Power Plant Engineering

Boilers

BoilersBoiler is an apparatus to produce

steam. Thermal energy released by combustion of fuel is used to make steam at the desired temperature and pressure.

The steam produced is used for:(i) Producing mechanical work by expanding it in steam engine or steam turbine.(ii) Heating the residential and industrial buildings(iii) Performing certain processes in the sugar mills, chemical and textile industries.

Classification of Boilers

1. Horizontal, vertical or inclined– The parts of a horizontal boiler can be inspected

and repaired easily but it occupies more space.– The vertical boiler occupies less floor area.

2. Fire tube and water tube3. Externally fired and internally fired – Eg. Babcock and Wilcox boiler, Stirling boiler etc– Eg. Cochran, Lancashire boiler etc.

4. Forced circulation and natural circulation– Ex. Velox, Lamont, Benson Boiler etc– Ex. Lancashire, Babcock and wilcox boiler

etc.

5. High pressure and low pressure boilers– Pressures above 80 bar and above are called

high-pressure boilers. Ex. Babcock and Wilcox, Velox, Lamont, Benson boiler

– Pressures below 80 bar are called low pressure boilers. Ex. Cochran, Cornish, Lancashire and Locomotive boilers

6. Stationary and portable

7. Single tube and multi-tube boilers

– The fire tube boilers are classified as single-tube and

multi-tube boilers, depending upon whether the fire

tube is one or more than one. Ex. Cornish, simple

vertical boiler

– and rest of the boilers are multi-tube boilers.

CLASSIFICATION OF BOILERS

1. Fire tube– Simple vertical boiler– Cochran boiler– Cornish boiler– Lancashire boiler– Locomotive boiler– Scotch boiler

2. Water tube

Water tube boilers

Horizontal straight tube

Longitudinal drum

Cross drum

Two drum

Three drum

Four drum

Bent tube Low head three drum

MERITS OF WATER TUBE BOILERS OVER FIRE TUBE BOILERS

1. Generation of steam is much quicker due to small ratio of water content to steam content. This also helps in reaching the steaming temperature in short time.

2. Its evaporative capacity is considerably larger and the steam pressure range is also high-200 bar.

3. Heating surfaces are more effective as the hot gases travel at right angles to the direction of water flow.

5. The combustion efficiency is higher because complete combustion of fuel is possible as the combustion space is much larger.

6. The thermal stresses in the boiler parts are less as different parts of the boiler remain at uniform temperature due to quick circulation of water.

7. The boiler can be easily transported and erected as its different parts can be separated.

8. Damage due to the bursting of water tube is less serious. Therefore, water tube boilers are sometimes called safety boilers.

9. All parts of the water tube boilers are easily accessible for cleaning, inspecting and repairing.

10.The water tube boiler's furnace area can be easily altered to meet the fuel requirements.

Demerits1. It is less suitable for impure and sedimentary

water, as a small deposit of scale may cause the overheating and bursting of tube. Therefore, use of pure feed water is essential.

2. They require careful attention. The maintenance costs are higher.

3. Failure in feed water supply even for short period is liable to make the boiler over-heated.

advantages of fire tube boilers are as follows.

I. Low costII. Fluctuations of steam demand can be met easilyIII. It is compact in size.

Comparison between ‘Fire-tube and Water tube’ BoilersS.No. Aspects Fire-tube boilers Water-tube boilers

1 Position of water and hot gases

Hot gases inside the tubes and water outside the tubes.

Water inside the tubes and hot gases outside the tubes.

2 Mode of firing Generally internally fired Externally fired

3 Operating pressure Operating pressure limited to 16 bar Can work under as high pressure as 100 bar.

4 Rate of steam production Lower Higher

5 Suitability Not suitable for large power plants Suitable for large power plants

6 Risk on bursting Involves lesser risk on explosion due to lower pressure

Involves more risk on bursting due to high pressure

7 Floor area For a given power it occupies more floor area For a given power it occupies less floor-area.

8 Construction Difficult Simple

9 Transportation Difficult Simple

10 Shell diameter Large for same power Small for same power

11 Chances of explosion Less More

12 Treatment of water Not so necessary More necessary

13 Accessibility of various parts

Various parts not so easily accessible for cleaning, repair and inspection

Various parts are more accessible

14 Requirement of skill Requires less skill for efficient and economic working

Requires more skill and careful attention

We are going to study…………

•Cochran Boiler•Lancashire Boiler•Locomotive boiler•Babcock Wilcox Boiler

Boiler Mountings1. Water level gauge2. Safety valve3. Steam stop valve4. Blow off cock5. Manhole and6. Pressure gauge

