Boilers

BOILERS - AN OVERVIEWBOILERS - AN OVERVIEW

Abstract:Abstract:The production of steam and its utilization have undergone radical changes over

the years through the pioneering efforts of scientists and engineers in the fields of fuel and combustion technology, boiler operation and power generation.

In this report let us go through the various elements of the boilers such as their components, accessories, mountings etc and other aspects like their classification, design, operation, maintenance, inspection etc. The design aspect is being highlighted.

Introduction:Introduction:Generally the term boiler is referred to as a device used for generating either

steam for power generation or hot water for heating purposes. But the term boiler has been defined by the Indian Boiler Act, 1923, as a closed pressure vessel with a capacityexceeding 22.75 lts used for generating steam under pressure.

Boilers are a part and parcel of any manufacturing unit. Without them no operations such as power generation, heating, and many others cannot be performed. And we know the result of what would happen. Such is the importance of boilers in the industry.

Classification:Classification:Boilers are classified on the basis of:

Mode of circulation of working fluid a) Natural circulation boiler: here the circulation of water takes place due to

the difference in density between the saturated steam and the feed water.

b) Forced circulation boilers: here the circulation of water takes place by the help of a feed pump.

Feed water tank

Boiler

Boiler

Feed water tank

Feed pump

Type of fuel used a) Coal fired b) Gas fired c) Oil fired d) Wood fired

e) Bagasse fired

Steam pressure a) Low pressure boilers – operating steam pressure < 1.021 atm.

operating pressure of hot water < 10.893 atm temperature = 394 K

b) Power boilers - conditions exceeding L.P.Boilers. c) Miniature boilers - I.D of shell not > 406.4 mm

- Gross volume not > 0.14158 m3

- Water heating surface not > 1.858 m2

- Working pressure not > 6.8 atm 1 atm = 1.033 kg/cm2

Furnace position a) Externally fired boilers: here the furnace is located outside the boiler. b) Internally fired boilers: here the furnace is inside the boiler.

Type of furnace a) Dutch oven boiler b) Open boiler c) Scotch boiler

d) Screened boiler e) Twin boiler

Shape and position of tubes Shape - a) Straight tube boiler b) Bent tube boiler

Position – a) Horizontal b) Vertical c) Inclined

Tube contents a) Fire tube boilers: here the tubes contain the flue gases and water surrounds

them. b) Water tube boilers: here the tubes contain water and the flue gases

surround them.

Mode of firing a) Fired boilers :here the firing is done with the help of fuel. b) Non fired boilers: here firing is done by the combustion products.

Nature of heat source a) Fuel fired boiler b) Waste heat boilers c) Electrical powered boilers d) Nuclear powered boilers

Boiler size a) Commercial boilers - heating surface = 11.98 to 331.756 m2

gross heat output = 300 to 10827 kJ/s

2

b) Residential boilers - heating surface = 1.486 to 27.313 m2

gross heat output = upto 30 kJ/s c) Oil fired boilers - gross heat output = upto 30 kJ/s

Nature of use a) Stationary boilers b) Mobile boilers

Materials of construction a) Low pressure heating boilers - cast iron or steel

b) Miniature boilers - copper, stainless steel etc c) Power boilers - special steels

Manufacturer’s trade name for e.g.a) Benson boiler b) La Mont boilerc) Babcock boiler d) Wilcox boiler

Essential Qualities Of A Good Boiler:Essential Qualities Of A Good Boiler: The essential qualities of a good boiler are as follows

Capable of quick start-up. Should meet large load fluctuations Occupy less floor space Should afford easy maintenance and inspection Capable of producing maximum steam with minimum fuel consumption. Light and simple in construction The joints must be accessible and away from direct flame impact. Tubes should be sufficiently strong to resist wear and corrosion. Mud and other deposits should not collect on heated plates. The velocity of water and other flue gases should be minimum.

Criteria For The Selection Of A Boiler: Criteria For The Selection Of A Boiler: The factors taken into account while selecting a boiler are

Power required to be generated. Operating pressure. Fuel quality and type. Water availability and its quality. Probable load factor. Location of the powerhouse or process plants. Cost of operation and maintenance. Cost of installation and erection. Availability of floor space.

