what is the purpose of providing fusible plug in boiler?
It is a pressure relief based on temperature. In many ways a more fool proof way to provide pressure relief with no moving parts. It works on the idea that for a given pressure in a boiler, there is a corresponding temperature, so if you know the temperature you also know the pressure. This is based on saturated steam and not super heated steam. Many pressure cookers used in kitchens use a similar safety feature.
Safety. Simple explanation. The fusible plug is just a hollow bolt filled with lead which melts if the water cannot absorb the heat generated in the firebox The boiler construction is an inner box for the fire. Surrounded by another box filled with water. The plug is screwed into the top of the firebox. When the fire is lit the heat boils the water. There is no problem if the water keeps the firebox covered. If the water evaporates and uncovers the top of the firebox then the lead melts and steam escapes through the now hollow bolt and puts out the fire. There has been instances where the water has uncovered the firebox top and heat in the fire box has weakened the firebox steel and boiler pressure has distorted the firebox until it has cracked and the violence of the steam escaping has caused the boiler to be torn apart
Fusible plugs in pressure vessels serve a similar function as a fuse in an electrical circuit. They are designed to fail before the temperature or pressure in a boiler reaches the point that it damages the boiler. A fusible plug in cheap compared to the cost of a damaged boiler.
Sounds like a safety measure so that the pressure will be released if the boiler gets too hot. Just a guess.
