler (A 3-way cock on the gauge and another cock on the boiler, connected with a syphon pipe is a standard practice). 2. Water level gauges: These are situated at either side of the smoke box, complete with steam, water and drain cocks. 3. Stop valve: This is the steam outlet to the process, normally fitted at the highest point of the boiler, and of an ordinary screw-down type valve. 4. Blow-down valve: This enables feed water sediments at the bottom of the boiler to be discharged. 5. Feed Cheek valve: This valve consists of one screw-down isolating and another non-return anti-suction valve is fitted either in one common or separate valve housing. 6. Safety valves: Lifts at a pressure above the maximum working pressure and is pre-set and locked to prevent unauthorised interference. Easing gear is provided for test purposes and also in case of emergency. 7. Salinometer cock: Used for drawing boiler water sample for testing.

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Note: 1. All boiler mountings will be explained latter in these notes. 2. Composite boilers will be taken after explaining water tube boilers.

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MARINE WATER TUBE BOILERS:

The Water Tube Boilers mainly for the following reasons replaced the Scotch Boiler: 1. Saving in weight: Water tube installations give a relative saving of 1: 3 when compared with

Scotch boilers on a basis of equivalent heating surface with the water at working level. 2. Higher pressures and temperatures: The advent of steam turbine propelling machinery called

for higher pressures and temperatures with the advantage of higher efficiencies, which enabled machinery size and weight to be reduced for a given output.

3. Greater Mechanical Flexibility: The Scotch boiler has poor circulation, particularly when raising steam, and is therefore prone to mechanical straining, however, the water tube boiler has rapid circulation and with its greater structural flexibility, this defect is avoided.

4. Rapid steam rising: Under normal circumstances the time recommended to raise steam to full working pressure from cold is three to four hours, but this may be reduced, when a hot boiler is used, to twenty minutes.

5. Saving in space: The good circulation, ability to withstand forcing and the higher pressures obtainable have led to a much smaller dimensional boiler when compared to the Scotch type.

6. Wider safety margin in event of explosion: Water tube boiler drums are protected from flame impingement or direct radiation and the small tube bore, in the case of rupture, gives a relative slow escape rate. An overheated furnace in a Scotch type boiler, when ruptured can empty the boiler contents almost instantaneously. In all these types the water circulation is natural, correct design ensures that the water speeds are adequate and each tube has sufficient circulation to prevent steam locks which in turn could cause overheating and tube failure. The direction of circulation in the tubes of a vertical tube water tube boiler is largely dependent on the external conditions. In a water tube boiler the tubes contain a mixture of steam and water, and due to the furnace fluctuations, particularly when maneuvering, a particular tube may function as a down-comer one minute and a riser the next. Since circulation depends upon specific gravity, the relative speed of circulation will, in a bank of tubes, be greatest in the front and rear rows, as between these rows exists the greater difference in specific weights. Efficient circulation is more easily obtained in a low then a high pressure water- tube boiler, as increase in pressure and temperature involves a leveling out in the difference in specific weights of steam and water, hence at higher pressure say above 30 bar, it is usual to assist circulation by fitting unheated external down-comers.

BABCOCK BOILERS: (Figure on page 29): The Babcock and Wilcox header-type boiler is essentially a robust and accessible unit. Early designs had a working pressure of approximately 12 bar at 221*C, 100 mm. tubes and three-passes, while later designs had a working pressure of 30 bar at 399*C, 21 mm. tubes and a single pass. Smaller tubes were possible due to the improvement in feed-water quality, and these are situated farthest from the furnace, where, space for space, they give a greater area of heating surface. It is normal to have a 15o rise between the front and back headers and the amount of heating surface per section is made greater or less by variation in the number of tubes, height of section or in the length of the tubes. The boiler pressure parts are made of steel, the headers being solid drawn and forged into a sinuous form, which gives a staggered configuration. All tubes are expanded and bell-mouthed in the normal manner.

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Early Babcock Header-Type Boilers:

A U-bend type of superheater is fitted which lies at right angles to the generating tubes of the boiler, and the headers are placed along one side. Internally welded-in, division plates give a number of steam passes. The front headers are anchored, while the back headers of these boilers are free to move on the rear structure, which allows for tube expansion when raising steam and contraction when shutting down. The air supply, after passing a preheater is lead through a double casing surrounding the sides and bottom of the furnace, hence to the boiler front. This gives a low radiation loss and insulates the refractory-lined furnace. To assist the separation of steam and water after leaving the return pipes, a longitudinal baffle is fitted in the steam drum, which allows the steam to rise to the internal steam collector in the drum, the water dropping. Wash-plates are fitted in the steam drum to prevent excessive movement of water such as encountered in heavy weather. Where units are of the high superheat type it is usual to have a coil type de-superheater located in the steam drums to de-superheat a quantity of steam for use in the saturated steam services, and also used in connection with an automatic superheat temperature-control system. This type of boiler may vary with the designed position of the individual units depending on the purpose of the boiler, but the basic arrangement varies only the position the economizers, air preheaters, superheaters and water-walls etc. Eventually steam conditions were raised to 41 bar and 454*C. Babcock and Wilcox Selective Superheat Boiler:(Figure on page 30): This is a two drum, bent-tube type boiler having an integral furnace at one side formed by an extended screen of tubes, which are part of the main circulating system.

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Cut-away view of Babcock Marine Boiler: Selective Super-heater Type:

Showing a single furnace and two set of dampers for adjusting: The gas flow is in two passes, achieved by a division wall, and the total gas flow is regulated by temperature control dampers, each pass having one set, which enables the desired steam temperature to be selected by adjustment of the dampers. The super- heat is in the second pass division and since this constitutes a low temperature zone, the tubes do not require to be manufactured from special heat-resisting steel. After the dampers, the combined gas flow passes through an economiser and air heater before entering the funnel. In this design, the main bank baffle is formed by chrome-ore on studded tubes 50.8 mm O/D and this requires very little maintenance. The superheater is arranged to give several steam passes, the steam outlet is situated in the cooler part of the gas stream near the rear of the boiler. This gives high steam velocities with the highest steam temperature in these tubes, which are comparatively low temperature gas zones, hence the metal temperatures are reduced and vanadium attack is lessened. Since the superheater elements are concentrated in one pass they are of necessity made into double instead of single U-tubes. The overall arrangement gives a wide range of temperature control than can be obtained with an attemperator and is simple to control. Generally under normal steaming conditions, the full range of control is unnecessary, but must be utilized when steaming at reduced temperature and pressure conditions. Also the low temperatures obtainable are useful when warming up and manoeuvring. A double casing of welded construction encloses the boiler, which prevents outward leakage of gas and soot while radiation losses are kept low.

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Babcock & Wilcox Integral Furnace Boiler:

Cut-away view of Babcock Marine Boiler – Integral Furnace Type:

A two-drum type boiler with the furnace at one side, formed by an extended screen of tubes, which are part of the main circulation system. Operating conditions may be up to 69 bar and 510*C with a capacity of 81500 kg/h. It should be noted that the temperature limit is dependent upon the material used for the superheater. The gases pass through a screen of two, three, four or five staggered rows of 50.8 mm O/D tubes before entering the superheater. Behind the superheater is the main bank of tubes which consists of about twenty staggered rows of 31.8mm O/D tubes as closely packed as practicable to ensure high gas speeds and heat transmission rates. Some of the screen tubes are used to form baffles which prevent hot gasses bypassing the superheater, impinging on drums, superheater headers, superheater supports and boiler casings. These baffles are formed with studded tubes. Where high steam temperatures are encountered the tubes, in addition to the usual expanding, are also seal-welded inside the headers. The superheat temperature is controllable within certain limits by fitting an attemperator and a de-superheater can be sited for the auxiliary services. The Integral Furnace and Selectable Superheat Boilers are similar, their main difference being in the method of superheat control. Drums of each boiler are of fusion, welded construction, and fitted internally with cyclone steam separators.

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Large unheated down-comers, fitted between top and bottom drums and to all water-wall headers are located within the double casing of the boiler ensure positive circulation. Babcock & Wilcox Radiant Boiler: The increase in steam temperature, feed temperatures, boiler efficiency, and the reduction in the quantity of excess air for combustion, which have occurred in recent years, all reduce the amount of boiler heating surface required. This reduction in the amount of convection heating surface required has caused boiler designers to study the radiant type boiler similar to that used for large stationary units in which the boiler bank is replaced by economizer surface. Such a design was installed in a 167,000 tonne tanker having machinery of 22,370 kw., the superheat outlet condition being 62 bar and 513*C. The burners are located in the roof of the furnace and the furnace is separated from the convection heating surfaces by a division wall of membrane construction. The gases leave the furnace through an opening at the lower end of the division wall and then pass upwards over multi-loop superheaters and an economizer to the air heater. Except where openings are provided to permit the superheater and economizer tubes to be welded to the headers, the walls of the furnace and boiler are entirely of membrane construction. For this particular application the design proved cheaper and occupied less floor space than the equivalent two-drum boiler. From combustion point of view the design is superior since a greater residence time is achieved than is possible in a conventional two-drum boiler. If higher pressures are required the radiant boiler has advantages since all tubes can be welded to the drum or headers and expanded joints are thus eliminated. The radiant boiler is also readily adapted for a reheat plant since it is relatively simple to arrange for additional rows of superheater tubes and to accommodate extra access spaces and soot blowers, the superheater design is not connected so intimately to the furnace design as in the two-drum boiler. This design of boiler is not suitable for use with a non-unidirectional turbine since complicated or inefficient arrangements are needed to control the flow through the re-heater, however, a further design is available in which the superheater outlet pressure has been raised to 100 bar, the steam temperature remaining at 513*C for both reheater and superheater. In this unit the gas passage is separated into two parallel paths by a membrane wall. One path contains the reheater, while the other has economizer surface. The gas flow over the reheater, and hence the reheat steam temperature is controlled by dampers above the reheater and economizer sections. A superheater is arranged in the reheater path and further superheater surface is provided in the path containing the economizer. When running astern, the dampers on the reheater side are closed to restrict the gas flow over the reheater and there is sufficient superheater surface below the reheater to ensure that, under these conditions, the gas passing to the reheater has been cooled to below the safe metal temperature of the reheat tubes, even when the maximum quantity of oil is fired. The superheater outlet steam temperature is controlled by an inter-pass attemperator. Babcock MR type: The MR boiler was introduced in response to marine industry demands for boilers to exhibit the highest possible efficiency and the lowest possible maintenance. It is a single drum radiant boiler of all welded construction in which all exposed refractory and all expanded joints and gaskets were eliminated. The membrane tube panel enclosure walls, in which adjacent 63 mm outer diameter tubes were joined by welding in a 12 mm wide mild steel strip, provided water cooling and gas tightness. The large fully water cooled furnace had a roof sloping at 5 deg to the horizontal, enabling the oil burners to be attached normal to the roof and yet fire down the long vertical axis of the furnace.

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These units were, with the use of the steam atomizing burners, able to achieve complete combustion within the furnace with as little as 3% excess of air, and an efficiency in excess of 90.7% on the gross calorific value was recorded when the units were fitted with rotary regenerative air heaters reducing the temperature of the funnel gases to 116*C. In the convection passage the widely spaced superheater tubes were aligned at right angles to the drum axis so that the products of combustion produced by the row of burners in the furnace roof were evenly distributed across the whole width of the superheater. This encourages effective use of the heating surface and minimises risk of hot spots due to maldistribution, which could adversely affect tube temperature. The lowest possible superheater tube temperature was further encouraged by arranging the primary superheater, containing the cooler steam, below the secondary, both being connected so that the steam progresses upward in parallel flow with the products of combustion. Inter-stage attemperation and control of superheat is achieved by a steam-boiler water heat exchanger in the drum. This single drum radiant boiler has a drum diameter of at least 1.5 m and it was possible to accommodate, in addition to the attemperator, a desuperheater to provide steam for auxiliary purposes.

Babcock MR Type Boiler:

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Sectional Sketch of Babcock MR Type Boiler:

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Sectional Sketch of Babcock MR Type Boiler: Path of Feed Water and Steam from drum Shown:

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Babcock M12 type Boiler:

Sectional Elevation of Babcock Type M 12 Boiler:

Plan View of Babcock Type M 12 Boiler:

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As already mentioned, preference for two drum boilers was sometimes stated and there were ships that did not provide space, notably headroom, for the radiant boilers then being used extensively in VLCC’s and container ships. It was to meet such situations that Babcock offered the M12, a bi-drum unit with primary and secondary superheaters and a fully water-cooled furnace similar to the Foster Wheeler DSD type. A fully water cooled furnace with membrane wall tube panels or tangent tubes backed with refractory and steel casings could be chosen, and the burners could be mounted on the roof or in the furnace front wall. The double superheater was arranged with the primary upstream of the secondary in the furnace exit gas stream, each being arranged with multiple steam passes with the hottest pass in parallel flow. Ample gas side access spaces were provided and steam temperature control was achieved by inter-stage attemperation. Babcock M21 type Boiler:

Sectional View of Babcock M21 Type bi-drum Boiler:

In order to simplify construction and to introduce a degree of standardisation, the M21 type, a bi-drum unit giving a choice of features such as, replaced the M12:

1. Single superheater. 2. Double superheater. 3. Tangent tubes, double casings. 4. Membrane tube panel enclosures. 5. Roof mounted burners. 6. Front wall mounted burners.

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The above could be combined ACE, or ACF, or ADE, or ADF, or BCE, or BCF, or BDE, or BDF. Each of these eight alternatives could be met with the same basic layout of the main boiler parts with the same overall dimensions simplifying drawing and ordering requirements. Babcock MRR type Boiler:

Babcock MRR Reheat Boiler:

In 1963 a power plant design study instigated by the ‘Esso’ Petroleum Company produced a set of marine propulsion machinery based upon the reheat cycle and which incorporated many novel features aimed at combining high efficiency and low maintenance. Out of this work came the Babcock MRR reheat boiler from which the straight cycle MR was soon to follow.

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The MRR is similar in construction to the MR except that the convection passage is divided into two parallel paths by a membrane tube wall, the gas flow over which is controlled by dampers located at the top, or gas outlet end of each path.

Babcock MRR Reheat Boiler Arrangement of Convection Chamber:

Primary and secondary superheater surfaces are arranged in each path. The reheater is above them in one path and in the other is an economiser with bare tubes connected so that any steam generated rises upwards with the water flow into the steam drum to be separated in the cyclones. The division wall is completely gas tight and the superheater surfaces are so proportioned that when the reheater path dampers are closed the small gas flow leaking through them is cooled by the superheater to a temperature well below the normal reheater tube operating temperature so that no damage to the reheater can occur when reheat steam is not flowing. In normal, ahead steaming, modulation of the dampers controls reheat steam temperature with- out significant disturbance in main steam temperature, which is, in any case, controlled by an attemperator inside the boiler drum. A significant step was taken during the latter part of the 1970s, when Stal Laval, in cooperation with Babcock, developed a very advanced propulsion system (VAP). Steam was generated at 125bar or higher depending upon the shaft power of the set and at a temperature of 500*C by a standard MR boiler and then rose to 600*C in a separate superheater immersed in an oil fired fluidised bed of graded sand.

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After expanding through the HP turbine the steam was to be reheated to 600*C in a second oil fired fluidised bed built in battery with the first and then returned to the IP and LP turbines. The combustion environment of the fluidised bed was intended to permit the achievement of 600*C or even higher without the problems afflicting conventional superheater and a full scale experimental fluidised superheater operated by Stal-Laval at Orebre in Sweden proved this to be so. The turbine and gearing developments were also demonstrated to the technical press. By the time this was all ready for the market the diesel designers had forged further ahead and the demand for steam ships was in decline so that no VAP plant entered sea service. Combustion Engineering Boilers:

Combustion Engineering Boiler Type: V2M-8:

The V2M-8 is a bi-drum boiler of the integral furnace type with a vertical superheater, with all welded furnace walls or with tangent tubes backed with refractory lined steel casings. Advantages claimed by the manufacturers include: the superheater is positively drained at all times regardless of the attitude of the ship; slag accumulation on the superheater tubes is minimised; and the general layout of the unit is such as to avoid pockets where explosive gas mixtures could accumulate, thereby ensuring effective purging prior to lighting up. Provision can be made for firing in the roof, front or side of the furnace.

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Type: V2M-9:

Combustion Engineering V2M-9 Type Boiler:

As boiler plant in general began to demonstrate improved reliability ship owners showed increased interest in the single main boiler ship philosophy. A single boiler, used in place of two boilers, would require less space, but could still have the same capacity. It could have a very large furnace so as to give a greater residence time affording the opportunity for improved combustion compared to two smaller units. Better access for maintenance would be more easily obtained and initial cost would be reduced. The radiant boilers previously described all exhibited these advantages and Combustion Engineering responded by taking a basic D type boiler and extending the furnace downwards and beneath the unit. This layout necessitated supporting the boiler unit at its mid height so reducing movement of the upper and lower extremities due to thermal expansion. Stability when mounted in the moving platform of a ship at sea was also improved. A double superheater and welded furnace walls were employed and the firing platform was beneath the lower boiler drum. A modification employed a tangential firing system, with burners mounted in each of the four corners aligned tangential to a circle at the furnace center. This gave increased turbulence and a longer spiral flame path before the products of combustion impinged upon relatively cool boiler and superheater tubes.

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Combustion Engineering V2M-9 Type later version with Tangential Firing Arrangement:

Type: V2M-8-LTG Reheat Boiler: The boiler and superheater are as for the V2M-8 but an additional furnace chamber is added on the side of the boiler-generating bank remote from the main furnace and superheater. This additional reheat furnace is provided with oil burners and the horizontal tube reheater is arranged above its outlet. In normal ahead mode products of combustion, from oil burned in the main furnace in sufficient quantity to achieve the desired degree of superheat, pass over the superheater and main generating bank entering the reheat furnace, where the balance of the fuel is burned raising the gas temperature by an amount sufficient for the reheater duty needed. In port or when manoeuvring astern, the burners in the reheat furnace are secured, and the products of combustion then reach the uncooled reheater tubes at a temperature low enough to avoid causing them damage. Type: V2M-8-Divided Furnace Reheat Boiler: A further derivative of the V2M-8, this reheat unit I has the main furnace divided by a membrane wall. Each of the two furnaces so formed are provided with oil burners mounted on the roof. The products of combustion from one of these furnaces pass over reheater tube surfaces arranged at one end of the boiler whilst from the other furnace the gases pass over superheater tube surfaces at the other end of the boiler. Both gas streams combine before passing over the main bank of generating tubes.

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Combustion Engineering V2M-8 LTG Reheat Boiler:

Combustion Engineering V2M-8 Divided Furnace Reheat Boiler:

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Differential firing in the two furnaces gives control of reheat steam temperature whilst the superheat is controlled by attemperation between stages of the double superheater. All welded furnace enclosure walls are used and the superheaters and reheater are all arranged in the near vertical position with horizontal inlet and outlet headers beneath. Kawasaki Heavy Industries Boilers:

Kawasaki BDU Boiler:

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BDU Type Boiler: This is a basic bi-drum integral furnace boiler, the Kawasaki version having a double horizontal tube superheater, and front-fired furnace constructed with tangent tubes backed with refractory lined steel casings. The bottom ends of the furnace exit screen tubes terminate in a separate header fed with sophisticated system would utilise a second control valve in place of the orifice with means provided to water from the lower drum. The bottom headers of the front, rear and side furnace walls are fed by unheated down-comers from the steam drum. Steam temperature is controlled by attemperation with a heat exchanger in the steam drum and a desuperheater in the lower drum provides auxiliary steam at a reduced temperature. The steam circuit associated with steam temperature control incorporates a control valve and a fixed orifice in a bypass line. Care is needed in sizing the orifice since if the control valve is wide open and the orifice is too large insufficient steam will pass to the attemperator and the final steam temperature may exceed safety levels. Conversely, should the orifice be too small the control valve will be closed in to establish the correct steam quantity to the attemperator and drum steam pressure may exceed the working level. A more sophisticated system would utilise a second control valve in place of the orifice with means provided to prevent it from being completely closed. The two valves under the influence of the steam temperature controller would operate in sequence to control the steam temperature even if operating conditions drifted away from design values. This avoids down time, which may be required to change the orifice plate. UF Type Boiler: This is a radiant type boiler unit with fully water cooled furnace and convection passage enclosure walls and is very similar in arrangement and construction to the radiant designs of the British boiler- makers, having primary and secondary superheaters with inter-stage attemperation.

a) Kawasaki UF Boiler: b) Superheat Control on UFE and UFC Boilers:

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UM Type Boiler:

Kawasaki UM Type Boiler:

In conformity with boilermakers elsewhere Kawasaki also offered a bi-drum unit incorporating modem construction methods with welded connections between tubes and headers wherever possible. The whole unit is enclosed in membrane wall tube panels and the oil burners are arranged in the furnace roof. There is an all welded vertical U-tube superheater immediately behind the furnace exit screen and generally simple tube shapes are used throughout the unit. The superheater construction is novel in that the U-tubes are made up into panels by being welded to stub headers at their ends, as shown in the figure on page 47. These are given a prior pressure test in the factory and then connected to the main headers by welded connecting tubes. As with the vertical tube superheater proposed by all the boilermakers offering this type of boiler unit the main support of the tube bundle is taken on the main headers at the bottom. Location and guidance of the superheater tubes is obtained by means of heat resisting alloy steel castings welded to adjacent boiler and superheater tubes.

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Method of locating Superheater Tubes from Boiler tubes:

The designatory letters defining Kawasaki boilers are supplemented by an 'E' if the final heat recovery is by economizer or by a 'G' if final heat recovery is by a gas to air heater; the UM type thereby becoming UME or UMG. UFR Type Boiler:

Kawasaki UFR and UF Reheat Boiler:

To provide for the adoption of the reheat cycle Kawasaki modified their UF type by arranging for the convection passage to have three parallel paths.

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As other boilermakers had done they divided the main convection passage into two parallel paths by means of a membrane tube wall with superheater surfaces on either side but reheater surface on one side only. As a departure from previous designs they introduced a third convection passage between the furnace and the main divided passage, as shown in the figure below.

Bypass Economiser System of Reheat Boiler:

This third or bypass passage contains economiser surface. Dampers at the outlet of the three convection paths could be adjusted to control reheat and superheat in the normal ahead mode. As usual when operating astern or in a port the dampers above the reheater are closed. In this design a double damper arrangement is used and the space between them can be pressurised with air to effectively seal the dampers preventing gas flow over the reheater. Since some gas always passes through the bypass passage, less heat is available for superheating and reheating. To compensate, the reheater is brought into a slightly hotter zone and additional superheater surface provided, with some primary surface above the reheater. UTR Type Boiler: A more simple solution to the problems posed by reheat was obtained by Kawasaki when they introduced this unit in which the bypass passage is eliminated. The resulting design, although exhibiting the same constructional detail as the UFR type, controls reheat and superheat generally in the manner adopted by the British boilermakers. UTR boiler sketches are shown on the next page.

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Kawasaki UTR Reheat Boiler on reheat condition:

Kawasaki UTR Reheat Boiler as on Non-reheat condition:

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FOSTER WHEELER BOILERS: ‘D’ Type Boiler:

Foster Wheeler ‘D’ Type Boiler:

This is an early bi-drum design in which the two drums are connected by a multi-row bank of small bore generating tubes, and three rows of larger bore screen tubes in front of a U-loop superheater. The furnace sidewall tubes extend upwards from a header at floor level, turn over to form the furnace roof and are connected to the steam drum. The furnace rear wall is water-cooled and the lower headers of this and the sidewall are fed with water from the lower drum. Unheated down-comer tubes connect the two drums. The front wall and floor of the furnace are refractory lined. The horizontal U- tubes of the superheater are connected to vertical inlet and outlet headers. Baffles are fitted inside the headers, requiring the steam to make several passes through the tubes, thus achieving the high steam velocity necessary to ensure safe tube metal temperature in service. Oil burners are fitted in the refractory front wall of the furnace and on leaving the boiler, combustion gases pass over further heat recovery surfaces such as economiser (heating feed-water) or air heater (heating combustion air). Steam soot blowers are fitted to give means of on load cleaning of boiler, superheater and further heat recovery tubes. ESD I and ESD II Type Boilers: In an attempt to combat the problems experienced with the early ‘D’ s, Foster Wheeler introduced the External Superheater ‘D’ in which the basic construction methods remained as for the ‘D’ type but the superheater was removed to a position behind the generating tube bank which was reduced in depth. This resulted in a reduced steam generating surface, an increased superheater surface and an increase in heat recovery surface beyond the boiler.

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Foster Wheeler ESD I Type Boiler: Sectional view and Superheater & attemperator arrangement: Finding itself in a cooler gas temperature zone compared to the ‘D’ type, the superheater exhibited a much greater rate of change of steam temperature with load and for this reason steam temperature control was adopted, even though design final steam temperature was only 450*C. In the ‘Mark I’ version steam temperature control was by means of a steam combustion air heat exchanger and in the ‘Mark II’ by damper control of gas flow over the superheater.

Foster Wheeler ESD II Type Boiler Flow Diagram & Sectional View:

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ESD III Type Boiler:

Foster Wheeler ESD III Boiler Internal View:

The ESD I and II designs still contained a good deal of refractory material in the furnace zone and very many expanded tube joints and gaskets. It was seen that maintenance could be reduced if these were reduced in extent or eliminated. In the ESD III the furnace was much enlarged and the bi-drum radiant approach appeared with the adoption of complete water-cooling and burners mounted in the furnace roof. This increase in radiant surface reduced the need for a large generating tube bank that, in this design, reduced to eight rows in staggered formation, formed from the lowest metre or so of the four rows of tubes separating the furnace from the superheater. The superheater was further enlarged, permitting wide gaps between the tubes. Steam temperature control, now used because of more advanced steam conditions, was achieved by use of a steam boiler water heat exchanger located in the upper drum.

Boiler Furnace tube Arrangement:

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Later Type Boiler Furnace Tube Arrangement:

Sectional View of Foster Wheeler ESD III Type Boiler:

Refractory was still not eliminated, but was largely shielded from direct radiation by close-pitched furnace wall tubes. Many expanded joints also remained. The superheater tubes, being arranged parallel to the drum axis, tended to be long, requiring intermediate support along their length, and this proved to be troublesome in service. Further steps were taken to address these matters and an improved version of the ESD III used gas tight, all welded mono-walls in place of refractory lined casings behind tangent tubes for the furnace, and extended mono-wall construction to the superheater pass.

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Later Foster Wheeler ESD III Type Boiler showing Mono-wall Construction:

The number of rows of tubes between furnace and superheater was reduced from four to two and the superheater was now aligned at right angles to the drum axis, the resulting shorter tubes not needing intermediate support. ESD IV Type: With final stage development of the ESD series we arrive at the single drum radiant boiler with complete mono-wall enclosure and mono-wall division between furnace and superheater. This further halves the number of tubes between furnace and superheater so that the lower ends of all the tubes forming the side walls and the division wall can now be accommodated in a header with all welded connections. Both refractory and steel casings are eliminated and the steaming economiser appears to compensate for loss of generating surface elsewhere.

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Foster Wheeler ESD IV Type Boiler:

DSD Type: To cater for those ship owners who stated a preference for two drum boilers of more conventional design, the DSD (double superheater D type) offered several advantages over the D type or even the ESD I to III. A fully water cooled mono-wall enclosure system could be used with burners mounted in the furnace roof giving good distribution of hot products of combustion to the vertically aligned superheater tubes. The primary and secondary superheater sections were behind a three-row furnace exit screen and were virtually self supporting, needing only to be located relative to adjacent boiler tubes. It was further claimed that the propensity for deposits to form would be reduced on vertical tubes and any that did would be more readily removed. Ample access around the superheaters was provided for this purpose. A conventional generating bank of small-bore tubes was provided, with external unheated down comers, and additional, external feeders supplied water from the lower drum to the bottom headers of the water wall circuits.

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Foster Wheeler DSD Type Boiler:

Foster Wheeler ESRD Type: Achieving the maximum efficiency from steam plant at sea requires the adoption of the reheat cycle and for this a special boiler type is needed. In the reheat cycle steam, after passing through the superheater and HP turbine, is taken back to the boiler and reheated before returning to the intermediate and low- pressure stages of the turbine. At sea, this is the sequence followed when in the ahead mode, but when manoeuvring astern or when steaming in to port, re-heated steam is not required. Under these conditions the reheater tubes will not receive a cooling flow of steam and so other means of protection are required. The ESRD is constructed in a manner similar to the ESD IV except that the convection passage containing the superheaters is divided into two parallel paths by a further mono-wall. Superheater surfaces are deployed in both paths but reheater surface is installed in one path only. Dampers at the exit from each path control the gas flow over the two paths, so that the gas flow to the reheater, and thereby the reheat steam temperature, can be controlled. In astern or harbour operation the dampers above the reheater path are closed.

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Cooling air is admitted to the space between the top of the reheater and the closed dampers, and passes downwards over the reheater, joining the combustion gases, which have crossed part of the superheater beneath the reheater, and exiting through a small permanent opening in the division wall. It joins combustion gases there, flowing upwards in the parallel path across economiser tube surfaces and out to further heat recovery equipment, used to ensure a high boiler efficiency at all times. Superheated steam temperature is controlled interstage by the use of a steam boiler water heat exchanger in the boiler drum.

Foster Wheeler ESRD Type Boiler:

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A:

B:

A: Ahead operation. B: Astern operation: Key: 1: Primary superheater. 2: Secondary superheater. 3: Reheater. 4: Bypass economiser. 5: Steel finned economiser. 6: Reheat cooling damper. 7: Reheat control damper. 8: Reheat shut-oft damper. 9: Attemperator.

Gas flow through ESRD boiler:

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SUPERHEAT CONTROL IN BOILERS: In Scotch Boilers the production of superheated steam is generally achieved by the use of small-bore steam carrying element U-tubes inserted into the gas carrying tubes. The element tubes are connected to the supply and return headers fitted at the front of the boiler. When raising steam the headers are first drained and the return header drain is left open to ensure circulation in the element U-tubes, the supply header drain being closed. Temperature control is by means of a mixing valve. The bulk of the steam passes through the superheater section, thus ensuring that no overheating of the elements occurs and some steam passes straight from the boiler to the return line from the superheater header to mix with (and hence reduce the temperature) the superheated steam as shown in the figure below.

Water tube boilers, however have a variety of methods to control the steam temperature. Among the more common methods used in marine practice are:

3. Damper Control: This method utilizes dampers to control the gas flow across the superheat sections and is used in the ESD II Babcock selectable superheat boilers as shown below.

4. Differential Firing: This type consists of two furnaces which are separated by a section of operating tubes or membrane wall, one with a superheater section and the other without, where gas passage could be dual, merging into one, or single. Such a design is the Babcock controlled superheat boiler, which has twin furnaces and a single uptake. The above arrangement is shown in the figure below.

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The third method of controlling the temperature is by means of a combination of (1) and (2): See figures below:

In Addition to the above methods of control, an attemporator is often used. This may be fitted in the airway passages or in the water space of the boiler drum. The air attemporator consist of elements formed by extended surface tubing connected to inlet and outlet headers. The steam from the first pass superheater is led to the second pass superheater via the attemporator, which is positioned in the air trunking after the air heater. A damper, which is opened, controls the airflow over the attemporator and the by-pass closed when a lower temperature is required.