• Working pressure 6.5 bar (max. Pressure 15 bar

• Efficiency 70 to 75%

The outstanding features of this boiler are :– It is very compact and requires minimum

floor area– Any type of fuel can be used with this

boiler–Well suited for small capacity

requirements– It gives about 70% thermal efficiency with

coal firing and about 75% with oil firing

1. Pressure Gauge. This indicates the pressure of the steam in the boiler.

2. Water Level Indicator. This indicates the water level in the boiler .

3. Safety Valve. The function of the safety valve is to prevent the increase of steam pressure in the Boiler above its design pressure.

4. Fusible Plug. If the water level in the boiler falls below a predetermined level, the boiler shell and tubes will be overheated.

Main parts ……

5. Blow-off Cock. The water supplied to the boiler always contains impurities like mud, sand and, salt due to heating, these are deposited at the bottom of the boiler, they have to be removed using blow off cock.

6. Steam Stop Valve. It regulates the flow of steam supply outside. The steam from the boiler first enters into an anti-priming pipe where most of the water particles associated with steam are removed

7. Feed Check Valve. The high pressure feed water is supplied to the boiler through this valve. This valve opens towards the boiler only and feeds the water to the boiler.

Fire – tube Boiler Lancashire Boiler

• It is stationary fire tube, internally fired, horizontal, natural circulation boiler.

• This is a widely used boiler because of its good steaming quality and its ability to burn coal of inferior quality.

• These boilers have a cylindrical shell 2 m in diameters and its length varies from 8 m to 10 m.

• It has two large internal flue tubes having diameter between 80 cm to 100 cm in which the grate is situated.

• This boiler is set in brickwork forming external flue so that the external part of the shell forms part of the heating surface.

Lancashire Boiler

Fire – tube Boilers

Lancashire Boiler

• It is commonly used in sugar-mills and textile industries where along with the power steam and steam for the process work.

• Used where large reserve of water and steam are needed.• Maximum working pressure is 16 bar.• Efficiency 50 to 70%

Advantages• The feed pipe projecting into the boiler is perforated

to ensure uniform water distribution.• Its heating surface area per unit volume at the boiler

is considerably large.• Its maintenance is easy.• It is suitable where a large reserve of hot water is

needed. • This boiler due to the large reserve capacity can

easily meet load fluctuations.• Super-heater and economizer can be easily

incorporated into the system, therefore; overall efficiency of the boiler can be considerably increased (80-85%).

LOCOMOTIVE BOILER

• Locomotive boiler is a horizontal fire tube type mobile boiler.

• The main requirement of this boiler is that it should produce steam at a very high rate. Therefore, this boiler requires a large amount of heating surface and large grate area to burn coal at a rapid rate.

• In order to provide the large heating surface area, a large number of fire tubes are setup and heat transfer rate is increased by creating strong draught by means of steam jet.

LOCOMOTIVE BOILER

Advantages1. Large rate of steam generation per square

metre of heating surface. To some extent this is due to the vibration caused by the motion.

2. It is free from brickwork, special foundation and chimney. This reduces the cost of installation.

3. It is very compact.4. Working pressure is 14 bar5. The pressure of the steam is limited to about

20 bar.6. Steam capacity 9000kg/h7. Efficiency 70%

BABCOCK WILCOX BOILER

The water tube boilers are used exclusively, when pressure above 10 bar and capacity in excess of 7000 kg of steam per hour is requires.

• Efficiency 60 to 80%• Working pressure 40

bar (max)• Steaming capacity

40000kg/h (max)

Features1. The evaporative capacity of this boilers is high

compared with other boilers (20,000 to 40,000kg/hr). The operating pressure lies between 11.5 to 17.5 bar.

2. The draught loss is minimum compared with other boilers.

3. The defective tubes can be replaced easily.4. The entire boiler rests over an iron structure,

independent of brick work, so that the boiler may expand or contract freely. The brick walls which form the surroundings of the boiler are only to enclose the furnace and the hot gases.

