Safety valve is a Boiler mounting for safety against high pressure. Fusible plug on the other hand is another Boiler mounting for safety. But Fusible plug is a safety mounting against high temperature due to low water level inside the Boiler.

Function of a Fusible plug

It protects the Boiler from explosion due to low water level. Low water level causes overheating of the heating surfaces and may results in damage to the boiler with potential catastrophic failure.

You are already familiar with two safety fittings in a conventional Pressure Cooker. They are Safety Valve (the whistle) and the Fusible Plug positioned under the lid handle. The Fusible plug in a pressure cooker is popularly known as fusible safety valve. You might have read the instructions that fusible safety valve will operate when: (1) the normal escape of steam is blocked and (2) there is less water in the pressure cooker and it boils dry, causing temperature to rise beyond the normal operating range. The plug in a boiler works on same principle of elevated temperature of boiler surface due to less water.

Location 

It is positioned in a suitable location within the boiler so that the fire in the furnace can be extinguished.

The fusible plug must be positioned above the furnace or combustion chamber (in the crown of the furnace) at the lowest permissible water level. Let us understand the working principle and construction.

Construction and Material

The plug is made of two materials:

• Plug Body and
• Fusible alloy.

Fusible alloy is the plug within a threaded plug body (brass, bronze, or gun metal). The Fusible alloy has a lower melting point than that of plug body. A common fusible alloy material is tin, which has a melting point of 232°C. The following diagrams of plug and plug body are self-explanatory.

How does it Work?

The threaded plug with plug body is fixed by nut and bolt to the boiler at the lowest permissible water level. One side of the plug is exposed to heat from combustion gases and other side is submerged in water of boiler drum.

If water level is normal, the fusible plug is surrounded by water. So, the water is always available for heat transfer. Therefore, temperature of fusible metal or alloy remains below its melting point. So, the solid plug remain in its position and normal steam generation process continues in boiler. What if there is no water for heat transfer?

If the water level falls below the lower limit in the boiler, the plug gets exposed to steam only. The heat of the plug is not carried away by water now. So, both plug-body and fusible alloy gets overheated. This results in melting of fusible alloy first. The core alloy is released from the plug-body and falls. This opens a hole between steam space and furnace. Steam rushes into the fire box and extinguish the fire. In absence of heat of combustion, the steam generation is stopped. Thus, the Boiler get protected from any further damage or bursting.

Conclusion

While Safety valve protects the boiler from over pressure, Fusible plug protects the boiler from over temperature.

Both Safety valve and Fusible plug are Boiler mountings for safety.

You can watch this short video to understand the construction and working principle.