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The boiler water attemporator is a simple heat exchanger of tubular design which has its water side directly coupled to the boiler steam by large bore pipes.

A controlled amount of superheated steam is passed through the U-tubes, the reduced temperature outlet steam from the attemporator being used for auxiliary purposes and also if required, for reducing the temperature of the main steam from the superheater.

Boiler Water Attemperator:

NOTE: The superheater is steaming at desired boiler output at all times, i.e. all the boiler steam passes through it.

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The Foster Wheeler ESD II type of boiler uses a further method of superheat control in which the heat input to the superheater is limited to the amount of superheat required, this being affected by providing the superheat itself with an outlet damper and also a damper controlled by-pass. The control unit, which is an upward flow economizer section through which all the feed water to the boiler passes, extracts excess heat from the gases as they pass through the by-pass section to the boiler uptake. When raising steam no feed water will be passing to the boiler, to prevent boiling off and possible damage occurring to the control unit, circulation through it is ensured by means of a balance leg connected to the water drum. A method of controlling the superheat temperature rarely used in marine application is where a separately fired superheater is utilized, and is mainly used in boilers having three drums with twin furnaces, the superheater being situated within either the middle or gas-outlet tube banks, and the superheat temperature being regulated by employing burners in the inner and outer furnaces.

Superheat Temperature Control by Air Attemperator:

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Diagrammatic Arrangement of Steam Connections for Boiler Fitted with

Attemperator:

HEAT RECOVERY EQUIPMENT FOR MARINE BOILERS: The general classification of heat recovery equipment covers heat exchangers, which are located such that heat is absorbed from the combustion gases, after the gasses have passed through the superheater and steam generating sections of the boiler. Tubular air heaters, Ljungstrom generative-type air heaters and economizers are the most usual types for marine boilers. Some designers have proposed various other combinations of indirect heat exchangers, such as an arrangement whereby heat is absorbed from the stack gas in one heat exchanger and then subsequently transferred to the combustion air through the medium of a secondary fluid and a second heat exchanger. In still another arrangement, the heat recovery equipment consists of a gas turbine, which absorbs heat from the combustion gases and converts the heat energy, which in turn, can be used to drive the forced draft fan or compressor. This refers to pressure-fired boilers in which the combustion air is delivered to the burners at a pressure of several atmospheres, in contrast to the much lower pressures used for conventional boilers. This type of boiler is primarily for application to naval vessel propulsion plants. The boiler drum pressure is a most important factor for the design of the heat recovery equipment, since the water in the generating tubes is essentially at the saturation temperature corresponding to drum pressure and that temperature represents a theoretical limit to which the generating tube bank can cool the combustion gases. In other words, for 44 bar drum pressure, the water in the generating tubes is 272*C and, therefore, if the main generating bank were provided with an infinite amount of heating surface, the gas temperature were provided with an infinite amount of heating surface, the gas temperature leaving could be reduced only to 272*C.

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In consideration of various practical limitations, it is customary to proportion the boiler design such that the gas temperature leaving the main bank is about 65*C above saturation temperature. With increase in the operating pressure, a greater proportion of heat must be absorbed by heat recovery equipment as indicated by the table, which shows saturation temperatures corresponding to various drum pressures. Under optimum conditions the heat recovery equipment cools the stack gas to a temperature level consistent with reasonable efficiency. TABLE:

Drum Pressure Bar:

Corresponding Saturation Temperature: *C.

17.3 208 31.0 238 41.2 254 58.7 276 83.0 298

ECONOMIZERS: The marine economizer consists of a series of horizontal tubular elements by means of which heat is recovered from combustion gas leaving the boiler and added to the incoming feed-water. The amount of heat absorbed in an economizer is dependant on the temperature difference between gas and water, the heat transfer rate and the amount of heating surface. It is usual practice to utilize some form of extended surface on tubes to increase the heating surface per meter of tube length. Economizers can be considered to fall into two categories: bare tube and extended surface types. The bare tube usually takes in varying sizes, which can be arranged to form hairpin or multi-loop elements. Due to the fact that the coefficient of heat transfer is relatively low on the gas side of an economiser tube as compared to that on the waterside, it is desirable to have some form of extended heating surface on the outside of the tube so as to increase the overall rate of heat transfer. Many different configurations of extended surface are used, and each has characteristics of a specific standard adopted by a particular boiler manufacturer. In turn, each type of extended-surface economizer has a heat transfer rate that is peculiar to the shape and finish of the surface, the material employed and the method of fabrication. Most marine economisers are designed for counter flow of gas and water, that is, water down through elements, and gas up outside of elements. This is done in order to take advantage of the greater temperature difference between the gas and water. Usually the average temperature difference is in the range of 93*C to 121*C. Under these conditions the heat transfer cannot be increased without incurring a substantial resistance to gas flow, and the use of extended surface to increase the heating surface per lineal metre is desirable. Whatever the arrangement of economizer, ample provision must be made to assure that the heating surface can be kept free of deposits. The co-existence of deposits and moisture will cause corrosion, and accordingly the spacing of the elements, the quantity and effectiveness of the soot blowers, and the design of the broaching above the economizers must all be carefully considered. Moisture can come from a number of sources, as follows:

1. Down the stack. 2. Leaks in economizers or boiler pressure parts. 3. Condensation of moisture in gases.

Although a leak in an economizer pressure part cannot go long undetected, a tremendous amount of corrosion/erosion damage can result in a very short time if a leak is allowed to persist.

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a) Mild Steel Stud Economiser Surface: b) Mild Steel Plate Fin Economiser Surface:

c) Cast Iron Gill Economiser Surface:

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Arrangement of Primary and Secondary Economisers:

Therefore, it is advantageous to design economisers with the minimum of joints and hand-hole plates consistent with proper provision for replacing an element. Generally, the header-tube joint is made up as a full welded joint, or, as an alternative construction, the elements are welded to nipples in the headers. While the economizer is designed principally for steady steaming conditions, proper consideration must be given to the design of the elements so that steam will not be generated within the economizers during manouvering. This means that there must be suitable water velocities and pressure drops which will assure adequate circulation to each element. Under most conditions there is sufficient differential saturation and feed water temperature to prevent the generation of steam. However, there may be some manouvering conditions, such as the sequence from stop to full ahead, in which steaming may become a problem. This is particularly true if the water level is high at the start of the manouvre and the boiler control system is of the single element (water level) type. Usually an economizer is installed at the boiler outlet adjacent to the steam drum with elements parallel to the drum. In most fire room arrangements sufficient space is available for a good economizer arrangement.

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However, to obtain the required economizer heat absorption, the economizer length, width and number of tubes high may have to be varied to obtain the best arrangement for the space conditions available and meet thermal performance conditions. In addition, the gas pressure loss permitted and water pressure drop must be considered. There are a few instances where the economizer tube length, that is, distance between sheets, is slightly more or less than the boiler bank width or depth. Today it is common practice to design marine economizers so that they can be by-passed if a leak occurs. Thereby allowing the boiler to remain in service until the necessary repairs can be made. This means that the economiser must withstand entering gas temperatures without any feed- water flow through the economiser tubes. With the economiser by-passed, the loss of efficiency results in an increased fuel-firing rate and increasing draft losses cause a higher fan load. Another important consideration, which applies to most merchant ship boiler designs, is that there is generally an increase of total steam temperature due to the higher gas flow and increased furnace temperature. AIR HEATERS: Most marine air heaters are of the horizontal type, which is preferred because of its simplicity and the ease of incorporating it into the boiler arrangement.

Cast iron Plate Type Gas/Air heater:

Plate Type: Plate type is usually of cast iron with integral fins on both air and gas side. TUBULAR AIR HEATERS: The tubular type consists of a suitable number of horizontal tubes expanded into and supported by tube shoots. The number, size, spacing and arrangement of the tubes are dependant on the available space and the mount of heat the air heater must recover from the flue gas.

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Horizontal Tube Gas/Air Heater:

The tubes and tube sheets are enclosed in a steel plate easing. The tubes are grouped in a suitable number of banks to allow for the installation of scat blowers and to provide access for cleaning. Air heaters have utilized tubes over quite a wide range. However, for most installations it is found that the smaller tube size allows for good tube arrangement with low box volume to minimise space occupied. This permits suitable tube spacing for effective cleaning of the gas passage area. At the same time there is a satisfactory heat transfer rate with an air pressure loss through tubes in keeping with design requirements. In marine installations most tubular air heaters are installed immediately above the boiler gas exit where the unit can be fitted into the boiler design. For a two-pass heater, air both enters and exits at the rear side of the boiler, presenting an efficient arrangement for transporting air to the fuel burners. The installations of air heaters usually results in exit gas temperatures exceeding 149*C at the normal operating conditions. With air inlet temperatures of 38*C and with counter flow arrangement this results in a tube metal temperature of approximately 93*C in the air inlet area. Under low boiler load conditions such as in port or in maneuvering, the exit gas temperature may fall below the dew points, resulting in air heater tube deterioration caused by sulfuric acid formation. Under some conditions sulfuric acid may condense on air heater surfaces even when metal temperatures are in excess of 149*C. Excessive moisture in flue gas contributes to air heater maintenance.

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The vapor may come from the following sources:

a) Fuel. b) Leak in boiler or economizer. c) Improperly drained soot blower lines. d) Soot blower leaks. e) Residue from water washing.

To minimise the effect of low exit gas temperature it is the practice to incorporate in the design an air heater by-pass, which, under low load conditions, prevents all, or a portion of the air from entering the air heater by means of a damper arrangement. REGENERATIVE AIR HEATERS: Regenerative air heaters for marine service are designed for horizontal or vertical flow of the air and stack gases. This type of heat exchanger is made of heating elements, which are contained, in a slowly turning rotor of cellular construction.

Diagrammatic Arrangement of Rotary regenerative Air Heater (vertical shaft arrangement) with

Gas and Air Counter Flow:

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Arrangement of Counter-flow Regenerative Air Heater with Stationary plates:

As the rotor turns, at about 4 rev/min, heat is absorbed continuously by the heating elements from the flue gas, while a like amount of heat is released simultaneously to the combustion air, as these fluids flow axially through the rotor. The rotor is enclosed in a gas tight housing, which is fitted at each end to make connections with the air and flue gas ducts. Dampened integral air and flue gas by-pass ducts are incorporated in the four corners of the structure. The air bypasses in parallel afford means of controlling cold and heating elements temperature during operation at reduced steaming rate. The flue gas by-passes in parallel, in combination with the air by-passes, afford the limiting overall pressure loss at steaming rates above that corresponding to normal power.

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In counter-flow air heaters the part where the flue gas enters and air leaves is the hot end, and conversely the part where the air enters and flue gas leaves is the cold end. These terms apply to the structure as a whole as well as the rotor and heating elements. The heating elements are stacked in the rotor in layers, usually two but seldom more than three. This feature permits replacement of the elements of the cold layer, which are those subject to possible deterioration, without disturbing the main body of heat transfer surface. The elements of the cold layer are packed in baskets convenient for handling. Reversibility nearly doubles the useful element life. The elements of the cold layer are fabricated from corrosion resistant low alloy steel as are also the baskets containing them. Heating elements of the hot layer can be shop packed in baskets or in the rotor. Shop packing simplifies field installation and assures control over tightness of the pack. Cleaning devices driven by power are furnished with the regenerative air heater to remove dust deposits from the heating surface. These cleaning devices usually installed in the gas outlet and air inlet ducts integral with the air heater. The gas side cleaner is normally used at sea for soot blowing, while the airside cleaner is primarily reserved for use in port. The airside cleaner blows the soot back into the furnace, thus avoiding objectionable smoke and dust, which is prohibited by harbour ordinances. The need for soot blowing can readily be determined through an observation port, which permits inspection of the heating surface while the air heater is in operation. The regenerative type of heat exchanger is small and light. This results from the fundamental nature of counter flow heat transfer, which permits significant reduction in heating surface. A factor to be considered in the selection of air heaters is that of metal temperature. In the regenerative type, tests and experience have demonstrated that the actual metal temperature is 11-17*C above the average of the air and gas temperatures. By contrast, in a tubular type air heater the measured temperature of tube metal ranged from 5 to 67*C below the mean of the air and gas temperature. This fact is of material significance in the selection of air pre-heaters if corrosion and plugging are to be avoided and it demonstrates one of the reasons why regenerative heaters may be operated with materially lower leaving gas temperature than tubular types. Installation of a regenerative type air heater aboard ship presents no problem. The vertical flow air heater can be located directly above the boiler outlet or in the area above the boiler and connected with conventional ductwork. From the top of the air heater on the gas side an uptake is led to the stack in the usual manner. Forced draft fans may be located conventionally and connected to the airside of the heater by means of ductwork. Various arrangements can be utilized to suit the conditions. Cooling air to be introduced to the casing can be taken from the forced draft fan discharge duct ahead of the heater. STEAM AIR HEATERS: The selection of the heat recovery equipment to be incorporated in the design of a boiler is influenced by the boiler operating conditions and the efficiency required of the boiler unit. These are related to the specification of a suitable steam cycle for a marine installation. The most suitable steam cycle for one installation may not be advantageous to another. Therefore, for certain installations an air heater may be best suited. Where feed temperature and pressure conditions permit, an economiser or a combination of an economiser and a steam air heater are used to obtain the boiler efficiency required. One of the methods used to improve the steam cycle efficiency is by means of the feed water heating system, and here the steam air heater serves the purpose of utilizing steam extracted from the feed water heater or auxiliary exhaust. This reduces the flow of steam to the condenser and improves cycle efficiency. Another method used is to extract low-pressure bleed steam from the main turbines.

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A steam air heater is comprised of small diameter tubes about 16mm O.D. They have sufficient size for the steam flow involved and are connected to inlet and outlet header boxes. The bleed or exhaust steam enters the inlet header box and flows inside the tubes, while the combustion airflows around the tubes. In the condensing of the bleed steam or auxiliary exhaust, heat is transferred to the air used for combustion. Since the condensing steam film coefficient of heat transfer is high, extended surface is used to advantage. Closely pitched tubes and fins can be utilised, as both the air and heating fluids are clean, eliminating entirely the necessity for cleaning. In most steam air heater installations, bleed steam at 0.7 to 3 bar is used. While a feed water heater uses steam at relatively high pressure the steam air heater uses auxiliary exhaust or low-pressure bleed steam. Utilizing the lower pressure bleed steam is a good method of improving cycle efficiency. Another point to be considered is that the auxiliary exhaust can be utilized to an advantage in a steam air heater while boilers are steaming in part, thereby improving low load boiler operating conditions. Suitable air temperature for good combustion is readily maintained, whereas with a tubular or regenerative type air heater a by-pass system around the heater is necessary to avoid low stack temperature. Steam air heaters do not improve the boiler efficiency since the heater does not serve to reduce the gas temperature at the boiler outlet. The steam air heater, however, is effective by improving the overall marine plant efficiency. For this reason it is the practice to use the steam air heater with an economizer, as the economizer will absorb heat from the boiler and thereby reduce stack temperature to meet the required boiler efficiency. ************************kv************************** AIR SUPPLY SYSTEM: A boiler combustion air supply system may be based an natural, forced, induced or balanced draught arrangements depending upon generally the particular requirements of the boiler design and the fuel used. Natural Draught: Refers to the draught caused by the natural convective current set up in the atmosphere and the boiler gas passages due to the temperature and hence the density differential between the surrounding atmosphere and hot combustion gases. Such marine installations are now obsolete. Forced Draught: Combustion air is forced, by means of a fan, into the boiler combustion chamber. Induced Draught: A fan is situated near the base of the stack and induces the combustion air into the combustion chamber. Balanced Draught: The balanced system comprises of a forced draught fan to supply the necessary air supply and velocity for combustion and an induced fan is used to exhaust the waste gases. Marine installations favor forced draught systems although for large installations the balance draught is favored. The increasing use of automatic combustion control has led to the increasing use of the much more compact high-speed, backward-blade type, forced-draught fan operating as a single unit, damper or speed controlled, to simplify and reduce the cost of control equipment. An induced fan would not be used where the gas temperature is above 200 degrees centigrade since this would create problems of clearance, distortion, erosion and the need for water-cooled bearings, which all increase the maintenance liability, thus they are seldom used marine installations.

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Typical performance characteristics for a backward-curved blade fan:

Draught Control of Air Supply System: In marine installations the control of the air supply to multi-burner installations, with the emphasis on simplicity and economy, tends towards a single draught source and a single method of control usually by damper or variable speed motor. The selection of a fan with the most suitable characteristics for the duty involved is an essential but, beyond this, efficient control of the fan is required to ensure the correct air supply to the register for efficient combustion. There are a number of methods of fan-control available, selection of which must be based on the evaluation of such factors as the type and range of the boiler and oil-burning equipment, speed and response of the equipment required to suit the changes in load, simplicity of operation, reliability, initial cost, operating cost, maintenance cost, and life expectancy of the equipment, not necessary in that order. Modern marine requirements of high fan pressures tend to favour constant speed electric or turbine-driven fans, damper controlled for simplicity.

Simple outlet damper control system:

A typical outlet damper-control system suitable for operating in conjunction with a semi-automatic control system modulating at or near the maximum continuous rating is shown above.

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The control of the fan outlet damper is such that the air flow is directly proportional to the fuel flow achieved by use of a fuel/air ratio controller which continually measures the oil and air flow in terms of pressure or pressure differential and corrects the damper position, from that initially governed by the master steam pressure controller, to one of optimum fuel/air ratio. By continually proportioning the oil and air flows in this manner, any change in the boiler air or exhaust gas resistances, due to burner shut down or possible fouling of the air heater or boiler gas passages, is immediately responded to with an appropriate damper readjustment. The outstanding disadvantage of the outlet damper control system is the method of throttle control of volume, which involves excessive pressure losses over the damper, and waste of power consumption. The pressure-flow curves show the difficulty of matching the register and boiler resistance characteristics with that of the fan and the extent of the pressure drops prevailing over the damper necessary to maintain optimum airflow. Pressure drops of this nature make the butterfly damper extremely critical to quantity variation and in consequence limit the range of affective control of the air supply and as a direct result the range of the burners. Simple outlet damper control is essentially confined to the small plants where power saving is fractional or to large plants in which power saving is of secondary importance and automatic modulation is over a very limited range at or near maximum continuous rating. Many marine installations favour this form of control on a basis of simplicity, initial cost and operation for prolonged periods at maximum continuous rating.

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SOME ARRANGEMENTS OF STEAM GENERATION IN MOTOR SHIPS: In all types of motor vessels, auxiliary steam demand varies depending on the Particular class of the ship; but all ships would require steam for at least the following needs:

(a) Heating fuel oil - storage tanks, settling tank, pre-heater for purifiers and main fuel injection system, service tanks.

(b) Heating lubricating oil - sump tanks, pre-heater for purifiers. (c) Heating water - for domestic requirements and engine room Cooling water tanks (warming

prior to starting). (d) Heating steam for ships galley and general purposes. (e) Heating steam for oily water separators.

There would be a slight variation due to other requirements in different category of vessels such as in Tankers, Bulk carriers, Fast-cargo liners etc. DRY CARGO SHIPS: These Ships are normally have 6000 – 8000 BHP and runs at 14 – 16 knots. Due to their frequent calling of ports and comparatively lower power, steam produced with the help of exhaust boiler is mainly utilized for heating and domestic purposes such as mentioned above. Steam demand at sea or in port remains around 2000 kg/hr at about 7 bars, which can be supplied by various arrangements such as:

1. A Spanner, Cochran or Thimble Tube composite boiler, which can be run on either oil or diesel engine exhaust gas.

2. Two independent low pressure boilers such an one all welded Package boiler and another exhaust gas economizer type unit at the base of the funnel being fed independently from the hot-well.

3. A small oil fired tank boiler (for port use) connected to a small waste heat unit in the funnel, the steam space of the tank boiler being used as a steam receiver at sea.

Spanner Composite Boiler:

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BULK CARRIERS: These are higher power vessels with longer operation at sea and the sea electrical load may be carried by one steam turbine-generator, which is powered by the steam from the exhaust boiler unit. At sea, the exhaust boiler also supplies steam for all the general requirements. Non self-discharging vessels' port electrical demand can easily be met by 2 diesel driven generators and all other heating and domestic requirements can be met by a small oil-fired tank boiler when in port. A typical arrangement may be as shown below:

FAST CARGO LINERS: These ships have high power engines with about 18000 BHP and service speed could be about 20 knots. Higher amount of waste heat is so available for recovery. This specialized class of ships may have a large steam demand to supply heating steam for a particular cargo to be maintained at an elevated temperature and depending on the type of vessel in use, the boiler arrangements would differ. A typical example may have a small auxiliary water tube boiler working in conjunction with a forced circulation exhaust-gas heat exchanger, the oil fired boiler acting as steam receiver at sea. A rotary change valve controls evaporation on gas.

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TANKERS: Apart from beating and domestic requirements such as mentioned in this page, auxiliary steam is maintained on motor tanker to run various deck machineries and utilized for cargo heating and pumping. The auxiliary steam demand in a steam turbine propelled vessel could run as high as 75% of the propulsion steam demand. So, the large steam demand in motor tanker requires one or sometimes two water tube auxiliary boilers to be fitted together with an exhaust-gas heat exchanger at the base of the funnel. Steam is always taken from the auxiliary boiler, which is oil-fired in post and acts as a steam receiver at sea. A forced circulation is used with the help of a circulating pump drawing from the lower water drum.

WASTE HEAT RECOVERY IN MOTOR SHIP: In the main propulsion engine of a ship, the chemical energy of the fuel is first converted to heat energy by combustion and this energy is then converted and utilized to rotate the crankshaft, doing useful work to propel the vessel. A large amount of available heat energy is not converted to work and is lost with the engine exhaust gas through the funnel; this may amount to 35% of the supplied heat energy and is considered a complete loss, which in the present day of energy crisis cannot simply be ignored or accepted. So, various types of waste heat boilers have been manufactured which can recover up to about 60% of the exhaust losses to the atmosphere. In all motor-ships there is a demand for steam for heating and steam can also be used to run turbo-generators to produce electricity. Type of waste heat boilers vary depending on the class of vessels, their power, the route the vessels ply etc. but most types of motor-ships would have a waste heat recoverable boiler system together with an independent oil-fired boiler for port and emergency use.

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Certain amount of heat is still lost through the funnel exhaust as the temperature of exhaust gas at the exit from the boiler must be maintained above the dew point of the gas; if not, there will be formation of a sulphurous acid vapour which would eventually deposit on the tube metal surfaces of the boiler and harmful corrosion would ensue. Slag deposits on the external of the tubes are a persisting problem due to the presence of sodium, vanadium and other harmful constituents. These deposits can also give problem by causing external corrosion of the tube surface, apart from restricting the gas passage. To run a waste heat boiler efficiently and without trouble, the boiler heating surfaces should be maintained in a clean condition by regular soot blowing and water washing, fuel treatment and purification should be correct prior to its burning and proper combustion condition should always be maintained. Corrosion by the water in circulation could also be a problem and various chemical treatments are recommended to keep the water in good and alkaline state to avoid such problem. A higher feed water temperature by employing feed heating and higher hot-well temperature also helps to keep the amount of dissolved gases specially oxygen within limits, which reduces the corrosion problem. Various arrangements are possible and they would be one or the other of the following: Natural Circulation: Composite Boilers: These are auxiliary boilers having arrangements for exhaust gas heat recovery within the same boiler unit. In most designs different tube banks are selected for oil firing and exhaust gas flow, so that the boiler can be simultaneously fired on oil or run on exhaust gas but an alternatively fired unit is also constructed where the gas passage is one and either the oil firing or the waste heat running has to be selected. Exhaust gas flow is usually controlled by dampers for flow through the boiler or by-pass to the funnel exhaust. Forced Circulation: (a) Exhaust gas boiler is coupled to the auxiliary boiler: (Figure on page No: 79): Water from the auxiliary boiler is drawn by means of circulating pump, which forces the water through exhaust gas heat exchanger (economiser, or boiler) and steam produced is returned to the steam space of the auxiliary boiler, the distribution of the steam is done from the auxiliary boiler. (b) Exhaust gas boiler can operate independent of auxiliary boiler: (Figure on page No: 79): In this arrangement, the exhaust gas boiler can be supplied with water directly from the hot well and steam can be distributed to the main steam line without returning to the auxiliary boiler. The circulating pump directly controls water level in the exhaust gas boiler. Most exhaust gas boilers are made tubular in modern practice because they are much more efficient than a tank type boiler. The tubes can be arranged in different tanks having an economizer a generating and a superheating section, each serving definite purposes. (c) Dual pressure forced circulation system: (Figure on page No: 79). This system is capable of producing steam at two different pressures at the same time by two independent circuits. The high-pressure circuit, which is termed as the ‘Primary’ system, operates as a closed cycle in the form of a water tube boiler having provisions for oil burning unit. The steam thus produced passes through a steam/steam generator (heat exchanger), secondary system, and returns to primary system. Circulation within the primary system is natural but that in the secondary system is assisted by means of circulating pump.

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Exhaust gases transfer heat to the secondary system and superheating arrangement is also provided. Contamination from the various heating systems of ship is restricted to the secondary system only. Primary system is virtually free of contamination risk and can be maintained in best of conditions.

(a): Exhaust Gas Boiler (b): Exhaust Gas boiler Coupled with Auxiliary Boiler: Independent of Auxiliary boiler:

Duel Pressure Forced Circulation System: (d) Advanced waste heat system: Heat utilization from main engine Jacket water and supercharge Air Cooler using a separate steam receiver makes it a system with higher thermal efficiency. See figure on page 80.

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Advanced Waste Heat recovery System:

Auxiliary steam system for motor vessels embodying close circuit water tube boilers, steam to

steam generators, waste heat boiler and turbo alternator:

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Arrangement of Engine Room with Main Engine Exhaust led to Composite Boiler:

Typical Exhaust Gas Heat Exchanger:

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Above figure on page number 81 shows a typical arrangement of exhaust gas heat exchanger circulating into water tube boiler which, acts as steam receiver at sea and can be fired in port or when required. Steam is supplied at 9 bar superheated to 340*C for turbo generator and other services, also steam, at 1.7 bar for heating and domestic use. **************************************Kv******************************************* BOILER MOUNTINGS: Boiler mountings are required for the proper working of the boiler. Those attached directly to the pressure parts of the boiler are referred to as boiler mountings. In general all these mountings must be carefully designed to perform their function correctly. They must be positioned so as to be readily accessible both for maintenance and operation, the later being performed either directly, or indirectly, by means of extended rods, spindles, etc. General descriptions of these mountings are as follows: SAFETY VALVES: These are fitted to protect the boiler from the effects of overpressure. The DOT demands that at least two safety valves are fitted to each boiler, but in practice it is usual to fit three safety valves-two on the steam drum, and one on the superheater outlet header. This latter valve must be set to lift before the drum safety valves so as to ensure a flow of steam through the superheater under blow off conditions. It is normally of the same basic type fitted on the drum. MAIN STOP VALVE: This is mounted on the boiler shell or superheater outlet header, and enables the boiler to be isolated from the steam line. If two or more boilers are fitted supplying steam to a common line, the stop valve on each boiler must be a screw down, non-return type. This is to prevent steam from the other boilers flowing into a damaged boiler in the event of a loss of pressure due to a burst tube. In some cases the main stop valve incorporates an automatic closing device, designed to operate in emergency Conditions, to shut off steam from the main turbines. AUXILIARY STOP VALVES: This is basically a smaller version of the main stop, fitted for the purpose of isolating the boiler from the auxiliary steam lines. Again these must be screw down, non-return type valves if necessary to prevent steam flowing back into the boiler in the event of damage. The valve will be mounted on the superheater outlet header. FEED CHECK VALVES: These are fitted to give final control over the entry of feed water into the boiler. They must be screw down, non-return valves so that, in the event of a loss of feed pressure, the boiler water cannot blow back into the feed line. Main and auxiliary feed cheeks are fitted. The main check is often fitted to the economiser inlet header; if not, like the auxiliary cheek, it will be mounted directly on the steam drum. Extended spindles are usually fitted so the cheeks can be operated from a convenient position. Care must be taken to ensure the valve can be operated easily and quickly, and that a positive indication of the open and closed positions for the valve is given. BOILER FEED WATER REGULATOR: The water level in a boiler is critical. If it is too low, damage may result from overheating; too high and priming can occur with resultant carry-over of water and dissolved solids into superheaters, steam lines, etc. Automatic feed regulators are therefore fitted to control the flow of water into the boiler and maintain the water level at its desired value.

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They are fitted in the feed line, before the main feed cheek. In most cases they use a float or thermal means of operation and thus must have a direct connection to the steam and water spaces as required. The regulator can be attached directly to the boiler shell, or alternatively mounted in an external chamber with balance connections to the steam drum, or boiler shell. In the case of water tube boilers with their high evaporation rate and small reserve of water the control of the water level is so critical that the classification societies demand that some form of automatic feed regulator must be fitted. WATER LEVEL INDICATORS: The DOT demand that at least two water level indicators must be fitted to each boiler. In practice the usual arrangement consists of two direct reading water level gauges mounted on the steam drum, and a remote reading indicator placed at a convenient control position. LOW WATER ALARMS: The classification societies demand that these should be fitted to reduce the risk of damage in the event of a loss of water in the boiler due to a burst tube or failure of the feed supply. In some cases they are mounted inside the steam drum, but many are mounted externally. Various types are available, either steam or electrically operated. Some versions also incorporate high water level alarms. BLOW DOWN VALVES: These are fitted to the water drum to enable water to be blown from the boiler in order to reduce the density. When the boiler is shut down these valves can be used to drain it. They usually consist of two valves mounted in series, arranged so that the first valve must be full open before the second can be cracked open; i.e. sufficient to give the required rate of blow down. In this way the seating of the first valve is protected from damage, so reducing the risk of leakage when the blow down valves are closed. These blow down valves discharge into a blow down line leading to a shipside discharge valve. SCUM VALVES: These should be fitted when there is a possibility of oil contamination of the boiler. They are, mounted on the steam drum, having an internal fitting in the form of a shallow pan situated just below the normal water level, with which to remove oil or scum from the surface of the water in the drum. These valves discharge into the blow down line. DRAIN VALVES: These are fitted to headers, etc., so enabling the boiler to be completely drained. They must not be used to blow down, only being opened when the boiler is shut down. AIR VENTS: These are fitted to the upper parts of the boiler as required to release air from drums and headers, either when filling the boiler or raising steam. SUPERHEATER CIRCULATING VALVES: These are fitted so that when raising steam they can first release air from the superheater, and then provide enough circulation to prevent overheating by allowing sufficient steam to blow off to the atmosphere or to a suitable drain system. They should only be closed when there is enough demand for superheated steam to provide the minimum circulation of steam required to prevent overheating. CHEMICAL DOSING VALVES: These are fitted to the steam drum to enable feed treatment chemicals to be injected directly into the boiler. SALINOMETER VALVES: These are fitted to the water drum to enable samples of boiler water to be drawn off so that the tests required for the control of the feed treatment can be carried out.

