Operations of Steam Boiler and Auxiliaries

Steam boilers can be considered a special type of closed loop wherein water is
heated to the boiling point. Steam boilers are manufactured in a variety of
configurations, but all consist of a chamber in which water is heated to the boiling point
under pressure by the combustion of fuel. Steam is delivered to heat exchangers or air
handler coils where the heat flows to the material being heated. As heat is lost during
this process, the steam condenses back to water and is pumped back to the boiler.

The water for this process must be purified, avoid foaming, and have the correct
pH chemistry to prevent scaling. To achieve these qualities, the makeup water needs
processing through filtration and chemical balancing before proceeding to the boiler.
These steps depend on the boiler manufacturer’s recommendations and the chemistry
of the makeup water.

On ships, boiler is used as a pressure vessel in which the water is heated to evaporate
and generate the steam and the unit is so arranged that the generated steam
accumulated in it. It is a steel pressure vessel in which water under pressure is
converted into steam by the application of combustion. In other words, it is simply a heat
exchanger which uses radiant heat and hot flue gases, liberated from burning fuel, to
generate steam and hot water for heating and processing loads.

Boilers are one of the essential equipment onboard ships. Its purpose is to
provide heating to the main diesel propulsion engine, to the bunker F.O. tanks (to make
it less viscous for transfer purposes as well as easy ignition).

To operate the boiler, first, the fuel oil pump takes suction from the diesel oil
service tank or heavy oil service tank through the three-way change-over valve. Then,
the pressurized oil is supplied to the boiler burner through a solenoid valve, the pilot
burner is fired first to light up the main burner, and a flame sensor senses the flame of
the pilot burner and actuates the fuel oil solenoid valve for the main burner. Next is,
feedwater pump takes suction from the hot well and discharges to the boiler drum
through feed water regulator, and the feedwater regulator maintains the water level in
the boiler according to the steam demand.
 Boiler Accessories
Boiler accessories are devices that are installed along the boiler and the
surrounding area to increase the efficiency of the boiler. These are not an indispensable
part of the boiler, and thus without installing these devices, the boiler operation can be
completed at low efficiency. These are auxiliary parts required for steam boilers for their
proper operation and to increased efficiency of the boilers.
Boiler Accessories are as under:
(1) Feed pump
(2) Economiser
(3) Superheater
(4) Air preheater
(5) Water Level Control

Feed Pump

It is used to deliver water to the boiler. A

feed pump may be of centrifugal type or
reciprocating type. But a double acting
reciprocating pump is commonly used as a feed
pump. 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.

The feed pump raises the feedwater to a pressure high enough for it to enter the
boiler. For auxiliary boilers, where small amounts of feedwater are pumped, a steam-
driven reciprocating positive displacement pump may be used.

Economiser

Economiser increases
the temperature of feed water
using waste of heat to flue
gases leaving the boiler. A
common application of
economizers in steam power
plants is to capture the waste
heat from boiler stack gases
(flue gas) and transfer it to the
boiler feedwater. This raises
the temperature of the boiler
feedwater, lowering the needed energy input, in turn reducing the firing rates needed for
the rated boiler output.

Economizer works as a heat exchanger by preheating a fluid which as a result

minimizes energy consumption. In a steam boiler, it heats the fluid and recovered
residual heat which in turn is used to preheat the boiler feed water that is ultimately
converted to superheated steam.

Economiser is a type of boiler which is installed in the exhaust passage of the

main engine and where the exhaust heat energy is utilised to generate steam from fresh
water. The exhaust of all cylinders is sent to the turbocharger, where some heat
enthalpy is utilized to supply fresh air as discussed above.

Super Heater

The function of super

heater is to increase the
temperature of the steam
above its saturation point.
Super heaters are heat
exchangers in which heat is
transferred to the saturated
steam to increase its
temperature. The
superheater is designed to
dry the steam, raising the
temperature whilst
maintaining the pressure at a predetermined set level. The superheated steam then
enters the outlet manifold that is protected from over pressure by a superheater safety
valve.

The superheater is a device which converts saturated steam or wet steam to dry
steam, and it is used in driving the lager turbines in the marine propulsion system. In the
superheating process the temperature of the steam is only raised, keeping the pressure
at a constant level.