BOILER

MOUNTINGS & ACCESSORIES

• Mainly seven (7) mountings are required and essential to a Boiler :1. Water level indicator. (Water gauge)2. Main steam stop valve.3. Pressure gauge.4. Feed check valve.5. Fusible plug.6. Blow down valve. (Blow off cock)7. Safety valve.

BOILER MOUNTINGS

Blow-Off CockFunction:The function of blow-off cock is to discharge mud and other sediments deposited in the bottom most part of the water space in the boiler, while boiler is in operation. It can also be used to drain-off boiler water. Hence it is mounted at the lowest part of the boiler. When it is open, water under the pressure rushes out, thus carrying sediments and mud.

• It is fitted on the boiler shell directly or through the short branch of pipe at lowest part of the water space.

• It consists of gun metal conical plug having a rectangular hole, spindle with yoke as shown in fig.

• The plug meshes accurately into the similar casing. The plug spindle is generally rotated by mean of spanner on the top of the yoke, two vertical slots are provided for fixing spanner.

CONSTRUCTION

• When rectangular hole of plug is brought in line with casing hole by rotating spindle, the water flow out. When the solid portion of the plug is in front of casing hole (this position shown in the fig.) the water cannot flow out of boiler.

Working

Fusible plug

• Function : It is use to protect

the boiler against damage due to overheating caused by low water level in the boiler.

• It is fitted on the fire box crown plate or over the combustion chamber.

• The fusible plug consist of two hollow guns and one conical plug a shown in figure. A hollow gun metal body is screwed to the fire box crown plate of boiler.

• Another hollow gun metal is screwed to the first body. Third plug is made from copper is locked with second plug by pouring metal in to the grooves provided on the both plugs.

CONSTRUCTION

In normal working condition, the upper surface of fusible plug is covered with water which keeps the temperature of the plug below its melting point while other end of plug is exposed to fire or hot gases. The low melting point (tin or lead) does not melt till the upper surface of plug is submerged in water. But in case of water level in boiler falls below the danger levels, the fusible plug uncovered by the water and get exposed to steam. This overheats the plug and the fusible metal having low melting point which melts quickly. Thus the third plug drops down and second hollow gun became open, the steam rushes into the furnace and puts out the fire (stop).

Working

Feed check valve

• Function: It controls the supply of water from the boiler when the feed pump pressure is less than boiler pressure or pump is stopped.

The feed check valve is fitted in 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 wheel as shown in fig. the outlet pipe of valve connected with and of delivery pipe of feed pump

Construction

Inlet and outlet pipe of valve expose different pressure at inlet of valve the feed pump pressure acts and outlet pipe of the 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.

WORKING

WATER LEVEL INDICATOR

Function:- It indicate the

water level inside the boiler vessel. It shows the level in the boiler drum.

Normally two water level indicator are fitted to the boiler. these are fitted at the front end of every boiler. Water level indicator consist three cock as steam cock, water cock, drain cock and glass tube. The steam cock connect or disconnects the glass tube with steam space. while water cock connect or disconnect the glass tube with water in the boiler. the drain cock is used to drain out the water in from glass tube at interval to ensure that the steam and water cock are clear in operation. The glass tube protected by means of a cover, and made of specially

CONSTRUCTION

WORKING

When steam cock and water cock opened, steam rushes from upper passage and water rushed from lower from passage to the glass tube. This will indicate the level of water in the boiler. Two ball are places at the junction of metal tube. Under normal operating condition the ball are kept. full line circle in case the glass tube is broken, steam will rushes from upper passage and water from lower passage due to pressure difference between boiler pressure at atmospheric pressure. the ball are carrier along the passage to and of glass tube and then closed passages. This position of ball sown in fig by dotted circle. Thus flow of water and steam out of boiler is prevented.

Steam Stop Valve

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

CONSTRUCTION The common type of steam stop valve. 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.

The lower ends of spindle connected to valve

upper and pass through gland and yoke, and connected with hand wheel. The gland is used to prevent the leakage of steam.

The spindle it rotated by help of 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 the clearance (passage) between valve and valve seat regulates the flow of steam out of boilers. in locomotive boilers, the supply of the steam is regulated by means of a regulator which is placed inside boiler cell and operated by a handle from driver’s cabin.

WORKING

Pressure Gauge

Function : It indicates the pressure of steam in boiler.