3

Boiler Auxiliaries:Boiler Auxiliaries:Boiler auxiliaries are the devices incorporated in the boiler circuit to boost up

the efficiency and performance of the steam generation plant and assist in the systematic and adequate operation of the boiler unit for prolonged periods.

The various boiler auxiliaries are as stated below Air preheater. Economizer. Super heater. De-super heater. Boiler feed pump. Forced draft and induced draft fans. Mechanical separator. Equipment tanks

Feed water tanks De-aerator Continuous blow down expander Drainage expander

Chemical dosing system. Soot blowers and wall blowers. Pressure reduction valve. Pulverizers and fuel firing system. Ash handling systems.

Boiler Circuits:Boiler Circuits:

All the above mentioned auxiliaries are a part and parcel of the boiler circuits. Some in one type of circuit and some in the other. Let us get a brief idea about these boiler circuits. There are basically six types of boiler circuits. they are: Fuel circuit. Air circuit. Gas circuit. Water circuit. Steam circuit.

Fuel Circuits:The fuel circuit consists of the following elements: Loaders : Used to lift the coal from ground level to elevated crusher

hoppers. Pulverisers : Sometimes the size of the coal is not as per the

requirement. At that time the coal crushers are put to work, the coal thus obtained is of uniform size also. The particle size obtained is of the order of 15-25 mm.

Magnetic Seperators : They are used to remove metallic objects so as to avoid serious damage to the crushers.

4

Coal Driers : It is very much essential to remove the moisture content from the coal, other wise it will lead to incomplete combustion. This is done by blowing hot air over the coal bed.

Feeders : this is the one that supplies the pulverised coal to the furnace. this may be done either by employing the natural or screw conveyer or vibratory feeder.

Fig 1. Block diagram of a Fuel circuit.

Air Circuits:The air circuits primarily consist of the following: Air filters : These are used when the air entering the feeder contains

impurities, unnecessary gases or suspended particles. Forced draft fans : These are used to push the air through the

combustion air supply system in to the furnace. Its discharge pressure must be high enough to equal the resistance of air ducts, air heater, burners or any other resistance between the fan discharge and the furnace.

Primary air fan : A branch line is taken from the FD fan to push the fuel inside the furnace.

Air Preheater : this is the device used for heating the combustion air by picking up the heat from the flue gases. The preheating of air helps in:

1. Igniting the fuel.2. Improving combustion efficiency by ensuring complete combustion.3. Reducing the flue gas temperature.4. Reducing the physical size of the boiler.

Raw coal Primary crusher Magnetic seperator

Pulverizer

Grinder Coal drierCoal bunkerFeeder

Boiler

F.D. fan

Ai

r

5

Fig 2. Block diagram of an Air circuit.

Gas Circuits:The gas circuits primarily consist of the following: Dust Collectors : These are used to collect the dust, which is carried

by the flue gases. If the dust is not eliminated from the flue gases, it leads to the erosion of the ducts, which may lead to the failure of the ducts. A rotary valve is used to discharge the fly ash from the dust collector. Efficiency is 85-95%.

Bag Filters : When the pollution norms are very stringent as in the densely populated regions bag filters are used. They are very costly.

Electro Static Precipitator (ESP) : They operate by charging dust particles as the gas passes through the electrically charged wires. This dust is attracted to, and collected on oppositely charged plates, which are periodically rapped and moved.

Induced Draft Fan: These fans suck the flue gases and force them through the chimney. Even a temporary stoppage of the boiler can necessitate the shutdown of the boiler. Here dampers are used to control the volume and the path of flow to prevent the jamming of the hot gases.

Chimney : This is a long, vertical cylindrical column used to discharge the flue gases at such a height that they will not create any problem to the surroundings. Its height is usually 20 mts.

Fig 3. The block diagram of a gas circuit.

F.D.fan Air Preheater Furnace

Boiler

Air

Boiler Air filter I.D.fan

Chimney

6

Water Circuits:The water circuits primarily consist of the following: Boiler feed pump: It is a device which pumps the water to the steam

drum via the economizer (for preheating). It is a usual practice to employ two pumps in case one gets clogged.

Economizer: It is a heat recovery equipment that picks up the heat from the flue gases and heats the feed water. They work under higher steam drum pressure than the working pressure of the boiler. They are located at the pump discharge side of the feed water circuit. According to the I.B.R an economizer is defined as any part of a feed pipe which is under pressure and through which feed water passes directly through a boiler and is exposed to the action of flue gases for the purpose of waste heat recovery.

