Fusible plug

The function of the fusible plug is to put-off the fire in the furnace of the boiler when the water level falls below an unsafe level and thus avoid the explosion which may take place due to overhearing of the tubes and the shell.

Description of fusible plug:

It consists of a hollow gun metal body screwed into the fire box crown plate. A hollow gun metal plug is screwed into the gun metal body by tightening the hexagonal flange in it. There is another copper plug locked with the gun metal plug by pouring a low melting point metal (lead) into the groove provided for the same.

Working of fusible plug:

During the normal operation, the fusible plug is submerged in water which keeps the temperature of the fusible metal below its melting point.

But when the water level falls below the top of the fusible plug, it is uncovered by the water. The fusible plug therefore melts by the heat of the furnace. Thus the copper plug drops down and is held within the gun metal body by the ribs. The opening so made allows the steam rush into the furnace and extinguish the fire. The damage to the fire box which could burn up, is avoided.

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Spring loaded safety valve (Ramsbotom Safety Valve):

Description of spring loaded safety valve:

It is loaded with a spring instead of weights. Hence it is called spring loaded safety valve. It consists of a cast iron body having two branch pipes P1 and P2. Two separate valves are placed over the valve seatings, which are fixed to the top of the branch pipes. A lever is placed over the valves by means of two conical pivots.
The lever is attached to a spring at its middle. The spring pulls the lever in downward direction. The lower end of the spring is attached to the valve body by means of a shackle. Thus the valves are held tight to their seats by the spring force.

Working of spring loaded safety valve:

When the steam pressure exceeds the normal working pressure , the valves rise up against the action of the spring and allow the steam to escape from the boiler till the pressure in the boiler reaches its working pressure.

The spring loaded safety valve is much lighter and compact compared with other safety valves. For locomotive or marine service, the safety valve should be such that it is unaffected by jerks and vibration likely to occur in such device. Hence spring loaded safety valve is preferred for locomotive and marine services, in addition to stationary boilers.

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Dead Weight safety valve:

The Dead Weight safety valve consists of a valve V which is made of gun metal to prevent rusting. It rests on the gun metal seat S and is fixed to the top of a vertical steam pipe P. The pipe has a flange F at the bottom for fixing at the top of the boiler shell.

A weight carrier C is suspended from the top of the boiler. It carries cast iron rings (i.e., weight W). the total weight must be sufficient to the keep the valve on its seat against the normal working pressure.

Working of Dead Weight safety valve:

When the steam pressure in the boiler exceeds the normal working pressure, it lifts the valve with its weight. The excess steam therefore escapes through the pipe to the atmosphere, until the pressure reaches its normal value.

It is the simplest type of safety valve; it is suitable for stationary boilers only, because it cannot withstand the jerks and vibration of mobile (marine) boilers. Another disadvantage of this valve is the heavy weight required to balance the steam pressure. Hence, it is not suitable for high pressure boilers.

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Safety valves

Safety valves are devices used to maintain a constant safe pressure inside the boiler. When the pressure inside the boiler increases, the excess steam will escape to the atmosphere through the valve automatically. Generally, a boiler is provided with two safety valves.

There are four different types of safety valves, viz.,

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

In the upcoming posts, I will write post about the four different types of safety valves.

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Pressure gauge

A pressure gauge is used to indicate the steam pressure of the boiler. It is also called as steam gauge. It is usually mounted in the front top of the steam drum.

Figure shows a commonly used pressure gauge known as bourdon type. It consists of an elastic metallic bourdon spring tube S of elliptical cross section and bent in the form of circular arc. One end of the tube is fixed at the block B. it is connected to the steam space of the boiler by menas of cock C. The other end is connected to a toothed sector T through a link L hinged at the point H. The sector is in mesh with a pinion P fixed on a spindle. An indicating pointer N is attached to the spindle to read the pressure on a dial gauge D.

Working:
When steam enters the elliptical tube, the tube section tries to become circular, which causes the other end of the tube to move outward. The movement of the closed end of the tube is transmitted and magnified by the link and the tooth sector. The magnitude of the movement of the sector is indicated by the pointer on the dial.

Note: Since the spring tube is surrounded by the atmospheric air, the pressure in the interior of the tube is above that of the atmosphere.
Hence absolute pressure = gauge pressure + atmospheric pressure.

Use the link to know more about Bourdon tube pressure gauge

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Water gauge or Water Level Indicator

Water gauge indicates the water level inside the boiler and is hence called as water level indicator. Usually two water gauges are fitted in front of boiler.

Description of water gauge:

It consists of a glass tube, two gun metal tubes and three cocks. The steam cock C1 is provided on the gun metal tube M1 which connects the glass tube with the steam space in the boiler. The water cock C2 is provided on the gun metal tube M2 which connects the glass tube with the water space. The gun metal tubes M1 and M2 are bolted to the boiler shell.

The drain cock C3 is used to drain the water from the glass tube at intervals to ascertain whether the gauge is in proper order or not. The glass tube is protected by means of a cover, made of specially toughened glass which will prevent any accident that may happen due to the breaking of glass tube.

Working of Water gauge:

The water gauge shows the level of water in the boiler drum. It warns the operator if the water level goes below a fixed mark, so that corrective action may be taken in time to avoid any accident.

For the observation of the water level in the boiler, the water and steam cocks are opened and drain cock is closed. The steam enters from the upper metal tube M1 into the glass tube and water enters from the lower metal tube M2 into the glass tube. Hence, water stands in the glass tube at the same level as in the boiler.

The junctions of the metal tubes and the glass tube are provided with two balls. In case the glass tube is broken, the balls are pushed to the top nd bottom ends of the glass tube. Thus the flow of both water and steam out of the boiler is prevented.

When the boiler is not working, the water gauge can be taken out from the boiler for cleaning purposes by removing the bolts.

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List of Boiler Mounting Valves and Gauges

To provide for intelligent and safe operation of a boiler, a full complement of gauges and safety devices should be provided.

These include:

• Water gauge (water level indicator)
• Pressure gauges.
• Safety valves:
  • Dead weight safety valve:
  • Spring loaded safety valve
  • Lever safety valve
  • High steam low water safety valve
• Fusible plug
• Feed check valve
• Stop valve
• Blow – off cock

In the upcoming posts, I will write about the above listed topics in detail.

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