Boiler ssasit

Steam BoilersSteam Boilers

IntroductionIntroduction

Steam boiler is a closed vessel in which heat produced by the combustion of fuel is utilized to produce steam from water, at desired temperature and pressure.

According to Indian Boiler Regulation (I.B.R.) a boiler is a closed pressure vessel with capacity exceeding 22.75 liters for generating steam under pressure. It includes all the mounting fitted to such vessels which remain wholly or partly under pressure.

The boiler used for power generation has considerably large capacity and steam is produced at high pressure to get high efficiency of turbine. The boiler used for process heating generate steam at a lower pressure. It is generally smaller in size and simpler in design.

Classification of BoilersClassification of Boilers

(1) According to geometric orientation of boiler:If the axis of the boiler is horizontal, the boiler is known as horizontal boiler,

Example : Lancashire Boiler

Locomotive Boiler

If the axis of the boiler is vertical, it is known as vertical boiler,

Example :Cochran Boiler

And,

If the axis of the boiler is inclined it is known as inclined boiler.

(2) According to relative position of water and hot gases :(a) Fire Tube boilers :- If the hot gases of combustion from the furnace pass through the tubes and water is surrounding the tubes is called water tube boilers.

(b) Water tube boiler :

If the water passes through the tubes and hot gases surrounding the tubes is called water tube boiler.

(3) According to location of furnace(a) Externally Fired boilers: Furnace is placed outside the boiler shell. Water tube boilers are always externally fired.

(b) Internally fired boiler: Furnace is placed inside the boiler shell. Most of fire tube boilers are internally fired boiler.

(4) According to method of water circulation :

(a) Natural circulation boilers: In this boiler, water flow take place naturally, by temperature difference of water. The low capacity boilers use natural circulation.

Example : Lancashire, Babcock and Wilcox boiler.

(b)Forced circulation boilers : In this boiler, water flow (circulation) is take place by a pump. High pressure boilers are forced circulation boilers.

Example : Benson boiler, La-mount boiler, Velox boiler.

(5) According to working pressure :(a) High pressure boiler: The working pressure of this boiler is higher than 25 bar. Example : Babcock and Wilcox boiler(b) Medium pressure boiler : The range of working pressure of this boiler is between 10 bar to 25 bar.Example : Lancashire boiler, Locomotive boiler(c) Low pressure boiler : The range of working pressure is between 3.5 to 10 bar.Example : Cochran and Cornish boiler.

(6) According to mobility of boiler :(a)Stationary boiler : This boiler cannot be transported easily from one place to another place. The stationary boilers are used for power generation or process heating in industries. Example : Lancashire, Babcock and Wilcox boiler.

(b)Mobile boiler : It is portable boiler and can be easily transported (moved) from one place to another place. This boiler is used in marine and locomotive. Example: Locomotive boiler.

(7) According to numbers of tubes in the boiler :(a)Single tube boiler: This boiler having only one fire or water tube for circulation of hot gases or water.

Example: Cornish boiler.

(b) Multi-tube boiler : This boiler having two or more fire or water tubes for the circulation of hot gases or water. Example : Locomotive, Cochran, Lancashire, Babcock & Wilcox

Advantages and disadvantages :(1) The heating surface of the boiler is greater.(2) Required a smaller floor area compared to fire tube boiler.(3) The fuel is completely burnt due to the large heating area.(4) A good and fast circulation of water can be possible, and the temperature is

maintained uniform throughout the different parts of the boiler.(5) Steam can be raised more quickly therefore it can be used for variable load.(6) The different parts of the boiler can be separated, therefore easier to transport.

Disadvantages of water tube boiler :(1) Scale deposition will cause overheating and bursting of tubes.(2) Proper control system is required to maintain constant level of water as there is small reserve of water.(3) The maintenance cost is higher than that of the fire tube boiler.

Water Tube Boiler Over The Fire Tube Water Tube Boiler Over The Fire Tube Boiler Boiler

Comparison :Water Tube Boiler and Fire Tube Comparison :Water Tube Boiler and Fire Tube Boiler Boiler

(1) Cylindrical shell: It is shell in which different parts of the boiler are enclosed and on which the different mountings are fitted.

(2) Grate : It is platform in the furnace on which fuel is burnt.

(3) Furnace (fire box): It is a chamber formed by the space above the boiler shell in which combustion take place. It is also called a fire box. This chamber contains hot gases produced by combustion of fuel.

(4) Fire hole : It is the hole through which coal is added to the furnace.

(5) Ash pit (ash pan): It is the area in which the ash of burnt coal is collected.

(6) Smoke chamber (smoke box): The waste gases are collected here and then releases to the chimney and then to atmosphere.

Basic parts of boiler (boiler details)Basic parts of boiler (boiler details)

(7) Man hole : It is a hole provided on the boiler shell so that a workman can go inside the boiler for inspection, cleaning and maintenance.

