Marine boiler

A steam boiler is a closed container designed to heat water until it

vaporizes and transforms into steam at a pressure that exceeds
atmospheric pressure. As defined by the IBR Act 1923 (Indian Boiler
Regulation), a steam boiler refers to any closed vessel with a
capacity greater than 22.75 litres, which is used exclusively for
generating steam under pressure.

 Boiler shell: A hollow cylindrical body made up of steel plates riveted or welded
together.
 Furnace: The part of the boiler where fuel is burned to produce heat that
generates steam.
 Heating surface: The boiler surface exposed to hot gases on one side and water
on the other.
 Water space and steam space: The volume of the boiler occupied by water is the
water space, while the remaining space used for storing steam is the steam
space.
 Flue gases: Hot gases produced by fuel combustion in the furnace, containing
water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2),
and complete/incomplete products of fuel combustion.
 SCREEN TUBES
 GENERATING TUBES
 WATERWALL TUBES
 DOWN COMER
Marine boiler
 Marine boiler

VERTICAL Boiler construction

The principal drawing of boiler is shown in Figure.

The steam drum is cylindrical with two flat plates of equal thickness. Because of
the internal pressure, the flat plates are mutually connected by vertical solid
stays. The steam drum is furnished with the necessary internal fittings to ensure
an even distribution of the feed water, of the circulation water from the exhaust
boiler and to ensure a sufficient dryness of steam.

The burner cone in the furnace opening is an integrated part and accordingly no
refractory are provided in the top of the furnace room.

Manholes are conveniently placed in both drums and inside the drums enough
space is available for inspection and maintenance.

The water drum is designed similar to the steam drum. The drum size gives
optimal space for the heating coil and easy access for inspection.

As standard the boiler is prepared for tack welding along the foundation of the
boiler shell. Some boilers may, however, be provided with four support feet
instead, one as fixed foot and the others with the possibility of thermal
expansion.

Both the furnace and the generating tube bank are located asymmetrically and
are separated by the screen wall. Besides the screen wall the furnace consists of
gas tight polygon shaped membrane walls. The generating tube bank consists of
vertical pin- tubes arranged in a staggered configuration. To avoid any risk of
vibration problems supports are arranged. The flue gas passes through the
deflected tubes at the bottom of the screen wall, up through the generating tube
bank and out through the smoke outlet box.

An effective circulation in the boiler is achieved by means of down comers.

The bottom tube plate is first covered with coat of insulation refractory and
above with castable refractory.

Access to the furnace is possible through the access door at the bottom of the
furnace. Inspection of the generating tubes is also possible through the
inspection door at the bottom of the furnace.

Inspection of the burner flame is possible through the two inspection holes
arranged in two heights on the furnace panel wall.

Manhole
 Marine boiler

Burner

Smoke outlet Steam drum

box

Inspection hole

Access door
Membrane walls

Furnace

PINNED TUBES
Screen
Generating wall
tube bank

Down comer

Access door

Water drum
Foundation

What is stay bar in boiler?

Stay bars are solid, round steel bars welded into the front and rear tube plates, and run
the full length of the boiler. Shorter stay bars run between the end plate of the return
chamber and the rear tube plate to help strengthen these sections.
 Marine boiler

Plain tubes
Plain tubes have thin walls, and their sole function is the transfer of heat from the flue gas to
the water. They are expanded into holes in the tube plates.

Stay tubes
The stay tubes have thick walls and are welded to the tube plates. They conduct heat from the
hot gases to the water, but also add strength to the boiler construction by holding the tube
plates in position.

Stay bars
Stay bars are solid, round steel bars welded into the front and rear tube plates, and run the full
length of the boiler. Shorter stay bars run between the end plate of the return chamber and
the rear tube plate to help strengthen these sections. Small holes, called tell-tale holes, about
4 mm in diameter, are drilled into the centre of the stay bars. Should the stay bars corrode at
the point where they fit into the tube plate on the water side of the boiler, the pressure in the
boiler will cause water to leak from the holes, indicating that the bars should be replaced.
Boiler mountings
Mountings

The following is a brief description of the most important items

of the boiler mountings. The components mentioned in this
section are referring to the general arrangement drawing of the
boiler.

Safety valves
Two safety valves are fitted to the pressure vessel of the boiler.
They are installed for security reasons, and designed to prevent
the boiler pressure from rising above the design value.

The safety valves must be supplied with waste steam pipes and
either expansion de- vices, or bellows.

Main steam valve

The main steam valve is a shut off valve. When closed, it
isolates the boiler from the main steam line. A check valve is
mounted after the main steam valve which prevents steam
from flowing backwards into the boiler.

The steam piping up to the main steam valve must be flexible

in order to minimise external loads on the main steam valve
and associated branch. The flexibility can be achieved with
loops build into the main steam line to take up heat expansion
from the boiler and steam piping or with an expansion joint
next to or close to the main steam valve.

By-pass valve
The by-pass valve is a shut off valve. The purpose of the valve
is to equalise the pressure between the boiler and the steam
system when the main steam valve is closed. Please note that
the by-pass valve is only supplied for larger boilers.

Feed water valve / feed water valve (sdnr)

Two groups of feed water valves are provided in the boiler.
Each group comprises a shut-off valve and a non-return valve.

The shut-off valve in the ordinary group must be open when the
boiler is in operation, or if the boiler is used as a steam drum.

The shut-off valves should be closed when the boiler is not in

use.
Water level gauges
Two local water level gauges are connected to the front of the
boiler, each gauge being provided with two shut-off valves and
a drain valve.

The shut-off valves, fitted at the top and bottom of the sight
glass, have a quick-closing mechanism to be used in case of
broken glass.

