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.

Read more...

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.

Read more...

Lever safety valve:

The advantage of level safety valve over the dead weight safety valve is that the heavy dead weight is replaced by a lever with a smaller weight.

Description of Lever safety valve:

It consists of a valve resting over a gun metal seat. The valve seat is fixed on a mounting block, fitted over the boiler shell. One end of the level is hinged to a rod of the mounting block, while the other end carries a weight. A short strut is placed over the valve.

Working of Lever safety valve:

The thrust of the lever with its weight is transmitted to the valve by the strut. When the steam pressure exceeds the safe limit, the upward thrust of steam lifts the valve from its seat and the lever with its weight. The excess steam escapes till the pressure falls back to the normal value. The valve then returns back to its original closed position.
The required weight W at the end of the lever for maintaining the pressure P in the boiler is obtained by taking moments about the hinged point.

i.e., PaL1 = WL2

Where a = area of the valve exposed to steam,
L1 = distance of valve centre from the hinged point, and
L2 = distance of the centre of the weight to the hinged point.
The lever safety valve is used in stationary boilers only.

Read more...

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.

Read more...