(8) Hand holes : It is a hole provided on the shell to give easy access for the purpose of cleaning the water tubes or some other internal parts of boiler. They are also fitted with covers.

(8) Mud box: It collects all impurities present in the water. It is at the bottom of barrel or shell. This impurities are removed time to time by help of blow off cock.

(9) Steam collecting pipe (Antipriming pipe): When the steam leaving the boiler, it contains certain amount of water. Antipriming pipe is used to separate water particles from the steam and to collect dry steam from boiler.

Characteristics of boiler: A vertical, multifire-tube, internally fired, natural circulated boiler.

Construction : The boiler consists of a cylindrical shell, hemispherical fire box, fire tubes and chimney. The hemispherical crown of boiler gives good strength to withstand against the pressure.

Cochran boiler Cochran boiler (Vertical multi-tabular (Vertical multi-tabular boiler) boiler)

The hemispherical shape of furnace can withstand high heat and is also useful to increase radiant heat transfer from the furnace to hemispherical furnace wall. The grate is placed at the bottom of furnace and ash pit is located below the grate.

Working : The water is supplied to the boiler through feed check valve and the Coal is introduced to the grate through the fire door.

The hot gases produced from furnace enters combustion chamber and passes through horizontal fire tubes.

Heat transfer take place from flue gases to water surrounded the tubes by convection.

The flue gases coming from of fire tubes enter into smoke box. Finally they discharge to atmosphere through a chimney.

The ash formed is collected in ash pit.

Advantages :(1) It is compact and portable boiler therefore minimum floor area is required(2) Initial cost of boiler is less.(3) It can be moved and set up readily in different locations.(4) Quick and easy installation.(5)Any type of fuel can be used (coal or oil)

Disadvantages :(1) Steam raising capacity is less due to vertical design.(2) Water along with steam may enter the steam pipe under heavy loads due to small steam space.(3)Efficiency is poor in smaller sizes.

Babcock Wilcox Boiler

Babcock and Wilcox water tube boiler Babcock and Wilcox water tube boiler

Construction:It consists of inclined water tubes, a steam and water drum, a mud box and super heater as shown in figure. The drum is connected to up take and down take header by tubes.

The water tube inclined to the horizontal about 15° or above to bring natural circulation of water.

The hand hole is provided in header for cleaning and inspection of tubes.

The baffles plates are provided in order to make the circulation of hot gases in sine wave form.

Soot doors (cleanout door) is provided to clean the outside of the water tubes and to remove the soot. Soot doors also help to access to the interior of the boiler.

Working : The water is fed into the drum through the fed check valve. Initially one half of drum is filled up with water. Due to gravity water passes through the short riser tubes, header and fills up the inclined water tubes.

The coal is introduced to furnace grate by help of stroker and the damper control the flow of air into the furnace.

Due to heating the density of water decreases. Low density water moves upward in water tubes. The water tubes just above furnace is heated comparatively at a higher temperature than the rest of it. Therefore low density water gradually converted to steam in their path and rises into the drum through the uptake header.

Thus a continuous circulation of water from drum to water tubes and water tubes to drum is maintained due to density difference of water and gravity, without any pump.

The steam then enters through the antipriming pipe and flows in the super heater tubes where it is further heated and is finally taken.

Advantages :

(1)The steam generation capacity of the boiler is very high, about 2000 to 40,000 kg/ hr.(2) Replacement of defective tubes is easy.(3) The draught losses as compared to other boilers is minimum.(4) It is used in power station for generating large quantity of steam.(5)Boiler is required less space area compared to fire tube boilers, and offers greater operational safety.

Lancashire Boiler

Lancashire boiler

Construction :

It consists of a cylindrical shell and two fire tubes. The cylindrical shell is placed over the brick structure.

The boiler have three passes for flow of flue gases.- One flue passes from inside of boiler, - second from below and - third from the side of boiler shell. These three passes are formed by brick work, one bottom flue and two side flue.

The grates are provided at the front end and inside of two main fire tubes.

Super heater is provided at the end of the main flue tubes in passage of flue gases. While an economizer is at the end of the side flues, before exhausting the gases to chimney.

Working :

The coal is introduced to the grate through fire holes. The combustion of coal takes place in presence of air which is regulated by damper. The combustion will produce hot gases.

Path of flue gases :

Grate Flue tube Bottom flue

Chimney Atmosphere

The hot gases from the grate pass to back end of the tubes and then in the downward direction (FT to BF). They move by the bottom flue to the front of the boiler where they are divided into two streams and pass into the side flues (BF to SF). They move along two side flues and enter the chimney and discharged to atmosphere.

