Table of content: |
1. Water level indicator |
2. Pressure gauge |
3. Steam stop valve |
4. Feed check valve |
5. Blow-off valve |
6. Fusible plug |
7. Safety valve |
Boiler Mountings:
- These are different fittings and devices which are necessary for the operation and safety of a boiler. These devices usually mounted over the boiler shell.
1. Water lever Indicator:
Function: The function of a water level
indicator is to indicate the level of water in the boiler shell constantly.
Construction: Fig. shows the details of a
water level indicator. It is fitted on all types of boiler. It is fitted at the
front end of the boiler so that it is visible to the operator.
Working: When the cocks D and E are opened,
steam rushes from the upper passage in B and water rushes from the lower
passage in C to the glass lube F. Both of them adjust their levels. This will
show the level of water in the boiler. In hollow gun-metal column G, the water
also stands at the same level.
In case, glass tube F gets broken, the steam
will rush from the upper passage and the water from the lower passage because
of the pressure difference between boiler pressure and atmospheric pressure.
This water carries the hollow gun-metal ball J from its normal position to the position shown by a dotted circle. This
will close the passage of water to the glass tube.
Now the water rises in the
hollow gun-metal column G. This water aided by the steam rushing through the
upper passage will carry the ball H from its seat and placed it to the new
position shown by a dotted circle. Hence the supplies of both water and steam to
the glass tube fare closed. The glass tube F can be replaced by closing two
cocks D and E.
The water level indicator used for high-pressure boiler should be provided with guards made of thick toughened glass
which cover the front and sides of the glass tube. The boiler, the attendant is
then protected from the flying fragments of glass when glass tube bursts.
2. Pressure gauge:
Function: The function of pressure gauge is
to measure the pressure exerted inside the vessel.
Working: The movement of the free end of the
elliptical tube is proportional to the difference between external and internal
pressure on the tube. Since the outside pressure on the tube is atmospheric,
the movement of the free end is a measure of the boiler pressure above
atmospheric i.e. gauge pressure.
The pressure gauge is connected with the
boiler's steam space through a U-tube siphon. The U-shaped siphon pipe contains
water which fills the Bourdon tube. The pressure of the steam acting' through
the water on the inside of the tube tries to make the tube circular.
One end is fixed, the free end moves. The
movement at the free end is proportional to the difference in pressure as shown
above. Because of pinion and gear arrangement, the slight movement of the
Bourdon tube is magnified considerably and the point gives a maximum deflection which can be read on the scale.
The water in the V-tube syphon prevents the steam from coming in contact with
the spring tube. This will reduce overheating and thermal stresses in the spring
tube. A three-way cock is provided for connecting the boiler pressure gauge to
the steam space.
3. Steam Stop Valve:
Function: The function of the steam stop
valve is to regulate the flow of steam from one steam pipe to the other or from
the boiler to the steam pipe.
What is a Junction Valve? - The steam stop
valve, when directly mounted on, the steam space of the boiler shell and
connected to the steam pipeline which supplies steam to the prime mover. It is
called a junction valve.
Working: When the hand-wheel is turned in the anticlockwise direction, the spindle is raised up. This will raise the valve
from its seat. Thus, a passage for the steam from the clearance between the
valve and valve seat is formed. In order to lower the valve, the handwheel is
rotated in a clockwise direction. This rotation will close the passage for
steam. This can be regulated by adjusting the position of the valve based on
the requirements. Under the normal working, condition the valve is open and
steam flows from the boiler to the steam pipe.
4. Feed Check Valve:
Function: It is used to control the supply of
water to the boiler and to prevent the escaping of water from the boiler when
the pump pressure is less or the pump is stopped.
Construction: This is a non- return valve
placed as near to the boiler as possible. It is shown in Figure. The lift of
the valve is controlled by an extension of the spindle of the screw-down valve
above it. It is the feed valve. It is the flange. It is bolted to the front end
of the boiler shell at a point from which an internal pipe leads the feed water
and distribute the same to the boiler. It is the handwheel. This is used to
control the positions of the valves. The construction above valve is similar to
that of the steam stop valve.
Working: Under the normal working condition, the
pressure on the feed pump side (connected to elbow) is more than the boiler
side pressure. This pressure difference lifts the check valve. To permit the
feed water to enter the boiler, the feed valve is lifted manually. Hence, the feed
water may enter the boiler. In order to control the supply of feed water to the
boiler, the position of the feed valve is controlled. In the event of failure
of the feed pump, the pressure on the water sump side reduces. The check valve will
be closed due to the higher steam pressure.
This will stop the backflow of water from
the boiler to the water sump. The check valve is to be replaced if it does not
give satisfactory results. The stuffing box is provided in the valve to stop
the leakage at the spindle.
5.
Blow-off cock Valve:
Functions:
(l) To discharge a portion of water when the
boiler is in operation to blowout mud, scale or sediments periodically.
(2) To empty the boiler, when necessary for
cleaning, inspection and repair. It is fitted on the boiler shell directly or
to a short branch pipe at the lowest part of the water space. This pipe is
known as blowdown pipe.
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 a 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 as shown in Fig. of the body, the clock
is closed.