Water Preheater: These are basically shell and tube type heat exchangers working at atmospheric pressure. They work on the principle of natural circulation.

Evaporator: This is the one where the generation of steam takes place. It is called saturated steam. Steam which is fully vaporized is known as saturated steam, there is no moisture content.

Superheater: It is used to superheat the saturated steam produced in an evaporator to a specified temperature. here the phase does not change.

De-superheater: it is installed to control the steam temperature by injecting water to the superheated steam.

De-aerator: It is a device which removes the dissolved gases in the feed water and hence prevents corrosion.

Chemical dosing: this is a process to remove the traces of dissolved gases which are still left after the process of de-aeration. There are two types:

1. L.P.dosing.2. H.P.dosing.

Fig 4. The block diagram of a water circuit

Feed pump Economizer

Superheater 1De-superheater

Superheater 2

Steam drum

Evaporator

7

Boiler Mountings and Accessories:Boiler Mountings and Accessories:

Boiler mountings are the fittings primarily intended for the safety of the boiler and control of the steam generation process completely.

The various mountings and accessories are as stated below. Pressure gauges. Safety valves (2 nos.). Water level indicators (2 nos.). Feed check valve. Steam stop valve. Fusible plug. Blow off cock. Man holes and mud holes.

Boiler Design:Boiler Design:

Whether the boiler is a drum or once through type, or whether it is an individual unit or a small part of a large complex, it is necessary in design to give proper consideration to performance required from the total complex of the steam-generating unit. Within this framework, there are some important items, which must be accomplished in boiler design. The items which are of importance are as stated below:

Determine the heat to be absorbed in the boiler and other heat transfer equipment, the optimum efficiency to use, and the type of fuel or fuels for which the unit is to be designed. When a particular fuel is selected, determine the amount of fuel required, the necessary or preferred preheated air temperature and the quantities of air required and flue gas to be generated.

Determine the size and shape required for the furnace, giving consideration to location, the space requirements of burners or fuel bed, and incorporating sufficient furnace volume to accomplish complete combustion. Provision must also be made for proper handling of ash contained in the fuel, and water cooled surface must be provide in the furnace walls to reduce the gas temperature leaving the furnace to the desired value.

The general disposition of the convection heating surfaces must be so planned that the superheater and the reheater when provided, are located at the optimum temperature zone where the gas temperature is high enough to afford good heat transfer from the gas to the steam, yet not so high as to result in excessive tube temperatures or excessive fouling from ash in the fuel.

Pressure parts must be designed in accordance with applicable codes using approved materials with stresses not exceeding those allowable at the temperatures experienced during operation.

8

A tight boiler setting or enclosure must be constructed around the furnace, boiler, superheater, reheater and air heater, and gastight flues or ducts must be provided to convey the gases of combustion to the stack.

Supports for pressure parts and setting must be designed with adequate consideration for expansion and local requirements, including wind and earthquake loading.

Fuel Characteristics: From these very characteristics, a boiler designer gets the knowledge of heat

value available from the fuel as well as tits specific properties such as: ash content and percent of volatile matter nature of ash and its fusion point unburnt fuel losses as carbon is lost and escapes through the flue gases the presence of such corrosive agents like sulfur and vanadium that will

dictate the flue gas exit temperature as well as the materials of construction of the heating surfaces of the boiler to avoid the problems of corrosion and slagging.

It is these characteristics that govern the fuel burning mechanism in a boiler, which in turn influences the boiler design.

It is usually possible to determine which fuel is the most difficult from the standpoint of combustion and ash handling, and the unit is therefore designed for the most difficult fuel that would be used. In the case of sulfur-bearing fuels, flue gas temperature is usually kept above the dew point to avoid sulfur corrosion of economizer or air heater surfaces. The efficiency of combustion is 100 minus the sum of the heat loses expressed in percentage. After the efficiency is calculated the fuel input rate is then determined by the formula

Wf = q / (Qh * eff)Where: Wf = fuel input rate, lb/hr

Qh = high heat value of fuel, Btu/lb eff = efficiency of combustion, %/100

The fuel input rate determines the furnace volume and design specifications. As the furnace design undergoes a change, so does the layout of the heat-absorbing surface of the boiler. However it can be safely concluded that the type of fuel burning equipment and the method of firing exercise much greater influence on furnace design than on boiler design.