Advantages:(1) Due to three passes of flue gases, the heating surface area per unit volume of boiler is large.(2) The fluctuations in load can be easily met by this boiler due to large reservoir.(3) Easy operation, low maintenance costs, easy to clean and inspect.(4) By use of economiser and super heater, maximum heat of flue gases is utilized, so efficiency of boiler can be increased.

Disadvantages :(1) Maximum working pressure is limited to 16 bars.(2) Due to brick work, more floor area is required.(3) Response of pressure build up is less.(4)The furnace is inside the tubes therefore the grate area is restricted.

Locomotive Boiler

Construction : The Locomotive boiler consists of a cylindrical barrel with a rectangular fire box at one end of barrel and a smoke box at the other end of barrel as shown in figure. Firebox entirely surrounded by water except for the fire hole and the ash pit which is situated below the fire box. The dampers control the flow of air to the grate. The fire box is connected with smoke box by series of fire tubes. The fire tubes are placed inside the barrel. Some of these tube are of larger diameter, in which super heater tubes are placed.

Working :

The coal is introduced through the fire hole into the fire box. The hot gases which are generated due to burning of the coal are deflected by an arch of fire bricks, so that walls of the fire box may be heated properly.

The hot gases pass from the fire box to the smoke box through a series of fire tubes and then they are discharged into the atmosphere through the chimney.

The heat of the hot gases is transferred into the water through the heating surface of the fire tubes. The steam generated is collected in a dome shaped chamber above the water surface.

Advantages :(1) High steam capacity(2) Low cost of construction(3) Compact and portable(4) Good response against fluctuating demandsDisadvantages :(1) There are chances of corrosion and scale formation in the water space due to the accumulation of sediments and the mud particles.(2) It is difficult to clean some water spaces.(3) It may be damaged by overheating during overloads.

High pressure boilerHigh pressure boiler

In applications where steam is needed @ 30 bar and @ generating capacity less than 30,000 kg/hr, the fire tube boilers are considerably cheaper than the water tube boilers.

But the steam requirement is above 30 bar pressure and higher rate, the fire tube boilers will be very bulky and difficult to transport to the site. The water tube boiler are not facing this problem.

Therefore, water tube boiler are generally preferred for high pressure and high steam generating capacity whereas fire tube boiler for low pressure and low capacity.

In the power plant, it is necessary to generate steam at a higher rate, high pressure and with higher efficiency. This requirement is fulfill by high pressure boilers.

Advantages of High pressure boilerAdvantages of High pressure boiler

(1)Due to forced circulation of water, evaporative capacity of boiler is increased and size of drum is reduced. (2)The tendency of scale formation is eliminated due to high velocity of water through the tubes.(3)Smaller diameter tubes are used, which increases heat transfer rate and reduces fuel consumption. (4)The cost of foundation, the time of erection and cost are reduced due to less weight of the tubes used.(5)Pressurized combustion is used which increases rate of firing of fuel thus increasing the rate of heat release.(6)These boilers are compact and hence less floor space is required.(7)Due to uniform heating of all parts, there is less chances of overheating.(8)The efficiency of plant is increased up to 40 to 42% by using high pressure and high temperature steam.(9)The steam can be raised quickly to meet the variable load.(10) A very rapid start from cold is possible.

Boiler mountings: These are different fittings and devices which are necessary for the operation and safety of a boiler. Normally these device are mounted over boiler shell. According to IBR the following mountings should be fitted to the boilers.(1) Two safety valves (2) Two water level indicators(3) A pressure gauge (4) A steam stop valve(5) A feed check valve (6) A blow off cock(7) Mud holes or sight holes (8) A man hole(9) An attachment for inspector's test gauge

Boiler Accessories: These are auxiliary plants or parts required for steam boilers for their proper operation and increases efficiency of the boiler.

Commonly used boiler accessories are as(1) Feed pumps(2) Injector(3) Economiser(4) Air preheater(5) Superheater(6) Steam seperator(7) Steam strap

Boiler mountingsBoiler mountings

Pressure Gauge:

Function: It is an instrument measure the pressure of steam in boiler.

Construction : The gauge is usually mounted on the front top of the shell or the drum. It is designed to read pressure in kgf/cm2 or bar, above atmosphere.

The circular bent bourdon tube of oval-cross section is closed at one end and the other end is connected with steam space of boiler through siphon (U-tube).

A bourden gauge with its interior mechanism as shown in Fig.

Working : When pressure is applied to inside of oval tube, It cross section tends to becomes circular, and free end of tube try to becomes straight, so turning the spindle and needle by the links and gearing.

Function: It is indicates the water level inside the boiler to an observer.

Water Level Indicator

Working: The water of the boiler comes into the glass tube through the lower tube and the steam through the upper tube. The water then stands in the glass tube at the same level as in the boiler.Two cocks are used to control the passage of between the boiler and the glass tube while the third cock is in used to discharge some of the water from inside the boiler to see whether the gauge is in proper order or not.