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At high pressures it is necessary to provide some means of preventing flash off taking place as the pressure over the sample is reduced to atmospheric. This is usually done by passing the Crater from the salinometer valve through a cooling coil which reduces its temperature to a value below 100*C. SOOT BLOWER MASTER STEAM VALVES: These are usually mounted on the superheater outlet header to ensure the superheater is not starved of steam while blowing tubes. In some cases two valves are fitted in series, with a drain valve between them, in order to prevent steam leaking into the soot blower steam supply lines when these are not in use. PRESSURE GAUGE CONNECTIONS: Fitted as required to steam drum, superheater outlet header, etc. to provide the necessary pressure reading. In addition suitable connections must be provided for the pressure sensing points required for automatic combustion control systems, etc. THERMOMETERS: Pockets must be provided in superheater headers, etc. for the fitting of either direct or remote reading thermometers.

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WATER LEVEL INDICATORS: Some Regulations: Concerning water level indicators:

1. Every boiler is to be fitted with at least two independent means of indicating the water level in it. One of which is to be a glass gauge. The other means is to be either an additional glass gauge or an approved equivalent device. (A set of not less than two test cocks will be accepted as the approved equivalent device mentioned above, for boilers having a design pressure less than 8 Bar or internal diameter less than 1.83m.) For water-tube boilers the approved equivalent device is to be other than the test cocks, but where a steam and water drum exceeding 3.96 m in length is fitted two glass gauges are to be fitted in suitable position.

2. The water level gauges are to be readily accessible and placed so that the water level is clearly visible.

3. The lowest visible part of the water level gauge and the lowest test cock (if fitted), are to be situated at the lowest safe w6rkirg water level.

4. The cocks of all gauges are to be accessible from positions free from danger in the event of the glass breaking.

5. Each of fired boiler is to be fitted with a system of water level detection which is to be independent of any other mounting and which will operate audible and visible alarms and shut of automatically the oil supply to the burners when the water level falls to a predetermined low level.

6. Water-tube boilers are to be fitted with two system of water level detection, which are to be independent of any other mounting on the boiler. Both systems are to operate audible and visible alarms and automatic shut-off device.

TUBULAR TYPE WATER LEVEL INDICATOR: (See Fig: 42 & 43). There are two of them fitted on to each side of the boiler shell to indicate at all times the correct water level inside the boiler. The top gunmetal body has the steam end cock, secured to the shell just above the normal water level. The bottom gunmetal body is fitted just below the normal water level and contains the water end cock. A drain cock assembly is fitted underneath the lower gunmetal housing. A toughened gauge glass is fitted between the upper and lower housing and indicates the water level when steam and water cocks are open and the drain cock is shut. The glass is held in place and sealed at each end by gland nuts tightened on to soft, tapered packing rings. The drain cock enables blowing through the glass for testing purposes.

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Directly Mounted Water Level Gauge:

Key: a) Fixed directly to boiler: b) Fixed to a large-bore bent pipe: c) Mounted on a hollow-column, end of which are connected by pipes to shut-off cocks on the top and bottom of the boiler: d) Mounted on a column same as ‘c’, but center part at the column is solid.

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Direct Water Level Indicator:

This type of Indicator can be used only for pressures up to 17.5bars: A ball valve at the lower end of gauge is provided to shut off the water in the event of a glass breaking. If water were allowed to escape, in the event of a glass fracture, the reduction in pressure would cause large volume of steam to flash off leaking water with possible harm to the personnel. Steam escaping from other end is not associated with the said increase in volume and so the amount of steam blowing out would be limited.

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Plugs are fitted on to the gunmetal bodies to allow for renewal of glass and clean the various passages. All three cock handles are arranged to point down-wards when in the working position that means, the steam and water end should be open and the drain shut when the handles lie vertically downwards. This is to prevent possible movement of the cock due to ship vibration and then indicate a false water level indication. The cock handles are many times fitted with extended handles so that they can be operated from a convenient position in the boiler room floor. The drainpipe from the gauge glass should never be connected to a common drain system as this makes the sound of blowing through the cock inaudible. The drainpipe is lead directly down and terminates just below the boiler room floor and the blow down could be clearly seen issuing out from the open end by simply lifting out a small inspection cover on the boiler roan floor. Plate-glass rectangular section guards are fitted to prevent injury in the event of the glass tube fracture. Difficulty in ascertaining correct water level and Faults in gauge Glass: If there is any doubt about the accuracy of the reading indicated by the water level gauge, blowing through the steam should test the gauge and water cocks in a proper sequence and not merely by opening the drain cook. A strong blow from each of the steam and water cocks, in turn, will unobstructed passages and establish that the level seen in the glass is the correct representation of the level of water inside the boiler. If, out of two gauge glasses, one shows normal level and the other shows an abnormally lower level, the glass which is showing a lower level must immediately be blown through and the correct condition ascertained. If both glasses show a lower level, the first job, which must be done, is to secure the fire and then only blowing through the gauge glass should check the correct boiler condition. Difficulty is often experienced in finding whether a tubular glass is full or empty this can be avoided by using the principles of refraction, for example, by placing a board, painted with alternate black and white diagonal stripes to appear to be bent to the opposite angle. In the absence of such indicator, a pencil or similar object, held at an angle behind the glass tube wall gives the same effect. A False water level indication can be caused by:

1. Choked or partially choked cocks and passages by sediment, packing or use of a glass, which is too long.

2. Cocks partially closed due to twisted cock handles or because the cock handles are not properly aligned.

3. A dirty glass or the glass guard can prevent easy water level indication even when the gauge is well lighted.

If the steam cock is choked, a vacuum will form in the upper part of the glass as the steam entrapped condenses and the water would rise and fill the glass. If water cock is choked, the, water level will slowly rise as the condensation of steam in the upper part of the glass gradually fills the tube. The Correct Blowing Through Sequence:

1. Close steam and water cocks and open the drain; after the glass content is cleared, nothing else should blow indicating both steam and water cocks are not leaking.

2. Open the steam cock - steam should blow through with a clear strong sound through the drain. Close the steam cock.

3. Open water cock - water should blow through similarly as in (2) close water cock. Close the drain cock.

4. Open the water cock again - water would fill up the glass. 5. Open steam end cock, water level drops to the level appropriate to what is inside the boiler.

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When renewing a new glass, blow through steam to warm-up the glass before opening the water cock otherwise, the glass may break. Before fitting a new glass, all the passages must be blown through to clear from sediments, glass pieces, etc.

Water Level Indicator for Medium Pressure Boilers: Although plate-type water level gauges can be used for low pressures in view of their greater cost, they usually only come into general use for pressures above a value of about 1750 kN/m2. The reflex glass shown in, Fig. (44) is an example of a type of water level indicator suitable for boilers working at a medium pressure range between about 1700-3000 Kn/m2. These gauges are supplied with gunmetal bodies up to pressures of 1750 Kn/m2, while for higher pressures forged steel bodies are used. The gauge is normally fitted directly to the boiler shell or steam drum. The isolating cocks in the steam and water connections, together with the drain cock, are of the asbestos sleeve type. In many cases extended rods, or chains, to prevent injury in the event of the glass shattering when blowing through the glass, can operate these cocks. However, the glass plates are so strong that this form of failure does not occur often, and even then the pieces of glass frequently remain in position; thus the fitting of an external guard is not necessary. The single-sided glass ingeniously makes use of the refraction of light so that, when illuminated from the front, the series of ribs at the back of the glass plate cause the light rays to be reflected back from the steam space and absorbed in the water space. This gives a bright silvery appearance to the former, while the latter shows dark. The strong contrast between the two enables the operator to see immediately the position of the water level. It also makes it possible to tell at a glance whether the glass shows completely empty or completely full of water (a condition which causes some confusion with many other types of gauge glass). A ball valve is normally fitted to the lower end of the gauge to shut off the water in the event of glass plate shattering and blowing out. The ball valve thus prevents the escape of water, with the resulting flash-off of large amounts of steam, making it difficult to shut off the gauge, and possibly causing injury. Another problem arises at higher pressures due to the fact that hot distilled water at high pressure erodes the glass away. This effect is even more pronounced if alkaline feed additives are being used. To resist this, a special borosilicate glass is used for the higher pressures, but can only give limited protection. For pressures above 3400 Kn/m2 some means must be used to prevent the water from coming into direct contact with the surf ace of the glass. Placing a sheet of mica between them usually does this. Due to the ribbed glass, the reflex-type gauge cannot make use of this form of protection, and so is not suitable for use with high-pressure boilers.

REFLEX TYPE SINGLE GLASS PLATE WATER LEVEL GAUGE:

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Water level indicator suitable for use on high-pressure water tube boilers: All boilers must have at least two independent means of indicating the water level, and in the case of high pressure water tube boilers working at values above 3400 kN/m2, these usually take the form of double-sided, plate-glass type water level indicators, with mica protection for the glass plates. This protection is necessary as at high pressures hot distilled water erodes the glass away, and unless a sheet of mica is placed between the glass and the water, attack take place quickly, indeed at the higher pressure ranges the glass will burst within a few hours if this protection is omitted. The general arrangement of a typical double-plate water level gauge is shown in the figure below.

Double Plate Type Water Level Gauge:

It consists basically of a hollow centerpiece with flats machined on each side to take the two plates of toughened glass. These are held firmly in place by means of a clamp plate.

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Care must be taken during assembly to prevent undue stresses being set up which will cause the glass plate to shatter when put into service. Thus the following procedure should be carried out. Strip down the faulty gauge glass. Discard the used glass plates, mica sheets, and joints. Make sure all joint faces are scrupulously clean. Cheek frame and cover plates for flatness; any warping can cause the glass to shatter. Build up the gauge, inserting the new joints, together with the mica sheets, in their correct sequence. The clamping bolts should be pulled finger tight onto the louver plate. Then starting from the centre, tighten these nuts in the order. Do not over tighten and pull up evenly, preferably using a torque spanner. The louvre plate at the back of the gauge is placed with its slots angled upwards so that it directs the light rays from the electric lamps in such a manner that the actual water level in the glass appears as a plane of light when viewed from below. A ball valve is fitted at the lower end of the gauge to shut off the water in the event of the glass plates shattering. It should be noted that some forms of double plate gauge glasses could be placed on the boiler upside down. This places the ball valve at the top of the gauge, where it rolls down and obstructs the steam passage, so causing a false reading. It is thus advisable to mark this type of fitting so as to clearly identify the top and bottom ends of the gauge. When installing a new gauge glass, first shut the steam and water cocks, and open the drain. Remove the defective unit and fit the new gauge. Leave it in this condition, with the steam and water, cocks closed and drain open, to heat up for some hours. Then just crack open the steam cock. After about twenty minutes follow up the clamp nuts in the correct sequence, preferably using a torque spanner. Then close the drain, and fully open the steam and water cocks, to put the gauge into operation. Do not stand directly in front of the gauge during these operations in case the glass shatters. Remote reading type water level indicator suitable for a high-pressure water tube boiler: Difficulty is often experienced in observing the water level as indicated by the direct reading water level gauges mounted on the steam drums of water tube boilers. Thus it is usually considered necessary to provide an additional means of indicating the water level at same point convenient to the starting platform or control room. This can be done by a remote reading indicator such as the (Igema) gauge shown in the figures below.

This consists basically of a U-tube, the two legs connected to the steam drum as shown. Red indicating fluid, which is insoluble in water, fills the lower end and remains there since its density is greater than

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that of the water. Above this fluid the two legs of the U-tube are filled with water; one being kept filled to a constant head by means of steam condensing in the un-lagged condenser. The level in the other leg corresponds to that in the steam drum. Thus the heads supported by the indicating fluid vary. As the water level in the drum rises so it tends to balance the constant head, and the indicating fluid rises in the glass. The opposite happens when the drum water level falls, the level of the indicating fluid in the glass also falling. The sharp contrast between the red indicating fluid and the water enables the operator to see the indicated water level at a glance. A completely empty or full glass is immediately obvious. When taking the boiler out of service, shut off the remote indicator by first closing the gauge isolating valve (3), then the steam isolating valve (1), and finally the water isolating valve (2). When opening up, first open the steam valve (1), then the water valve (2), and finally the gauge-isolating valve (3).

Igema Type Remote Reading Water Level Indicator:

If the remote indicator is connected to the same balance connections as one of the direct reading water level gauges, it is important that the remote indicator is isolated before the water level gauge is blown through. Otherwise water may be drawn out of the legs of the U-tube so causing a false water level to be indicated by the remote reading gauge. After cleaning, etc. the following procedure should be carried out to refill the indicator. First close the isolating valves (1) and (2) on the boiler, and the regulating screw (4). Remove all filling plugs.

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Then pour in the indicating fluid through the indicator filling plug (5) until the lower part of the U-tube is completely filled, fluid overflowing at the filling plug. Close the gauge-isolating valve (3) and replace the filling plug (5). Then slowly pour distilled water into the water filling plugs (6) on top of the dirt traps until it overflows. Replace the filling plugs. Finally pour water into the top filling plug (7), until again it overflows the plug is then replaced. The remote reading gauge glass should now show completely red. Leave it in this condition until full boiler pressure has been raised. When the boiler is under steam open the steam valve (1), then the water valve (2) followed by the gauge-isolating valve (3). Leave for about 15 minutes to settle down, then crack open the regulating screw (4) and slowly bleed off excess indicating fluid, dropping the level about 6 mm at a time, with about fifteen minutes between, until finally the level of indicating fluid at the centre of the glass corresponds to the water level at the centre of the direct reading water level gauge glass. Although subjected to boiler pressure, the remote indicator glass is not at high temperature and very rarely gives trouble. However, the apparatus should be cleaned out about once every six months. The indicator should be isolated, drained, and the flushed through with clean water. The indicator must never, under any circumstances be blown through either with steam or water. The glass is illuminated from behind, access to these lights being obtained by removing the sheet metal easing at the back of the gauge. **************************kv************************ SAFETY VALVES: The function of a safety valve is to prevent excessive pressure from building up in a steam boiler. It prevents boiler pressure from rising above a certain predetermined pressure by opening to allow excess steam to escape into the atmosphere when that point is reached, thus guarding the boiler against possible explosion. Each boiler must have at least one safety valve but, if the boiler has more than 46.4 sq m of water heating surface, it should have two or more safety valves. In any case, a safety valve should have a capacity such as to discharge all the steam the boiler can generate without allowing pressure to rise (accumulation) more than 10% above the maximum allowable working pressure, which is normally stamped on the boiler. Safety valves are to be set to operate (set pressure) at a pressure not exceeding 3% over the maximum allowable working pressure (maximum design pressure) of the boiler. When a boiler is equipped with a superheater and with safety valves on both the superheater outlet and the steam drum, the safety valve on the superheater should open first. This maintains adequate flow of steam through the superheater and prevents the superheater elements from being damaged by high temperature gases, which would result it all the steam, were to be discharged directly through the drum safety valves. The safety valve on the superheater outlet header is to be set at a pressure not more than: Set pressure of drum safety valves -- 0.35 kg/cm2. To prevent starvation of superheater element and subsequent overheating, the safety valve on the superheater is also made to reseat (close) last, having the longest ‘blow-down’ amongst all the other safety valves fitted.

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The difference between the set pressure (opening) and closing pressure (re-seating) of a safety valve is called ‘Blow down’ or ‘Blow back’. Too much blow down causes wastage of steam, too less would give an unstable condition and it should he limited from a minimum of 1 to a maximum of 4% of the set pressure. The minimum blow down (ASME-code) should not be less than 0.14 kg/cm2. Types of safety valves: The most commonly found safety valves, known as spring loaded safety valves, are available in various categories.

1. ‘Ordinary lift safety valve’ in which the valve member lifts automatically a distance of at least one twenty-fourth of the bore of the seating member, (or the lift: D/24), with an overpressure (accumulation) not exceeding 10% of the set pressure.

2. ‘High lift’ type, with a lift = D/16, accumulation not exceeding 10% of the set pressure. 3. ‘Improved high lift’ type, with a lift D/12, accumulation not exceeding 10% of the set pressure. 4. ‘Full lift’ valves with a lift = D/4, accumulation not exceeding 5% of the set pressure. 5. ‘Pilot operated safety valve’, the operation of which is initiated and controlled by the steam

discharge from a pilot valve which is itself a direct operated safety valve. Two independent pilot device systems shall be provided for each main safety valve. The lift of the main valve shall be achieved with an over pressure not exceeding 5% of the set pressure.

Spring loaded Safety Valve for Low Pressure Boiler:

Description of low lift safety valves: For low-pressure boilers, the most common valves found today are the improved high lift type of safety valves, which are developed from the ordinary spring-loaded valves. In a spring loaded valve, the valve lid is subjected to boiler pressure, which tends to open it against the compression of a heavy spring, the tension of which can be adjusted by a compression nut screwed into a bush in the top cover. When boiler pressure exceeds spring pressure, the valve opens and waste steam escapes to atmosphere.

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The lip on the periphery of the valve disc gives additional uplift once it is raised from the valve seat by steam pressure. This additional uplift helps to counteract the increase in spring load as the spring is compressed by the valve lifting. Description of high-lift and improved high-lift safety valves:

Improved High-Lift Safety Valve:

In the case of the ‘Cockburn’ high-lift and improved high lift valves, a further additional lift is obtained through the pressure in the waste steam space acting on a piston connected to the valve spindle. Thus the waste steam energy is utilised to assist the valve lift.

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Arrangement of Uplift Piston Cockburn Improved High-lift Safety Valve:

Line Diagram of Improved High Lift Safety Valve:

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With reference to above figures, the waste-steam pressure acts on area 'A' of the piston member, moving vertically in a loose or floating ring held down by the pressure on the annular area D. In the event of piston member and loose ring adhering, the combination is still operative -- the loose ring also lifts with the spindle. A split compression ring is fitted to fill the gap between he collar of the adjusting nut and top of the cover push this prevents any alteration of setting and to further safeguard the adjustment, a cap is fitted over the spindle. Through slots in the cap and spindle a cotter is pad-locked in place. Adequate clearances must be maintained at the cotter, above the top of the valve lid and t the top of the spindle so that the valve can open freely. The easing gear associated with this type of valve is also compulsory. This helps in lifting the valve by hand, when in emergency, from either a remote or local position. The forks on the easing gear shaft fit under a collar on the cap. Turning the shaft lifts the cap until the cotter lifts the valve spindle and the valve against the spring compression to relieve the boiler pressure.

A safety valve, which has, in recent years, gained in popularity because of its relative simplicity and consequent low cost, is illustrated above. These valves are currently fitted to auxiliary tank type and water tube boilers operating below 35bar.

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High lift valve of this type consists of hardened chrome steel balls between the spindle, valve disc holder and the valve disc. This arrangement ensures correct alignment for safe operation and proper sealing of the valve under all conditions. Single ring 'blow down' or closing control is an additional feature of this valve. The blow down ring can be adjusted up or down and serves dual purpose. On initial valve lift, steam deflected by the blow down ring acts on the enlarged part of the valve disc and creates additional upward or opening thrust giving quick and additional lift. Secondly, when the valve is closing the blow down ring, creates a retarding effect with the waste steam cushioning the final valve seating. Raising or lowering the ring giving an increase or reduced blow down respectively thus controls degree of blow down. The Hopkinson ‘Hylif’ Safety Valve:

Hopkinson ‘Hylif’ Single Spring Safety Valve:

Materials: Body: cast steel; Valve: Platnam; Valve guide: Platnam; Seat guide: Platnam; Valve spindle: Stainless steel. Composition of Platnam: Nickel: 54%; Copper: 33%; Tin: 13%; Iron: 0.5%; Aluminium: 0.3%. These valves, incorporating a full-lift feature, are designed for working pressures of up to 900 lb/sqin; the arrangement being as shown in the figure above. When the steam pressure rises to the set pressure, the valve discharges with a small lift on the principle of the ordinary safety valve. This initial opening allows the escaping steam to exert its pressure over the full area of the bottom of the valve and increases the lift until the bottom face of the valve has entered the valve guide, at this point the escaping steam is deflected downwards by the bottom edge of the guide, and the consequent reaction pressure lifts the valve to its full open position (see detail). At this final stage of valve lift the discharge area between the scat and the valve is claimed to be equal to the net area through the scat throat, and the discharge capacity is at its maximum. When the discharge pressure has been relieved the valve begins to close; as it emerges from the valve guide the reaction pressure ceases and the valve shuts down cleanly with out slimmer.

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Full bore safety valve (relay operated type):

Full Bore Safety Valve:

Conventional spring-loaded valves have limitation to deal with steam at high pressures and temperatures. Full bore relay operated safety valve is well suited for high pressure boilers, having no springs on the main valve to be affected by high temperature steam. The control valve (relay) seat is small and is less liable to distortion. The boiler pressure acting on the main valve provides positive closing of the valve, which is reverse condition to that of a spring-loaded valve. The discharge capacity is four times that of an ordinary spring-loaded valve, of equivalent size. For the valve to operate, first the pilot valve lifts at its set pressure and blanks-off the parts leading to the atmosphere. The steam pressure then builds-up and acts upon the operating piston attached to the main valve spindle. This piston has about twice the area of the main valve and the forces set up cause the main valve to open by one-quarter of its diameter, so giving full bore conditions for the boiler steam to the atmosphere through the waste steam pipe. When the excess pressure has been relieved from the boiler, the pilot valve closes, so opening the ports that vent the operating steam to the atmosphere. The escaping steam helped by the valve's return spring, closes the main valve. For superheater application, the control valve can be fitted on the steam drum so that it operates on saturated steam. In this way, the control valve spring and the opening piston are protected from the high temperature superheated steam. The main valve, of course, is fitted on to the superheater outlet header.

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Up to a pressure of 100 bars, this valve gives satisfactory result except that the material of the valve parts is then of high alloy-steel. Safety valves of each boiler may be fitted to a common chest connected to the boiler with only one connecting neck. Each safety valve chest is to be drained by a pipe fitted to the lowest part and led with a continuous fall to a tank, clear of the boiler. No valves or cocks are to be fitted to these drain pipes. Adjustment of safety valves on tank boilers:

1. Before attempting to adjust the safety valves of any boiler, it is essential that the accuracy of the boiler pressure gauge to be verified. There should be at least two pressure gauges to be verified while the safety valves are being adjusted.

2. Consider that the valves have been assembled correctly without the top hood and the easing gear. Cheek the drains and waste steam pipes and make sure that those are clear.

3. Steam pressure is raised and the boiler put on banked fire, light load to maintain steady pressure during setting process.

4. Screw down each compression nut a few turns more than the previous setting. 5. Raise the boiler pressure and maintain the blow-off pressure at which the safety valves are

required to be set. 6. Adjust one valve at a time. Slacken the compression nut slowly till the valve on test lifts under

the set steam pressure. 7. Stop firing immediately when the valve lifts. Keep tapping the spindle top softly for the valve to

sit back smartly and remain seated in place. This setting of the valve would be slightly less than the blow-off pressure.

8. Restore water level if necessary and start raising the boiler pressure back. 9. Try-out again for lifting (floating) of the safety valve to check if the pressure at which the valve

lifts and sits back is correct; if not, re-set the valve with the same procedure. 10. The valve setting is done with a bit of trial and error procedure and with practice can be achieved

fairly quickly. 11. On valves with blow-down control, the blow-down ring is initially set at a particular position as

per the manufacturer's instructions and fine adjusted during the floating of the safety valve. 12. Once set, the valve adjusted, should be gagged (by means of special tool) and the other valve

should now be floated and adjusted with the same procedure as the first one. Note: The gag fitted should only be finger tight on the spindle and should never be fitted when the boiler is cold, to avoid any chance of the spindle bending due to thermal expansion as the temperature rises.

13. The gag from the first valve is now removed. 14. Next the boiler is fired again to cheek that both the safety vales adjusted open and close

simultaneously. 15. A caliper measures distance from the bottom of the compression nut to the top of the safety valve

cover bush and split ring is made and fitted in that space. 16. Next the hood is locked to the spindle with a cotter key and the easing gear is fitted into place. 17. Both the safety valves are now manually lifted by means of the easing gear to ensure its correct

operation. 18. A padlock then locks one end of the hood cotter key.

For the floating of the safety valve, the surveyor brings along his own pressure gauge - this helps in checking the accuracy of the boiler pressure gauges.

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Accumulation Test: The pressure rise in excess of working pressure is termed accumulation pressure. It is a requirement that when initially installed, accumulation tests are to be carried out on the safety valve of boilers. For cylindrical and unfired boilers - during a test of 15 minutes with the stop valve closed and under full firing conditions the accumulation of pressure is not to exceed 10 per cent of the design pressure. During this test no more feed water should be supplied than is necessary to maintain a safe working water level. For water-tube boilers - test to be carried out with the stop valve closed and under full firing conditions for a period not exceeding 7 minutes. The accumulation is not to exceed 10% of the design pressure. Where accumulation test might endanger the super-heaters or economisers, the test may be omitted provided that the safety valves fitted are of the approved type. *********************Kv************************* BLOW DOWN VALVES AND BLOWING DOWN OPERATION: The Bottom Blow-down Valve is connected to the lowest part of the boiler and is used on occasions when boiler is required to be emptied or for periodic short or flash blow-downs to remove accumulated deposits from the boiler drums; this keeps the boiler water density, alkalinity and chloride level under control. This valve must be fluid-tight at all pressures and parallel slide valve design is so chosen on-account of its reliability in this respect. Here, two parallel discs slide between parallel seat faces. A light spring holds the discs against the seats when the valve is not under pressure. When the valve is closed, and under pressure, the disc at the outlet side is held in contact with its seat by the pressure. On being opened, the discs slide over the seat faces until they are completely clear of the bore of the seat, thus giving an unobstructed passage through the valve and causing a minimum pressure drop. The sliding action, in opening and closing, removes from the seat faces any grit or sediment, which might otherwise build up, and cause leakage past the seat faces.

The Surface Blow off Valve, located at the top of the boiler is used to blow lightweight impurities from the water line. The valve is connected to a scum pan or a slotted pipe, which is fitted inside the boiler shell or drums and placed at the normal water level. The Surface Blow Valve should be used wherever evidence of foaming or oil is noted in the gauge glass. These valves should never be left unattended when they are in use and never more than one half a gauge glass should be blown down to the desired half-glass level.

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Blow Down Arrangement: A blow down piping system will have a Shipside blow-down cock or valve, which would discharge the boiler effluent overboard. This blow-down cock is connected to the bottom blow-down valve via a non-return valve. The non-return valve prevents any back flow either from other discharging boilers or from the sea (in case of vacuum formation in the boiler and skin cock left open). Blowing Down Procedure (For a Auxiliary Boiler 10 bar working pressure): To blow the boiler right down, the following should be done in sequence.

(a) Take the boiler out of service. (b) Allow pressure to fall to about 4 bars. (c) Open shipside cock. (d) Open scum valve and scum boiler before blowing down and then close scum valve. (e) Open blow down valve and continue to blow down. (f) Stop blow down when noise level falls, pressure is observed to be low and the pipe next to the

blow down cock gets cold. Close the bottom blow down valve. (g) Then close the shipside cock. (h) Wait till pressure in the boiler is near atmospheric and open the air vent to prevent vacuum

formation inside the boiler. It is always advisable to open shipside cock first and close it last, so as to avoid undue pressure rise on the blow down line, which is very liable to corrosion. Blowing down to reduce density and sludge is also regularly carried out but these lasts only for a few seconds and can be carried out when the boiler is running on load. Very careful attention has then to be paid to the water level and it is always advisable to reduce the boiler load as much as possible and increase water level in the gauge glass before a short blow down exercise is carried out. The shipside valve has a special guard fitted so that its operating spanner cannot be removed when the cock is in operation and the spanner can only be removed when the cock is fully shut. This arrangement prohibits the possibility of the cock remaining open after the blowing down operation has been over. The spanner is kept fixed to a hook on the adjacent bulk- head frame next to the cock for ready use. If the vessel is in port, check that ships side is clear in way of the outlet from the blow down line and inform the deck department that blowing is to be carried out.

Bottom Blow Valve: Ship side Blow down COCK: **************************Kv**************************

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Steam Stop Valve:

Cast Steel Main Stop Valve: Cast Steel Self-Closing Main Stop Valve for Tank Boilers: for Water Tube Boilers: At one time steam stop valve and safety valve chests were made of cast iron, but the liability of this material to fracture under shock loading, which, may occur from water-hammer and other causes, has resulted in its use being superseded by steel. Main stop valves for ordinary tank boilers are usually of the screw-lift type, whereas in the case of water-tube boilers non- return valves are normally fitted-the reason for the differentiation is seen when a comparison is made of the evaporative power and water capacity of the two types of boiler. If screw-lift stop valves were fitted to each of a battery of four water-tube boilers and while steaming hard a serious tube burst occurred, the contents of the four boilers (6 tons each against 30 tons for a Scotch boiler) could very soon be lost through the ruptured tube. The non-return or self-closing stop valves fitted to water-tube boilers act, therefore, as a safeguard against loss of water. Contrary to the observation that non-return or self-closing valves are normally fitted as main stop valves in water-tube boiler installations, it must be mentioned that the Liberty class vessels built during the last war are fitted with screw-lift-type valves. The reason is that when non-return valves are used to supply steam to a reciprocating prime mover they are very prone to damage through hammering. Types of Steam Stop Valve: The types of main stop valve in general use are legion, and it is not proposed to detail them all. The main stop valve as fitted to the ordinary tank-boiler shell is normally a right-angled cast-steel globe valve with a pressed-in pinned gunmetal seat, the gunmetal lid guided in the seat by wings or a center pintle and the screwed spindle attached to the valve by a nut and collar, working in an external bridge on the chest cover. The material of the valve lid and scat is Monel metal in the case of stop valves used in conjunction with superheated steam. In all cases the stop-valve chests must be fitted with ample drainage arrangements. The main stop valves of water-tube boilers, mounted on the superheater outlet header, operate under high temperature and pressure conditions, 850*F. and 850 lb/sq.in. frequently being used. In view of the high temperatures to which these valves are subjected, it is important that suitable materials are used in their construction. Under the combined effect of high temperature and stress some materials alter their physical properties and progressively ‘flow’ or ‘creep’ in a manner

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similar to that of an extremely viscous fluid. The materials used must, therefore, have creep strength in excess of their service loading throughout the operating temperature range. For stop valves dealing with superheated steam temperatures up to 800*F. it is usual for the valve chest to be made of normal cast steel, with a forged or cast-steel cover, the valve lid and seat being either Monel metal, stellited steel or stainless steel, according to temperature conditions. When the steam temperature is above 800*F. heat resisting alloy steels are used, 0.5% molybdenum cast steel for the valve-chest cover and seat, stainless steel for the valve lid and creep-resisting steel for the cover studs. Securing of Valve Seat: The valve-seats are secured in the chests in several ways:

(a) The seat is made with a slight interference fit, pressed in cold, after which the chest is peened over the top edge of the seat.

(b) The scat is screwed in with a fine thread, the top of the scat having a collar, which lands on a

facing in the chest.

(c) The seat and guide for the valve are combined in one unit, which is secured by set bolts to the

valve chest, and is thus easily removable.

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FEED CHECK VALVE: The basic duty of the feed valve is to shut off the flow of water to the boiler when necessary but there must also be an arrangement to prevent reversal of flow through the feed line from the boiler towards the pump side which way cause serious damage to the line. So there are two types of designs available- one is a stop valve which includes a device for preventing a reversal of flow even when the valve is open and another comprises two valves one shut-off and one cheek valve, all housed in one single chest.