6. Fusible Plug:
Function: Its function is to protect the
boiler against damage due to overheating for low water level.
Where it is fitted? - It is fitted in the crown plate of the furnace or firebox at the appropriate place.
Working: In the normal working conditions of
the boiler, the fusible plug is full, submerged underwater. This is so
because, under normal condition, the proper water level is maintained and can be
checked by a water level indicator. Under the circumstances, the heat from the
fusible plug is being conducted to water. This keeps the fusible metal at an
almost constant temperature, much below its melting point.
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 overheat the fusible metal. The plug falls down along with
the fusible metal making a hole. The steam and water, being under pressure
immediately reach to the firebox and extinguish the fire.
The attendant is cautioned against
overheating and burning of the fire tubes etc. Before starting the boiler again,
the operator fixes a new fuse. The fusible plug should be renewed generally
after a period of about two years as they are liable to become defective over a
long period of use. One plug is provided for each furnace.
7.
Safety
Valves:
Function: It is a device, attached to the
steam space of the boiler shell, which opens automatically to discharge some
steam and to prevent the steam pressure inside the boiler to exceed.
There are four types of safety valves
generally fitted on a boiler:
(a) Dead-Weight Safety Valve
(b) Lever-Weight Safety Valve
(c) Spring Loaded Safety Valve
(d) High Steam and low-water safety Valve
The spring-loaded safety valve is used with
locomotive and marine boilers. The other type of valves is used with stationary
boilers.
(a) Dead-weight Safety Valve:
This valve is used only with the stationary type
of boilers. It is the most elementary type of safety valve.
Working: The valve will stick to its seat, as
long as the pressure exerted on the valve is less than the pre-determined
value. When the pressure exerted by the steam on the valve becomes more than
the predetermined value, the valve is lifted from its seat. The pre-determined value of pressure is adjusted by the force acting from above the
weights etc. Because of the lifting of the valve, the steam will go to the atmosphere through the discharge pipe. The valve comes back to its seat when
the upward steam pressure is less than the downward force exerted by the
weights etc. This will help to maintain constant pressure inside the boiler
shell.
Merits of dead weight safety valve:
l. Simple in design.
2. Give quiet and satisfactory operation.
3. From the pressure adjustment viewpoint it
cannot be easily tempered.
Demerits:
l. Unsuitable for any boiler subjected to
vibration and movement (i.e. Locomotive boiler).
2. Because of the requirement of a large amount
of weight to balance steam pressure this valve is not suitable for use on the high-pressure boiler.
(b) Lever loaded safety valve:
For larger size safety valve lever loaded
safety valve is used. Weight is not put directly on the valve but provided at the
end of the lever. In this type of valve, a pivot pin is put on fulcrum which has a
lever. As shown in Fig. the valve is located with another pivot pin. At the far end
of the lever, weight is suspended. When steam pressure exceeds the limiting pressure
dead weight with a lever is raised and steam exits from boiler shell until
boiler pressure and limiting pressure becomes the same.
The force acting by a steam on valve = P✕𐍀/4✕D*2
= a/b✕m✕g
where,
P = Max. working pr.
V = Diameter of valve sheet opening
a/b = leverage
m = mass suspended at the end of lever
This type of safety valve is used in
stationary boilers.
(c) Spring-loaded safety valve:
Deadweight safety valve and lever loaded
safety valves are not used in marine and locomotive boiler because of
vibrations. Fig. shows Rams bottom spring-loaded safety valve. Body of valve is
of C.I and Y shaped. Valve sheets are located on both ends of the pipe. Spring
ends are fitted on shackle provided in the valve body. The upper end of spring is
fitted with lever pivot. The lever is forged, both valves are fitted with the lever as
shown in the figure. Both valves remain fit on valve sheet due to the spring tension
force.
When steam pressure exceeds the limiting
pressure then upward force exceeds limiting pressure(force) of spring &
valve lifts from valve sheet & excess steam come out until steam pr.
(boiler pressure) becomes equal to spring pressure. Then valve fits on the valve
sheet. Boiler attendant checks valve position & working by operating lever
up and down manually. Links provided on both sides of spring keeps the lever in
position during breakage of spring.
(d) Combined low water level and high steam
safety valve:
It provides safety to the boiler in the
following two conditions.
(I) When working pr. of steam boiler exceeds
limiting pressure then the valve operates and excess steam rushes outside.
(2) When the water level in a boiler decreases
then safety level, in this condition valve operates & steam comes out with a hissing noise, which makes attention to the operator.
As shown in Fig. one lever loaded safety
valve is given in such a construction & another valve is provided in the main
valve. When pressure exceeds then the main valve (high-pressure valve) lifts on the
sheet & excess steam rushes outside.
When the water level goes down than a safe level in a boiler drum, then balancing disturbs between float weight and balancing
weight & dead weight. Hence float weight goes towards the downside. In such
condition, the lever end of balancing weight side goes up. The upward force is created
due to projection. Hence low water raises from sheet & steam comes out from
the drum with a hissing noise, which warns boiler attendants for low water level.
Also read: IC engine types and Working
Boiler Mountings: Types, Functions and working
Reviewed by Harish
on
June 19, 2020
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