What is the purpose of a fusible plug in a boiler and how does it work?

The purpose of a fusible plug in a boiler is to provide a last-resort safety device to prevent catastrophic failure in the event of dangerously low water level. It works by melting at a predetermined temperature, allowing steam and water to escape and extinguish the fire, preventing overheating of the boiler's pressure-containing parts. A fusible plug is typically a threaded brass or bronze plug with a core made of a low-melting-point alloy, usually a lead-tin alloy. This plug is installed in a part of the boiler that is normally submerged in water, such as the crown sheet of a fire-tube boiler. When the water level drops to a dangerously low level, the fusible plug becomes exposed to the hot combustion gases. The temperature of the plug rises rapidly, and when it reaches the melting point of the alloy core, the core melts away, creating an opening through the plug. The high-pressure steam and water inside the boiler then rush out through this opening, extinguishing the fire and reducing the boiler pressure. This prevents the boiler tubes and other pressure-containing components from overheating and potentially rupturing. Fusible plugs are a simple but effective safety device. However, they are a one-time-use device and must be replaced after they have activated. Also, they are not a substitute for proper boiler operation and maintenance, including maintaining adequate water level and regularly inspecting the boiler for other potential problems. For example, if a boiler's feedwater pump fails and the water level drops below a safe level, the fusible plug will melt, releasing steam and water and preventing the boiler from exploding.