fusible plug for boiler safty

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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• Function: To extinguish fire in the event of water level in the boiler shell falling below certain specified limit.
• It protects fire tubes from burning when the level of the water in the water shell falls abnormally low and the fire tube or
crown plate which is normally submerged in the water, gets exposed to steam space which may not be able to keep it
cool.
• It is installed below boiler's water level.
• When the water level in the shell falls below the top of the plug, the steam cannot keep it cool and the fusible metal
melts due to over heating. Thus the copper plug drops down and is held within the gunmetal body by the ribs. Thus the steam space gets
communicated to the firebox and extinguishes the fire. Thus damage to fire box which could burn up is avoided.
• By removing the gun metal plug and copper plug the fusible plug can be put in position again by interposing the fusible
metal usually lead or a metal alloy.