Gas Flow Characteristics:The gas flow through the boiler is effected by the differential pressure

between the combustion products in the furnace core and the flue gases at the boiler exit. This pressure difference, called draught (draft) may be affected by natural

9

means or by mechanical means to supply the necessary primary and secondary air to sustain and control fuel combustion.

Depending upon whether this draught is produced naturally (by chimney effect) or by mechanical means (by installing induced draught fans, forced draught fans or both), the boiler design is altered accordingly. The quantity of excess air supplied in the form of secondary air influences the boiler capacity as well as the furnace temperature.

Again, higher boiler efficiency needs combustion air to be preheated and therefore, an air preheater is to be installed almost invariably in the convective shaft of the boiler furnace, and that means a further draft loss, which must be taken into account in the overall design of the steam generator.

Steam Requirements:The quality of the steam required is another important consideration the

designer has to make (i.e.) whether the steam required should be wet, dry or superheated. If wet steam is required then the designer may do away with the separator and superheater. If 99.5% dry steam is required then he has to opt for suitable steam separators. The incorporation of a superheater and reheater becomes obvious if superheated steam or steam reheating is required downstream, e.g., in the turbo alternator.

Fig 2. Steam quality limit for nucleate boiling in smooth and ribbed tubes as a function of mass velocity.

The steam requirements are taken into consideration and after that the steam flow, steam pressure, and temperature and the boiler feed water temperature are

10

determined, the required rate of heat absorption, Q, is determined from the equation:

Q = w’ (h2’ - h1’) + w” (h2”- h1”)

Where: Q = rate of heat absorption, Btu/hr w’ = primary steam or feed water flow, lb/hr w” = reheat steam flow, lb/hr h1’ = enthalpy of feed water entering, Btu/lb h2’ = enthalpy of primary steam leaving the superheater, Btu/lb h1” = enthalpy of steam entering reheater, Btu/lb h2” = enthalpy of steam leaving reheater, Btu/lb

Furnace Design:The furnace volume must be sufficient to maintain the necessary heat release

rate and furnace temperature while the combustion space should be sufficient to contain the flame so that it does not directly hit the water walls. The rate of steam generation and the heat release rate, which in turn govern the size and shape of the furnace, nature and materials of construction of the furnace walls and disposition of the heat-absorbing surface in the radiant and convective shafts of the furnace.

When pulverized coal or Cyclone-Furnace firing is used, the walls in which the burners or cyclones are located must be designed to accommodate them along with the necessary fuel and air supply lines.

Where fuel is burned on stokers or hearths, the size of the furnace is usually set by providing a plan area based on a specified release of heat per sq. ft of base area per hr. The furnace must also be proportional so that combustion is completed with due regard to the factors of temperature, turbulence and time.

Preheated air is beneficial in obtaining an adequate combustion temperature, and is required for pulverized coal or Cyclone-Furnace firing, as well as for residual or heavy oils. Turbulence is primarily a function of fuel-burning equipment, and its importance lies in supplying air, not only to individual fuel particles but also to any unburnt or partially burnt gases until combustion is completed. The time factor is fulfilled primarily by providing sufficient furnace volume so that the combustion gases remain in the furnace long enough to assure complete combustion.

The furnace bottom design becomes a major consideration during the design of coal-fired boilers. They may be pulverized coal fired or stroker or grate fired. For pulverized coal firing, the furnace bottom should be cone shaped to drop all molten slag to be carried off mechanically, pneumatically or by water. In the case of stroker fired boilers, high pressure water jets are directed upon the sash and the molten slag as they spill over the chain at the furnace bottom. The lack of proper furnace design for ash and slag removal may result in excessive slagging of water walls impairing the heat transfer characteristics and performance of the boiler.

Most of the modern boiler furnaces have all water-cooled walls. This not only reduces maintenance of the furnace walls but also serves to reduce the gas temperature entering the convection bank to the point where slag deposits and

11

superheater corrosion can be controlled by sootblowers. The wall tubes of the furnaces are spaced close centers to obtain maximum heat absorption. Membrane walls with refractory lining are used in the lower furnace walls of cyclone-fired units.

