Introduction:
In a boiler, the water level needs to be maintained at a safe limit to ensure the safe operation of the furnace. If the water level falls below this limit, it can lead to overheating of the furnace and potentially cause a fire. To prevent such accidents, a device called a fusible plug is used in the boiler.

Explanation:
A fusible plug is a safety device that is installed in the furnace of a boiler. It consists of a brass plug with a tapered hole filled with a low melting point alloy or fusible metal. The fusible metal used in the plug has a melting point lower than the critical temperature of the boiler.

Function:
The fusible plug acts as a safety valve and helps prevent the boiler from overheating and catching fire when the water level falls to an unsafe limit. When the water level in the boiler drops below the required level, the exposed plug becomes overheated due to the high temperature of the furnace.

Working:
When the temperature around the fusible plug reaches the melting point of the fusible metal, it melts and creates an opening in the plug. This opening allows the high-pressure steam inside the boiler to escape, which results in a sudden drop in pressure and extinguishes the fire in the furnace.

Significance:
The fusible plug is an essential safety device in a boiler as it helps prevent catastrophic accidents by automatically shutting down the furnace when the water level falls to an unsafe limit. By extinguishing the fire, it prevents the overheating of the boiler and the potential explosion that could occur due to the build-up of pressure.

Conclusion:
In summary, a fusible plug is a crucial safety device used in boilers to prevent fires when the water level falls below a safe limit. Its function is to melt and create an opening when exposed to high temperatures, allowing the steam to escape and extinguish the fire. The fusible plug plays a vital role in ensuring the safe operation of boilers and preventing accidents.