A pressure relief valve is a means to control pressure that may build up in a system, be they from expansion due to overheating of a liquid, or a pressure drop resulting from water hammer, or even a collapsing or disturbed part of the system. The parts of a pressurized system are built to resist pressure, up to a certain value, because a column of liquid flowing is often the result of pressure differentials. If a vacuum forms, for example, the pressure spike my be such that parts of the system fail, break, or are damaged, rendering the entire system either useless or nearly useless. In order to prevent this, the pressure may need to be relieved. Relief of pressure generally requires that the vacuum, or the sealed system be ejected into the atmosphere, which is far less pressurized. Release into the atmosphere disperses the built up pressure. This is how pipes often burst: the need to relieve pressure.
A pressure relief valve takes the principle of relief and makes the function manageable, by directing the pressurized liquid or gas out of a prepared exit, one that can resist the sudden expulsion of pressurized liquid or gas without bursting. To do this, the resistance it produces against the pressure must be less than other parts in the system, pressure following the path of least action.
So the pressure relief valve is a valve usually with a spring and a seal over a seat and an aperture that opens into atmosphere. The spring is made to be as elastic or as rigid as the maximum pressure that the system can withstand. Thus pressure is always acting on the spring, but the spring is always too rigid for the seal to leave its seat. Once pressure is great enough to reach the maximum value, the spring’s elasticity requires that the spring compress and the seal be lifted from its seat. When the seal is removed from its seat, the liquid or gas in the pressurized system is ejected from the system into the relief valve and out the aperture as the pressure is dispersed into the atmosphere. Once this pressure is stabilized to below the maximum value that the system can withstand, the seal is reseated because the spring extends.
As a word of caution one must be careful with a pressure relief valve. The liquid or gas ejected from a pressure relief valve may be either highly pressurized, very hot, or both, depending on the system in play. Because of this, safety ought be exercised by anyone or anything nearby, when and if the valve is relieved.