1. Role of the Fusible Plug: The
Fusible plug is a safety device fitted to the crown of the furnace or the top of the combustion chamber. It consists of a hollow gunmetal body filled with a low-melting-point alloy (such as tin or lead).
2. Principle of Operation:
• Normal Condition: As long as the water level is above the plug, the water absorbs heat from the plug, keeping the temperature of the alloy below its melting point.
• Low-Water Condition: When the water level drops below the plug, the plug becomes uncovered and is exposed directly to the steam and intense heat of the furnace.
• Safety Action: Without water to cool it, the temperature of the alloy rises rapidly until it melts. This leaves a hole through which high-pressure steam and water rush into the furnace,
extinguishing the fire and preventing the furnace crown from overheating or collapsing.
3. Analysis of Other Devices:
• Economiser: A boiler accessory (not a mounting) used to preheat feedwater using flue gases.
• Safety Valve: Protects the boiler against
excessive pressure, not low water.
• Blow-off Cock: Used to remove sludge or drain the boiler.