Pressure Relief Valves
Pressure relief valves are common household safety devices. In fact one resides on the hot water heater in your home. Your automobile may also employ a similar safety device on the air conditioning system. If for some reason these products produce a dangerous high pressure, enough to injure persons or damage property, these valves will open and relieve that excessive pressure to outside of the closed sealed system.
Hot water heaters are typically a very reliable source for heating water in the home and commercial purposes. But every so often these gas and electric devices can have problems with the thermostat and control circuits that can cause the heater to "run away" or over heat. When this occurs the water inside the sealed hot water tank will begin to boil.
You may have witnessed this when boiling water in a covered pan on the stovetop. The boiling water will produce steam. The steam will begin to rattle the cover from the higher pressure inside the pan or vessel. The higher pressure inside the covered pan wants to escape from its sealed surroundings.
The same thing can occur in a hot water heater if the heating elements or gas heater does not shut off when told to do so by the controlling thermostat. The hot water piping and water heater tank can begin to build excessive pressure due to the boiling water inside the heater. This building pressure must go somewhere, either through an open faucet or by bursting a water pipe.
All pressure relief valves have an internal spring that holds a stop valve in place. This stop valve remains closed under normal pressure operating conditions. When the water pressure or steam pressure builds beyond the springs capacity to hold back the liquid, that internal pressure forces the relief stop valve to open and the liquid or steam can escape from the closed system.
Regardless of the application, the pressure relief valve operates on the same principle whether the material is water, oil or even air. Most applications for pressure relief valves are for excessive high pressures. There are some low-pressure relief valves that can be used as a warning identification system for critical operations such as aircraft hydraulics that aid in flying an airplane.
