3. Boilers
Boiler 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.
4. 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 tube
3. 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. 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.
6. CLASSIFICATION OF BOILERS
1. Fire tube
– Simple vertical boiler
– Cochran boiler
– Cornish boiler
– Lancashire boiler
– Locomotive boiler
– Scotch boiler
2. Water tube
8. 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.
9. 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.
10. 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.
11. Demerits
1. 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.
12. advantages of fire tube boilers
are as follows.
I. Low cost
II. Fluctuations of steam demand can be met easily
III. It is compact in size.
13. Comparison between ‘Fire-tube and Water tube’ Boilers
S.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
14. We are going to study…………
• Cochran Boiler
• Lancashire Boiler
• Locomotive boiler
• Babcock Wilcox Boiler
15. Boiler Mountings
1. Water level gauge
2. Safety valve
3. Steam stop valve
4. Blow off cock
5. Manhole and
6. Pressure gauge
• Working pressure 6.5 bar (max.
Pressure 15 bar
• Efficiency 70 to 75%
16. 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
17. 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 ……
18. 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.
20. • 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
21. 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%
22. 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%).
23. 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.
25. Advantages
1. 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 bar
5. The pressure of the steam is limited to about
20 bar.
6. Steam capacity 9000kg/h
7. Efficiency 70%
26. 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)
27. Features
1. 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.
29. • 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
30. Blow-Off Cock
Function:
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.
31. • 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
32. • 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
33. Fusible plug
• Function :
It is use to protect
the boiler against
damage due to
overheating caused
by low water level in
the boiler.
34. • 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
35. 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
36. 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.
37. 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
38. 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
40. 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
41. 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.
42. Steam Stop Valve
Function:-
It regulate the flow of
steam from boiler to
the steam pipe or
from one steam pipe
to the other.
43. 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.
44. 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
46. • 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
47. • 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
48. 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
49. 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
50. 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.
51. 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
52. 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.
53. 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
54. 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
55. 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.
56. • 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
57. 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.
58. • 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
59. 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
61. :Boilers Accessories:
These are auxiliary plants or parts required
for steam boilers for their proper operation
and to increased efficiency of the boilers
63. 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
65. 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.
66. 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
67. 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:
68. Function: Economiser increases the temperature of
feed water using waste of heat to flue gases leaving
the boiler through chimney.
Economiser
69. 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:
70. • The function of super heater is to increase the
temperature of the steam above its saturation
point.
Super heater
71. • 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
72. 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.
73. 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.
74. • 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
75. 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.
76. 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.
77. • 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
78. • 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