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In the combined stop & Check valve, a dashpot arrangement is provided. When the valve is opened, the valve head, not being connected to the spindle, is lifted from its seat by the water pressure at the inlet and is free to re-seat itself independently in the event of a reversal of flow. It is very important that the contact of the valve and seat is fluid tight and very accurate machining work and fitting is done. These valves are always fitted with the spindle vertical so that there will be less chances of misalignment. These could be either angle-type or straight- type valve.

Combined Feed Check and Shut-off Valve:

Screw-Down Non-return Feed Check Valve:

With two independent valves in one chest, care should be taken when overhauling the valves to see that the attachment of the shut-off valve lid to its spindle is good since if the valves becomes detached it will stop any feed entering the boiler.

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Marine Boiler Fuel System: The vast majority of steam ships will currently be fuelled with a residual type of oil, which is likely to be of high density and viscosity. The reception and storage of this aboard ship is of importance with regard to successful boiler operation. Oil is fed to the boiler via pumping and heating equipment so that the oil arrives with the correct viscosity and energy for the atomising system in use. The pipe system conducting the fuel is arranged so that prior to lighting up, adequately heated oil can be recirculated providing a supply at suitable conditions as soon as light up is attempted. Flexible connections between the burner front manifold and the individual burners must be kept as short as possible to limit the amount of cold oil within. In addition the pipe system will include fuel flow control valves actuated by the automatic combustion control system, isolating valves, quick closing valves actuated by safety devices such as low water level and individual burner shut off valves. The choice of combustion equipment and design of combustion chamber are complementary to the attainment of the three ‘T’s -time, temperature and turbulence- to a degree necessary for good combustion. Time is required for combustion air and fuel to mix and to burn completely within the confines of the combustion chamber. Modern marine boiler plant provides for this with larger furnaces than were previously found. Its most advanced form is in radiant boiler designs with roof-mounted burners firing down the long vertical axis of the furnace with an outlet to one side at the bottom. High temperature is needed to vaporize the fuel and to ensure rapid ignition. Most combustion equipment incorporates some form of bluff body to create a low pressure area and recalculation zone to drawback some of the atomised and ignited fuel into the path of incoming fuel spray, creating a stable area of high temperature. Turbulence is necessary to aid mixing of fuel and air so that complete combustion can be achieved without the need for more air than that required to consume the combustibles carbon, hydrogen and sulphur. The arrangement of the air admission apparatus is important. The apparatus comprises the air inlet trucking, the wind box containing the air registers and the air registers them selves controlling the air to each burner. The design of the furnace chamber is also of some importance as, for example, in the Combustion Engineering tangentially fired furnace and in other cases where arches or similar projections have been made into the furnace with the object of encouraging turbulence. Generally speaking such devices have proved unnecessary as shown by the success of radiant boilers where sufficient time and turbulence have been obtained with burners firing, substantially, vertically downwards. Simple furnace shapes and circular air register designs are widely available. Of further importance for efficient, complete combustion of heavy fuel oil is the design and performance of the fuel atomiser. There are a number of systems available involving the energy either contained in the pressurised fuel itself or in a separate atomising agency. Using the pressure of the fuel is the oldest and simplest method, but, since oil flow rate through the atomiser is proportional to the square root of the oil pressure, a very high maximum pressure is required if a large turn down is needed. Oil pressure at 70bar or more has been used but a value of 20bar is much more common. This will only allow a turn down to 70% of maximum oil flow at an oil pressure of 10bar, below which atomisation is seriously impaired. In this case in order to reduce the boiler output below about 70% of maximum, burners have to be shut down in sequence and relit when load increases again. At one time such a practice was common and great skill was achieved by firemen in anticipating the number of burners needed on each occasion. With modern, automatically controlled plant where there may be no one on the firing platform it is necessary from a safety aspect to have all burners firing at all times. This means that a turn down of at least 10:1 is desirable otherwise at very low steam demand it may be necessary to dump excess evaporation to the condenser with consequent fuel wastage.

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To achieve a very high turn down ratio without using excessive maximum oil pressure requires an atomising system where the energy in a separate atomising medium is used. One such system utilises a spinning cup rotated at high revolutions per minute by an electric motor. Oil is fed at low pressure onto the inside face of the cup, the spinning action of which causes the fuel to progress down the slightly conical surface and shear off the rim in a fine spray. Sufficient energy for atomisation is provided by the electric motor for all rates of oil flow. Another system uses a separate atomising fluid. Steam is the usual choice when available although when lighting up before steam becomes available compressed air may be used. Steam at 10bar or so is used with a maximum oil pressure of about 20bar. The steam and fuel mix within the atomiser just prior to the point of discharge, where the energy released by the steam shears the fuel into an extremely fine mist. This provides a simple system with no moving parts. Operation with a constant steam pressure and a varying oil pressure gives adequate turn down with the high quality atomisation necessary for complete combustion of heavy oil fuel with a minimum of excess air.

Foster Wheeler ESD Roof-Fired Mono-wall Oil/Natural Gas Boiler:

Natural gas as a fuel at sea will be found on ships designed to transport liquefied natural gas in insulated tanks. Since it is carried at virtually atmospheric pressure the liquid gas must be cooled to about 180*C, any heat leakage into the cargo resulting in some ‘boil off’. The degree of boil off will depend on the design of the tanks, the tank insulation, the nature of the voyage and weather conditions. This will be collected, heated and compressed then fed to the boiler through special gas burners, providing steam for all purposes.

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It is a requirement to always burn a pilot quantity of fuel oil and so if steam demand is greater than can be met from the boil off gas the oil quantity is increased. Use of gaseous fuels requires special care with regard to safety aspects and the requirements in this respect are covered by the classification societies. Most boiler types would be suitable for use with natural gas, the main consideration being that there should be no risk of pockets of explosive gas mixtures forming within the unit and that no such mixtures should leak into the machinery spaces. Top fired radiant boilers would appear to be less acceptable in this respect but this has not proved to be the case as arrangements are made to vent the top of the furnace into the uptakes via the division wall. This is in any case desirable even with oil fuel firing, as in certain circumstances small pockets of combustibles could accumulate in that zone. Prevention of leakage into the machinery spaces can be achieved by use of all welded enclosure walls or by double casings with combustion air between. Fuel lines to the burners are double pipes with inert gas in the annulus at a pressure greater than the fuel gas, which will be about 1 bar. A ventilation hood connected to an extractor fan is arranged above the firing platform so that there is a continuous sweep of air across the burner zone for discharge outboard with gas detection devices. During the 1980s a number of coal fired ships were built. There was much speculation concerning how best to deal with this renewed interest in a fuel, which had lost favour when oil first became plentiful and cheap.

Babcock Marine Radiant Coal fired boiler: Coal Fired very Advanced Propulsion: The predominant means of burning coal ashore was established in central power stations where the coal was pulverised to a fineness of 70% less than 75g and fired in a burner, which could also handle oil fuel as a support fuel.

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Translating this to the marine environment presented problems, as the grinding mills were bulky, heavy and susceptible to vibration. Indeed it was not clear whether the ship would adversely affect the mills or vice versa. An alternative was to pulverise ashore and bunker in this condition. This would have required the bunkers to be kept under inert gas as coal in a finely divided state presents a spontaneous combustion risk. At the same time great interest was being shown in combustion of coal in a fluidised bed. This had potential advantages for burning coal at sea in so far as combustion residues could be more easily dealt with and a wider range of coal types consumed. However, there were but few examples of this technology in use ashore and marine industry was not ready to adopt any process so important to the success of the ship if it had not already gained acceptance elsewhere. This left industrial experience ashore, of which there was a great deal where coal was burned on mechanical stokers. Bearing in mind the likelihood that coal quality would vary between bunkering ports and that a good response to changes in load demand was a requirement, the favored choice for use at sea soon became the spreader stoker. We will not go in to details of coal burning ships in these notes.

Section of Marine Bi-Drum Coal Fired Boiler:

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Combustion of fuel in Boilers: The combustion of a residual fuel oil in a boiler furnace takes in a number of stages. The oil is first heated in steam or electric fuel oil heaters. This reduces its viscosity and makes it easier to pump, filter, and finally to atomize. However it must not be overheated at this stage, otherwise a process known as 'cracking' occurs, leading to carbon deposits, and the formation of gas in the fuel oil lines, etc. The gas, due to its large volume, reduces the mass of oil passing through the burner, which in turn leads to a possible reduction in the steaming rate of the boiler owing to the reduced amount of fuel actually burnt. This gasification can also cause instability in the combustion process itself, resulting in a fluctuating flame formation. The heated oil is now passed through the burners where it is atomized; this process breaks it up into a fine spray of droplets, so presenting a very large surface area of oil to the combustion processes. The droplets formed are of two main types, i.e. very fine particles consisting of the lighter fractions of the fuel, which form a fine mist, and slightly larger droplets formed by the heavier fractions of the residual fuel. The burner also imparts rotational energy to the fuel so that it leaves the burner tip as a hollow, rotating cone formed of fine droplets of oil. The combustion stage itself can now commence, and in a boiler furnace a type of combustion often referred to as a 'suspended flame' is used. For this a stream of oil particles and, air enters the combustion zone at the same rate at which the products of combustion leave it. The actual flame front therefore remains stationary, while the particles pass through it, undergoing the combustion process as they do so. The combustion zone itself can be sub-divided into two main stages; these are referred to as the primary and secondary flames. PRIMARY FLAME: Heated Combustion Air:

Combustion in a Boiler Furnace:

For the oil to burn, it must be raised to its ignition temperature, where continuous vaporization of the oil required for its combustion takes place.

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Note this temperature should not be confused with the flash point temperature of the oil, where only the vapour formed above the oil in storage tanks, etc. will burn. The ignition or burning temperature should normally be at least some 20*C above this value. For the reasons already stated this ignition temperature cannot be obtained in the fuel oil heaters, and therefore the heat radiated from the flame itself is utilized so that, as the cone of atomized oil leaves the burner, the lighter hydrocarbons are rapidly raised to the required temperature by the heat from the furnace flame; they then vaporize and burn to form the primary flame. The heat from this primary flame is now used to beat the heavier constituents of the fuel to their ignition temperature as they, together with the incoming secondary combustion air, pass through the flame. The stability of the combustion process in the furnace largely depends upon maintaining a stable primary flame and, to ensure it is not overcooled, a refractory Quarl is usually placed around it so as to radiate heat back to the flame. The primary flame should just fill the Quarl. If there is too much clearance excessive amounts of relatively cool secondary air enter the furnace; too little and the heavier oil droplets impinge on the Quarl and form carbon deposits. Another important factor for the formation of the primary flame is that it must be supplied with primary air in the correct proportion and at the right velocity. In the case of air registers using high velocity air streams this is done by fitting a tip plate which spills the primary air over into a series of vortices, as indicated in the figure. This ensures good mixing of the air and fuel and, by reducing the forward speeds involved, helps to maintain the primary flame within the refractory Quarl. SECONDARY FLAME: The larger oil droplets, heated in their passage through the primary flame zone, then vaporize and begin to burn. This, although a rapid process, is not instantaneous, and so it is essential that oxygen is supplied steadily and arranged to mix thoroughly with the burning particles of oil. An essential feature for the stability of this suspended secondary flame is that the forward velocity of the air and oil particles must not exceed the speed of flame propagation. If it does the flame front moves further out into the furnace and the primary flame will now burn outside the quarl with resulting instability due to overcooling. Careful design of the swirl vanes in the air register can be used to create the required flow patterns in the secondary air stream. The secondary flame gives heat to the surrounding furnace for the generation of steam. Sufficient time must be given for complete combustion to take place before un-burnt oil particles can impinge onto tubes or refractory material. This usually entails the supply of a certain amount of air in excess of the theoretical amount required for complete combustion if these practical considerations could be neglected, and unlimited time taken for the mixing of the air and fuel. The actual amount of excess air supplied depends upon a number of factors, such as the design of the furnace, the efficiency of the combustion process for the condition of load, etc., but will in general reduce the boiler efficiency to some extent due to the heat carried away by this excess air leaving the funnel. It can also lead to increased deposits in the uptakes due to the increased amount of sulphur trioxide that will form from sulphur dioxide in the presence of excess oxygen. Fuel Oil Burners: A pressure jet oil burner forms a simple robust unit, widely used in marine boilers. The basic assembly consists of a steel tube, or barrel, to which are attached swirl and orifice plates; these are made of a high grade or low alloy steel, and are held in place by a cap nut. The complete unit is clamped into a burner carrier attached to the boiler easing. This both holds the burner in its correct position relative to the furnace, and also permits the, supply of fuel through an oil tight connection.

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Key: 1: Brick retaining ring; 2: Closing flange; 3: Sprayer carriage; 4: Burner base plate; 5: Sprayer body; 6: Tip plate; 7: Carrier tube; 8: Coupling block; 9: Shut-off cock; 10: Centre tube; 11: Annular port; 12: Flexible hose; 13: Hose coupling; 14: Air vanes; 15: Air shut-off tube; I6: Air shut-off runner; 17: Air shut off handles; 18: Observation port; 19: Sight door; 20: Brick throat; 21. Wildish bolt.

Modern Water-Tube Boiler Front or Register: Some form of safety device must be fitted in order to prevent the oil being turned on when the burner is not in place. The oil is supplied to the burner under pressure and, as it passes through, the burner performs two basic operations. First it imparts rotational energy to the oil as it passes through angled holes in the swirl plate. The rotating stream of oil thus formed is then forced under pressure through a small hole in the orifice plate, which causes the jet to break up into fine droplets. This latter process is referred to as atomization, although each individual droplet of oil is formed of vast numbers of atoms. As the final result of these operations a hollow rotating cone, formed of fine particles of oil, leaves the burner tip. Many variations of design exist for the swirl and orifice plates. The choice of the actual design used often depends upon the means employed by the manufacturer to carry out the accurate machining processes required for these items. In this type of burner control over the throughput of oil is obtained in two ways, by varying the oil supply pressure and/or by changing the diameter of the hole in the orifice plate. Limitations exist which prevent either method being used as the sole means of control over a wide range of throughput.

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Y-Jet Steam atomizer:

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The ratio of the maximum to minimum oil throughput of the burner is known as the turn down ratio of the burner, and in the case of pressure jet burners this can be stated in terms of the square root of the ratio of the maximum to minimum oil supply pressures. In all pressure jet burners however, a minimum supply pressure in the order of 700 kN/m2 is necessary to ensure efficient atomization is maintained. At the same time various practical considerations limit the maximum pressure to about 7000 kN/m2, thus the turn down ratio with this type of burner is limited to a value of about 3.5. If a wider range of turn down is required a system incorporating a number of burners is used, which controls the overall turn down on the basis of the number of burners in operation, or changing the orifice size in addition to the variation in supply pressure considered above. However, while this system is convenient for manual operation, it is not suitable for automatic control due to the need to change orifice sizes when the oil supply pressure reaches its upper or lower limits. The burners must be kept clean and care should be taken during this operation not to damage or scratch the finely machined surfaces of the swirl and orifice plates. The latter should be renewed as the orifice wears beyond a certain amount. This should be checked at regular intervals by means of a gauge. After cleaning make sure all the various parts are correctly assembled. Any oil leaks must be rectified as soon as possible as they can lead to fires in the air register or double casing of the boiler. Burners not in use should be removed otherwise the heat from the furnace will cause any oil remaining in the burner barrel to carbonize. Rotating cup type of fuel oil burner:

A rotating cup oil burner atomizes the oil by throwing it off the edge of a tapered cup being rotated at high speeds of between 2000-7000 rpm by either an air turbine driven by primary combustion air, or by an electric motor driving the cup shaft by means of ‘vee’ belts.

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The basic assembly consists of a tapered cup fitted onto the end of a central rotating spindle mounted on ball or roller bearings. The fuel oil is supplied to the inner surface of the cup through the hollow end of the spindle. Here centrifugal force causes it to spread out evenly into a thin film, which then moves out along the taper until it reaches the lip of the cup, where the radial components of velocity cause it to break up into fine particles as it passes into the surrounding air stream. Thus like a pressure jet burner this type of burner performs two functions: first, supplying rotational energy to the oil, and then breaking it up into fine particles. The final result is a hollow rotating cone of oil droplets leaving the burner.

Spinning Cup Bearer:

High oil supply pressure is unnecessary as this pressure plays no direct part in the atomization process, and only sufficient pressure to overcome frictional resistance to the flow of oil through the pipes is required. Thus this type of burner can be used with a gravity type oil fuel supply system. The oil throughput is controlled by a regulating valve placed in the fuel supply line, and thus can easily be adapted to automatic control. Here the wide turn down ratio available with this type of burner is a great advantage. Values of over 10: 1 are possible. The diameter of the cup must be large enough to handle the required throughput, and there must be sufficient taper and rotational speed to ensure the oil is thrown off with the desired velocity. These factors govern the maximum oil throughput of the burner; the minimum throughput is limited only by the fact that sufficient oil must be supplied to maintain a continuous film of oil inside the cup so as to provide a stable primary flame. The flame produced by a rotating cup burner tends to be long and cigar shaped, although a shorter flame can be obtained by careful design of the swirl vanes in the air register, so as to direct the flow of air in such a manner as to give the desired flame shape.

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In the smaller units it is possible to supply all the combustion air through the burner itself, the air flowing through the space between the rotating cup and the fixed casing. However, in most cases only the primary air, which in this type of burner is used mainly for atomization, is supplied in this way. It only forms about 10% of the total air required, the remainder being delivered through a secondary air register, to which it passes by means of a separate air duct with its own forced draught fan. This type of burner is difficult to design for very large throughputs, and still give the required flame shape, and so while very suitable for auxiliary boilers with their relatively small outputs, they are not in general use for main water tube boilers. Here if more than one burner is to be fitted, the complication inherent in each rotating cup burner, with its own drive motor, makes other systems of atomization more suitable. Also rotating cup burners cannot be used in roof-fired boilers. Steam blast jet type fuel oil burner: With automated control systems it is advisable to avoid extinguishing and re-igniting burners while manoeuvring, etc. It is also impracticable to change the size of the atomizing tip automatically. Thus simple pressure jet burners with their limited turn down ratios on a single orifice size are not suitable, since it is necessary to use a type of fuel oil burner with a large turn down ratio. Various forms of these wide range burners are available, and one type in common use is the blast jet burner.

Key: 1: Atomiser handle. 2: ‘O’ Rings. 3: Covering door. 4: Locking ring. 5: Distance piece. 6: Atomiser outer barrel assembly. 7: Atomiser inner barrel assembly. 8: Cap nut. 9: Sprayer plate. 10: Atomiser tailpiece. 11: Safety shut-off valve body and seat. 12: Atomiser body. 13: Coupling yoke. 14: Hand wheel and spindle.

‘Y-Jet’ Steam Atomiser: These atomize the oil by spraying it into the path of a high velocity jet of steam or air. Although either medium can be used, steam is usually both more readily available and economical at sea. Compressed air is therefore seldom used, except when lighting up from cold. ‘Y’- Jet Type: In this the steam flows along the central passage, and is then expanded through a convergent divergent nozzle, where its pressure energy is converted to kinetic energy resulting in a high velocity jet of steam. Oil sprayed into this jet is entrained by it, being torn to shreds and atomized in the process. The exit ports are arranged tangentially, thus giving the necessary swirl to the oil droplets in order to form the hollow rotating cone of fine particles of oil needed for the efficient combustion of a residual fuel oil in the boiler furnace. However, the flame shape is not so clearly defined as those obtained with pressure jet type burners due to the entrainment of air by the high velocity steam. This enables simple air registers to be used. There is no need to fit the usual swirl vanes for the secondary air stream- only a venturi shaped throat and tip plate are required. Varying the oil supply pressure controls the throughput of oil. Since the atomizing effect is not obtained directly by the use of, pressure, the same limit is not imposed on the use of very low oil supply pressures as with simple pressure jet burners; large turn down ratios of up to 20:1 are therefore available with blast jet burners without having to resort to unduly high pressures.

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The oil supply pressure ranges from about 140-2000 kN/m2, with corresponding steam pressures of 140-1500 kN/m2. Care must be taken to use only dry steam, any water present having a chilling effect, which could cause flame instability. The steam may be obtained directly from the boiler, the pressure being dropped to the required value by passing it through reducing valves. Alternatively it may be obtained from an auxiliary source such as a steam to steam-generator. Excessive use of steam can be caused by incorrect setting of the burner, or by leakage across the joint faces in the atomizing head of the burner, and in some versions gaskets are fitted to prevent this. Steam is left on all the time the burner is in operation, even when the oil is turned off, in order to cool the burner and prevent any remnants of oil in the burner passages from carbonizing. Safety shut off valve is fitted to the burner carrier; these are opened by projections on the burner so that oil and steam are automatically shut off when the burner is removed. Air register suitable for the supply of combustion air to the furnace of a water tube boiler: The term air register is applied to the assembly of vanes, air swirler-plates, etc. fitted within the double casing of the boiler in way of each burner position, in order to supply the air required for combustion in the correct manner. The width of the easing at this point is determined by the design of the register used. The register performs the following functions: it divides the incoming combustion air into primary and secondary streams, and then directs these streams so as to give the air flow patterns necessary for the efficient mixing of the air with the hollow rotating cone of oil particles leaving the burner. Another important duty performed by the register is to regulate the amount of air supplied to the individual burner. Earlier types of air registers dealt with large amounts of air flowing at low velocities whereas later types, for corresponding amounts of fuel, admit smaller amounts of air moving at much higher velocities. Carefully designed swirl vanes are used to direct the air as required. Many variations in design exist, and here any constructional details apply only to the register shown in page 111, but some, or all of the components shown, are common to all types of air register. The combustion air must pass through the air check in order to enter the register. In some cases the swirl vanes themselves being rotated about their axes until they touch form the check, so shutting off the airflow to the burner. However, in most cases some form of sliding sleeve is used. The air check may be operated by hand, usually being placed in a fully open or fully closed position. However, in the case of wide range burners, especially where automated combustion control systems are fitted, a pneumatic means of operation is used. In some cases the cheek may be placed in an intermediate position in order to adjust the air supply to individual burners. Baffles are then often fitted to separate the air into primary and secondary air flows; these are concentric within the cylindrical register. The inner primary air stream flows along until it reaches the tip plate fitted at the end of the burner tube. Here the air impinges on the back of the plate, and then spills over to form a series of vortices which have the effect of reducing the forward velocity of the air and so helps to retain the primary flame within the quarl. Various designs of tip plates are used ranging from circular flat plates to more complex swirl vanes, but all perform the same basic function; i.e. the formation of the vortices, which is so important in modern air, registers with their high velocity airflows. The outer, secondary air stream passes over swirl vanes which cause it to rotate as it passes through the quart, so giving better conditions for the mixing of fuel and air in the secondary flame zone. Again by careful design the air flow pattern can be made to form a series of vortices. In this way the forward velocity of the burning oil particles is reduced, giving a longer period for combustion to take place within the furnace. The secondary airflow also has some influence on the flame shape; this is especially the case with rotating cup type burners.

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A small amount of cooling air is often allowed to flow between the inside edge of the tip plate and the atomizing tip of the burner. This amount of air must remain small otherwise it can upset the vortex system formed by the tip plate. It is important that the air check forms a tight seal in order to prevent combustion air entering the register when the burner is not in use, otherwise thermal shock caused by the relatively cool combustion air leaking through can damage the refractory quarles. An insulated front plate must be fitted in some cases to prevent over heating of the boiler front due to radiant heat from the furnace penetrating through the gaps between the swirl plates, etc. In some types of registers, especially those used with simple pressure jet burners with their small turn down ratios, very little adjustment of the relative positions between the various vanes, air-swirler plates, etc. can be carried out. In others a whole range of minor adjustments may be carried out to suit different fuels, conditions of load, etc. It may be noted that, in the case of steam jet burners, the steam provides additional energy for the mixing of the air and fuel, and. the swirl vanes for the secondary air stream may be omitted from air registers used with this type of burner. Boiler fuel oil system: Figure on page 118, shows the diagrammatic lay out of a semi-automatic fuel oil system, which makes use of wide range burners operating on a variable oil supply pressure. The basic layout consists of a ring main supplying the individidual burners by a series of dead legs. Oil is pumped into the settling tanks, and any Water allowed settling out. This can then be drained off by means of spring-loaded drain cocks. When required for use, the high suction valve on the settling tank is opened and oil allowed passing to the cold filters fitted on the suction side of the fuel oil service pump. Due to the high viscosity of the unheated residual oil only a coarse filter, just sufficient to prevent damage to the pump, can be used at this stage. This consists of a positive displacement pump operating at a constant delivery pressure. A spring-loaded relief valve fitted on the discharge side of the pump allows any excess oil to spill back to the suction side of the pump in the event of over pressure. After leaving the pump, the oil temperature is raised in a fuel oil heater. This is done in order to lower the viscosity of the oil, making it easier to filter and finally to atomize. The correct oil temperature is maintained by means of a thermostat placed in the outlet from the fuel oil heat or, which controls the supply of steam to the heater. The hot filters fitted after the heaters are normally of an auto-clean type; in some cases they are constantly rotated by electric motors. These filters provide a fine filtration. This prevents wear and chokage of the fine passages in the atomizing tip of the fuel oil burners. The heated and filtered oil now passes through an automatic pneumatically operated control valve which varies the oil supply pressure to the burners in response to, variations in the main steam pressure transmitted to a master pressure controller. The combustion air controller also varies in order to maintain the correct ratio between the amounts of fuel and air supplied to the furnace. Two emergency valves are now fitted; the first is a manually operated quick shut off valve, which enables the fuel oil to be shut off by hand from the boiler very rapidly in case of emergency. The second is a shut off valve with a steam actuator, which operates to shut off, the fuel oil in the event of loss of water in the boiler. The oil is now ready to enter the individual dead legs supplying oil to the burners. An isolating valve and a safety cock, or similar device, is fitted to each leg. To enable the oil temperature in the system to be brought quickly up to and then maintained at the desired operating value, a re-circulating valve is fitted which enables oil to be circulated through the ring main back to the pump suction. This valve is closed as the burners are brought into operation.

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Fuel Oil System for Boilers:

In the system shown above the individual burners are ignited by means of a paraffin torch, but in many cases automatic igniting devices are fitted. In this case it is advisable that a flame failure alarm should also be fitted. The system uses gas oil to flash up from cold. Air pressure supplied to the gas oil storage tank is used to force the oil through to the burner. The use of this oil continues until sufficient steam has been generated to enable the fuel oil heaters to be put into operation so as to raise the temperature of the residual fuel oil to the value required for its combustion. All necessary pressure gauges, thermometers, air vents, etc. must be fitted for the proper operation of the system.

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Safety fittings include quick closing valves-operated from outside the engine room fitted to the suction lines from the settling tanks, which enable fuel to be shut oil from the system in case of emergency. There is also an emergency stop fitted to the fuel oil service pump. All oil lines and fittings containing heated oil should be placed above the plates, in well-lit situations, so that any leakage can be easily detected. The system can easily be changed to manual control by simply bypassing the automatic control valve, and controlling the oil supply pressure by means of a hand jacking wheel on the spring-loaded relief valve governing the discharge pressure from the fuel oil service pump.

Line Diagram for Boiler Fuel Oil system:

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Refractory materials used in boilers: The basic requirement of boiler refractory materials is that they should contain the heat generated in the furnace. They must therefore have good insulating properties and be able to withstand the high temperatures to which they will be exposed. They must also have sufficient mechanical strength to resist the forces set up by the weight of adjacent brickwork, etc.; to withstand vibration; and the cutting and abrasive action of the flame and flue dust. The materials must also be able to expand and contract uniformly without cracking. At the present time no single refractory material can be used economically throughout the boiler, and the temperatures to which they will be subjected generally govern the choice of suitable materials for various parts of the boiler. The material from which these refractory are manufactured can be grouped into three main types:

1. Acid materials: Clay, silica, quartz, sandstone, gamister. 2. Neutral materials: Chromite, graphite, plumbago, alumina. 3. Alkaline or base materials: Lime, magnesia, zirconia.

It should be noted when choosing suitable materials that care must be taken to ensure acid and alkaline substances are kept apart as, at high temperatures, they can react to form salts which destroy the effectiveness of the refractory. These refractory materials are available for installation in one of two basic forms: 1. FIREBRICKS. These are formed into bricks and then fired at high temperatures in special kilns. 2. MONOLITHIC REFRACTORIES. These are supplied in an unfired state, installed in the boiler, and fired in situ when the boiler is put into service. This form of refractory can be subdivided into:

(a) Moldable refractory: This is used where direct exposure to radiant heat takes place. It must be pounded into place during installation.

(b) Castablerefractory: Placed behind water walls and other parts of the boiler where it is protected from direct exposure to radiant heat. It is installed in a similar manner to building concrete.

(c) Plastic chrome ore: This, bonded with clay, is used in the construction of studded water walls. It can resist high temperatures, but has little mechanical strength, and is pounded onto steel studs welded to the tubes. These studs provide both strength and means of attachment for the refractory.

All the forms of refractory materials previously mentioned must be securely attached to the boiler easing and, in addition to the above-mentioned studded tubes, various types of bolts, clips, and keys are used for this purpose. To prevent undue stresses being set up in the refractory, ample expansion spaces must be provided. Care must be taken to ensure these spaces do not become blocked in any way as this can cause the refractory to break away from its attachments and bulge out, with the danger of possible collapse. Refractory materials impose limits upon the time required for raising steam; the greater the amount of refractory, the slower the steam raising process must be in order to prevent damage to the refractory material. Air checks should be closed immediately the corresponding burner is shut off; otherwise the relatively cold air impinging upon the hot refractory causes a surface flaking, known as spalling, to take place. This leads to a reduction in wall thickness. Flame impingement must be avoided as it leads both to a build up of carbon deposits, and to damage caused by carbon penetrating into the refractory.

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Another form of damage is caused by impurities in the fuel, mainly vanadium and sodium salts reacting with the refractory material to form a molten slag, which then runs down to the furnace floor. This both reduces the wall thickness, and leads to a build up in the level of the furnace floor, which can eventually interfere with the shape of the flame. Soot blowers: In order to maintain the gas side heating surfaces of a boiler in a clean condition and so prevent an undue build-up of deposits, which can lead to corrosion and/or uptake fires, soot blowers are fitted. Soot blowers consist of two main parts, the head or chest, and one or more nozzles attached to a tube or spindle made of heat-resistant steel. An operating mechanism fitted in the chest rotating and, in some cases, retracting the nozzles as required. The efficiency of the blower depends upon the conversion of pressure energy in the blowing medium to kinetic energy; this results in a high velocity jet of fluid impinging upon the sooted surfaces. Although in a few cases the soot blowers operate with compressed air, the majority of marine soot blowers use steam. Superheated steam is normally used to maintain dry conditions, and also to ensure that the superheater is not starved of steam while blowing is in progress. Soot blower steam lines should be sloped so as to be self-draining to a suitable drain valve. These are often automatic to ensure they remain full open when the steam is shut off, closing when the steam is turned on, but not until sufficient steam has been allowed to blow through in order to warm the lines. The reason for leaving the drain full open, when the master valve supplying steam to the soot blowers is closed, is to prevent any leakage past this valve leading to a build up of pressure, which would force steam to leak past the blower seal rings and so cause corrosion of the blower nozzles. When air or high-pressure steam is being used, a double shut off valve is fitted in addition to the seal rings to ensure the air or steam only enters each soot blower as it actually commences its blowing sequence.