Convection Boiler Surface:The cost of furnace wall cooling surface is relatively higher than that of the

boiler surface, therefore the furnace size and surface are limited to the amount required to lower the gas temperature entering the convection tube banks sufficiently to avoid ash deposits.

The first few rows of tubes in the convection bank may be boiler tubes widely spaced to provide gas lanes wide enough to prevent plugging with ash and slag and to facilitate cleaning. In many large units they are used to support the furnace rear wall tubes. These screen tubes (widely spaced boiler tubes) receive heat by radiation from the furnace and by convection and radiation from the combustion gases passing through them.

Design of the boiler surface after the superheater will depend upon the particular type of unit selected, desired gas temperature drop, and acceptable gas pressure drop through the boiler surface. The object in the design of convection heating surfaces is to establish the combination of tube diameter, tube spacing, length of tubes, number of tubes, width and depth, and gas baffling that will give the desired gas temperature drop with the permissible pressure drop.

Heating surface and pressure drop are directly interrelated since both are primarily dependent on gas mass velocity. So if there is an optimum gas mass velocity it results in the optimum combination of heating surface and gas pressure drop.

Gas turns between tube banks generally add draft loses with little or no benefit to heat absorption and hence designed for easy flow.

Convection Banks:Under this category come the aspects like tube spacing and arrangement, tube

diameter, penetration of radiation, and effect of lanes.

Tube spacing and arrangement: In addition to heat absorption and resistance to gas flow, the other important

factors to be considered in establishing the optimum tube spacing and arrangement for a convection surface are lagging or fouling of surfaces, accessibility for cleaning, and space occupied. A large longitudinal spacing in relation to the transverse spacing is usually undesirable, since the length of the flow path for the calculated surface may be excessive.

There are some important points to be considered regarding the arrangement of tubes:

1) The tubes must be son arranged that they will not be subjected to excessive bending-moment stresses in carrying the weight of the tubes, drums, other

12

parts which they support, and contained water. When the unit is bottom-supported, the tubes must satisfy column requirements.

2) The holding strength of the tube seats must not be exceeded.3) Provision must be made to accommodate the required expansion of pressure

parts. For a top-supported unit, the hanger rods must be designed in such a way that they swing at the proper angle, and they must be long enough to take the movement without excessive stresses in either the rods or the pressure parts. Bottom-supported boilers should be anchored only at one point, guided along one line, and allowed to expand freely in all other directions. To reduce the frictional forces and resultant stresses in the pressure parts, roller saddles or mountings are desirable for bottom-supported heavy loads.

Tube Diameter:For turbulent flow, the heat transfer conductance is inversely proportional to

some power of the tube diameter. The tube diameter should be held minimum for most effective heat transfer, however this optimum tube diameter may require an arrangement that is expensive to fabricate, costly to install and difficult to maintain in operating condition. A compromise between heat transfer effectiveness and manufacturing, erection, and service limitations is thus necessary in the selection of tube diameter.

In oil fired marine boilers of high rating, 1-in. O.D tubes are use in boiler banks beyond the screen tubes. The high heat absorption rates in the furnace necessitate an augmented quantity of circulating water. For this reason the screen and water wall tubes are usually have 11/2 in. outer diameter.

Fig 3. General effect of convection tube arrangement on volume occupied, amount of surface, draft loss, and floor area for selected conditions fixed.

Penetration of Radiation:A convection bank of tubes bordering a furnace or a cavity acts as a black

body radiant heat absorber. Some of this heat however radiates through the spaces between the tubes. The effect of this penetration is especially important in establishing tube temperatures for superheaters located close to the furnace or high temperature cavity.

13

Effect of Lanes:Lanes in tube banks, formed by the omission of a row of tubes, may

decrease the heat absorption considerably. These passages, in effect, act as bypasses to the hot gases through the banks. Although the overall efficiency decreases, the high mass flow rate and gas weight through the lanes increase the absorption rates of the remaining tubes.

Lanes must be eliminated both within tube banks and between tube banks and walls. This is not always possible for e.g., with superheaters space must be allowed for additional surface to satisfy future increases in steam temperature. In these cases an alternative arrangement is made to satisfy the need of space.

Heat Transfer Characteristics:All the three modes of heat transfer – radiation, conduction and convection

enter into the equation, either alone or in combination during theoretical calculations of water walls, superheaters, economizers and air heaters.