• The gauge is usually mounted on the front top of the shell or on the drum. It is having dial which graduated to read pressure in bar, above atmosphere A bourdon tube gauge with its interior mechanism is shown in fig. The circular bent bourdon tube of oval-cross section is close at one end and connected at the other end to steam space of boiler through siphon (U-tube).

• The siphon is filled with water which prevents hot steam from entering the pressure gauge and keeps the gauge cool. Closed end of bourdon tube is attached to a toothed quadrant with help of link and pin. This quadrant meshes with a small pinion on the central spindle.

CONSTRUCTION

• When pressure is applied to inside of oval bourdon tube. It cross section tends to becomes circular, and free end of bourdon tube try to becomes straight, so turning the spindle by the links and gearing. This causes the needle to move and indicate pressure on graduated dial.

Working

Function : Safety valves are located on the top of the boiler. They guard the boiler against the excessive high pressure of steam inside the drum. If the pressure of steam in the boiler drum exceeds the working pressure then the safety valve allows blow-off the excess quantity of steam to atmosphere. Thus the pressure of steam in the drum falls. The escape of steam makes a audio noise to warm the boiler attendant.

Safety valves

There are four types of safety valves

1. Dead weight safety valve. 2. Spring loaded safety valve 3. Lever loaded safety valve 4. High steam and low water safety valve

Safety valves

Dead weight safety valve

CONSTRUCTION

It consists of valve V is placed upon a valve seat which is fix upon a long vertical pipe having a flange at the bottom for fixing at the top of the boiler. The weight carrier D suspended from the top of the valve which carries cast iron rings (weight) as shown figure. The total weight must be sufficient to keep the valve on it seats against the normal working pressure.

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

The construction of this valve is simple. It gives satisfactory operation for low pressure and stationary boilers. It is not suitable for moving boilers as the force of the weights should always work vertically downward. It is also not suitable for high pressure boiler as the weight becomes to large.

Working

Lever safety valve

• The main disadvantages of dead weight safety valve as heavy weights required for high pressure boiler is eliminated in a lever safety valve by use of a lever.

The valve rests over the gun metal seat which is fixed to a mounting block. The mounting block is fixed upon the boiler. One end of the lever is hinged while the other end carries a weight W as shown in figure. The thrust of the lever with its weight is transmitted to the valve by a short strut. The position and amount of weight W decides the safe pressure limit.

CONSTRUCTION

When the pressure exceeds the normal limit, the upward force on valve is become higher than the downward thrust on valve due to weight W on lever. Thus the valve lifted from it seat with it weight and excess steam will come out of the boiler.

This safety valve used only for stationary boiler. It used at a pressure higher than that of dead weight safety valve.

Working

Spring loaded safety valve

• There are various types of spring loaded safety valve use on different boilers.

Construction: It consists of a cast iron body having two

branch pipes. Two separate valves V are placed over the valve seat as shown in figure. A lever is placed over the valve by means of two pivots. The lever is held tight at its proper position by means of a spring. One end of spring is connected with the lever while other end with the body of the valve. The valve is kept on it seats with help of spring force.

• In the normal condition, the downward force due to spring is higher than upward force applied by steam. The valve is closed due to spring force. When steam pressure exceeds the normal limit, upward force due to steam pressure is became higher than downward force due to spring. Thus the valves are lifted from their seats opening the passages for steam to release out of boiler.

Working

High steam & low water safety valve

Functions: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 in Cornish or Lancashire boiler.

• This valve is combination of two valves and serve two purpose. It consists of valve V resting on the valve seat and the valve U loaded with the weights W rests on the valve Vas shown in figure.

• Inside the boiler ,a lever L2 hinged at the fulcrum F2 . One end of lever L2 attached to a float E and other end carries balance weight w.

CONSTRUCTION

When steam pressure rises above the normal pressure of the boiler, the valve V lifted along with valve U and excess steam is released out of boiler. The weight W1 and w decides the limit of normal steam pressure. If W1 is more, the valve V to lifted at higher steam pressure.

When float E submerged in water in normal condition, the lever L2 is balanced about fulcrum F2 and valve U is remained close. When water level in boiler falls below a normal level, the float with its weight moves downward and left side part of level L2 moves upward. So that knife edge pushes the spindle which opens the valve U with weight W and steam escapes through specially constructed passage causing a loud noise. This alerts operator to start the feed water pump to increase the level of water in the boiler.

