Fusible plug Diagram, Working | Boiler Mounting and Accessories

Fusible plug Diagram, Working | Boiler Mounting and Accessories


Boiler mountings are the devices mounted on the boiler surface for safe operation, efficient working, control steam generation and easy maintenance of the boiler.

In accordance with the Indian boiler regulation the following mountings should be fitted to the boilers.

Read more : Introduction to Boilers | Classification Of Boilers

Function of Fusible Plug : 

To protect the boiler against damage due to overheating for low water level.

diagram of fusible plug
diagram of fusible plug


It is fitted on the fire box crown plate or over the combustion chamber at its approprirate place.

A common form of fusible plug is illustrated in Fig. It consists of a hollow gun metal body screwed into the fire box crown: The body has a hexagonal flange to tighten it into the shell. A gun metal plug having a hexagonal flange is screwed into the gun metal body. There is another hollow gun metal plug separated from the metal plug by an annulus of fusible metal. The fusible metal is protected from fire by flange on the hollow gun metal plug.


Under normal condition when the water-level in the boiler shell is normal, the fusible plug is fully submerged under water. In this case, the heat from the fusible plug is being conducted to water which keeps the fusible metal at an almost constant temperature and below its melting point. But when the water level falls below the fusible plug, it gets uncovered from water and is exposed to steam. The heat conduction from the fusible plug to steam is very little compared with that to water. Hence fusible plug becomes overheated and it melts with the result that the hollow gun metal plug falls down making a hole. The steam and water being under pressure immediately rush to fire box and extinguish the fire.

The fusible plugs should generally be renewed after a period of about two years as they are liable to become defective over a long period of use (because they are subjected to heat on one side and scale deposits on the other).

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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