Radiation heat transfer is prevalent in the core (hottest part) of the furnace and the transfer of radiant energy to the boiler tubes is dependent on the luminosity of the flame and the amount of heat absorbing surface exposed to the flame.

The rate of radiant heat absorbed by the water wall is: qrad = E*A*F*S*[(T1)4- (T2)4]

Where: qrad = radiant heat absorbed by water wall, W

E = emissitivity of the flame A = area of cross section of radiant heat absorbing surface, m2

F = view-factorS = Stefan-Boltzmann constant = 5.67*10-8 W/m2K4

T1 = absolute temperature of flame, KT2 = absolute temperature of radiant heat absorbing surface, K

The coefficient of radiative heat transfer is given by:

hrad = qrad / (A*T) Where: hrad = coefficient of radiative heat transfer, W/ m2K

qrad =radiant heat absorbed by water wall, W T = temperature difference between the core and inner dia, K

The radiative resistance is given by:

Rrad = 1/ hrad*A

Rrad = radiative resistance

14

hrad = coefficient of radiative heat transfer, W/ m2K A = area of cross section of radiant heat absorbing surface, m2

Heat transfer in the second vertical shaft and the horizontal duct of the furnace take place entirely by convection. The rate of heat transfer from hot flue gases to the heat absorbing surfaces out in this zone is given by:

qconv = hconv *A* T

Where: qconv = heat absorbed by convection, W

hconv = coefficient of convective heat transfer, W/m2K T = temperature difference between the hot gases and walls, K

The convective resistance Rconv is given by:

Rconv = 1/(hconv*A)

Heat transfer by the mode of conduction takes place through the wall thickness of the tubes as well as across the scale or depositions on both the inside and the outside of the tube surface. The rate of conductive heat transfer through a wall is given by:

qcond = k*A*(T/X)

For a composite wall, qcond = k1*A*(T/X)1 +k2*A*(T/X)2 + k3*A*(T/X)3

and conductive resistance is,

Rcond = (1/A)[ (X1+X2+X3) / (k1+k2+k3) ] Where:

k1 = thermal conductivity of the scale, W/mK(T/X)1 = temperature gradient across the scale, K/mk2 = thermal conductivity of the tube wall, W/mK

(T/X)2 = temperature gradient across the tube wall, K/m k3 = thermal conductivity of the slag deposition, W/mK(T/X)3 = temperature gradient across the slag, K/m

If all the three processes take place simultaneously then the overall resistance to the heat flow is given by:

R = Rcond+Rconv+Rrad

Therefore heat flow rate,

q = T/ R =U*A*T

15

where, U is known as the overall heat transfer coefficient.

A boiler designer must consider the internal and external fouling factors while designing the superheaters, waterwalls, economizers, etc.

Heat transfer to water:a) Water-film conductance:

The film conductance for water in economizers is so much higher than the gas side conductance that it is neglected in determining the economizer surface.

b) Boiling-water conductance:The combined gas side conductance (convection plus intertube

radiation) seldom exceeds 30Btu/sq ft, hr, F in boiler design practice. The film conductance for boiling water (10,000Btu/sq ft, hr, F) is so much larger that it is generally neglected in calculating the resistance of heat flow.

c) Effect of oil or scale:Water and steam-side scale deposits interpose a high

resistance to the flow of heat. The additional temperature drop required in maintaining a given fluid temperature inside the tube, as the thickness of the scale increases, leads to a high metal temperature and ultimate failure. The high heat absorption rates in furnace enclosure tubes of high capacity boilers make it essential to prevent the formation of scale to assure continuity of service. Good feed water treatment and proper operating practices prevent deposition of scale and other contaminants.

Heat transfer to steam:In the design of superheaters, the steam film constitutes a significant

resistance to the flow of heat, and although this resistance is much lower than the gas-side resistance, it cannot be neglect6ed in computing the overall resistance to heat flow or the heat transfer rate. It is particularly significant in calculating superheater tube temperatures, since the temperature of the inside tube wall is equal to the steam temperature plus the temperature drop through the steam film.

It is imperative to prevent the scale deposits in superheater tubes because of the magnitude of resistance to heat flow in the steam flow and the elevated temperatures at which the superheater tubes operate. Even an extremely thin layer of scale forms an insulating barrier, which together with the steam film, may be sufficient to cause overheating and ultimate failure of a tube.

