What is a Fusible Plug?

There are a variety of safety gadgets put in place to ensure the safety of mechanical machines. A fusible plug, also known as a threaded fusible plug, is one such device commonly used in steam engines. It ensures that should water levels recede to dangerous levels, operators will be alerted by a loud stream of steam, allowing them to take action.

Created with brass, bronze or gunmetal, fusible plugs are formed with a tapered hole. This hole runs throughout the entire length of the plug. It is then secured with a metal sealant, such as tin or lead. When put to use, a fusible plug is attached to the top plate, or crown sheet, of the steam engine's firebox.

Once in place, the fusible plug dips an inch into the water space above it. It works by heating up when its tip is no longer submerged in water, indicating that levels are too low. As it overheats, its sealant melts, allowing it to become a simple whistle, much like that on a teapot. This is considered a last resort, since such a low water level at this point would already prove to be dangerous.

The danger lies in the heat of the steam engine firebox, particularly that of the flue gases. They can reach up to 1000 °F (550 °C), which is perilous to both the engine and humans. Since many fireboxes are made of copper, which can melt at this temperature, this heat makes the firebox vulnerable to collapse. If this occurs, the entire machine can explode as a result.

Richard Trevithick invented the threaded fusible plug in 1803. After one of his own boilers exploded during a loss of water, he was intent upon making a detection device that could offer a means of prevention. He was especially invested in the project due to his advocacy of high pressure steam engines. Due to his invention, this form of steam engine was approved, and is still in operation today.

Though helpful, threaded fusible plugs can fail. Should the sealant become oxidized or encrusted as it ages, it can damage the reliability of the plug. These conditions can increase the melting point of the metal, making it fail to react when heated. Some fusible plugs have failed to activate even after being exposed to temperatures exceeding 2000 °F (1100 °C).

Because of this threat, standards have been established to prevent these conditions. In some areas, fusible plugs must be replaced after thirty to sixty days of use. In others, the limit is 500 hours.