In the superheater zone the products of combustion were still at a high

temperature and deposits from impurities in the fuel condensed out on the tubes,
reducing heat transfer and steam temperature. Eventually gas passages between the
tubes would become so badly blocked that the forced draught fans would be unable to
supply sufficient air to the burners, combustion become impaired and the fouling
condition accelerated. Sodium and vanadium compounds present in the deposits
proved very corrosive to superheater tube causing frequent repeated failure. Due to the
fouled conditions, there was a loss of efficiency and expensive time-consuming cleaning
routines were required.

Air Preheater

Air Preheaters is the Shell and

tube type Heat Exchangers used for
preheating the air which is fed to the
boiler or furnaces/kilns for combustion
of fuels. Air Pre heater’s primary
objective is to extract the waste heat
from the flue gases leaving the boiler.

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.

The steam coil air pre-heaters are used in the majority of boilers to keep the cold
end temperature higher during cold start-ups. The recuperative and regenerative air
pre-heaters are for heating the combustion air to the boiler. Designers select these air
pre-heaters depending upon the economics, the heat to be recovered from the flue gas
leaving the economizer in the boiler, the capacity of the plant, etc.

Water Level Control

The water is supplied to the boiler water

drum through feed water pump. For an
automatic operation, the feed water pump
must know when to start and when to stop. If
the feed pump runs continuously, it may
overflow the boiler which can cause priming or
if the pump does not come up with the
demand, the water level will reduce below the
required, leading to severe thermal stresses
and overheating of the boiler. The feed water system is provided with a level control
system, which commands the start and stop of the feed water pump to maintain the
boiler water level.
Fuel System

The fuel oil system is designed to

deliver fuel oil to the boilers at
temperatures and pressures suitable for
satisfactory combustion. The system
basically consists of settling tanks, suction
and discharge strainers, pumps, heating
coils, seven valves to properly meter fuel
and other assorted hardware. There are
various piping systems, provided for
bunkering, storage, transfer, offloading
and treatment of fuel oils.

Boiler Mountings

Mainly seven (7) mountings are required and essential to a Boiler:

1. Blow down valve. (Blow off cock)
2. Fusible plug
3. Feed water check valve.
4. Water level indicator. (Water gauge)
5. Main steam stop valve.
6. Pressure Gauge
7. Safety valve

Blow down valve. (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.

Construction: 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. 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.

Working: 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
the water cannot flow out of boiler.

Fusible Plug

Function: It is use to protect the boil against damage due to

overheating caused by low water level in the boiler.

Construction: It is fitted on the fire box crown plate or over the

combustion chamber. The fusible plug consists 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.

Working: 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).

Feed Water 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.

Construction: The feed check valve is fitted in water space of

the boiler just below the normal level of the water. It consists of
non-return valve, water inlet pipe, out let pipe, spindle, gland and wheel. The outlet pipe
of valve connected with and of delivery pipe of feed pump

Working: 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.

Water Level Indicator

Function: It indicate the water level inside the boiler vessel. It

shows the level in the boiler drum.

Construction: Normally two water level indicators are fitted to

the boiler. these are fitted at the front end of every boiler. Water
level indicator consist three cocks as steam cock, water cock,
drain cock and glass tube. The steam cock connects or
disconnects the glass tube with steam space. while water cock
connects 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.

Working: When steam cock and water cock opened, steam rushes from up per
passage and water rushed from lower from passage to the glass tube. This will indicate
the level of water in the boiler. Two balls are places at the junction of metal tube. Under
normal operating condition the ball is kept. full line circle in case the glass tube is
broken, steam will rush from upper passage and water from lower passage due to
pressure difference between boiler pressure at atmospheric pressure. the ball is 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 consists 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.

Working: The spindle it rotated by help of hand wheel due to rotation of spindle the
valve moves 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.

Pressure Gauge

Function: It indicates the pressure of steam in boiler.

Construction: 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.
Working: When pressure is applied to inside of oval bourdon tube. Its 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.

Safety Valves

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
an audio noise to warm the boiler attendant.

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.

Working: 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 too large.