Design of Pressure Parts:Boilers have achieved the safety and reliability, which they now have through

the use of sound materials and safe practices for determining acceptable stresses in tubes, drums and other pressure parts. Boilers are always designed to applicable codes. In each case, the stress allowable depends on the maximum temperature to which the part is subjected, and therefore it is important that the pressure parts be so

16

designed that the design temperatures are known and not exceeded in operation. In boilers the material temperatures are normally designed to be only a few degrees above the saturation temperature corresponding to the boiler pressure.

In boiler tubes this is accomplished by providing sufficient water to avoid the occurrence of a DNB, or departure from nucleate boiling. So adequate supply of water must be provided for each tube, and this is particularly important in furnace and screen tubes where the heat input is high. Steam drums have thick walls, and hence it is necessary to limit the heat flow through them to avoid excessively high thermal gradients. Where the heat input through a drum would be too high, because of high gas pressure or velocity, insulation may be provided on the outside of the drum. In a drum type boiler equipment is provided in the steam drum for the reduction of moisture and the solids in the steam to acceptable values.

The boiler safety valve constitutes a very important item in the safety of modern boilers. By law, the boiler design pressure must not be less than the safety valve relief pressure. The operating pressure in the boiler, in turn, depends upon the pressure required at the point of use and the intervening pressure drop.

Boiler Settings:The term boiler setting refers to all the walls that form the boiler and furnace

enclosure, and includes the lagging and insulation of these walls. Casing is sheet or plate attached to pressure parts for the purpose of supporting the insulation or forming a tight enclosure. Lagging is an outer covering over a wall for the purpose of protecting insulation or improving appearance.

Design Requirements:Settings must safely contain high temperature gases and air. Leakage, heat

loss and maintenance must be reduced to acceptable values. A number of factors require consideration in the design of settings. They are as listed below:1) Enclosures must withstand the effects of high temperatures, ranging upto

3500F in some cases.2) The action of ash and slag must be considered from the following point of

viewpoints:i) Destructive chemical reactions between slag and metal or refractory

can occur under certain circumstances.ii) Accumulations of ash on the water walls can significantly reduce heat

absorption.iii) Ash accumulations can fall from a height and cause injury to personnel

or damage to apparatus.iv) High-velocity ash particles can erode the pressure parts.

3) Enclosures must be designed for high pressures and differential expansion of component parts.

4) Supports must be designed to accommodate the effects of thermal expansion, temperature and pressure stresses, and wind and earthquake loadings appropriate to the plant site.

17

5) The effect of explosions as well as implosions must be taken into account to lessen the probability of injury and damage.

6) Vibrations caused by combustion pulsations and the flow characteristics of gas and air must be limited to acceptable values.

7) The insulation of the enclosure must limit the heat loss to an economic minimum.

8) The surface temperature or the ambient air temperature must not cause discomfort or hazard to the operating personnel.

9) Enclosures must be gas-tight to minimize leakage into or out of the setting.

10) The design must be adequate to meet the corrosive effects of ash and gases.

11) Setting of outdoor units must be weatherproof.12) Settings must be designed for economical fabrication and erection.13) Serviceability, including access for inspection and maintenance, is

essential.14) Good appearance, consistent with cost and maintenance requirements, is

always desirable.

This was about the distinct features of boiler design. After the boiler has been designed it is fabricated, erected and then put to use. Now let us see the various aspects in the boiler operation, inspection and maintenance. These are much more important because only when they are put to use u get the output for which they have been designed.

Boiler Operation, Inspection and Maintenance:Boiler Operation, Inspection and Maintenance:

Boiler operation is a very important aspect. Even the smallest of the errors and it may prove fatal to the operating personnel as well as the surroundings. There are some conditions specified before hand to ensure safe operation of the boiler. The principal characteristics that are taken into consideration in describing the operating conditions of the boiler are:

Average efficiency of a boiler for a particular operating period. Net efficiency of boiler at rated load. Availability factor, i.e., the ratio of operation time and reserve time to the

calendar time. Operation factor, i.e., the actual operating time of the boiler to the length of

the calendar time (month, year) considered. Capacity factor, i.e., the ratio of the total steam generated during operation

time to the probable steam generation during the calendar time at the rated steam generation time.