Function : To regulate the flow of steam from boiler to the steam pipe or from one steam pipe to the other.

Steam Stop Valve

Construction :

The flange of valve body bolted to the boiler at the highest part of steam space.

It consist of valve seat and nut. The main body of valve is made of cast iron and valve seat made from gun metal.

When steam stop valve is placed directly over the boiler and connected to the steam pipe line is called the Junction valve. If it is placed near to prime mover than normally called steam stop valve.

Working :

The spindle is rotated by help of a hand wheel. Due to rotation of spindle the valve move up and down.

When the valve sits over the valve seat, the passage of steam is completely closed.

The steam passage may be partially or fully opened by moving the valve up, help of rotating the hand wheel.

Function: It is used to control the supply of water to the boiler & to prevent the escaping of water from the boiler when the pump pressure is less or pump is stopped.

Feed Check Valve

• Construction: The feed check valve is fitted in the water space of the boiler just below the normal level of the water. It consist of non-return valve, water inlet pipe, outlet pipe, spindle, gland and hand wheel. outlet pipe of valve connected with boiler and inlet pipe connected with end of delivery pipe of feed pump.

• Working : Inlet and outlet pipe of valve is exposes different pressure. At inlet of valve the feed pump pressure acts and outlet pipe of valve the boiler pressure act.

When feed pump is in operation, the pressure on the feed pump side (inlet) is more than pressure on the boiler side (outlet). This pressure difference lifts the non return valve, and allows water flow into boiler.

Blow-off cock :

Function : The blow-off cock or valve performs the two functions.(1) To discharge periodically a portion of water which contains mud, scale or sediments at bottom of boiler vessel while boiler is in operation. (2) To empty the boiler when necessary for cleaning, inspection and repair.

Working: In order to operate the valve, the rectangular slot is brought in line with the passage of the body. This is possible by rotating the plug with the help of wheel.

When the slot is placed in this position, the cock is opened and all the impurities, mud, sediments etc. Start flowing out of the boiler and they are removed.

When the slot is brought at right angles to the passage of the body, the cock is closed.

Function: Its function is to protect the boiler against damage due to overheating for low water level. It is fitted in the crown of the furnace or firebox at appropriate place.

Fusible plug

Working:

 In the normal working conditions of the boiler, the fusible plug is fully submerged under water.

When the water level falls below the fusible plug, the plug gets uncovered from water. The upper portion of the plug gets exposed to the steam space.

The steam cannot keep the plug cool. This will over heat the fusible metal. The plug falls down along with the fusible metal making a hole.

The steam and water, being under pressure immediately reach the firebox and extinguish the fire.

Safety Valves :Function : To release the excess steam when the pressure of steam inside the boiler increases higher than the safe pressure (maximum pressure). Safety valve are used to maintain safe pressure inside the boiler.

As soon as pressure of steam inside the boiler increases higher than the safe pressure, safety valve automatically opens and excess steam rushes out into the atmosphere still pressure drops down to normal value.

Safety valve is generally mounted on the top of the shell. All boiler should have at least two safety valves.

The safety valve may be classified as

(a) Dead weight safety valve(b) Lever safety valve(c) Spring loaded safety valve(d) High steam and low water safety valve.

Dead weight safety valve :

Construction : A valve is placed upon a valve seat which is fixed upon a long vertical pipe having a flange at the bottom for fixing at the top of the boiler.

The weight carrier suspended from the top of the valve which carries cast iron rings (weight).

The total weight must be sufficient to keep the valve on it seat against the normal working pressure.

• Working:

When the steam pressure exceeds the normal limits, this high pressure steam creates upward force on valve, thus valve lift with its weights and the excess steam escapes through the pipe to the outside.

• Advantages :

(1)The construction of valve is simple.

(2)It gives satisfactory operation for low pressure and stationary boilers.

• Disadvantages :

(1)It is not suitable for moving boilers as the force of the weights should always work vertically downward.

(2)It is not suitable for high pressure boiler as the weight becomes too large.

High steam and low water safety valve

Function : This valve serves the following purposes.

(1)The steam automatically escapes out when the level of water falls below a normal level.

(2)It automatically discharges the excess steam when the pressure of steam rises above a normal pressure.

This valve is generally used at the Cornish or Lancashire boiler.

Working:When steam pressure rises above the normal pressure of the boiler, the valve V lifted along with valve U (hemispherical valve) and excess steam escapes out.

When the water level is fall below the predetermined value, float moves downwards and valve U is move up.This will provide special passage to the steam for the whistling.

Construction : This is a device which combination of two valve and serve two purposes. It consists of valve V resting on the valve seat and the valve U (hemispherical valve) loaded with the weights, resting on the valve V as shown in fig.

Inside the boiler, a lever L1 is hinged at the fulcrum. One end of lever L2 attached to a float E and other end carries weight W.

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