Thermal expansion causes pressure to increase in hot substances, which can be dangerous. Thermal relief valves allow excess pressure to escape, and are required for safety. They work based on pressure, triggered by temperature. Early valves were simple, and modern valves vary in size depending on the substance. They are commonly found in systems that rely on hot water, as water-based pressure is more dangerous than steam-based pressure.
As a substance increases in temperature, it also increases in size, which causes a subsequent increase in pressure when contained. This process, called thermal expansion, could easily destroy equipment based on hot steam or water. To counteract thermal expansion, people use thermal relief valves. These valves allow excess pressure to escape from a system rather than causing an explosion. A thermal relief valve is common on many household and commercial appliances, as the law requires its use as a basic safety precaution.
Although it’s called a thermal relief valve, it works based on pressure. As the temperature increases, the internal pressure also increases: it is this pressure that triggers the valve. The thermal relief valve came into common use with the steam boiler, a machine that uses heat to turn water into steam, which creates a positive pressure within the boiler. This pressure pushes the steam out and past a wheel, which spins from the force of the passing steam. The turning generates power, and the steam is contained, allowed to cool, and returned to the boiler.
These machines were prone to overheating and explosion if not monitored closely. Early thermal relief valves allowed water to escape when the pressure inside the boiler got too high. Early valves were very simple, usually relying on a plug that was forced up and down when the pressure got too high. Even though it was very rudimentary, this is still the basis for many modern thermal relief valves.
A thermal relief valve varies in size depending on whether it releases water or steam. A water release valve is usually much smaller than a steam release valve. This is because pressure is based on quantity. In a steam system, most of the volume is taken up by air, while in a water system it is just water. This means that to clear the same volume of material, much more air must be removed.
Most of the time, a person will find a thermal relief valve on the part of a system that relies on hot water. Although parts of systems that run on hot steam also generate excess heat and pressure, they are less likely to have complete valve systems. When the air is pressurized enough, it returns to a liquid state; in liquid form, it takes up less space, which takes the pressure off. It takes a significant amount of pressure to get water back to a solid state, far higher pressure than most equipment. This generally makes water-based pressure more dangerous than steam-based pressure.
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