ANS:C - Fusible plug

The safety device used to protect the boiler when the water level falls below a minimum level is called a fusible plug. A fusible plug is a small, hollow cylindrical device installed in the crown sheet (the top part of the firebox) of a boiler. It is typically made of brass or bronze and filled with a fusible metal such as lead or a lead alloy. When the water level in the boiler drops to a dangerously low level, the exposed part of the fusible plug becomes overheated due to the absence of water. The fusible metal inside the plug melts at a predetermined temperature, usually around 165°C to 175°C. Once the fusible metal melts, it releases pressure inside the boiler by allowing steam to escape through the hole in the plug. This sudden release of steam alerts the boiler operator or shuts down the boiler to prevent further damage or catastrophic failure due to overheating. The other options listed:

• Blow down valve: Used to remove sludge and sediment buildup from the boiler.
• Blow off cock: Used to remove impurities from the boiler water by draining a small quantity of water.
• Safety valve: A device that automatically releases excess steam from the boiler to prevent pressure buildup and potential explosion.