Old Type Soot Blower:

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High Pressure, Automatically operated soot Blower:

It is operated by an electric motor driving through a reversing gearbox. The motor is started in its correct sequence from a central control consol, moving the blower through its correct blowing procedure until a cam arrangement in the gear box switches it off, and as it does so it gives a signal for the next blower to start. A handle can be fitted to the square at the end of the operating screw to enable the blower to be operated manually. The type shown in the figure is retractable, so first the operating screw extends the nozzle to its blowing position. As it comes towards the end of its travel, the cam opens the double shut off valve, allowing steam to pass through to the blower chest, where the seal rings have now moved to a position, which allows the steam to enter the admission ports in the sliding spindle. The steam passes along this spindle to the convergent divergent nozzle where the expansion process-taking place results in a high velocity steam jet. A guide pin moving in a slot causes the nozzle to be rotated through its blowing are while the steam is on. To prevent high-pressure steam eroding tubes in high temperature regions of the boiler, restriction or orifice plates, fitted in the steam supply passage to the individual soot blowers, reduce the pressure as required. Cooling air is sometimes supplied to the wall box tube. This air is normally taken from the forced draught fan discharge before the air heater.

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In its simplest form a steam soot blower consists of a headpiece, including a valve, mounted external to the heat exchanger. Extending from this into the gas passage is a tube or lance fitted with nozzles through which the steam discharges. An electric or pneumatic motor attached to the headpiece causes the lance to rotate and when the nozzles come into a position where the discharge of steam will impinge on the area to be cleaned a cam operated valve opens in the head to admit steam from the soot blowers steam piping system. As the lance continues to rotate, bringing the nozzles clear of the heating surfaces, the cam allows the steam valve to close. In such a soot blower the lance is permanently in the gas pas- sage and, apart from a small quantity of purge air admitted to prevent combustion products from entering the head, is un-cooled when not in operation as shown in the figure below.

Multi-Nozzle, Rotating Element Soot Blower:

When fully inserted lance withdraws after making a half turn so that the outward spiral cleaned is out of phase with the in- ward enabling the whole area to be exposed to the cleaning effect of the steam jets as shown below.

Rack Soot Blower Cleaning Pattern:

The head of the soot blower is supported on steel work outside the boiler and the lance is traversed along a rack with the steam being supplied through a telescopic tube, as shown on the next page. Retractable or rack type soot blowers, so called because of the rack traversing mechanism, have proved extremely successful in hot gas zones due to their improved life span and superior cleaning effect.

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They are naturally more expensive than the simple, fixed head, rotating element blowers and they occupy a good deal of space outside the boiler. For these reasons their use is confined to those areas where the simpler type has not proved satisfactory and that is basically in areas where the gas temperature exceeds 750*C.

Long Retractable Soot Blower:

Boiler Operation: Process of raising steam from cold on a Scotch boiler: If the boiler has been opened up for cleaning or repairs cheek that all work has been completed, and carried out in a satisfactory manner. Ensure that all tools, etc. have been removed. Examine all internal pipes and fittings to see that they are in place, and properly fitted. Cheek that the blow down valve is clear. Then carry out the following procedure: Fit lower manhole door. Check external boiler fittings to see they are in order. See that all blanks are removed from safety valves, blow down line, etc. Fill boiler with water to about one-quarter of the water level gauge glass. If possible hot water heated by means of a feed heater should be used. The initial dose of feed treatment chemicals, mixed with water, can be poured in at the top manhole door at this stage if required. Then fit top manhole door. Make sure air vent is open. Set one fire away at lowest possible rate. Use the smallest burner tip available. By-pass air heater if fitted. Change furnaces over every twenty minutes. After about one hour start to circulate the boiler by means of auxiliary feed pump and blow down valve connection, or by patent circular if fitted. If no means of circulation is provided, continue firing at lowest rate until the boiler is well warmed through especially below the furnaces. Running or blowing out a small amount of water at this stage will assist in promoting natural circulation if no other means is available. Continue circulating for about four hours, raising the temperature of the boiler at a rate of about 6*-7*C per hour. Water drawn off at the salinometer cock can be used to cheek water temperature below 100*C. At the end of this time set fires away in all furnaces, still at the lowest rate. Close the air vent. Nuts on manhole doors, and any new joints should be nipped up. Circulating the boiler can now be stopped, and steam pressure slowly raised during the next 7 to 8 hours to within about 1 00 kN/m2 of the working pressure.

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Test the water gauge. The boiler is now ready to be put into service. About 12 hours should be allowed for the complete operation provided some means of circulating the boiler is provided. If circulation cannot be carried out, the steam raising procedure must be carried out more slowly, taking about 18 to 24 hours for the complete operation. This is due to the fact that water is a very poor conductor of heat, and convection currents leaving the water below the furnace cold will carry heat from the furnace up. This will lead to severe stresses, being set up in the lower sections of the circumferential joints of the boiler shell if steam raising is carried out too rapidly, and can lead to leakage and 'grooving' of the end plate flanging. If steam is being raised simultaneously on more than one boiler, use the feed pump to circulate each boiler in turn, for about ten minutes each. Procedure for opening up a Scotch boiler and its’ inspections: Empty the boiler, preferably by allowing the boiler to cool down, and then running or pumping out. If there is not sufficient time for this, allow boiler pressure to fall to 300-400 kN/m2 and blow down. When pressure is off the boiler, open the air vent and allows the boiler to cool down. When the boiler is cool, make sure there is no vacuum in the boiler; this should be done by opening the drain cock on the water level gauge glass in case the air vent is choked. Then commence to open up the boiler by first removing the top manhole door. To do this, slacken back the nuts holding the dogs, but do not remove them until first breaking the joint. This precaution should be taken in the event of pressure or vacuum existing in the boiler. The nuts and dogs can then be removed, and the door removed. Depending upon the weight of the door, it may be necessary to rig a lifting block to the door in order to do this. The opening should then be roped off, and all personnel warned to keep clear. The bottom door can now be removed, again taking care when breaking the joint in case water is still above the sill of the door. If this should be the case, pump out before removing door. It is important that this sequence be followed as, when the lower door is removed, it allows a through draught and hot vapour rising through the top door may scald anyone standing over the hole. Hot vapour can remain in a Scotch boiler even after a considerable period of time allowed for cooling down. With the doors removed, allow the boiler to ventilate before attempting to enter. Do not allow naked lights near the boiler until it has ventilated due to the danger of explosive gas in the boiler. If in doubt, use a safety lamp to test the atmosphere in the boiler is safe to breathe before entering. A preliminary internal inspection should be carried out before cleaning is commenced to cheek the general condition. Note scale deposits and any special points. Plug the orifice to the blow down valve to ensure it does not get choked during cleaning operations, and place guards over the manhole landings to ensure they are not damaged. The boiler can now be cleaned, and any internal work carried out. When all work is completed, a full internal examination must be carried out. It is advisable to keep a record of the boiler, consisting of a drawing on which any troubles, repairs, etc. can be shown, and a book in which remarks regarding scale formation, corrosion, deformation, etc, can be kept. Cheek to see all cleaning, has been carried out efficiently, especially where the tubes enter the tube plates. See that all tools and other articles have been removed from the boiler, paying special attention to combustion chamber top, tube nests, and bottom of boiler. Make sure all openings are clear, taking special care with the water level gauge connections to ensure they are clear and free from deposits. Make sure all internal pipes and fittings have been replaced correctly, and are securely attached. The guards can be removed, and the faces of the manhole doors and landings inspected to see they are clean and undamaged.

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Remove the plug from the blow down valve orifice. Replace the lower manhole doors, using a new joint. Operate all boiler mountings and see they work correctly. Leave in a closed position, except for water level gauge steam and water cocks, and air vents. Hydraulic testing of a Scotch boiler: New boilers having a design pressure in excess of 690 kN/m2 together with their components, must be subjected to a hydraulic test at a pressure = (1.5 x design pressure + 350) kN/m2 upon completion. For boilers working at pressures below this value the test value is 2 times’ design pressure. The test must be carried out in the presence of an authorized surveyor, who upon satisfactory completion of the test will stamp the boiler with the official DOT stamp if it is for a passenger vessel, or if a classification society surveyor is concerned, their official stamp will be used. The surveyor's initials are also put on alongside the stamp, which is usually on the bottom front plate, near the furnace. Boilers, which have undergone structural repairs, must be subjected to a hydraulic test at a pressure at least equal to the design pressure. The surveyor may call for a hydraulic test at any survey, the test pressure being to the surveyor's requirements. The procedure for such a test is carried out as follows. Close or blank off all openings. Measuring tapes may be placed around the boiler, and deflection gauges in the furnace. Lagging should be removed as required to facilitate inspection of joints, etc. The boiler is then completely filled with water the air vent being left open until water shows to ensure no air is trapped inside. It should be noted that the use of hot water places the boiler closer to working conditions, but may scald in the event of failure if the water is hot enough to flash off into steam with the resultant drop in pressure. The force pump, and test gauges can now be fitted. The gauge glasses should be shut off if the test pressure is to be above the design pressure. The readings on the measuring tapes, and deflection gauges should be noted. The boiler can now be pressurized by means of the force pump. Care should be taken to ensure that the pressure rises smartly in response to the pumping action; if it appears sluggish, open the air vent to remove any air remaining in the boiler. Listen carefully during application of pressure in case any combustion stays, etc. fracture. Then examine all joints, especially if these are riveted. Cheek flanges for cracks. All flat surf aces should be checked with a straight edge for signs of bulging due to stay failure, overheating, or thinning of the plate. Look for signs of leakage at tell-tale holes in the combustion chamber stays and welded compensating rings. Examine all tube ends for signs of leakage. Cheek, and note readings on measuring tapes, and deflection gauges. The test pressure must be maintained until the surveyor has completed his examination, and must in any case be kept on for at least ten consecutive minutes. The pressure can then be released. Readings on the measuring tapes and deflection gauges should again be checked to ensure they have returned to their initial values. The boiler can then be emptied, and examined inside and out. Procedure for closing up, and then raising steam on a water tube boiler: Before closing up the boiler inspect the internal surfaces to ensure they are clean, all openings to the boiler mountings clear, and tubes proved to be free of obstruction by means of search balls, flexible wires, air or water jets. Replace any internal fittings, which have been removed, checking to ensure they are correctly positioned and secured. The header hand hole plugs and lower manhole doors are now replaced. Operate all boiler mountings to ensure they work freely, leaving all the valves in a closed position.

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Check the gas side of the boiler is clean and in good order. Make sure the soot blowers are correctly fitted, and operate over their correct traverse. Operate any gas or air control dampers fitted to ensure they move freely for their full travel. Leave them closed or in mid-position as necessary. The boiler easing doors are now replaced. Open the direct reading water level gauge isolating cocks, together with all boiler vents, alarm and pressure gauge connections. The superheater drains are also opened. Cheek that all other drains and blow down valves are closed. Commence to fill the boiler with hot de-aerated water. At this stage the initial dose of chemical treatment can be added through the top manhole doors, which are then replaced. Continue to fill until water just shows in the water level gauges. Close any header vents as water issues. Remove the funnel cover, and ensure that all air cheeks operate correctly and that the forced draught fans are in working order. If gas air heaters are fitted they should be by-passed. Check the fuel oil system to ascertain it is in good order. Start up the fuel oil service pumps and check for leaks. The boiler is now ready to commence raising steam. Heat the fuel oil to the required temperature, using the re-circulating line to get the heated oil through the system. If no heat is available for this, use gas oil until sufficient steam is available to heat the residual fuel oil normally used. Start the forced draught fan, and with all the air checks full open purge the boiler, making sure any gas control dampers are in mid-position so giving a clear air passage. Carry out a final check to make sure water level gauge cocks are open, water is showing in the glass, and that steam drum and superheater vents are open. Now close all the air checks except for the burner to be flashed up, this being done by means of ignition equipment or a paraffin torch. Use the lowest possible firing rate. Adjust the air supply so as to obtain the best combustion conditions and check that, as the boiler heats up, the water level in the gauge glass begins to rise. After about one hour steam should show at the drum and superheater vents and, when issuing strongly, open the superheater-circulating valve and close the air vents. When the steam pressure has reached a value of about 300 kN/m2, blow through the water level gauges to ensure they are working correctly. The isolating valves on the remote reading water level indicator can now be opened, and the indicator placed in service. With the steam pressure at about 1000kN/m2

follow up the nuts on all new boiler joints. At a pressure of about 1400kN/m2 open the drains on the auxiliary steam lines, crack open the auxiliary stop valve and warm the Auxiliary line through. Now close the drains and fully open the auxiliary stop valve. Various auxiliary equipment such as fuel oil heaters, turbo-feed pumps, etc, can be put into service and, provided this entails a flow of steam through the superheater, the superheater circulating and drain valves are closed. Bring the boiler up to working pressure, keeping the firing rate as steady as possible, and avoiding intermittent flashing up. Check the water level alarms. Open the main steam line drains, and crack open the main stop valve and warm through the main steam line. Then close the drains and fully open the main stop valve. The procedure from flashing up to coupling up at full working pressure should take about four to six hours. Only in emergency should it he carried out more rapidly. If new refractory material has been installed, carry out the procedure more slowly. At all times during the raising of steam the superheaters must be circulated with steam to prevent them overheating. If the temperature of the superheaters goes above the permitted value for the boiler, reduce the rate of firing. NOTE: Due to the great variety of water tube boiler designs in use, the foregoing procedure is only to be taken as a guide; for example, header boilers with their greater amount of refractory material will require about eight hours to reach full pressure. Thus the engineer should always follow the procedure laid down for his particular boiler, which may vary in detail from the basic principles previously stated. ************************Kv**************************

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Emergency operation due to fire: Fires can occur in the air heaters, economisers, super-heaters and exhaust gas heat exchangers. These heat-extracting units are all situated in the path of combustion gases and under certain conditions, fortunately rare, can experience disastrous fires. These fires are of two types:

1. Soot fires. 2. Hydrogen fires.

Soot fires: The ignition of an accumulation of soot, rich in carbon, caused by poor combustion either at start or when operating at low load for prolonged periods, can when supplied with the necessary oxygen be the source of a fire sufficiently intense to melt and burn steel. Air heaters, with their thin steel plates or air tubes and an abundance of oxygen, can, unless kept clean, be very susceptible to this kind of damage. Immediately after lighting-up the boiler and during periods of low load operation, the temperature of the gas leaving the air heater should be closely observed. Any sudden rise in this temperature can be an indication of fire in the heater. In the event of such a fire in the gas passage:

1. Shut-off fuel supply to all the burners. 2. Shut down the F.D. and I.D. fans. 3. Close all air-inlet dampers. 4. Determine the location of the fire and flood the area with water. (Do not use water spray). 5. Do not use the soot blowers provided. (This blows carbon dust in suspension, which may cause a

serious explosion). 6. After the fire has been extinguished and the unit has been cooled, thoroughly clean the unit and

make such repairs as may be necessary. Hydrogen fires: Instances have occurred in which the tubes of water tube boilers, superheaters, economisers and exhaust gas heat exchangers have, as a result of an intense fire, literally melted and run away in streams. Sometimes, in the case of vertical tubes, they have melted and flowed back into their headers to solidify. These fires were subsequent to the overheating of tubes, which were short of water or steam. Dissociation of steam into hydrogen and oxygen by heat alone requires temperatures in the region of 2500*C. Iron will however burn in steam with the production of free hydrogen at much lower temperatures of about 700*C. For example, if a superheater is severely overheated due to insufficient steam circulation, the tube material may ignite at about 700*C and, burning in the steam, produce free hydrogen. The iron will continue burning independently of any supply of oxygen from the air, and the hydrogen produced by the reaction will burn on coming into contact with air. This means that once such a fire has started, there are likely to be two fires burning simultaneously; one, iron burning in steam and the other, hydrogen burning in air. This combined fire being self- supporting and probably lasting until the supply of steam is exhausted. The conditions necessary for the Initiation of a hydrogen fire are as follows:

1. Tube metal temperatures of over 700*C. 2. Tubes with some steam content. 3. The presence of a catalyst in the form of a carbon ash.

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If a soot fire starts in a finned tube exhaust gas heat exchangers or boilers, and if the unit is on fire and if it is not being circulated with steam or water, the intense heat of the soot fire, rich in carbon, may initiate a hydrogen fire. Unfortunately, an early indication of a hydrogen fire in a boiler is not there. A hydrogen fire stops only when the supply of steam/water is exhausted. Furnace explosions: Furnace explosions or on a smaller scale ‘blow backs’, generally occur when volumes of oily vapour and air, present in a furnace in explosive proportions are ignited; sudden admission of air to fuel rich burner flame may well produce the same result. These explosions should not occur in boilers fitted with automatic sequential controls, as these, apart from controlling the fuel to air ratio, also ensures adequate purging before ignition. Even in the best-designed systems, however, automatic light-up failures do occur, and it is then, when going over to manual control, often in a hurry, that the wrong action is sometime taken, resulting in an explosion. As a precaution, failure to obtain ignition at the first attempt must be followed by adequate purging. Precautions against furnace explosions:

1. Purge furnace thoroughly before light-up. 2. Check and clear the furnace of flammable deposits. 3. Purge furnace after every unsuccessful light-up attempt. 4. Soot blowers to be operated only at or above 50% of the normal rated out put of the boiler. 5. The furnace suction should be increased when soot blowing, sufficiently to prevent dust blowing

in to the boiler room. 6. Operation of regenerative type of air heaters at low loads for extended periods can result in

unstable burner flame due to the dilution of the burner air with flue gases being volumetrically replaced in the cells of the heater. Operating with above normal excess air can minimize this.

7. Instructions for boiler operations, both in instruction manuals and on notice near the boiler, should additionally contain adequate warning regarding extra precautions necessary with degraded logic systems and over-rides on use. Operators must be sure they understand all the implications of such instructions and act upon them.

8. Check the condition of the igniters and flame scanners, to ensure that they are in good working order.

9. Automatic fuel oil shut off should, as a routine, be tested to ensure that the fuel valves operate efficiently for fault conditions. (e.g. flame failure and combustion air failure).

10. Membrane walls are now a common and generally accepted feature of boiler design but as a consequence of this form of construction, the furnace is largely enclosed in a rigid shell.

An explosive disruption of this shell may cause considerable structural damage to the boiler causing its contents to be suddenly released into the boiler house thus presenting greater danger to personnel. Such boilers are provided with extensive safety devices and alarms which must be maintained at high efficiency and which should not be over-ridden unless absolutely necessary. *************************************************************** Acknowledgement: Boiler manufacturer’s manuals, J.H. Milton’s Marine Steam Boilers, The Institute of Marine Engineers publications:

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Marine Steam Turbine Notes: For BE (Marine Engineering) Cadets.

Prepared By: Prof. K. Venkataraman. CEng; FIMarE; MIE. Turbines are ideal prime movers for high power steady state unidirectional operation, thermal efficiency and therefore fuel economy is dependent on initial and final steam conditions. This means high-pressure boilers with high degree superheat and probably reheat coupled with a high degree of vacuum on the exhaust system. Multi-stage feed heating would also be essential for maximum efficiency. For marine use powers are small in turbine terms, variations in speed and load must be possible, occasionally at short time impose design variations most of which involve some reduction in thermal efficiency. These modifications involve additional clearances to allow for unequal thermal expansions, some attempt to equalise the thermal insertion of rotor and easing, simplification of the astern power arrangements and; simplification of stage feed heating. Considerable ingenuity has been employed in marine turbine designs. Provided operating instructions are followed, turbines are extremely reliable, especially on long haul voyages. Boilers have given most problems and repair costs are high. Fuel costs have resulted in conversion of steam to diesel plant but coal firing and large power requirement may bring back turbine plants in large ships. Gearing is an essential part of turbine propulsion. Gear ratios tend to be high in the order of 30/1 to 60/1 this allows turbine speeds to be high, giving appropriate blade and steam speeds for turbine efficiency combined with low propeller speed for propulsive efficiency. Gears are usually, double helical, double reduction with primary epicyclic trains in many cases. Electric power may be obtained from alternators driven from intermediate gear shafts. There are many factors involved in the design of a turbine for a particular application, and these may be generally grouped as follows:

1. The maximum ahead power needed to provide the desired ship's speed. 2. The relative amounts of time spent at maximum power and reduced cruising powers. 3. The turbine throttle steam pressure and temperature. 4. The steam cycle arrangement, together with the number and location of extraction points and

corresponding steam flows. 5. The turbine exhaust vacuum for design purposes. 6. The type of power transmission to the propeller. 7. The astern operating requirements. 8. Space limitations of the engine room arrangement. 9. The importance of machinery weight and size,

Steam Conditions: As steam pressures increase, their specific volumes decrease, therefore, the nozzles and blades become smaller and less efficient. A limiting pressure is reached for every capacity of turbine at which the gain due to the improvement in pressure is offset by the decrease in internal efficiency. Thus, higher initial pressures may be used more effectively on large turbines. Below are the initial pressures recommended as practical upper limits for various sizes of propulsion units:

Rated Shaft Power (KW): Initial Pressure (Bar): 2000 - 3000 27 3000 – 7500 40 7500 - 15000 60

15000 100

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Increasing the initial steam temperature will also reduce the heat rate and steam rate, and since the temperature increase affects the specific volume only a relatively small amount, the machinery physical dimensions are changed only slightly improving the economy of small and large turbines about equally. For a given initial pressure, there is a minimum initial temperature below which the moisture content in the low-pressure end of is sufficient to cause undesirable erosion of turbine blades and loss of stage efficiency. This is often accepted as twelve percent content in the exhaust. Exhaust Vacuum. A moderate vacuum of 723 mm Hg has become generally accepted as a design basis for merchant propulsion turbines and is based upon a reasonable economic compromise when considering the worldwide variation in seawater temperature and the size, weight, and the cost of turbines and condensing equipment. Low seawater temperatures permit a high vacuum, while high seawater temperatures may limit the attainable vacuum and therefore it is difficult, at the construction stage, to ascertain that a particular vessel will remain within specific trade routes throughout its useful life. Hence the design for a moderately good vacuum. If the trade of the vessel is to be in low temperature waters permanently an economic evaluation study should be carried out to compare the increased cost and weight of turbines designed for higher than standard vacuums, against the cost of standard vacuum installations. It should be noted that the specific volume of steam increases rapidly as the vacuum is improved e.g. from 710 to 735 mm Hg, the specific volume practically doubles. In order to accommodate this increased volume the flow areas at the exhaust end must be proportionally increased. A good vacuum also reduces the ‘windage’ losses when using the astern turbine, which could otherwise limit the allowable speed or period of astern operation due to overheating. When the rated full power, initial steam conditions, and exhaust vacuum have been selected, it is possible to establish the steam rate which may be expected from well-designed equipment. Fig. 1 gives typical non- extraction steam rates for merchant type, geared turbine units designed for optimum performance at full power with a proper balance between efficiency, size, weight and cost, where the reduction in steam rate with improved steam conditions may be seen.

Generally, merchant vessels operate at full power during their service life and therefore performance at partial loads tends to be less important. A typical variation in steam rate at fractional powers that is representative of turbine designs, which incorporate no special features to enhance partial power performance other than first-stage nozzle control, is shown in Fig. 2. (Partial power performance is used almost exclusively for naval combat vessels as is therefore shown for comparison only).

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Machinery Arrangements: The steam turbine is essentially a high-speed machine, whereas the propeller is most efficient at low speeds and it is therefore necessary to install some form of reduction gearing. The most common geared steam turbine arrangement is the compound unit consisting of a high-pressure turbine and a low-pressure turbine driving a single fixed-pitch propeller through reduction gears and shafting. Complete astern turbine is generally provided in the low-pressure turbine easing. An alternative to this is that, all the ahead and astern blading may be provided in a single casing at a small sacrifice of 1 to 2 percent in efficiency. This arrangement offers a number of advantages including:

(a) Decreased space, (b) Decreased weight, (c) Lower initial cost, (d) Reduction in pipe runs, (e) Simpler seating.

However, it is limited to shaft powers up to approximately 15,00OKw. At least three arrangements of the low-pressure turbine and condenser are in use:

1. The low-pressure turbine is supported by longitudinal girders, forming an integral part of its lower casing the girders are supported by foundation structure at the forward end and by the gear casing at the aft end. This arrangement permits the condenser to be hung from and located below the turbine and has the advantage that thermal expansion of the condenser does not affect the turbine gear alignment.

2. When the condenser supports the turbine, due to thermal expansion the turbine centerline will rise with respect to the pinion. A flexible coupling is fitted to allow for this misalignment.

3. A ‘single plane’ arrangement where the condenser is located forward of the low-pressure turbine such that the turbine exhausts axially into the condenser. This reduces the overall height of the unit but increases its length.

Number of Stages: Modern steam installations allow large heat drops in the turbine. If a single stage were used, the theoretical steam velocity would be extremely high for good efficiency, and also there are practical limits to the blade speed. To avoid low speed ratios, it is common practice to split the total heat drop among a number of stages placed in series. The total blade speed needed in a turbine may be obtained either by having large diameter wheels operating at low rotational speed, the latter preferred because of the reduced weight, size and cost. Turbine Control: A number of means are available to control the steam flow through the turbine and hence the output power.

1. Throttle Valve Control - the simplest method of control, where at full power, with the value fully open there is little pressure drop. As the valve is closed to reduce the steam flow, its pressure drop increases; consequently, a throttling or constant enthalpy process occurs at the valve and causes a thermodynamic loss since there is a decrease in the available energy per Kg of steam. Fig. 3 shows the reduction in available energy as a result of throttling.

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2. Throttle Valve plus Hand Control Valves - throttling losses at reduced powers are minimised

by dividing the total nozzle area into groups. One nozzle group generally has about half the total nozzle area and is controlled by the throttle value, while the throttle valve and hand control valves control the remaining groups.

3. Bar Lift Control Valves - this essentially consists of a bar, which lifts the nozzle box valves in a

pre determined pattern depending on the steam flow required. 4. By- pass Valves - when the full chest pressure is applied to all first-stage nozzles, the steam flow

is limited by the total nozzle area and the pressure drop across the nozzles. In order to further increase the steam flow, valves may be installed which allow most of the steam to by-pass the first stage and enter a later stage where the nozzle area is large enough to pass the desired flow.

5. Variable Boiler Pressure - consideration has been given to this method of control but the response to the system is too slow, the efficiency is low at slow speeds, and the control system required do not justify its use.

6. Over speed and Low Oil Pressure Protection - In order to protect the turbine in the event of over speed a governor is fitted which, via relays will cut off the steam flow if the turbine over speeds. Similarly, if the lubricating oil supply fails relays again stop the steam flow. Trips are not to be confused with governors, which control the turbine speed within predetermined limits and are generally fitted to turbine-electric propulsion units.

Blade Erosion: From the heat chart it is seen that, the expanding steam in a turbine passes over the saturation line into the wet region. However, expansion takes place so rapidly that there is actually a small delay before moisture particles begin to form. This effect, known as super-saturation, results in dry steam being present at a temperature lower than the saturation temperature, and is an unstable condition. In an ordinary nozzle, expansion requires about 0.0002 seconds, and the moisture, which forms in the low-pressure stages of the turbine, is very finely divided. Thus, some of the finely divided moisture particles strike the metal surfaces and form small drops, which are subsequently swept along with the steam flow.

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The last rows of blades, having the greatest peripheral speeds, are then subject to a barrage of high velocity water droplets. If a blade has a microscopic inter-granular crack, the water tends to penetrate the crack, and from other cracks the propagation begins inwards until a small particle of metal breaks away. Repeated, this leads to erosion, pitting and deterioration of the blade tip. To offset this stellite tips may be brazed to the leading edge of the blades. Figure 4 shows the acceptable tip speed versus moisture at stage inlet:

From the above, it may be surmised that the ideal blade material should have:

1. A high yield strength at all operating temperatures. 2. Good ductility. 3. High fatigue strength. 4. Good resistance to corrosion. 5. Resistance to erosion due to wet-steam. 6. Machineability. 7. High internal damping capacity to absorb vibrational energy.

Several Corrosion resisting steels are available with 12-13 percent chromium content, and a small amount of molybdenum added to give creep resistance at elevated temperatures. Main unit rotors may be built up or gashed from a solid forging. For temperatures above 390*C, the forging material usually contains 0.5 per cent molybdenum, and for higher temperatures may contain various amounts of vanadium, chromium and nickel. Casings: Casings may be manufactured from cast steel or fabricated and made in two halves, their flanges ground and bolted together. As the temperature of the turbine changes, the casings must be free to expand and contract freely e.g. a single cylinder unit will expand lengthwise 6 to 9 mm and proportionally in other directions. To accommodate this expansion in a longitudinal direction, the casing is rigidly fixed at its gear end and allowed to move in a sliding foot at the forward end. Alternatively, a deep flexible I-beam, installed with its longitudinal axis athwartships, an arrangement that allows for free fore-and-aft movement while positioning the unit vertically and athwartships, supports the forward end of the casing.

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Glands: It is not practical to reduce steam leakage to zero in the turbine i.e. ensure that all steam flow passes through the turbine blading since this would involve packing in contact constantly with rotating parts. Marine installations favour the labyrinth type packing as shown in the figure below.

In this type the rotor has a series of ridges, machined on its diameter and an annulus, in the form of a number of segments, with protruding fins, which match the ridges on the rotator. The segments are held in position by a combination of steam pressure and springs. Each small clearance between the fins causes a pressure drop, effectively throttling the steam. In the event of contact between packing and rotor, little damage is caused since the packing is relatively soft and has low frictional resistance e.g. 6Pb - 13Ni - 65Cu for temperature of less than 470*C and for greater temperature a 12 percent chromium corrosion-resisting steel strip or a 22 percent nickel ductile iron casting. Leakage of steam through diaphragms in the impulse turbine is approximately 1.5 to 2.0% average. At the high pressure end, the leakage is most critical and this steam is often bled off and fed to some lower pressure stage of the turbine, while the high specific volume steam towards the lower pressure stages, presents lesser problems. These seals are also provided at the extreme easing ends, where gland steam at a pressure slightly above atmospheric, is utilised to prevent air leakage into the condenser. Another form of packing is carbon rings, cut in segments, held together by garter springs and situated in a special housing but maintenance problems encountered prevent them being widely used for main propulsion unite. OPERATION: The usual precautions are to be taken when going ahead, but prolonged astern operation should be avoided. Generally, design allows continuous astern operation (70 percent of ahead speed) for one hour, but if the temperatures of the crossover pipe and high-pressure turbine exceed allowable values, the speed should be reduced. Note: If the inlet steam temperature is constant, the astern exhaust temperature will rise with drop in speed since the exhaust is superheated and the turbine efficiency decreases. In the event of damage to the high-pressure turbine, the low-pressure turbine may be connected to the live steam line direct and the high-pressure turbine isolated. While damage to the low-pressure turbine, alters the exhaust pipe arrangement, however, no astern power will be available if the astern blading is incorporated in the low-pressure stage. Steaming under these conditions is limited to about 70-75 percent of normal ahead rating.

