Description
Introduction
Fusible plugs are a type of warning device employed on compressed air systems, well heads and steam systems. They are considered a last line of defense and are often used to avert a catastrophic failure occurring e.g. explosion. Whist safety valves protect machinery items from over pressure, fusible plugs protect machinery items from over temperature.
Fusible Plug Materials
Fusible plugs contain a fusible alloy housed within a brass, bronze or gun metal threaded plug body. The material chosen for the fusible alloy has a predetermined melting point that is lower than the plug body material’s melting point. A common fusible alloy material is tin, which has a melting point of 232°C (450°F).
How Fusible Plugs Work
The fusible alloy within a fusible plug is designed to melt at a lower temperature than the surrounding plug body. If the plug is exposed to elevated temperatures, the fusible alloy will melt and a straight or tapered channel will be formed lengthwise through the plug body. The open channel allows the system pressure to vent directly to a predetermined location. For compressed air systems, this location is usually just the space surrounding the plug. For boilers, it is more likely to be the furnace or firebox. The fusible plug body is often shaped so that a whistling noise is created as the system pressure vents, this acts as a local audible alarm to system operators.
Fusible plugs are installed within boilers to reduce the risk of a boiler explosion as a result of low water firing. A low water level scenario in a boiler would lead to over heating of the heat transfer surfaces and resultant damage to the boiler would occur (with potential catastrophic results).
Boiler fusible plugs are classed as steam actuated or fire actuated, and can be installed on both fire tube boilers and water tube boilers. It’s also possible to classify boiler fusible plugs as being either fire side or water side plugs. Fire side plugs are screwed from the fire side to water side. Water side plugs are screwed from the water side to fire side.
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