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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. In order to counteract thermal expansion, people use thermal relief valves. These valves allow excess pressure to bleed out of a system rather than cause an explosion. A thermal relief valve is common on many home and commercial devices, as the law requires their use as a basic safety precaution.
Though it is called a thermal relief valve, it operates based on pressure. As temperature increases, so does internal pressure—it is this pressure that cases the valve to trigger. The thermal relief valve entered common usage with the steam boiler, a machine that uses heat to turn water into steam, which creates a positive pressure inside the boiler. This pressure forces the steam out and past a wheel, which turns from the force of the passing steam. The turning generates power, and the steam is contained, allowed to cool and put back into the boiler.
These machines were prone to overheating and explosion if not monitored very carefully. The earliest thermal relief valves allowed the water to come out when the pressure become too high inside the boiler. The early valves were very simple, usually relying on a plug that would be forced up and out when the pressure became 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 based on whether it releases water or vapor. A water relief valve is usually much smaller than a steam release valve. This is because pressure is based on quantity. In a steam-based system, much of the volume is taken up by air, while a water system is just water. This means to get rid of the same volume of material, a lot more air needs to be removed.
Most of the time, a person will find a thermal relief valve on the portion of a system that relies on hot water. While portions of systems that run on hot steam will also generate excess heat and pressure, they are less likely to have full valve systems. When air is under enough pressure, it reverts to a liquid; in liquid form, it takes up less space, which removes pressure. It takes a significant amount of pressure to cause water to revert to a solid, a pressure far in excess of most equipment. This generally makes water-based pressure more dangerous than steam-based.
When air is under enough pressure, it reverts to a liquid. In a steam system, it's not the air (if there is any inside) that reverts to a liquid; it's the steam itself. Steam is water in gaseous form.
"In liquid form, it takes up less space, which removes pressure." It does not remove pressure because pressure is the only thing keeping it in liquid form. Remove the pressure, and you get steam (with constant energy above water's saturation temperature).
"It takes a significant amount of pressure to cause water to revert to a solid, a pressure far in excess of most equipment." Not really, considering modern car radiators allow water to remain a liquid even at temperatures above 212
degrees. A radiator cap is only designed to hold, typically, around 12-16 psi. The pressure will depend on how much energy the system has.
"This generally makes water-based pressure more dangerous than steam-based." Not really. As soon as the water comes out of the pressurized system, it becomes steam in atmospheric pressure. There are other factors that come into play that make them both equally as dangerous.
Also, I wouldn't want to operate a solid system (water under pressure) without some form of steam volume. Steam is compressible, it gives you an expansion volume. Water is not. A small temperature change in that solid system results in a drastic pressure change, enough to lift a relief or damage the system.
There is a large difference between a pressure relief valve and a thermal relief valve, as the name suggests!
Although they can be combined into one valve, a thermal valve will operate at a preset temperature regardless of pressure to discharge the hot fluid, introduce a cooling supply, or even quench a solid fuel fire.