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.
