What is the function of a fusible plug?

• A. To regulate pressure
• B. To melt when the water level drops below the level of the plug
• C. To filter contaminants
• D. To control temperature

Answer: (B)

The function of a fusible plug is primarily to melt when the water level drops below the level of the plug. This mechanism is a critical safety feature, particularly in steam boilers or pressure vessels. When the water level in a boiler falls too low, the temperature of the surrounding metal increases significantly. A fusible plug, typically made from a metal alloy with a low melting point, serves to prevent overheating and potential catastrophic failure by melting and allowing the steam or pressure to escape.

By doing so, the fusible plug acts as a safeguard against dangerous situations, such as an explosion due to superheated steam or hot spots that could cause structural damage. Other options, while relevant to safety and equipment operation, do not align with the specific function of a fusible plug. For instance, regulating pressure and controlling temperature generally involve valves and thermostatic controls, while filtering contaminants relates to the use of filters or screens in fluid systems. Therefore, melting in response to a dangerously low water level is the precise role that distinguishes a fusible plug in pressure vessel safety.