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TYPES OF STEAM TURBINES: Impulse Type: A small purely impulse turbine will expand steam to exhaust pressure in a single set of nozzles, such that the velocity of the steam leaving the nozzles is very high. To obtain maximum power from this steam jet on a single row of moving blades, the blades must move at about half the velocity of the jet. In order to reduce the resulting high rotative speed, but-maintain efficiency, the high velocity may be absorbed in several steps known as velocity compounding. This takes place in the first stage of the turbine. A further method of reducing rotor speed while maintaining efficiency is to decrease the velocity of the jets by dividing the drop in steam pressure into a number of stages, known as pressure compounding. Since each subsequent stage consists of a single row of stationary nozzles and a single row of moving blades it is equivalent to mounting several single-stage impulse turbines on a common shaft.

IMPULSE TURBINE:

If Vb = V1; Steam cannot overtake blade. Therefore no deflection:

If Vb = Zero, no work is done.

STEAM STRIKES THE BLADE WITH A RELATIVE VELOCITY V1 – Vb. RELATIVE VELOCITY = V1 - Vb : RELATIVE OUTLET VELOCITY = - (V1 – Vb).

Abs. LEAVING VELOCITY = RELATIVE VELOCITY + VELOCITY OF THE BLADE. = V2 = -(V1 – Vb) + Vb = -V1 + 2Vb.

IF K.E OF LEAVING STEAM = ZERO (Max. Work), V2 = 0; Therefore V1 = 2Vb or Vb = 0. 5 V1.

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STEAM NOZZLES:

Critical pressure ratio = 0.57 0.55.

Adiabatic Expansion of steam in a nozzle:

Pr: Bar:

dhs kJ/kg.

C m/s

V dm3/kg

A mm2

7.0 0 0 272.68 ∞∞∞∞ 6.0 31.5 251.0 312.0 1243.2 5.0 63.0 355.0 366.03 1031.2 4.0 103.3 454.0 445.18 980.2 3.0 153.6 554.0 573.08 1034.4 2.0 221.7 666.5 818.67 1228.9 1.0 330.9 815.5 1510.613 1852.4

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IMPULES BLADES:

REACTION BLADES:

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Velocity Diagrams: ‘αααα’: Nozzle Angle (Should be minimum).

ββββ1: Blade Entrance Angle. ββββ2: Blade Exit Angle.

Velocity diagrams for a single-row impulse turbine stage:

VA1: Absolute entrance velocity. VA2: Absolute exit velocity.

VR1: Relative entrance velocity. VR2: Relative exit velocity.

VB: Absolute blade velocity. Vw: Tangential component of velocity.

Va: Axial component of velocity.

Convenient method of drawing velocity diagram:

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SP: Velocity of whirl (Does useful work).

SQ: Axial thrust (Gives end-thrust). PQ: Actual change of velocity of steam through the blade.

For Impulse Stage:

For Reaction Stage:

V’

A1 ≈≈≈≈ ½ VA1. Vw2 ≈≈≈≈ ½ Vw1. VB = V’B. VR2 ≈≈≈≈ V’

R2. V’B ≈≈≈≈V’

A1. Work done at reaction stage is about half of the work done in an impulse stage if the mean blade velocity is the same for both stages (VB = V’

B).

Pressure Compounded Impulse Turbine:

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CURTIS TURBINE:

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SIMPLE IMPULSE TURBINE: (Delaval Type).

PURE REACTION TURBINE:

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DE-LAVAL IMPULSE TURBINE:

Disadvantages:

1. Very high Turbine speed. 2. Excessive centrifugal stresses. 3. Large reduction gear ratio required. 4. High steam-exhaust velocity. 5. Inefficient and impractical for Marine propulsion.

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PRESSURE COMPOUNDING:

‘Rateau’ Pressure compounded Impulse Turbine.

Advantages:

1. Smaller pressure drop causes lower steam velocity with lower blade velocity. 2. Better efficiency than Velocity-Compounded machine.

Disadvantages:

1. Increased initial cost. 2. Increased length of the turbine. *******************Kv**********************

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‘CURTIS’ VELOCITY COMPOUNDDED TURBINE:

Advantages: 1. Smaller casing pressure. 2. Reduced pressure stresses. 3. Gland length reduced. 4. Cheaper casing material. 5. Reduced turbine length. Disadvantage: 1. Lower efficiency. *******************Kv*********************

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PRESSURE-VELOCITY COMPOUNDED IMPULSE TURBINE:

Advantages: 1. Reduced turbine speed due to controlled steam velocities. 2. Reduced length of turbine. 3. Reduced centrifugal stresses. 4. Lighter and cheaper construction. 5. Reduced pressure on glands as compared to pure pressure compounding. *******************Kv*********************

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CONSTRUCTIONAL DETAILS OF A TYPICAL TURBINE: General arrangement: The usual arrangement of the turbine is as a two cylinder machine as shown in the figure on first page, with a high Pressure (HP) cylinder and a low pressure (LP) cylinder located side by side with the out puts feeding through flexible couplings into a gearbox (a cross compound turbine). The power developed by each cylinder is usually about equal to allow the best optimisation of the gearbox. To achieve the best turbine efficiency it will be apparent that the best solution is to employ a small diameter high speed HP turbine, where as the LP turbine will require long blades, which will limit the speed to give acceptable levels of centrifugal stress. For a 24 MW turbine the HP turbine would have a blade/nozzle ring diameter of about 500 mm and a rotor speed of about 6,500 rev/min, whereas the LP rotor speed would be about 3,500 rev/min. The cross compound arrangement allows the designer to select these differing speeds, and in some cases when maximum efficiency is being sought with high steam conditions, triple reduction gearing is used for the HP turbine, and also, in the case of reheat turbines, for the IP turbine. This allows very high rotor speeds of 12,000 rev/min to 14,000 rev/min to be employed with the very small diameter turbines necessary to give good efficiency with very high-pressure steam. After expanding through the HP turbine the steam passes to the LP turbine via the crossover connections, which will have a flexible bellows to allow for the difference in expansion of the HP and LP cylinders as they heat up to, normal working temperature. If an IP turbine is included the steam connections will be HP to IP and then to the LP. The turbines are supported on seating formed within the bottom structure of the ships hull. One end of the turbine will be made a fixed point by the use of a dowel on the turbine center line or else by a combination of transverse and axial keys as shown in the figure below.

Turbine casings: HP and IP casings are made from low alloy steel castings and the material must have adequate temperature/strength and creep properties for the inlet steam temperature employed. It must be appreciated that the strength of the steel at the operating temperature may be half that at the ambient temperature.

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Thermal stresses are set up during the period of warming through and working up to full power, and might cause cracking although the designer will have minimised this danger by making the casing as symmetrical as possible and avoiding sharp cornered recesses. Design of the main casing joint. The main joint between the top and bottom half casings must be designed to avoid danger of leakage because if a leak occurs the ‘wire cutting’ action of the steam will rapidly make it worse. The flanges must be perfectly flat with a good surface finish and the joint is made metal to metal with the addition only of a thin smear of a good graphite compound. The bolting is designed to counteract the steam pressure in the casing opening the inner edge of the joint, but since the effect of temperature is to cause the stress in the bolts to relax, and reduce with time, the initial tightening of the bolt has to be great enough to allow for this. Typically, the manufacturers will call for the bolts to be tightened to give 0.15% strain and this will give an adequate residual stress after 30,000 operating hours to still hold the inner edge of the joint closed, as shown in the ‘Stress relaxation of main joint bolts with temperature’ curve below.

If the turbine were continuing in service the bolts would require tightening. Small bolts are ‘flogged-up’ or tightened with torque increasing spanners, but bolts larger than 60 mm are usually made hollow to allow the insertion of an electric heating element. After making it finger tight, the bolt is heated up to temperature and the nut rotated a calculated angle to achieve the required force in the bolt. Measuring the bolt extension after it has cooled can check this. In order to maintain the correct alignment of the top and bottom half casings a number of the flange bolts will be ‘fitted’ bolts and identified to their holes, or else dowels might be used. Location of the casing: An important aspect of the casing design is the location to the bearing pedestals by vertical and horizontal keys as shown in the figure below. The casing is supported as closely as possible to its horizontal centerline to avoid vertical expansion causing a loss of concentricity between the casing and rotor.

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This is necessary because, in order to minimise steam leakages and maintain efficiency, the HP and IP turbines will operate with small radial running clearances of about 0.3 mm between the rotor and the stationary components. The rotor is located by its bearings and the keys locate the casing to the bearing pedestals. Their job is to maintain the casing concentric to the rotor whilst allowing free thermal expansions to take place as the metal temperatures increase from ambient to full working temperature. Because of the high temperatures, the HP and IP casings will suffer some degree of thermal distortion, particularly during start up. If the bearing pedestals were rigidly attached to the ends of the casing, these distortions could cause misalignment of the journal and thrust bearings and the usual practice is to support the casing with paws (Sliding feet) which extend from the casing joint flanges as seen in the figure below.

Sectional arrangement of HP turbine:

Sectional arrangement of LP turbine:

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Low-pressure casing: (See figure above). The LP casing is usually fabricated from mild steel plate and as the operating temperatures are low very little stress relaxation will take place in the main joint bolting under normal ahead steaming conditions. The exhaust chamber has to be a rigid structure to withstand the atmospheric load of nearly 11 tonnes/m2 under vacuum conditions. The astern turbine element is located in the LP exhaust space, the reason being that under vacuum conditions only a very small loss is caused by the ‘wind-milling’ of the astern blades when the ship is moving in the ahead direction. The highest temperatures are experienced in the LP cylinder during prolonged astern running as a result of the ‘wind-milling’ action of the long ahead blades under these conditions. In particularly severe cases the high temperature can cause such distortions of the easing that the main joint opens causing air to leak in and lower the vacuum, which worsens the ‘wind-milling’ effect and a deteriorating snowballing situation develops which can only be improved by stopping engines and going ahead at moderate power levels to cool things down. Turbine rotors: The HP and LP rotors are manufactured from very high quality mono-bloc forgings of low alloy steel. The HP rotor material is chosen to give good strength and creep properties at the high inlet steam temperatures. The LP rotor material is of high strength to with stand the high centrifugal stresses generated by the long last row blades and it also has to avoid any tendency for brittleness at the relatively low temperatures, which exist at this exhaust end of the machine. The rotors are described as 'mono-bloc' as the discs or wheels, which carry the blades are integral parts of the forging and not the separate shrunk-on components which were necessary in earlier times when it was not possible to make the forging large enough to machine the rotor out of the solid. Problems sometimes arose in service by these discs becoming loose during temperature transients, changing position, and causing the rotor to lose its original balance and so vibrate. Critical speeds/ Rotor response: A major design aim is to ensure that no ‘critical speeds’ occur in the normal speed range. Strictly, the term ‘critical speed’ should apply only to the theoretical concept of a rotor having rigid supports. The critical speed is then the speed, which is in resonance or coincides with a natural frequency of the rotor, which in turn depends on the distribution of weight along the rotor and its stiffness. In practice the rotor is supported upon the oil film in its bearings and this has two major effects. The oil films act as springs and lower the speed at which resonance occurs, in addition providing damping in the same manner as the hydraulic shock absorbers on a motorcar suspension system. Theoretically, the rigidly supported rotor will develop infinitely large amplitude of vibration at its critical speed. When running in journal bearings it will be seen that the speed at which resonance occurs has reduced and also most importantly the damping effect of the bearing limits the size of the vibration amplitudes. Turbine designers calculate the dynamic characteristics for the rotors using a complex computer programme. Turbine blading and diaphragms: Blades: The turbine blades convert the energy in the steam to mechanical power and to minimise the losses referred to earlier it is essential that they be in good condition with a high surface finish. Boiler deposits can precipitate out onto the blades, usually at a point in the expansion where all the superheat has been lost and the steam is approaching the saturated condition, and these deposits can have a very serious effect upon the turbine performance.

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The emphasis must be upon maintaining a very high level of feed water purity and monitoring the turbine steam continuously for dissolved solids. Apart from their effect upon turbine efficiency these contaminants are corrosive and can cause blade failures due to the stress corrosion effect upon highly stressed blades. Types of blades and roots:

Examples of Turbine Blades:

Various types of blades are illustrated in the figure above and the difference between a first stage blade (5) and a last stage blade (1) again indicates the enormous expansion in volume of the steam as it passes through the turbine. The blades are attached to the rotor disc by various types of blade root, the choice being dictated by the size of the centrifugal stress, which will vary according to the rotational speed of the turbine and the size and weight of the blade. Blades (3) and (4) in the illustration are short blades fitted to an HP turbine of conventional design running at about 6500 rev/min and the blade root is a fir tree or straddle type. All HP turbine blades are fitted with a band of stainless steel shrouding riveted to the blade tips by a riveting tang formed integral with the blade.

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Blades (2) and (3) are fitted in an LP turbine running at 3600 rev/min and it will be noted that the larger blade (2) has three sets of locating lands in the root compared to two sets in the smaller blades because of the higher centrifugal stress set up by the longer blade.

Runner Blade Root Fixing: Fie Tree Root:

Runner Blade root Fixing: Multi Fork Root:

The large last blade (1) is heavy and has a tip speed approaching 500 m/s when the LP turbine is running at its maximum speed of 3600 rev/min. In this case a multi-forked root is used in which the very high centrifugal force is resisted by a number of axial pins (three in this case).

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The two types of root are further illustrated in the figure given below, which also shows how the fir tree rooted blades are entered onto the rotor by means of a blade 'window' which is closed by a special closing blade having a pinned root. Very high-speed turbines could use the serrated roots used on blades (7), (8) and (9), but the blades may even be machined integral with the rotor if the stresses are too high for any type of root to cope with. Blade (6) is an example of a relatively short blade, which is subjected to such high steam stresses that it has to be made very wide, and the resulting high weight called for the use of a multi-fork root.

First Row Blade in a 2-row ‘Curtis’ Wheel:

Shrouding and Lacing Wires for Turbine Blades:

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Reaction Turbine Blade Root Fixing:

Impulse Turbine Blade Root Fixing:

Blade vibration: The blades have to be designed to avoid any vibration resonance throughout the operating speed range. This is achieved partly by the proportions of the blades themselves-the relationship of the blade height to its chordal width. A long, slender blade will have lower natural frequencies than a short wide blade. (Refer to figure on page 151): In addition the blades can be joined together in groups, by means of shrouding in the case of blades (4), (5) and (7), these blades having a tang formed on their top band, which is used as a riveting attachment for the shrouding band. Blades (1), (3), (8) and (9) have drilled holes through which lacing wire can be fixed while the blades are being located on the rotor. Various numbers of blades can be grouped together to give the required vibration characteristics. The shrouding band also provides an axial and sometimes radial seal against steam leakage as discussed earlier. Blades are manufactured in various grades of 12%-13% chrome steels to give the required property depending on the position of the blade in the turbine. Good high temperature strength and resistance to creep is required at the HP turbine inlet end whilst high strength is required for the highly stressed long LP blades. The high tip speed of the LP last blades coupled with the fact that the steam will be significantly wet at this stage means that they have to be protected against water droplet erosion. Erosion shields made of wear resistant material such as stellite are brazed to the outer section of the leading edge.

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Diaphragms: Each stage of the turbine consists of a circle of fixed nozzles and a row of blades attached to the rotor. The first stage nozzles are contained in a nozzle plate, which forms the end wall of the steam inlet belt of the turbine cylinder and all the other sets of stage nozzles are located in diaphragms. The typical features of a diaphragm are shown in the figure below from which it will be seen that the nozzles are contained in two half rings which fit into the top and bottom halves of the turbine casing. There is a significant pressure drop across the diaphragm and an inter-stage gland minimises leakage of steam, which bypasses the nozzles and represents a loss. This gland has to be centralised to the rotor accurately and this is done by adjustment to the side keys, which also support the diaphragms, and to the top and bottom keys. The diaphragms are located in grooves machined in the easing. The pressure drop acting upon the face areas of the diaphragms generates a considerable force, which holds the diaphragms against the downstream faces of the casing grooves. The diaphragms are usually given a considerable axial clearance in the grooves to prevent them becoming jammed in by an accumulation of boiler deposits, and each half is then located in the grooves by 3 or 4 adjusting pegs. Diaphragms are distorted in service by the pressure drop across them and the maximum deflection of about 1.5 mm takes place at the joint face adjacent to the gland bore.

Typical Diaphragm:

To prevent the top half diaphragms dropping out when the top half cylinder is lifted they are fitted with retaining plates, which are screwed into recesses in the top half main joint face.

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Radial keys are fitted into the mating joint faces of the two diaphragm halves to minimise steam leakage at this joint. The last 2 or 3 LP diaphragms maybe made of high-grade cast iron such as meehanite or spheroidal graphite types of iron, this choice of material being made to provide the best resistance to water droplet erosion from the wet steam. All the other diaphragms are made of steel of various grades to suit the operating conditions from molybdenum vanadium steel at high temperature locations to plain carbon steel where the temperature is below 400*C. Couplings: Flexible couplings are used on the drive shafts transmitting the turbine output to the gearbox. They have to be capable of absorbing misalignment caused by hull movements and the expansions of the turbine supports and the gearbox and also of accepting axial expansions of the turbine rotor. The alignment instructions usually make allowance for these expansions, for example by making the cold setting of the turbine end coupling higher than the gearbox coupling if it is calculated that the upward expansion of the gear-case is greater than that of the turbine supports at full working temperature. If membrane type couplings are used they might be pre-loaded in the cold state to allow for subsequent rotor expansion.

Toothed Membrane Coupling:

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Membrane Coupling:

The couplings may be either the toothed or membrane type illustrated in the figures above respectively. The latter use packs of thin stainless steel laminations to transmit the drive and provide the flexibility to absorb misalignment. Membrane couplings have two main advantages in that they do not require lubrication and they maintain an accurate location of the quill shaft center relative to the turbine rotor, so avoiding imbalance. Gear couplings can be prone to fretting at the tooth contact positions and care must be taken to ensure that lubricating oil sprays are accurately positioned. Sometimes centering rings are used to ensure the concentricity of the quill shaft. REDUCTION GEARING FOR TURBINES: The development of propulsion gearing has been one of a continuous improvement and refinement in materials and in manufacturing techniques and equipment to provide greater reliability and longer life. The Introduction of welded gear cases, gears and higher hardness pinion and gear materials with the attendant higher gear tooth loading, have all contributed to the reliability of the modern reduction gear, with its low noise level, make it completely accept- able in the engine room.

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Gear Arrangements: Most early designs incorporated devices to minimise the effects of bending, torsion, and inaccurate machining and alignment, which subsequent designs have proved unnecessary.

Figures represented are:

a) Single reduction single input: Used mainly for alternators and circulating pumps. b) Single reduction, double input: Often used for the second reduction gear unit of a double

reduction system. c) Single reduction, double input, three-bearing pinions: Similar to (b) and considered

obsolete. d) Double reduction, double input, articulated: Most commonly used for cross-compound

turbine installations, where the power is approximately equal from each input shaft Also known as a 'tandem' arrangement.

e) Double reduction, double input, nested: Similar to (d). f) Double reduction, single input, locked train: The two intermediate shafts take equal

power, and are, therefore, smaller than would be a single intermediate element. The overall size and weight are reduced, but more components are required and (timing) is necessary. Also known as a 'dual tandem' arrangement.

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g) Double reduction, double input, locked train: Standard for high powered naval ships, and

increasingly used for high-powered merchant ships because it minimises the total weight and assembly size.

h) Single reduction, planetary: Mainly used for turbo-alternator reduction, however, has been fitted to main turbine drive first reductions.

Casings: It is of the utmost importance that a fabricated gear case be structural stiff to prevent any measurable deflection under varying load conditions, while alignment of the bearings must ensure even tooth loading. To minimise the effect of ship's strains during loading and in a seaway, the gear is supported at two to four points to the ship structure.

Bearings: The conventional Babbit-lined, steel-shell sleeve bearing is used throughout and has proved to have an extremely long life. Clearances for this type of bearing are:

a) High speed: 0.002 to 0.003 mm/mm of journal diameter. b) Second reduction gear: 0.001 mm/mm of journal diameter. c) Intermediate: 0.001 to 0.0015 mm/mm of journal diameter.

It will be seen that the journal bearings must accurately position the gear and pinion journals to keep their axes precisely parallel. Thrust bearings may be of the pivoted shoe or plain collar type and of sufficient capacity to take the loads at all times. Where the structure allows, the main thrust bearing is housed forward of the second reduction gear, with the thrust housing an integral part of the gear casing. This location has two main advantages:

a) The diameter of the thrust bearing can be smaller since the shaft portion does not have to transmit torque.

b) The thrust collar can be a separate piece that can be readily removed over the end of the gear shaft in the event of replacement.

However, for higher powers and where greater stiffness is required for the thrust bearing foundation, the thrust bearing is located aft of the reduction gear. The seating for the thrust in this design is entirely independent of the main gear case and propeller thrust has no distorting effect on the gear casing. Couplings: Thermal distortions in the gear and turbine casings and their supporting structure create a relative movement of the turbine rotor and high-speed pinion axes that introduces an angular offset at one or both coupling elements. To accommodate any misalignment two types of flexible coupling are available:

a) Toothed: Where two gear tooth elements mesh and are separated by a length of shafting or a sleeve.

b) Membrane: A relatively thin hollow shaft with flanges at either end. Gearing Defects:

a) Gearing defects or failures in service are almost invariably due to a combination of factors such as poor design, unsuitable materials, poor gear cutting, incorrect heat treatment, incorrect alignment, either permanent or transient, inadequate lubrication and cooling, and over-loading, especially shock loading. Marine gears are prone to three major defects:

a. Pitting. b. Tooth Fracture. c. Scuffing.

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1. Pitting. In Marine gears, the most prevalent type of failure, where the causes are most complex, but now generally accepted as a local fatigue failure, which is brought about by the cyclic action of applied and released excessive local contact pressure, combined with the sliding and rolling action between mating teeth, usually in the presence of a lubricant. There is evidence that pitting is proportional to the tensile strength of the steel, to increase with oil viscosity and to be adversely affected by surface roughness. Severe pitting may call for gear replacement. 2. Tooth Fracture. The second most common type of tooth failure is tooth fracture, caused by bending stress under heavy loads. Essentially a form of fatigue failure, it almost invariably is started from a crack at the fillet of the root of the tooth, which propagates inwards and slightly downwards, then rises again until fracture is complete. 3. Scuffing. Sometimes called scoring, is defined as a tearing or breakdown of metal surface in the direction of sliding. It is caused by a local breakdown of the lubricating oil film between the mating sliding surfaces where the surface asperity peaks melt, resulting in local fusion or welding. Less common defects encountered in service are:

Defect: Cause: 1. Abrasive wear: Solid particles in lubricating oil. 2. Interference wear: Root and tip tooth in contact. 3. Corrosive wear: Electro-chemical corrosion. 4. Plastic flow: Loads beyond yield point. 5. Flaking: Heavy loads on casehardened gears. 6. Case-crushing: Soft-core collapses under load.

Surface Finish: Calculations for gear teeth are usually based on load per unit length, which assumes that the load is perfectly and evenly distributed over the whole tooth face in contact. However, in practice, even with the most sophisticated machining methods, the entire load is carried by the surface asperities, which constitutes a relative small proportion of the unit length. Also, in theory, only line contact occurs while in practice it occurs over a narrow band due to elastic deformation. This shows the need for an accurate tooth profile having the requisite surface finish. Operating Problems: In addition to the aforementioned marine turbine gearing problems, there are a number of operational problems, which occur and may be of a serious nature. Briefly, the more important are: 1. Primary Corrosion: This is frequently defined as, the oxidation of iron and steel in a moist or wet

atmosphere. During erection many parts of the turbine are exposed to humid atmosphere for prolonged periods resulting in the formation of common iron rust. Chemically cleaning and coating surfaces with a temporary corrosion inhibitor until the unit is finally erected can effectively reduce it.

2. Secondary Corrosion: An electro-chemical corrosion, it may be cause by weak, water soluble, organic acids formed as a result of oxidation of the turbine oil. Modern oxidation inhibited turbine oils have virtually eliminated this problem. The above phenomena may be accelerated if the gear-case ventilating system is not efficient or is incorrectly positioned.

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3. White-metal Bearing Corrosion: It is postulated to be an unusual form of electro-chemical

corrosion, attacking tin-based white-metal bearings, where only the tin matrix is attacked. Eventually the oxidised layer becomes extremely hard e.g. originally the Vickers hardness figure is approximately 30, and this has been known to reach values of 350+ (a typical mild steel value is 130). Thus the bearing becomes much harder than the journal or thrust collar. There is no apparent pattern of attack, but to reduce the effect, the turbine oil should be kept completely free from aqueous contaminants.

Thrust Bearing Failures: These may be grouped into two classes:

1. ‘Wire-wool’ failure where the thrust pads are damaged, but the thrust collar is severely scored, and is self-propagating from an extremely hard scab embedded in the white metal.

2. The thrust collar is virtually undamaged except for fine scoring/surface cracks, due to over- heating, but the thrust pads are severely damaged.

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Steam Turbine Unit Feed Water System: Definition: A feed system may be described as that part of the thermodynamic steam cycle which lies between exhaust steam leaving the engine and entry of feed water to the boiler, which includes all auxiliary equipment and fittings. The functions of any feed water system are:

1. The transfer of water from the condenser to the boiler (fundamental purpose). 2. Regenerative feed heating (for economy In operation). 3. De-aeration of the feed water (primarily to protect the boiler).

The main components of a typical feed system are:

1. Condenser. 2. Air ejectors. 3. Condensate extraction pump. 4. Low pressure feed heater(s). 5. De-aerator. 6. Main feed pump. 7. High pressure feed heater(s). 8. Drain cooler. 9. Glands condenser. 10. Boiler feed regulator. 11. Closed feed controller. 12. Re-circulating valve. 13. Evaporating plant.

Types of System:

1. Open feed system. 2. Closed feed system.

The former is not now installed in modern tonnage for main use, but may still be found onboard earlier ships.

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Open Feed System:

Simple Feed System with Direct-Contact Feed Heater:

CLOSED FEED SYSTEM: As steam conditions increased, operational troubles due to corrosion difficulties increased too, and the major corrosion factor was the dissolved oxygen in the feed water, which increases very rapidly with rise in boiler tube operating temperatures. Thus the feed water must be purged of this oxygen by optimum de-aeration, but such feed will, if permitted, absorb gases more readily than nondeaerated water, and, therefore, the feed circuit should be completely airtight, with strict attention being paid to those sealing glands operating at sub-atmospheric conditions and the elimination of open collecting tanks.

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Line Diagram of Closed Feed System:

Prior to 1940, America and Germany were researching into these corrosion problems and making considerable inroads towards its solution. In the U.K., the firm of G & J Weir, Ltd., can claim much of the credit, having tackled the corrosion problem for almost 100 years. One of their early suggestions towards overcoming corrosion was the de-aeration of all make-up feed water before introducing it Into the feed circuit, therefore, removing dissolved oxygen, carbon dioxide and other active corrosive gases from the make-up feed.

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The closed feed system is fundamentally concerned with preventing air from coming into contact with the feed water by using a closed circuit, thus preventing any absorption of air by the feed water, and so eliminating corrosion from the boiler and its associated equipment. This arrangement provides a flexible feed system under all steaming conditions and is shown diagrammatically as follows:

Typical Closed Feed System:

Typical Feed System of a Modern Tanker:

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CONDENSER: Low-pressure steam from the turbine exhaust condenses on the surface of the tubes in the form of a film. The film will be at a temperature between the cooling water temperature and the saturation temperature of the steam (i.e. the condensate is under-cooled). Under cooling is undesirable since it represents an unnecessary loss of heat from the system and it increases the amount of air the condensate can hold in solution. The steam flow is arranged to descend to the bottom of the condenser and then rise again, thus heating the descending water droplets regeneratively. Air is invariably present in the condenser, either through leakage from the atmosphere or in solution in the various drains, which are returned to the condenser. Theoretically, water at its saturation temperature can hold no air in solution; thus the air, which enters the condenser will tend to blanket the condenser tubes and inhibit heat transfer, and also to raise the condenser pressure if it is not removed continuously. This air is removed from the condenser by the air ejector, which takes its suction from the air-cooling section, of the condenser. The lowest temperature cooling water is circulated through the air-cooling section to reduce the amount of vapour withdrawn with the air by the air ejector. The steam enters and flows down the central space while the cooling water flows first through tube nest A and finally through tube nest B. Section C tube nest is the air cooling section. The condenser is used exclusively in turbine practice where it may be under-slung from L. P. turbine to allow for free expansion and its weight carried on spring chocks on the tank top. A perforated baffle may be fitted to stop impingement on the top rows of the tubes.

Regenerative Condenser.

Constructional and design features of condensers: Most marine condensers are of the axial in-plane type. The condenser can also be located beneath the LP turbine exhaust and this arrangement is sometimes used for VLCC machinery where there is no limitation on the height of the plant.

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This type of condenser is illustrated below, which show longitudinal and cross sections of a typical example and also serves to show the design features common to all condensers.

Longitudinal Section Through Condenser:

Cross section through Condenser:

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It is undesirable to suspend the whole weight of the condenser and the entrained cooling water from the LP turbine on the structure would need to be made very much stronger to avoid an unacceptably large vertical deflection. Thus, under-slung condensers usually either have spring supports as shown in the figure above, or else they can be mounted solidly on seating with a flexible bellows connection to the LP turbine exhaust. Spring supports are designed to cope with the range of vertical thermal expansions, which can occur in service. The cold setting procedure is laid down by the manufacturer, and usually consists of adjusting the height of the condenser after filling with cooling water by means of jacking screws provided on the spring bases. The aim is to establish a prescribed gap between the top flange of the condenser and the corresponding flange on the turbine exhaust. When the flanges are pulled together when making the joint a calculated proportion of the weight of the condenser is transferred from the springs to the turbine. The reason for doing this is to enable the springs to be able to absorb the maximum downward thermal expansion which can occur, which will be under the emergency operating condition when vacuum has been lost and the condenser casing has become very hot. If the springs had been incorrectly designed or set a large upward force could result under these conditions, which could lift the LP turbine with obvious hazardous effects. The disadvantages of using a solidly mounted condenser with a connecting bellows are:

1. The bellows could fail and cause loss of vacuum. 2. The turbine is subjected to an additional vertical load when vacuum is established and

this is equal to ‘Turbine exhaust area x Atmospheric pressure’. This is a significant load and is approximately 10,500kg/ m2. The condenser consists of a welded steel shell into which flows the exhaust steam from the turbine. Several thousand tubes, usually of about 25mm diameter pass through the shell as shown in the illustration, and these are arranged in a specific pattern decided upon by the designer. The cooling water flowing through these tubes removes the latent heat of the steam, so achieving the condensation process, but this also releases air and traces of other incondensable gases. These incondensable gases can congregate inside a badly designed condenser masking some of the tubes and preventing the full vacuum being achieved. The pattern in which the tubes are arranged is designed to assist in the removal of these gases. In the illustrations it will be seen that the tubes are arranged in two large groups called tube bundles. In the centre of each bundle is an air collection duct, which runs the whole length of the condenser and the steam flowing radially into the bundles drives air into these ducts. At the left hand end of the condenser (longitudinal section) these ducts connect to an air removal duct after passing through a batch of tubes, which are screened from the steam in the condenser by baffle plates and act as an air-cooling section. By cooling the air its volume is reduced and the air pump or air ejectors are made more effective. Both ends of each tube are located into a tube plate, usually made in rolled naval brass, and bolted to each end of the condenser shell. The tubes are expanded into the holes in the tube plates by the use of rolling tools or else by high-pressure hydraulic equipment. If the same material is used for the tubes and the tube plate they can be welded using specialised automatic orbital welding heads. The tubes have to be capable of withstanding the corrosive action of seawater, particularly when it is polluted, and a common choice of material is aluminium brass, which has a good reputation in marine applications. The effect of erosion is minimised by limiting the velocity of the water in the tubes and the table below gives the allowable water velocities for five commonly used tube materials. If tubes become partially blocked with debris the local water velocity may exceed these limits and cause erosion of the tubes.