The pipes from the drain cocks on the water level gauge must
lead to an open drain, visible for inspection.

Blow-down valve
Two blow-down valves are mounted at the bottom of the boiler
body. If connected to a separate drainage system the valves
are of the ball type. When connected to a common drainage
system two valves are provided in each group, one shut-off
valve and one shut-off/non-return valve. The shut-off function is
for security and the non-return function prevents steam/water
from flowing into an empty boiler by mistake.

Air valve
The air/ventilation valve located on top of the boiler is a shut-off
valve. It is normally closed except when the boiler is being filled
or completely drained. The end of the drain pipe from the air
valve must be visible in order to determine when water or
steam is coming out.

Scum valve
The scum valve is mounted at the top of the boiler body. In the
event of scum in the boiler, this scum can be blown off from the
water surface by opening this valve. If connected to a separate
drainage system the valve is of the ball type. When connected
to a common drainage system two valves are provided, one
shut-off valve and one shut-off/non-return valve.

Valves for WHR inlet and outlet

The boiler is provided with two circulation valves for inlet and
outlet connections to the exhaust gas boiler(s).

The valves should be shut-off valves, and depending on

deviations in the system, these valves can also have a non-
return function.

Sample valve
A sample valve is installed enabling connection to a sample
cooler for taking test samples to perform boiler water analyses.
 Inspection hole
Two small inspection holes are provided in the furnace wall to
enable inspection of the burner flame. A proportion of the air
supply is bled off from the burner fan to cool the window of the
inspection hole, and prevent soot deposits.

Hand holes

Towards the bottom of the shell, both at the sides and at the
ends, are hand holes and hand hole covers. The hand holes are
used for inspection and cleaning purposes.

Manhole
Two manholes placed at the boiler top and boiler bottom allow
inside inspection of the steam/water drum.

On top of the boiler is the oval-shaped manhole, which allows

access to the water side of the boiler for inspection, cleaning or
repairs. The manhole is closed with the manhole cover, and the
joint is sealed with either a graphite impregnated asbestos
gasket, or a spiral wound gasket. The manhole cover seals on
the inside face of the manhole, thus using the boiler pressure to
assist with sealing pressure. The manhole and cover are oval
shaped so that you can bring the cover out through the
manhole. It is important that the mating surfaces of the
manhole cover and the manhole are in good condition to ensure
effective sealing.

After starting a boiler from cold, it is important to re-tighten the

manhole nuts once the boiler is at operating temperature as they
will tend to loosen with thermal expansion.
 Manhole with cover in position

Access doors

Access to the furnace and smoke outlet box are possible

through the access doors placed at the bottom of the furnace
and at the smoke outlet box respectively. Both access doors
enable inspection of the generating tube bank.

Smoke boxes At both the front and the rear of the boiler, in
line with the tubes, are the smoke boxes. Their purpose is to
collect and route the hot gas through the tubes and out to the
grit arrester. The smoke boxes have removable doors which
allow access to the tubes for cleaning or inspection. The doors
are sealed with asbestos rope, and the doors held in position
with clamps. After a cold start the clamps must be checked for
tightness, as thermal expansion will cause them to loosen.

Front-left smoke box Smoke box door

clamps

Drain for furnace

 The furnace bottom is provided with a socket for drain of the
washing water.

Anti-priming plate Inside the shell, at the top where the

steam leaves the boiler, is a drilled plate called an anti-priming
plate. This breaks the vortex caused by steam leaving the
boiler, and helps prevent boiler water leaving with the steam - a
condition known as "priming".

Position of anti-priming plate inside the steam space of the boiler

Sparge pipe
On the inside of the boiler stretching about one metre across, is the
internal feed water pipe, or sparge pipe as it is commonly known. It is
drilled along its length at the bottom with numerous holes to assist
with distribution of the relatively cold feed water and avoid localised
cooling of the tubes. The holes are inclined to block quickly should
hardness be present in the feed water, as the rapidly temperature
change induces precipitation of hardness. The blockage is indicated
by increasing run time of the feed pumps, and frequent low water
alarms as the feed rate progressively decreases.
Sparge pipe - note scale deposits at holes

Soot blower steam supply pipes

Inside the boiler you will also find the soot blower steam supply pipes.
They are fitted through the side of the shell and through the rear tube
plate, and extend to the highest steam space in the boiler to feed dry
steam to the soot blowers. This steam supply is an exception to the
rule that steam can only be drawn through the crown valve .

Thermal insulation
A large part of the boiler surface is insulated to minimise the loss of
heat to the surroundings, as well as to reduce the surface
temperatures for reasons of safety. The most common insulating
material is mineral wool, which is applied in semi-rigid mats, held in
place with stainless steel wire strapping. The insulation is clad with
either galvanised steel, polished aluminium or stainless steel.

The cladding must have removable sections over the circumferential

and longitudinal weld seams, and the nozzle welds, so that these can
easily be exposed for inspection. It is good practice to leave an
unlagged section at the bottom of the boiler down its entire length, so
that any water which may enter the lagging from say, leaking fittings
or leaking roof, will drain out of the lagging at the lowest point. Also,
because the top of the boiler is exposed to light foot traffic to the ID
fan and safety valves, it is good practice to ensure that the lagging
and cladding is strong enough to sustain the traffic without damage
or degradation.

Lagging and cladding being applied to a boiler

Difference between Boiler Mountings and

Accessories
Boiler mountings include safety devices such as safety valves and
water level indicators, which ensure the safe operation of the boiler
by regulating pressure and water levels.

Boiler accessories are additional components that enhance the

efficiency and performance of the boiler. These accessories may
include economizers, super heaters, and feed water pumps, among
others.

While mountings focus on safety, accessories optimize the

functionality and energy efficiency of the boiler.