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Fuse plugs are safety devices used on steam engines to alert operators of dangerous water levels. Invented by Richard Trevithick in 1803, they work by heating up and melting their sealant when water levels are too low. However, they can fail if the sealant oxidizes or scales, so standards have been established to prevent this.
There are a variety of safety devices implemented to ensure the safety of mechanical machines. A fuse plug, also known as a threaded fuse plug, is one such device commonly used on steam engines. It ensures that if water levels recede to dangerous levels, operators will be alerted by a strong current of steam, allowing them to take action.
Created using brass, bronze, or gunmetal, fuse plugs are formed with a tapered hole. This hole extends through the entire length of the plug. It is then secured with a metal sealant, such as tin or lead. When put into use, a fusible plug is connected to the top plate, or crown sheet, of the combustion chamber of the steam engine.
Once in place, the fusible plug is submerged one inch into the water space above it. It works by heating up when its tip is no longer submerged in water, indicating that the levels are too low. As it overheats, its sealant melts, allowing it to become a simple hiss, much like that on a teapot. This is considered a last resort, as such a low water level at this point would already prove dangerous.
The danger lies in the heat of the steam engine’s combustion chamber, particularly that of the combustion gases. They can reach up to 1000°F (550°C), which is dangerous for 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 happens, the entire machine can explode as a result.
Richard Trevithick invented the threaded fuse plug in 1803. After one of his own boilers blew up during a water leak, he intended to make a detection device that could offer a means of prevention. He was especially invested in the project because of his advocacy of high-pressure steam engines. Due to his invention, this form of steam engine was approved and is still in operation today.
Although useful, threaded fuse plugs can fail. If the sealant oxidizes or scales as it ages, it can damage the plug’s reliability. These conditions can increase the melting point of the metal, causing it to not react when heated. Some fuse plugs did not activate even after being exposed to temperatures over 2000°F (1100°C).
Due to this threat, standards have been established to prevent these conditions. In some areas, fuse plugs must be replaced after thirty to sixty days of use. In others, the limit is 500 hours.
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