Working

Boiler:Accessories:

:Boilers Accessories:

These are auxiliary plants or parts required for steam boilers for their proper operation and to increased efficiency of the boilers

(1) Feed pump(2) Injector(3) Economiser(4) Air preheater(5) Superheater(6) Steam separator(7) Steam trap

:Boilers Accessories: :are as under:

Function:

• The feed pump is a pump which is used to deliver feed water to the boiler.

• Double feed pump is commonly employed for medium size boilers.

Feed Pump

Types of Feed pumps

Feed Pumps

Reciprocating pumps

Simplex

Duplex

Triplex

Rotary Pumps Centrifugal pumps

Reciprocating Duplex Pump

Duplex pump is very common steam driven reciprocating pump. It consist of two steam cylinder placed side by side. There are two steam ports for each of the cylinders.

Function : Injector is used to feed water in the boiler .

It is commonly employees for vertical boiler and does not find its application in large capacity high pressure boilers .

Injector

It consists of spindle, a steam cone ,a combine cone, a delivery cone and a handle. The lower end of the spindle works as the valve and the upper end is fitted with the handle. The rotation of the handle cause an upward or downward movement of the steam cone.

Construction:

Function: Economiser increases the temperature of feed water using waste of heat to flue gases leaving the boiler through chimney.

Economiser

Economiser consist of a large vertical cast iron or steel water pipes, one end of vertical pipes is connected with vertical pipes A and other end with B. Hot flue gases from boiler pass over the vertical tubes .The heat transfer take place from flue gases to cold water rising in vertical tubes.

Construction and working:

• The function of super heater is to increase the temperature of the steam above its saturation point.

Super heater

• To superheat the steam generated by boiler.• Super heaters are heat exchangers in which heat

is transferred to the saturated steam to increase its temperature.

Superheated steam has the following advantages :i) Steam consumption of the engine or turbine is reduced.ii) Losses due to condensation in the cylinders and the steam pipes are reduced.iii)Erosion of turbine blade is eliminated.iv) Efficiency of steam plant is increased

Working

Steam Separator

A Steam separator, sometimes referred to as a moisture separator, is a device for separating water droplets from steam. The simplest type of steam separator is the steam dome on a steam locomotive. Stationary boilers and nuclear reactors may have more complex devices which impart a "spin" to the steam so that water droplets are thrown outwards by centrifugal force and collected.

Steam separator is important to remove water droplets from steam because:

In all engines, wet steam reduces the thermal efficiency

In piston engines, water can accumulate in the cylinders and cause a hydraulic lock which will damage the engine

In thermal power stations, water droplets in high velocity steam coming from nozzles (or vanes) in a steam turbine can impinge on and erode turbine internals such as turbine blades.

• It consists of inlet and outlet steam pipes, baffles and water drain cock. The steam enters at the inlet pipe and flows down it strikes the baffles as a result it gets deflected but as the water particles have the grater weight , inertia and density falls to the bottom of the separator as a result steam and water are separated.

Construction and Working

Steam Trap

A steam trap is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam. Most steam traps are nothing more than automatic valves. They open, close or modulate automatically. Others, like venture traps, are based on turbulent 2-phase flows to obstruct the steam flow.

Construction and working: steam trap is a disc or short solid pipe nipple with a

small hole drilled through it installed at the lowest point of the equipment. Since steam condensate will collect at the lowest point and live steam is about 1200 times greater in volume than this hot liquid, condensate is effectively removed and steam is blocked. However, the vast majority of steam traps in current operation are of the mechanical or thermostatically operated design.

Mechanical and thermostatic steam traps basically open when condensate and inert gases need to be removed, and close when there is only steam present.

The three important functions of steam traps are:1 Discharge condensate as soon as it is formed. 2 Have a negligible steam consumption. 3 Have the capability of discharging air and other non-condensable gases.

• An air preheater is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.

Air Preheater

• Air preheater is installed between economizer and chimney.

• It consists of large number of tubes which are arranged in the path of the flue gases. Hot flue gases enters into the tube from the top of the shell and leaves from the bottom of the chimney. The inlet air at room temperature is admitted into the shell at the lower end with the help of fan. The air passes upward around the tubes in the opposite direction of the flow of the hot gases. Baffles are provided in the Air-preheater. The soot hoper provided at the bottom is used to collect soot during cleaning operation of the tubes

Air preheater

construction and working