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Another danger is that polythene sheet might be drawn into the cooling water intakes and block off a section of the tube plate, so causing an increase of water velocity in the unaffected tubes. Table for allowable water velocities:

Material: Typical well thickness (mm): Maximum velocity (m/s): Admiralty brass. 1.2 2.0

90.1 0 Copper nickel. 1.2 3.0 7.30 Copper nickel. 1.2 3.5

Titanium. 0.7 or 0.5 5.0 Stainless steel 316. 0.7 4.0

The condenser will have been designed to make full use of the maximum allowable velocities listed in the above table as the heat transfer on the waterside of the tubes is greatly improved by increases in water velocity. On the other hand, increased velocity will require more power to drive the cooling water pumps and an economic balance has to be aimed for. The steam enters the condenser at high velocity bringing with it water droplets; at the end of the expansion in the turbine about 10% of the steam will have condensed into water. Quite often a protective grid is fitted, to prevent erosion of the top layers of tubes from the high-speed impingement of these water droplets. The high steam speed can have another effect, which is to cause vibration of the tubes in a similar manner to the ‘singing’ of telephone wires in a high wind. The tube support plates, sometimes called sagging plates, are arranged so that the natural frequency of the tubes over the span between the support plates is higher than can be induced by the steam flow. The support plates have another function, which is to increase the strength of the condenser shell in resisting the vacuum collapse pressure which amounts to 11 tonnes for every square metre of shell surface. At the end of each condenser, water-boxes are provided to which are connected the cooling water inlet and outlet pipes. The prime function of the water-boxes is to obtain an even distribution of water flow through all the tubes. They are welded fabrications in mild steel protected from corrosion by a rubber lining or an internal coating such as coal tar epoxy paint. Inspection doors are provided to allow regular inspection of these protective coatings and also to investigate any suspected tube to tube-plate joint leaks. If any leaks are found, special ‘bungs’ are available to blank off the affected tubes since saltwater leaks into the condensate will have a disastrous effect upon the boiler. The feed water purity is continuously monitored and problems with the condenser would be indicated by the associated alarm.

Condenser with Side Regenerative Passage: Axial Flow Condenser:

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Under Slung Regenerative Condenser with Centre Regenanerative Passage:

Condenser Tubes, Tube plate & Packing: Impingement Attack at Tube Inlet:

Tube Bell-mouthed and Plastic Insert Fitted to Avoid Impingement Attack:

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Apparatus used to Test for Leaky Tubes:

AIR EJECTOR: High-pressure steam is expanded through a convergent-divergent nozzle into a chamber through which it passes with velocities of the order of 1200 m/s. This high velocity steam entrains air and water vapour from the condenser and carries It through a venture section where the mixture is diffused to a higher pressure. The mixture then passes to a condenser (the cooling water for which is the discharge from the condensate extraction pump - thus none of the heat of the steam is lost from the cycle) where most of the steam is condensed. The compression of the air and steam from condenser pressure to atmospheric pressure is usually carried out in two or three stages, each stage taking its suction from the condenser of the previous stage. At atmospheric pressure the water from the air ejector is led to the atmospheric drains tank and the air is vented to atmosphere.

Line Diagram of Air Ejector:

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Key: 1: First Stage Steam Chest. 2: Pipe plug. 3: Nozzle. 4: Nozzle Gasket. 5: Nozzle Ring. 6: Nozzle Gasket. 7: Mixing Chamber. 8: Diffuser. 9: 2nd Stage Steam Chest. 10: Pipe plug. 11: Nozzle. 12: Nozzle Gasket. 13: Nozzle Ring. 14: Nozzle Gasket. 15: Mixing Chamber. 16: Diffuser.

Two-Stage Ejector with Surface Inter and After Condenser: CONDENSATE EXTRACTION PUMP: This is the first stage feed pump, it is a multi-stage centrifugal pump designed to operate under the very low suction head conditions obtained at outlet from the condenser. Pump speed is approximately 1000-1400 rev/min, which gives satisfactory operation when handling “near boiling” water with the low static head available in ships, normally about 1m. To minimise heeling conditions it is essential that the extraction pump be fitted as near the centerline of the condenser as the installation will permit. FEED HEATERS (High pressure & Low pressure): Demarcation of feed heaters in the closed feed systems generally lies on which side of the feed pump they are fitted. Those fitted before the feed pump, are termed low-pressure heaters, while those after the feed pump are termed high-pressure heaters. The feed heating system is a very important factor in the design, operation and heat economy of ship's machinery. Considerable research has been carried out to determine the most economical final feed temperature it is advisable to carry for a given boiler pressure and use is often made of calculated estimating curves to enable determination of the most economic final feed temperature, bearing in mind consultation with the boiler designer must take place especially when economisers and/or air pre-heaters are to be fitted to obtain the best thermal efficiency.

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Generally, the auxiliary exhaust steam is led to the L.P. heater and the drains from the H.P. heater is cascaded into the L.P. heater also, whereas bled steam is used for heating in the H.P. heater. For marine work it is usual to have bled steam belts cast integrally in the turbine cylinder casing at the requisite stages, which give the correct bled steam pressures for the feed heaters. The temperature differential between the saturation temperature of the incoming bled steam and the temperature of the out/going feed from the heater is usually 6-9*C, which covers possible fluctuations in turbine nozzle pressure and keeps the heaters to a reasonable size. Low-pressure heaters operate at a pressure of 1 to 3 bar i.e. extraction pump discharge pressure. High-pressure heaters on the discharge side of the feed pump operate at 10 to 15bar above boiler pressures. In certain heaters, an automatic bypass valve operates in the event of a tube failure, which would flood the heater and cuts the heater out of the feed system, bypassing the damaged unit and permitting the feed water to flow direct to the boiler feed regulator.

High Pressure Heater:

DEAERATOR: Usually all the boiler feed water is subjected to the de-aerating effect of the main condenser, however, a second stage of deaeration takes place in an open type feed heater known as the deaerator. The water is raised to its saturation temperature by mixing it with bled steam at the same pressure. A series of perforated trays in the deaerator cause the water to be broken up into small droplets facilitating the release of any small bubbles of air. An air ejector similar to that fitted on the main condenser then removes the air and a small quantity of steam. The lower part of the deaerator shell is merely a storage tank of deaerated feed water to supply sufficient feed water to the boiler during load changes. The best position in the engine room for the de-aerator is on the floor level, which enables feed heater drains etc. to be returned by gravity. A booster extraction pump is then fitted at the deaerator outlet discharging into the feed pump suction, thus maintaining a pressure and preventing cavitations in the feed pump easing due to flashing.

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Line Diagram of Deaerator:

When the deaerator is fitted in an elevated position it is essential to fit a closed storage tank underneath it with a common connection to the deaerator to ensure that a suitable capacity of feed will be available to prevent any trouble at the feed pump suction when operating under varying loads. The height of the deaerator above the feed pump suction is determined by the feed water outlet temperature to insure that flashing will not occur. MAIN FEED PUMP & HIGH PRESSURE FEED HEATERS: The main feed pump compresses the feed water from the deaerator pressure (about 3 or 4 bars) to the boiler pressure. The feed water is then heated in closed type bled steam heaters, whose drains are usually cascaded into the deaerator. Systems usually have a main, auxiliary and a harbour service pump, the latter usually a reciprocating pump while the former are usually centrifugal with steam or electric prime movers.

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DRAIN COOLER. A heat exchanger to ensure hot drains from the various units in the closed feed system are at a suitable temperature before entry into the main feed tank without overheating the tank. The feed condensate is increased in the order of 6*C depending on operating conditions. GLAND CONDENSER. Primarily the purpose is to maintain a sub-atmospheric condition in the gland pockets of the main turbine glands. It incorporates its own single-stage air ejectors, the vapour being condensed and returned to the main condenser via a looped sealing pipe to overcome the difference in pressure between the two compartments. The air is released to the engine room through a vent pipe. BOILER FEED REGULATORS. The Weir robot feed regulator is one of the more popular types of feed regulators fitted to marine boilers. It is situated on the steam drum front and consists basically of a float chamber, float and valve. When the boiler water level falls below normal the valve arrangement, initially activated by the float, allows feed to pass into the boiler drum. A small by-pass enables hand feeding to be carried out, if necessary. CLOSED FEED CONTROLLER. Fitted to base of condenser and maintains the required water level in same, by means of a float controlled valve which permits water to flow into condenser at low level and out (to feed tank) at a high level. The valve design allows minor fluctuations in the condenser level to take place without continuous movement of the valve. RECIRCULATING VALVE. Used during starting up, manoeuvring or shutting down operations, when it ensures the circulation of condensate through the steam air ejector and gland condenser for the purpose of condensing the operating steam. EVAPORATING PLANT. The evaporator consists of a large shell with heating coils at the lower end and a baffle arrangement at the top. Heating in small plants may be direct from line steam, but the degrading of high pressure boiler steam is not acceptable in large installations due to the high cost of fuel, and, therefore, is bled from a suitable stage of the main turbines. This in turn involves a large heating surface due to the smaller mean temperature difference available for operation. The evaporator is usually fed with sea water at a rate twice that of the distilled water being made, the excess feed is removed, carrying with it the concentrated salts left behind by the vapour which has been boiled off. As sea water has a constituency 3% by weight of salts in solution, the constant discharge of brine has a salts concentration of about 6% by weight. Forcing of the evaporator leads to excessive scale formation on the heating surfaces although the latest plant has built-in features, which automatically prevent this event. This gives an extended operating life between major de-scaling maintenance. Older plants may have chemically treated feed in order to reduce sealing. The feed water produced has a contamination of less than 2ppm total dissolved solids. In line with modern trends virtually the whole feed system can be built-up from package units which offer economy of space occupied, reduction in weight, elimination of extensive interconnection pipe-work and simplification of layout in the shipyard drawing office.

****************************Kv**********************************

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MARINE STEAM TURBINE LUBRICATION: The marine steam turbine aroused active interest toward the latter part of the nineteenth century when Parsons and DeLaval, working independently in England and Sweden around 1885, developed their principles of blade design and perfected their ideas for the generation of power in stationary service. It was not until several years later that ways and means to step down turbine speeds to propeller shaft speeds were made practicable in the form of the herringbone or double helical reduction gear. Then the geared turbine proved its adaptability conclusively, and indicated definite economic and maintenance advantages. Credit for its development should in turn be given to DeLaval, Westinghouse, Melville and Macalpine. When the latter two investigators demonstrated the balanced thrust characteristics of the herringbone gear and proved the practicability of designing for large speed-reduction ratios, the marine steam turbine became an actuality. At about the same time, research in air pump and condenser design enabled use of greater vacuum at the turbine exhaust. This led to marked improvement in operating efficiency, heat transfer, and steam economy by enabling more complete expansion of the steam. The generation of power with steam is relatively simple. Heat developed from combustion of fuel with air is used to raise the temperature of water in a closed system until the water boils and forms steam. Since water increases in volume about 1700 times when changed to steam, the generation of steam in a confined chamber creates pressure. Steam under pressure is then piped to the turbine and discharged through suitable nozzles into the turbine blades. The steam expands and releases its energy at high jet velocity to create rotation of the turbine rotor. From the turbine, the steam exhausts into a condenser where it is liquefied and then returned to the boilers as feed water. Turbines are high-speed machines with output shaft rotational speeds of 4000 rpm or more. In contrast, and with the exception of special high-speed vessels, the normal ship's propeller operates at speeds between 75 and 120 rpm. Reduction of the high turbine output speed to the working speed of the propeller might be accomplished either by means of mechanical gear reduction units or electrical reduction. Ships may contain three turbines: High-pressure (H.P.), Intermediate-pressure (I.P.), and Low-pressure (L.P.). Each turbine runs at a speed best suited to its blades and steam velocity. Various turbine arrangements are used. In multiple propeller systems, each screw may be driven by a separate turbine (or combination of turbines) and reduction gear unit. For single screw ships, several combinations may be used in which one or several turbines are connected to the reduction gear. A common turbine arrangement in geared reduction units consists of a high-pressure and low-pressure turbine each connected to a common reduction gear. Exhaust from the high-pressure turbine is piped to the low-pressure turbine. A cross-compound 19,000 shaft horsepower (shp) steam turbine and articulated gear reduction unit is shown in the figure on the next page. Speed Reducers: As previously mentioned, mechanical gear reduction units or electrical reduction may be used to lower turbine speed to the working speed of the propeller shaft. The development of reduction gears for marine propulsion service has an interesting back- ground. Years ago when the change from paddle wheel to screw propeller took place, engineers were concerned chiefly with the problem of securing the necessary increase in propeller revolutions.

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Top: Westinghouse 19,000 shaft horse power Steam turbine: Bottom: De Laval Steam Turbine:

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There was no satisfactory means available whereby a sufficiently high speed could be secured in the slow engines of those days to permit them to be coupled directly to propellers. Accordingly, the propeller shaft revolutions had to be increased by means of gearing. Gradually, as improvements in mechanical design were effected, engine and propeller speeds became so synchronized that gearing was no longer necessary. The advent of the high-speed turbine, however, brought gearing again into consideration, but this time the gears performed an exactly opposite function, in that they served to reduce the speed transmitted to the propeller. Direct drive from the turbine to the propeller was tried at first, when the desired speed was not so low as to impair the efficiency neither of the turbine nor so high as to affect the efficiency of the propeller. It was soon appreciated, however, that most economical operation could be gained by using some means of speed reduction to enable the turbine and propeller to operate at their most efficient and natural speeds. Marine reduction gears may be single or double reduction units. As the single reduction units require a very large main gear to permit reduction down to the propeller speed, the double reduction units are more universally used. The gears are the double-helical or herringbone type. Double reduction gears may be further grouped into three general types (see figures below):

a) Nested gearing, b) Articulated gearing, c) Locked-train gearing.

Schematic views of four types of double-reduction, two-pinion marine propulsion gear sets:

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The elements of marine propulsion gears are identified in the above figure:

De Laval DLT-M Locked-train Gears:

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EPICYCLIC REDUCTION GEARS: Epicyclic reduction gears have also been used in recent years. The main difference between epicyclic and conventional parallel-shaft reduction gears is that in the conventional gear train the axes of the gears are fixed, but in an epicyclic train at least one axis moves around another axis, which is fixed. The basic elements consist of a central sun gear, an internally toothed ring gear, a planet carrier and several planetary gears.

Stal-Laval Steam Turbine Reduction Gearing System: A good example of a modern geared marine turbine installation is shown in the figure above. As in other installations, the high rotational speed (in the order of 6000 rpm) of the turbines must be reduced to the working speed of the propeller shaft. To obtain a propeller speed of about 103 rpm, Stal-Laval employ Allen-Stoeckicht planetary type gears for the first reduction. The torque from the turbines is transmitted to the sun wheel through quill and flexible couplings. A second reduction is then obtained through conventional double helical type reduction gears.

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LUBRICATING OIL SYSTEMS: Lubricating systems in marine steam turbine service differ according to the method of drive and the type of turbine used. Before the development of the modern governor and high-ratio speed reduction mechanisms, oiling systems were considered solely from the viewpoint of lubrication. The principal requisites of a lubricant were proper viscosity to afford good lubrication, and sufficient flow to dissipate the heat. There was no connection between the oiling and governing systems; the dependence was placed on alarms, gages and the like to warn the engineers in case the oil supply was interrupted. Lubricating oil was furnished by gravity, by a direct pressure system, or by a combination of both. In the modern turbine, however, the governor and bearings are on the same lubrication system. An emergency tripping device automatically shuts down the turbine, should the oil supply be cut off or delivered at too low a pressure. Experience has shown that the lubricating oil system is one of the most vital parts of a turbine installation.

Schematic Diagram of Lubrication System (De-Laval DLT-M Main Propulsion Unit):

The lubrication system for the De-Laval DLT-M main propulsion unit is illustrated in figure above.

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Gravity Oil System: The general layout of this, the simplest lubricating system, comprises the settling tank, the gravity drain tanks, oil pump, oil cooler, oil strainer, oil purifier and reserve oil tanks. A typical system is illustrated below.

Gravity Lubricating System:

Recommended Practice for Design of Marine Propulsion Turbine Lubricating System:

Pumps are necessary to return the oil to the filters or purifiers, coolers and overhead gravity storage tanks, which are located sufficiently above the operating floor to develop from 10 to 15 pounds of static oil pressure at the bearings. Thirty feet (9 meters) constitutes the usual minimum height at which any gravity tank should be placed above the operating floor.

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When the oil has passed through the bearings or gears and drained into the sump, it is rested, strained and subjected to filtration and centrifugal purification, after which it is cooled and pumped back to the overhead gravity tank for re-circulation. All or only a part of the oil may be so purified, according to the amount of oil in the system. On larger installations, some five to twenty percent of the oil may be by-passed to the centrifuge for eight or more hours each day. Gravity oiling is preferred because:

1. There is a steady, uninterrupted flow of oil, even with momentary pump failures or speed variations, whereas with the pressure system the flow may be more or less pulsating if a reciprocating pump is installed.

2. The oil is returned to an overhead gravity tank, where it is allowed to rest momentarily, and any air, which may be entrained, has an opportunity to escape.

3. It affords a factor of safety due to the fact that if the oil pump stops, there will be a few minutes' supply of stand-by oil in the gravity tank to keep the bearing lubricated until the spare pump can be started or the main machinery stopped.

The success of a gravity oiling system depends largely on the pipeline going directly from the tank to the equipment, with a minimum number of bends, and of sufficient diameter to insure adequate gravity flows in the system. The total amount of oil in a system varies according to the size of the turbine, the capacity of the gravity and sump tanks and rate of circulation. This latter must obviously be dependent upon the design of the lubricating system, the size of the turbine, the prevailing bearing temperatures, the oil pressure and viscosity, size of piping and the provision for free return of the oil. In system circulating oil at a rate of 150 gallons (approx. 568 liters) per minute, if the oil were to repeat this cycle at five-minute intervals, 750 gallons (approx. 2840 liters) would be required. Direct Pressure System: Continuous circulation of oil by direct pressure (see figure in the next page) likewise has its advocates and advantages. Generally it is less expensive to install than a gravity system since less equipment is required. In operation, the lubricating oil pumps draw the oil from a level considerably above the bottom of the sump, drain tank or purifying system, as in the gravity system, and deliver it under pressure through coolers to the turbine bearings. No reserve tank is provided; consequently, as the oil drains back to the sump tank, the pumps again pick it up and the cycle repeated. Any direct pressure system must have ample capacity in order that the re-circulation of oil will not be too frequent; it also must have adequate means for cooling the oil. Where units are operating with too small a quantity of oil in the system, the circulation may be so rapid that there is but little time for it to settle and cool. Under such conditions not only may the bearings and gears suffer from too low viscosity and poor settling or purification, but also the oil will deteriorate from oxidation more rapidly. To function properly, the pressure system should always pump oil in excess of the amount required and a relief valve or other means is provided to by-pass the excess oil back to the sump tank. An advantage of the direct pressure system is that it is possible for the engineer to obtain practically any desired pressure or volume to the bearing by regulating the relief valve. He should he careful, however, not to return too much oil to the sump through the spring-loaded relief valve, as this might cause excessive turbulence and enable the oil to take up air and develop foaming. Turbine builders furnish data as to oil capacity and rate of circulation for their systems and their recommendations should be followed.

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Recommended Practices for the Design of Marine Propulsion Turbine

Pressure Lubricating System: Oil Sump Tank: The sump tank must he placed sufficiently below the turbine to insure complete drainage of the turbine and gear easing, regardless of how the vessel may be rolling or pitching. Oil backing up in a gear case will cause excessive heating of the gears and, if the case becomes flooded, the turbine may have to be shut down to release the oil from the upper portion of the case where it may be held by ‘windage’ from the gears. The bottom of the oil tank should slope slightly in a fore and aft direction for drainage and the suction lines to the oil pumps should be located some distance above the bottom, leaving the lower portion of the tank available for collecting sediment and water for subsequent removal. Discharge lines should be at the opposite end of the tank from suction to allow for settling of any foreign matter or water and to permit escape of any entrained air. This also promotes complete oil circulation by eliminating ‘dead pockets’ and assures that only good clean oil is drawn into the circulating system. The capacity of sump tanks varies according to the size of the turbine; the must be sufficiently large to allow the oil to come temporarily to rest, free itself from air, and settle out impurities and water. An air vent to release air and vapor from the tank is provided; also a float indicator to show the height of the oil in the tank. Perforated baffle plates can be installed in the sump tank to prevent excessive splashing. It is general practice for the oil line to the sump tank to be placed so that the oil enters the tank at or slightly below oil level and at lowest possible velocity.

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Lubricant Requirements: A suitable turbine oil should have the correct initial viscosity, be resistant to oxidation and sludging, prevent rusting, be non-corrosive to turbine parts, be able to free itself rapidly from air and water, and resist foaming. The modern steam turbine oil must:

1. Transmit the governor-varied impulses to the control mechanism, properly lubricate its moving parts, and keep it free from rust and all deposits, to assure full sensitivity at all times.

2. Lubricate the bearings of the turbine and also the generator bearings of power unit and reduction gears where they are part of the mechanical system.

3. Act as a cooling medium for bearings and gears. 4. Prevent the formation of rust and of sludge deposits within the confines of the lubrication

system. For the modern, high-speed steam turbine directly connected to an electric generator, the accepted oil viscosity range is from 140 to 300 ‘Saybolt’ Universal Seconds (30-66eSt) at 100* F (38* C). Lubricating oil viscosity ranges for which the turbines are designed are specified by the manufacturers, and are related to the speed of the turbines and the anticipated bearing temperatures. In marine service where the turbine is geared down to the propeller shaft speed, a higher viscosity is necessary, since the oil must serve the dual purpose of lubricating the bearings and the gears. To meet these conditions the accepted ‘Saybolt’ Universal viscosity range is from around 250 to 560 seconds (54-12leSt) at 100* F. Here again the range is subdivided to provide an oil around the lower limit-from 250 to 350 seconds (54-7heSt); and one within the higher range-from 375 to 560 seconds (81-12leSt). These approximate ranges are in line with the turbine builder’s specifications, which depend upon design factors such as speed, gear tooth load, bearing clearances and expected temperatures of the oil at the gears and bearings. Extreme Pressure Type Oils (EP oils): After the conclusion of World War II, the trend in marine reduction-geared turbines, particularly in Naval service, was toward the transmission of greater power through smaller sized gear trains. Such units impose load-carrying requirements which cannot be met by normal premium quality turbine oils and require the use of oils having sufficient so-called extreme pressure or EP properties to prevent scoring of new gears during their break-in period, and to prevent scuffing in critical designs during service. They have all the good qualities normally associated with a good turbine oil plus greater load- carrying ability, which is attained through the incorporation of selected "EP agents." These oils are suitable for use in auxiliary turbine units as well as the main propulsion unit where they are required. They also can be used to advantage in units of older design, particularly those in which gear wear may be a problem. Lubrication of Gears: When reduction gears were first applied to marine service, their problem of lubrication was immediately noted to be quite different from that of direct-drive turbine installations. The ideal method seemed to require two type of complete lubricating systems, one carrying low viscosity oil for the turbine bearings and the other high viscosity oil for the gears.

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However, this duplication of equipment, together with additional space and cost requirements, was not practical in marine practice, so a compromise was sought. Selecting one type of lubricating oil, which would effectively serve both turbines and gears, solved the problem. This is customary practice today. Viscosity requirements must be carefully studied. If the viscosity is too low, the oil may not be able to withstand the prevailing tooth pressures, and surface wear will follow. There is a slight friction in the meshing action in all gears. The duty of the lubricant is to prevent or reduce to a minimum the amount of metallic contact, which may result from this action. In marine service, where the gears are served by the same lubricating system as the turbine bearings, the oil must have sufficient viscosity at the operating temperature to successfully withstand these high tooth pressures and prevent wear. It must also remain completely fluid and free of air entrainment. For this reason, drains from gears should always be led to the sump tank at or slightly below the operating level and at lowest possible velocity (as over a plate) to allow the oil to free itself of any entrained air without splashing and picking up more. This use of the same lubricant for both gears and bearings requires a certain increase in viscosity over that required for direct-drive turbines in order to insure effective lubrication of the gears. The oil must not be so high in viscosity, however, as to lead to the development of abnormal internal friction within itself as it passes through the bearing clearances. Most new vessels being built to operate in warm waters are being equipped with a small extra cooler for the gear oil, or are piped up in such a manner that oil going to the reduction gear-spray nozzles can be further cooled to insure a higher viscosity oil to the bearing clearances. However, oil for reduction-geared turbines should not be of higher viscosity than necessary because higher viscosity oils tend to separate less readily from water and also tend to increase bearing temperatures. In regard to lubrication of epicyclic reduction gears, it has been found that these units use the same type and qualities of oil, and demand the same degree of oil filtration, as do conventional parallel shaft gears. For marine service a viscosity in the range of 250 to 560 ‘Saybolt’ Universal Seconds (54 to 12leSt), at 100* F (38* C) is generally suitable. ***********************************Kv****************************************** Acknowledgement: The American Society of Mechanical Engineers, New York: ‘Texaco’ Marine Machinery Lubrication: Kv/BE/ST/02/04.

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Line Diagram of Turbine Gravity Type Lubricating System:

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Steam Turbine Control: There are two basic ways of controlling the steam to the turbine. 1. Throttle control: This utilises single valve to control the flow of steam through the complete power range. This means that if a ship operates at low power for significant periods of time the steam entering the turbine has been heavily throttled and this adds to the losses involved in running at a point well removed from full power for which the turbine will have been designed.

A Control System for Throttle Valve:

2. Nozzle control: In this case the inlet nozzles to the HP turbine are divided into groups with each group supplied from its own individual control valve. In this case the valves are closed sequentially as power is reduced so that throttling only takes place on the steam being supplied to one group of nozzles so minimising the losses. The most efficient way of operating at reduced power is at a so called ‘nozzle point’ where one or more of the nozzle valves are completely closed to give the desired reduction in power, but those remaining in service are all fully open.

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Control systems: A typical control system for a throttle-controlled turbine is shown in the figure in the previous page. The ahead, astern and guarding valve is all located in an integral chest separate from the turbine and connected to it by steam pipes connecting to the ahead and astern turbines respectively. The guarding valve is either open or closed and is a safeguard against steam being admitted simultaneously to the ahead and the astern turbine. It is normally closed and only opens when the signal is received for the astern valve to open.

Typical Arrangement of Valve Gear (Ahead position):

In the case of single screw ships there would usually be a provision for emergency steaming whereby if a problem had arisen with the HP turbine, this could be decoupled from the gearbox and a temporary steam connection made from the steam chest to the LP turbine. All the valves are of the single seating diffuser type-the diffuser being the tapered section down- stream of the valve, which enables an efficient pressure recovery to be made from the kinetic energy produced as the steam passes the valve seat at high speed. The valves are actuated by hydraulic servos supplied with oil from a module consisting of motor driven positive displacement pumps, and filters giving fine filtration.

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The servos are linked to the ship's bridge control system by an actuator, which moves a pilot valve, which in turn lets high-pressure oil into or out of the servo cylinder to open or close the steam valve. In the scheme illustrated there is a linkage between the pilot valve and the servo spindle, which is designed to improve the overall control characteristics by providing an approximate linear relationship between the input actuator movement and the speed of the ship. This linkage is better illustrated in the isometric sketch above. The same figure also shows the use of a stepper motor as the link with the ship's control system. A neater method would be to use a linear electric actuator. The sketch also shows the speed limiting actuator, which protects the turbine from over speeding if a coupling should fail or the ship's propeller should come out of the water in rough seas. A speed governor is shown in the figure below and this provides the sensitive oil supply to the speed limiter. The system diagram also shows an over-speed trip, which gives the final protection against over-speeding the turbines by trip closing all the steam valves if the speed exceeds a set value. This over-speed trip valve is solenoid operated and is triggered by the signal from a solid-state electronic over-speed trip unit.

Governor used for Speed Control of Turbine:

To avoid the inconvenience of spurious trips the over-speed trip unit has three independent circuits which sense the turbine speed by means of pick-ups mounted in the turbine pedestal, and receiving a speed signal by the pulses produced by a toothed wheel. A trip is initiated only if two out of the three circuits indicate that the tripping speed has been reached. In the case of nozzle governed turbines the ‘Ahead turbine’ will be controlled by a number of control valves, usually between four and eight, located in a steam chest, which is an integral part of the HP turbine. These valves may be operated by various means such as individual hydraulic actuators, or by a camshaft, which is rotated by an electric or hydraulic motor, or a ‘bar-lift’ mechanism. The overall control scheme would follow similar lines to those described above for the throttle control system. Control valves: Figure below illustrates a section through a typical control valve and its enclosure called a ‘chest’, which have various design features, introduced to over come operational problems. The steam chest is subjected to rapid temperature rise during start-up and every attempt is made to minimise the likelihood of cracking by employing large radii at corners and a constant section thickness.

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The combined valve seat and diffuser is manufactured in a Moly-Van material with stellite facings on the seating area to resist steam cutting at small valve openings. The location of the valve seat into the steam chest has a degree of flexibility designed into it. The reason for this is that the large mass of the chest compared to the seat results in different rates of expansion during the warm up period and with out some flexibility some crushing of the material beyond the elastic limit would take place and the scat would become loose. A major problem in the design of steam valves is the danger of damage due to steam buffeting. In severe cases this can cause very high frequency vibration, which results in extremely rapid wear of the valve spindles. The design features to combat this phenomena are the fitting of a flow straightener around the valve, radial keys to locate the valve and the use of hardened nitrided valve spindles and spindle bushes, both of which would be made in Nitralloy material. The force required to open a single seated valve can be very large as it is established by the full inlet steam pressure acting upon the area within the contact circle on the seats. In order to reduce this a balancing effect is obtained by the use of a pilot valve, formed at the end of the valve spindle, which connects a balance chamber, positioned above the valve, with the steam pressure which exists downstream of the valve seat. Thus the first movement of the valve spindle establishes a state of pressure balance before the main valve lifts off its seat and significant amounts of steam start flowing into the turbine. Some steam leakage takes place between the valve spindles and its bushes and leak-off connections are provided to direct this into the turbine gland steam system.

Section through Marine Propulsion Turbine Control Valve and Steam Chest:

Monitoring and Data Logging of Steam Turbine: Various operating parameters are continuously monitored to maintain safe operation and to maximize the overall efficiency of the steam turbine plant. Suitable measuring sensors are usually installed to measure HP shaft thrust wear; LP shaft thrust wear; HP shaft eccentricity; LP shaft eccentricity; differential expansion HP turbine; differential expansion LP turbine; bearing pedestal vibration.