Average and maximum time of a campaign (i.e., the operation time to failure).

18

Regimes of operation:There are two regimes of operation of the boiler, they are the steady regime

during which the steam parameters change insignificantly with time and the unsteady regime in which there fluctuations of steam parameters due to internal and external disturbances. The factors that are responsible for these disturbances are listed below:

Factors responsible for internal disturbances are the variation of:

Factors responsible for external disturbances are the variation of:

Flow rate of boiling feed water.

Temperature of boiling feed water.

Fuel consumption rate. Combustion airflow rate,

etc.

Steam pressure in the steam main.

Load of the turbo alternator.

The degree of opening of start-up and shutdown device.

Boiler Shutdown:In the process of boiler operation one of the most important processes is boiler

shut down there are three basic principle types of shutdowns. They are: Emergency shutdown

Shutdown for repairing jobs with cooling of the whole or part of the boiler unit.

Shutdown for repairing jobs or to reserve without cooling of the boiler and steam pipelines.

The emergency shutdown is carried on when any one of the following things which are listed below happen:

Explosion in the furnace damaging the brickwork or pressure parts. Flame extinction in the furnace. Deformation of the pressure parts that might invite explosion and endanger

the operating personnel. Failure to ensure reliable boiler operation because of bad visibility, fire and

danger of explosion. Non-permissible rise of superheated steam temperature. Failure of feed pumps. Failure of both water-level gauges for drum type boilers and feedwater flow

meters for once-through boilers. When the water level in the drum drops below the safety mark or in the case

of once-through boilers, the supply of water I interrupted for more than 30 sec.

Rupture of tubes in the water-steam path. Fuel burning on the hat recovery zone. This is accompanied by abnormal

rise of temperature of the flue gases.

19

Inadmissible pressure drop of gas or fuel oil behind the control valve. When there is no steam flow through the steam reheater.

Boiler tube failure (leakage): Due to some reasons there may be leakage in the boiler tubes. This is also

referred to as tube failure. Tube leakages, if minor, can be detected by the loss of working fluid from the system, by the noise produced from the leak and also in case of boiler water chemicals. The procedure to be adopted in case of a leakage are:

Boiler is to be shutdown and cooled. Boiler drum is to be drained. Inspection and detection of leakages has to be carried out. Leaks are to be repaired and leaky tubes are to be replaced.

Prec

autions for non-drainable superheaters:Non-drainable type of superheaters are very hazardous and especially during steam raising. There are certain precautions to be taken at that time. They are:

Temperature of the superheater tubes must not be allowed to raise above the higher allowable limit .

No abnormal temperature difference between any two parts must be allowed.

During start-up, the tube metal temperature is kept below the temperature that the tubes attain at the maximum designed capacity.

The firing rate of the tubes must be controlled to avoid accumulation of condensate in the superheater coils.

Inspection process and its importance:The inspection of the boilers is carried out according to preplanned schedules

and n a regular basis to detect defects, locate the deterioration of material, abnormal wear, etc., so that these can be rectified to avoid serious damage. The inspection is generally carried out both during the operation and shutdown at least once in a quarter.

During the loading of the boiler, one should inspect to ascertain the following things:

20

Feed systems, signals, interlocks, regulating devices, instruments, auxiliaries, furnaces and boilers functioning properly and satisfactorily.

The boiler operation is in conformity with the instructions specified by the boiler suppliers, Govt. regulations, safety rules, etc., with respect to the working parameters, viz. Steam temperature, pressure, water level, draft losses, etc.

Equipment is kept clean, tidy and in workable condition. Floors and passages are clean, adequate provision is made for fire fighting.

Skilled and well informed operators are manning the boiler.The processes of operation, inspection and maintenance are thus very important

for good life and efficiency of the boiler and also to prevent hazards.

Conclusions:Conclusions:

From the above report we get a bird’s view of the various aspects related to boilers. The process of the design of a boiler is very complex and the designer has to pool up all the considerations and constraints, which play a major role in the performance of the boiler after it has been erected. After the erection process the tasks of operation inspection and maintenance play a vital role in ensuring the proper working of the boiler.

Bibliography:Bibliography:

Boiler Operation Engineering – P.Chattopadhyay Boiler design - Collected papersA text book of Heat transfer - Sachdeva

21

C:\WDir\boilers.doc

22