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It should be noted that shaft eccentricity is a measurement of the orbit in which the turbine shaft is rotating and differential expansion relates to the different rates at which the turbine rotor and easing expand because of the differences in mass and surface area. The output signals from all the above may be fed to direct reading instruments in the control room or else fed into a computer data logging system. In the latter case the readouts could be obtained either as a printout or else as a display on a screen in which the current reading would appear superimposed on a schematic of the propulsion plant. Additional data such as vibration levels, oil and bearing temperatures, steam temperatures and pressures at various points in the cycle, cooling water temperatures, shaft speed and power etc; can all be fed into the data logging system and called up for display on the screen in schematic form by inputting the appropriate code on the keyboard. The turbine can be tripped by means of a solenoid trip valve, which drains signal oil from the control valve operating mechanism resulting in rapid closing of the steam valves. This trip can be initiated by one of the following:

1. The turbine over-speeding above a set limit, usually 10% above the normal maximum. 2. The loss of bearing oil pressure. 3. The loss of condenser vacuum. 4. Thrust bearing failure. 5. Other factors, such as high vibration, or rotor differential expansion, could be arranged to

trip the turbine if required by the owner and/or the turbine supplier. Turbine alarms: An alarm and annunciator panel might be provided which will give an audible alarm and an illuminated indication of the cause for the alarm being initiated. Alternatively the alarm system can form part of the computer data logging and control system in which case the fault identification would appear on the VDU when an alarm is initiated. Gland Steam System of a Marine Propulsion Turbine: The turbine glands have two main functions:

1. To minimise leakage from the shaft ends and between stages during normal running. 2. To seal the ends of the rotors when raising vacuum.

It is not possible to totally seal the rotating shafts and the presence of a running clearance means that some leakage of steam will always take place. This is minimised by having a number of baffles, which have a small radial clearance, usually of the order of 0.25- 0.40mm, with constellations on the rotor to form a number of restrictions. This arrangement gives rise to the name labyrinth gland. This can be seen in the figure on page 192, which illustrates typical gland assemblies, which are located at the inlet and exhaust ends of an HP turbine. It will also be seen that the gland is divided into a number of sections, four at the inlet end and three at the exhaust end, and that external connections are made to the spaces (called pockets) between these sections. The function of these connections can be seen in the figures on pages 192 and 193, which illustrate the conditions in a gland system at standby and at full power. The outer pockets are connected to a small gland steam condenser, which maintains them at a slight vacuum, about 50 mm water gauge below atmospheric, in order to prevent any steam leaking into the engine room. The adjacent pockets connect to the gland steam receiver which is maintained at a pressure just above atmospheric (to prevent any air leaking into the system) by control valves which either feed steam to the receiver or else leak it off to the main condenser.

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Under standby conditions it will be seen from figure on page 193 that 0.12, kg/s of steam is being supplied and this is used to 'pack' both the HP turbine and LP turbine glands to maintain the vacuum. When running at full power a considerable quantity of steam, 0.289 kg/s, leaks from the inner pocket of the inlet end gland and as this steam is at full inlet temperature it is used to contribute to the turbine output by being passed back into the turbine. In this case it rejoins the turbine at the HP exhaust and combines with the main flow of steam in the LP turbine. The gland condenser maintains the outer pockets at a slight vacuum and the adjacent pockets direct leakage steam to the gland receiver as also does the equivalent pocket in the LP turbine inlet end gland. A vacuum exists at the LP turbine exhaust so packing steam is always required by the astern end LP turbine gland and this is sup- plied from the gland steam reservoir. Surplus steam is passed to the main condenser under the control of the reservoir pressure control valve. It will be noted from the rotor gland figure below that the gland rings or segments are located against a shoulder by a spring and can be pushed radially outwards from their normal position. During start-up conditions considerable thermal gradients can be set up in the easing which can cause distortions greater than the radial clearance at the glands and the rubbing which results would cause severe local heating and bending of the rotor if the glands were solid and immovable rather than spring supported.

Rotor Glands & Gland Diagram:

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Key: Q = Quantity, kg/h: p = Pressure, bar: t = Temperature, *C: Vs = Specific volume, m3/kg:

Gland Steam at Stand-by Conditions:

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Gland System at Full Power Condition:

Additionally the glands are manufactured from materials, which give the minimum heating effect when rubbed. These are commonly nickel aluminium bronze, or, occasionally, carbon when the operating temperature is less than 425*C, and a high chrome iron (17% Cr) at higher temperatures. The inter-stage glands fitted in the diaphragms are of similar design, but require many fewer baffles as the inter-stage pressure drops are only a fraction of the pressure drop across a shaft end gland. Gland clearances are measured by removing the top half casing and rotor and inserting lead wire across the baffles at the top and bottom positions and measuring the thickness of the indentations after the rotor and top half casing have been dropped back into position. Long feeler gauges at either side take clearances. Temporary wedges are used to hold the spring loaded gland segments against their locating shoulders when these clearance checks are being made.

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TURBINE OPERATION: Turbine operation is based upon the principle of obtaining maximum reliability for the propulsion plant, minimising the amount of maintenance work that is required, and maximising the operating efficiency. To safeguard reliability of the plant the following sources of damage should be watched at all times.

1. Water erosion: Considerable amounts of water are condensed during start-up, the latent heat of the condensed steam being absorbed in heating up the pipe-work and turbine casings to their working temperature. The steam main should be gradually pressurised and warmed through using the blow-down drain until the full rated pressure is obtained at the turbine stop valve, with at least 50*C superheat before admitting steam to the turbine. The turbine should then be warmed through, with vacuum established and gland steam applied, by sequentially opening and closing the ahead and astern valves for short durations just sufficient to cause the shaft to rotate, all turbine casing drains being open. This warming through procedure would be incorporated into an automated start-up system, as would the rate of build up of power from a cold start. If the machinery has been in service within 12 hours or so there would be no need to follow this routine. 2. Thermal stress: The greatest danger of excessive thermal stresses being imposed on the turbine with subsequent damage in the form of cracks is likely during start-up from cold. The aim is to warm through gradually, following the manufacturer's instructions. An important aspect to understand is that heat transfer through a wetted surface will be 30 to 40 times greater than through a dry surface and this very rapid heating can cause high thermal stresses and distortion in the turbine casing. If, for example the turbine is on low power, a vacuum will exist within the turbine through the LP cylinder and up to the first few stages of the HP turbine which will be the only stages producing power. If the throttle valves are suddenly opened to obtain a large increase in power output, the pressure distribution inside the turbine is changed dramatically so that much higher pressures are established in the HP turbine reducing to a vacuum only in the last few LP stages. The inside walls of the turbine casing would have initially been dry and at the saturation temperature corresponding to a vacuum, which would be significantly lower than the saturation temperature of the high pressure which has been established suddenly. The cold metal surface initiates condensation and the subsequent wet conditions promote very high rates of heat transfer, with the risk of distortion and cracking referred to earlier. 3. Vibration: One source of serious vibration is a rotor becoming bent, and a bend of only 0.04 mm is sufficient to cause rough running. A rotor can have either a temporary or a permanent bend. A temporary bend is created if a rotor is allowed to remain stationary in a warm or hot turbine. Thermo-syphon effects in the space inside the casing result in the top becoming hotter than the bottom and this results in the rotor becoming bent in a convex upwards shape. The condition can be eliminated and avoided by always ensuring that the rotors are barred round if the casings are hot. A permanent bend can result from heavy gland rubs, although modem glands are designed to minimise the damage caused by rubbing. A rub is caused either by attempting to run with a rotor having a temporary bend as described above, or else because the casing has distorted as a result of attempting to increase power from cold too rapidly with the resulting effects described in the previous paragraph.

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The effect of a rub is to cause intense local heating of the rotor, which causes the bend to increase and create a snowballing worsening of the situation with the rotor rotating in a ‘skipping rope’ mode. The situation can only be avoided by rapidly reducing power on the first onset of vibration and attempt to roll out the bend by running the turbine at no more than 400 rev/ min. Vibration may also be the result of damage to the rotor blades or shrouding, the loss of blades or shorouding, or to bearing instability. 4. Damage to journals and thrust bearings: The section on bearings described the small dimension of the oil films established in the journal and thrust bearing, the result of which is that virtually any foreign matter in the oil will cause scoring which can reduce the effectiveness of the bearing in establishing an oil film. Thus a cumulative effect can take place, which can lead to a bearing failure. The point to be observed is to maintain scrupulous cleanliness whenever any part of the oil system is opened up.

Maximising efficiency: To maximise efficiency the aim is to run the plant in the manner intended by the designer. The inlet steam conditions should be kept at the prescribed values, as should the vacuum. Any fall off of the latter can lose about 4% inefficiency for every 25 mm loss of vacuum. There is no gain in operating at a vacuum better than the designed value as the turbine last stage of blades cannot take advantage of the lower pressure and the expansion to the lower pressure will take place in the condenser with no contribution of additional power. Any throttling at the turbine control valves should be minimised. If the turbine has multiple control valves the ship should be operated at a power level, which allows all the valves in service to be virtually fully open. If the turbine has a single control valve some designs provide separate small groups of inlet nozzles, each controlled by a manual isolating valve. These groups can be brought into service as required to maintain the service speed, depending on the cleanliness of the hull. This avoids the main valve operating in a throttling mode when the hull is clean, in order to allow the reserve of power needed to maintain speed with the hull in a fouled condition. Bled steam pressures should be maintained at the design values. If the stage pressures show significant variation it is likely that the turbine blades have either become coated with boiler salts or else have suffered some degree of damage. In either case the flow area through the blades has been reduced and this causes the change in pressure distribution in the turbine. As the condenser cooling water and the condensate and feed water pass through the various stages of feed heating, temperature rises should be monitored to ensure that the tubes are not fouled and in need of cleaning. OPERATING PROCUDERES OF A TURBINE PLANT: For efficient turbine running, the following must first be done before the vessel can be put to sea:

1. Propeller is free to turn. 2. Lubrication provided. 3. Cooling water circulation through condenser provided. 4. Turbines warmed through properly and spinning on steam. 5. Appropriate vacuum is brought up. 6. Correct Boiler-steam pressure maintained. 7. Turbine drains are kept open.

Now, starting with the warming up of turbine, we would proceed to have the turbine manoeuvred in port and then to sea and also try to understand the techniques of stopping the turbine.

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WARMING THROUGH: The object of warming through is:

1. To keep the turbine rotor straight. 2. To have no deformation of the easing. 3. No undue thermal stress to set up due to difference of temperature between outside and inside

portion of the easing and flanges. 4. To avoid excess condensation which may cause localized temperature stress and moisture

collection. When starting, if steam is supplied to a cold turbine rotor which is stationary there will be wide variation in temperature between the top and the bottom surface and the rotor would 'hog' or bend upwards. To ensure even temperature gradient across the rotor, it must so be rotated during the warming up period. The second objective to avoid casing distortion is due to the tendency of the steam to flow to the upper part of the easing and condensate fall to lower part. In L.P. casings, the difference of temperature between top of the casing and the cold condenser flange attachment due to the low-pressure situation prevailing cannot be totally avoided. This problem can be overcome by turning the rotor continuously to keep the warming steam adequately mixed and evenly distributed from top to bottom. The third objective to keep thermal stresses at a minimum cannot be truly solved, but slow gradual heating with just enough vacuum for the warming steam to flow and controlled amount of gland steam with rotor turning on turning gear, somewhat keeps the problem under control. Double easing construction, pre-heating of flange bolts etc, all endeavors to ease the situation. Condensation is avoided and got rid of by having easing and stage drains open and maintaining a flow of steam by having a low vacuum of about 0.16 bar (200mm Hg). WARMING UP PROCEDURE: Let us assume steam can be admitted to the turbine nozzles via a by- pass "warming up" valve.

1. Keep all casing and stage drain valves open. 2. Open gland steam and maintain reservoir pressure at 0.05- 0.1bar. 3. Start auxiliary circulating pump for condenser circulation. 4. Open air-ejector steam to a lower pressure setting to raise a slight vacuum 0.3- 0.5bars. 5. Start condensate pump at low speed to maintain condenser level and keep the re-circulating v/v

open for ejector cooling. 6. The main lubricating oil pump has now been started. 7. As the rotor is turned on the turning gear, the vacuum starts building up and to be controlled

within 0.3- 0.5bars by controlling ejector steam valve. 8. The L.P. inlet temperature when shows 82*C in about half hour. 9. The warming procedure is complete and the plant to be now ready for manoeuvring. 10. Take out turning gear and shut off warming steam valves. 11. Give steam spinning on 'ahead' and 'astern' intermittently for about 10 minutes (say 5 rpm)

before the plant committed ready for the voyage with full vacuum on. MANOEUVRING CONDITION: 1. Manoeuvring is done as per the telegraph order by closing or opening the ahead or astern

manoeuvring valves. 2. It is most times necessary to break the shaft by admitting astern steam when going from ‘ahead’ to

‘stop’ order. 3. Keep an eye on the vacuum gauge while manoeuvring and maintain correct vacuum. 4. Keep an eye on boiler water level when very quick or long astern manoeuvres are conducted. 5. Maintain lubricating oil temperature of 46- 49*C at the exit from the lubricating oil cooler.

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FULL AWAY:

1. Shut turbine drains and feed pump re-circulating valve completely. 2. Shut astern manoeuvring valve. 3. Open turbine bled steam valves as required. 4. Carry out round check for oil temperature, casing temperature, oil flow, various stage pressures,

any noise or Vibration.

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Typical Steam Cycle of a Steam Turbine Plant:

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MAIN PR0PULSION TURBINE BREAKDOWN: Emergency connections for complete breakdown of HP Turbine: In case either the H.P. or L.P. Turbine becomes inoperative due to vibration or other obvious signs of distress, pipe connections are provided to permit the faulty turbine to be disconnected and the other to operate alone. Suppose the H.P. Turbine becomes inoperative: One should all ways follow the maker's instructions for isolating the turbine, but the following is typical procedure in most ships.

1. Slowdown and stop the turbines after getting bridge permission. 2. Remove the coupling cover at aft end of the U.P. Turbine and break the connection between

turbine rotor and pinion by removing the coupling sleeve. 3. Replace the cover. 4. Close the receiver pipe from high-pressure steam outlet to L.P. Turbine by means of the blank

flange provided. 5. Blank off the main steam inlet and gland steam connections of the H.P. Turbine. 6. Remove the blank from the emergency flange on the steam inlet pipe. 7. Connect the emergency steam line and restricting orifice between the emergency flange and the

emergency inlet flange provided on the L.P. Turbine inlet belt. 8. Use de-superheated steam according to maker's instructions and operate the L.P. Turbine taking

care to avoid overheating of the turbine.

Emergency Connection for Complete Breakdown of HP: Turbine:

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TROUBLE SHOOTING GUIDE*: MARINE STEAM TURBINES (Oil system):

Problem or Symptoms: Possible Causes or Factors to Consider:

1. Drop in Oil Pressure. Cheek oil pumps and oil lines for obstructions. Switch to reserve system or change to oil feed by gravity. If oil pressure cannot be restored, shut down system.

2. Excessive Emulsification or Sludging of oil. Cheek for presence of excessive water due to leaks at the gland seals or leaks in the oil cooler. Investigate lines in oil system for presence of dirt and/or metallic particles. Cheek oil strainers to make sure they are removing foreign matter.

3. Foaming. Foaming may be due to excessive air in the oil system. All petroleum oils normally contain some air in solution, approximately 10% by volume, but excessive air may result from air leakage into the oil suction lines, splashing of oil from return lines in the reservoir or insufficient residence time for the oil in the reservoir.

4. Hot Bearings. This may be due to insufficient oil; foreign matter in the oil, misalignment or bearings too tight, insufficient oil cooling, leaky seals or the oil viscosity is too high.

5. Pitting and/or Rusting of Gear Teeth. Check gear alignments, oil flow rates, make sure proper oil is being used, and check inlet and outlet oil temperatures.

6. Sudden Overheating. Cheek all oil supply lines. 7. Vibration. Check for misalignment. If necessary, raise

rotor to proper position by adjusting bearings and balance the rotors.

*This list is by no means all-inclusive but may be of assistance in diagnosing difficulties that may be encountered. Main Turbine Turning Gear: When a turbine is shut down in the hot condition it needs to be rotated slowly to prevent thermal distortions taking place. The barring or turning gear which provides this rotation is an auxiliary, motor driven worm reduction gear mounted on the gearbox and can be brought into mesh with one of the input pinions or first reduction wheels. Suitable protection is provided to prevent any attempt to do this when the machinery is rotating under steam. One type of turning gear is shown in the sketch on page No: 201.

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Steam Turbine Turning Gear:

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Acknowledgements: Various turbine manufacturer’s instruction manuals, Institute of Marine Engineers publications; The running and Maintenance of Marine Machinery. Kv/BE/ST/06/04.

End of Steam Turbine Notes For BE (Marine Engineering) Cadets.

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Some Model Question Bank for Marine Boilers:

1. Sketch a double butt strap joint for a multi-tubular tank boiler. State why this must be the strongest joint in the shell.

2. Discuss the need for compensation for holes cut in the shell of a boiler. State the regulations concerning this compensation. Show methods of compensation that call be used. Sketch a manhole door, and show the position of these doors in the shell of a Scotch boiler. Why must a door cut ill the cylindrical portion of the shell is placed in a certain way?

3. Discuss the reasons for the limitation of pressure imposed upon tank type boilers. 4. Show the reason for the staying of any flat surfaces in a pressure vessel. How can the use of stays

be avoided? 5. State the regulations relating to the materials used in boiler construction. To what tests must

these materials be subjected? 6. Sketch and describe a Scotch boiler. Indicate overall dimensions, and plate thickness. 7. Describe the construction of the shell and the final assembly of a Scotch boiler. 8. Describe the construction of the end plates of a Scotch boiler. Show with the aid of sketches the

scarfed joints used with riveted construction, and state why they are necessary. 9. Discuss the reasons for the use of corrugated furnaces in Scotch boilers. Show some of the forms

of corrugation, which can be used. 10. Sketch and describe the furnace and combustion chamber of a Scotch boiler. Show clearly how

the furnace is connected to the end plate and combustion chamber, and how the flat surfaces are supported.

11. Describe the smoke tubes fitted in a Scotch boiler. State clearly how these tubes are attached to the tube plates.

12. Sketch and describe a vertical smoke tube boiler suitable for auxiliary purposes. 13. Sketch and describe a vertical tank boiler with vertical smoke tubes. 14. Discuss the reasons for the general adoption of water tribe boilers in place of the Scotch boiler

for the supply of main engine steam. 15. Describe the construction of the drums for a water tube boiler. 16. Discuss the functions of drums and headers as used in water tube boilers. 17. Give the various types of tubes used in water tube boilers, together with their main functions. 18. Sketch and describe header type water tube boiler. Give tube sizes and show the position of the

superheater. Indicate the gas and water flows, and give the gas temperatures from furnace to funnel.

19. Sketch and describe a two drum bent tube boiler. Give tube sizes and show the position of the superheater. Indicate the gas and water flows, and give gas temperatures from furnace to funnel.

20. Sketch and describe a controlled superheat boiler of a two-furnace type; give tube sizes and show the position of the superheater. Explain how the superheat temperature is controlled. Indicate gas and water flows for the boiler.

21. Sketch and describe a selectable superheat boiler. Give tube sizes, and show, the position of the superheater. Explain how the superheat temperature is controlled. Indicate the gas and water flows for the boiler.

22. Sketch and describe an ‘ESD type I’ boiler. Give tube sizes and show the position of the superheater. Indicate the gas and water flows and give gas temperatures from furnace to funnel.

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23. Sketch and describe an ‘ESD II’ type boiler. Give tube sizes and show the position of the

superheater and the control unit. Indicate the gas and water flows, and give gas temperatures from furnace to funnel.

24. Sketch and describe an ‘ESD III’ type boiler. Give tube sizes and show the position of the superheater. Describe the method of superheat control used in this type of boiler. Indicate gas and water flows, and give as temperatures from furnace to funnel.

25. Describe the methods of tube attachment used in water tube boilers. 26. Discuss the reasons for the use of higher pressures and temperatures in modern steam plant. 27. Discuss the need for control over superheat temperatures in water tube boilers. State methods by

which this control can be achieved. 28. Discuss, with the aid of sketches, forms of superheater suitable for use with Scotch boilers. 29. Sketch and describe a superheater as fitted to a header type water tube boiler. State the materials

used, and tube sizes. 30. Sketch and describe a superheater as fitted to a D-type water tube boiler. State tube sizes and

materials. Show the method of tube support. How is the superheater protected from overheating? 31. Sketch and describe the superheater fitted to a selectable superheat water tube boiler. State the

tube sizes, and show the method used to support them. How is the superheater protected from overheating?

32. Sketch and describe a multi-loop type superheater. 33. Sketch and describe both water, and an air-cooled attemperator. Explain the purpose for fitting

these to a water tube boiler. 34. Discuss the reasons for the use of de-superheaters. Describe how a de-superheater operates. 35. Sketch and describe an economiser suitable for use with a water tube boiler. 36. Discuss with respect to the efficient operation of economisers the importance of maintaining

both the gas and water temperatures at their correct values. 37. Sketch and describe a tubular type air heater as fitted in the uptake of a water tube boiler.

Discuss the limitations to the use of gas air heaters. 38. Enumerate the boiler mountings fitted to a water tube boiler. State the purpose of each fitting and

any special design features involved. 39. Sketch and describe a water level indicator suitable for boilers operating at low pressures. 40. Describe how you would test a water level gauge of tubular type to ensure it is indicating the

correct level. State the various causes for a false reacting. 41. Enumerate the faults to which direct reading gauge glasses of tubular type are subject. Explain

the effect these faults will have upon the level indicated in the glass. 42. Sketch and describe a water level indicator suitable for boilers working at the medium pressure

range. 43. Sketch and describe a type of water level indicator suitable for use on high-pressure water tube

boilers. 44. Sketch and describe a remote reading type water level indicator suitable for a high pressure water

tube boiler. 45. Sketch and describe an ‘Improved High Lift Safety Valve’. 46. Sketch and describe a ‘Full Bore Safety Valve’ operated by means of a pilot control valve.

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47. Explain why all easing gear is fitted to safety valves. Show by a sketch the easing gear and how

it is operated. 48. Explain how a safety valve is set to correct set pressure after all overhaul and survey. 49. Describe the process by which a residual fuel oil is burnt in a boiler. 50. Sketch and describe a pressure jet fuel oil burner. State how the throughput of oil is controlled. 51. Sketch and describe a rotating cup type of fuel oil burner. 52. Sketch and describe a steam blast jet type fuel oil burner. Discuss the reasons for the use of this

type of burner in preference to a pressure jet type fuel oil burner. 53. Sketch and describe an air register suitable for the supply of combustion air to the furnace of a

water tube boiler. 54. Describe a fuel oil system from the settling tanks to the burner of the boiler. 55. Discuss the use of refractory materials in boiler. 56. Sketch and describe a soot blower suitable for fitting to a water tube boiler and state why they

are fitted. 57. Describe the process of raising steam from cold on a Scotch boiler. 58. Describe the procedure for opening up a Scotch boiler. What inspections should be carried out

before the boiler is again boxed up? 59. Describe a procedure for closing up, and then raising stems on a water tube boiler. 60. Explain in detail the types of fires associated with boiler furnaces and uptakes. 61. Explain in detail how furnace explosions occur and state the precautions to be taken to prevent

such explosions. 62. What is a composite boiler? 63. What is the purpose of a steam generator? 64. What are the media used for cleaning the boiler tubes? 65. What is the purpose of hydraulic test? 66. How often boilers are surveyed? 67. Sketch and describe the operation of a Cochran exhaust gas and composite boiler. 68. Sketch and describe the remote water level indicator. What maintenance is required? 69. Specify with reasons the precautions taken with auxiliary boilers during:

a. Flashing up from cold. b. Blowing down the boiler and removal of the doors prior to internal inspection.

70. Explain how a cylindrical boiler is hydraulically tested. a. What inspection is to be carried out before and after testing? b. At what pressure is it tested and how long would it be kept under pressure? c. When and why are such tests carried out?

71. What is a donkey boiler? 72. Why are soot blowers used? 73. What value of discharge coefficient may be used for full bore relay operated safety valve? 74. What is a retractable soot blower? 75. What is the purpose of hammer test before hydraulic test of a boiler? 76. When do you close the shipside cock in the blow down process? 77. How is soot and ash cleaned from heating surfaces of a Marine boiler? 78. What is the frequency of external inspection for Marine boilers? 79. Describe the special features of furnace and Combustion chamber of a composite boiler.

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80. Draw a sketch of a remote water level indicator. 81. With the help of a neat sketch, explain the salient features of an improved high lift safety valve

of a marine boiler. 82. Describe in detail the procedure following the various stages of commissioning, steam raising

and full steam condition of a marine boiler. 83. What steps are taken when shortage of water is noticed in a boiler? 84. Explain the procedure of tube renewal in the case of a composite boiler. 85. Why Automatic Feed Regulators are incorporated in a boiler? 86. What are the two main types of Water Tube Boilers? 87. What is the inspection procedures adopted for a boiler. 88. What are the properties of Feed Water? 89. Explain the function of Main Stop Valve. 90. With the help of a neat diagram, explain the working of a boiler with multiple forced

circulations. 91. Explain the working of a Blow off valve. List the precautions to take with the Blow off valves. 92. What defects would you look for when making an internal and external inspection of a steam

Boiler? 93. Discuss the use of compressed air, steam and waterpower for driving water tube cleaners. 94. What are the special features of a Lamont boiler? 95. Describe the importance of stay tubes in Marine boilers. 96. State the special features of full bore type safety valve. 97. What is the principle of operation of an automatic feed regulator? 98. Why is soot blowing necessary? 99. What is the sequence of pre-commissioning procedures followed in a Marine boiler installation? 100.Explain, with schematic diagrams, the operation of three-element high and low water level. 101. Discuss the procedure of tube renewals for a water tube boilers. 102. How is the distillate fuel fired in an automatic ignition burner of an aux boiler? 102. What is the function of the Electromagnetic valve for ignition system of a boiler and how does

it operate? 103. What is an air ejector and why is it used in a boiler feed system?

Sample Question Bank for Marine Steam turbine: 1. Write in detail regarding the following:

a) Advantages of the Marine Impulse Turbine. b) Disadvantages of the above Impulse Turbine. c) Advantages of a Marine Reaction Turbine. d) Disadvantages of the Reaction Turbine.

2. A: Give the design considerations of the following Steam Turbine Components. a) Valve boxes, b) Nozzles, c) Casing, d) Rotors, e) Blades.

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B: Give in detail the selection of materials for the following Steam Turbine Components.

a) Steam valves and nozzles, b) Turbine casing, c) Turbine blades.

3. Explain in detail the functions of the following: a) Impulse Turbine Diaphragm with Nozzles. b) Labyrinth Gland or Packing.

4. Sketch and describe how a blade is fitted to turbine rotor. Explain also the forces acting on these blades and why proper clearances should be maintained. 5. With reference to turbine rotors explain in detail why:

a) Excessive rotor speed is not desired. b) Excessive axial movement is not desired.

6. Sketch and describe a flexible coupling suitable for the drive shaft transmitting the turbine output to the gearbox.

7. With reference to gland steam system of a marine propulsion system explain in detail the following: a. The main functions of the gland steam. b. How gland sealing is achieved when the turbines are stationary during warming

up. c. How gland sealing is achieved when the turbines are running at full speed. d. What are the effects of gland steam failures on the turbines.

8. Sketch and describe a gland steam system for marine steam turbine propulsion system consisting of a High pressure Turbine and one Low pressure Turbine.

9. Sketch and describe turbine gravity lubricating oil system suitable for a Main Stem Turbine Propulsion system.

10. Explain in ten detailed steps the warming up procedure of the Main Steam Turbine Plant consisting of a High pressure, a Low pressure and an astern turbine.

11. Explain in detail 5 important points each of “Maneuvering Conditions” and “Full Away” conditions of the Main Steam Turbine Propulsion Plant.

12. Write short notes on the following: i. Emergency tripping devices that may be fitted to the turbine propulsion plant.

ii. “Guardien Valve” as fitted to the maneuvering valve of the turbine plant. iii. Purpose of the turbine “Turning Gear”.

13. Sketch and describe any one of the reduction gear arrangements suitable for the main propulsion steam turbine unit. Explain why Marine Propulsion Turbines require reduction gears fitted to them while land based power station steam turbines do not require these reduction gears.

14. Pertaining to closed feed system of a main steam turbine plant explain in detail the following: i. The function of a condenser.

ii. The function of Air Ejector. iii. Feed Heaters. iv. De-aerators.

15. Sketch and describe a closed feed system suitable for a large main propulsion steam turbine system. The sketch should include a water tube boiler, all the relevant in line units and a turbine unit consisting of one HP and one LP turbine. Note: Cadets should request and get from the Head of the Department for Up-dated Question Bank every year as

the above bank is only a sample question bank:

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Some photographs are shown in the following pages for Cadet’s Understanding of

Marine Steam Turbine Installations:

Stal-Laval Advanced Propulsion Machinery:

Large HP. Turbine Rotor lifted off Lower Casing:

Note the side guides fitted on the lower casing to lift the rotor true to centerline of all blade wheels:

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Large LP. Turbine Rotor being lowered in to Casing:

Note the following:

1. Lifting cradle beam having adjustable mechanism to keep the rotor shaft centerline true- horizontal irrespective of blade wheel on the rotor weight distribution.

2. Rotor guide brackets could be seen. 3. Aster wheel can be seen. 4. On the left of the picture on the lower casing one could see a round rod stay. These are the guide

rods one in each corner for positioning the turbine upper casing after the rotor is lowered, to close the turbine unit. After fitting these guides are removed and stored on ship.

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QE 2 HP. Turbine During Assembly:

Note the following:

1. Muff coupling tooth wheel at the end of rotor. 2. Thrust color on the rotor. 3. Impulse blading and end shrouds. 4. Arrangement of diaphragms in the lower turbine casing. 5. Turbine rotor main bearing area. 6. Labyrinth steam gland area. 7. Casing holding down bolts in the lower casing.

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QE 2. LP. Turbine During Assembly:

Note the following:

1. LP. Blading arrangement. 2. Cross compound, Two casing, Double-flow arrangement could be seen. 3. HP. Turbine fully assembled and connected to reduction gearbox could be seen. 4. Main reduction gearbox could be seen. 5. Note the cap-nuts holding the lower and upper HP. turbine casing.

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Bottom Half of Casing of LP. Turbine in QE 2:

Note the following: 1. Diaphragm assembly in the lower casing. 2. Astern Turbine section at the after end. 3. Note engineers fitting the rotor guide brackets. 4. Upper casing lowering guide rods could be seen. 5. Reduction gearbox side inspection doors could be seen. 6. Reduction gearbox lubricating oil inlet pipes could be seen. 7. HP. Turbine foundation structure to the ships hull could be seen. *************************************Kv******************************************

End of Marine Boilers & Steam Engineering: Kv/AMET/BE/06/2004.