Essentially, a PRV should be installed anywhere there is a need to relieve pressure for any reason.
The hot water tank in your home has a PRV. It is set up so that if the pressure inside the water tank gets too high, the PRV will open and vent a stream of hot water and steam.. This usually means all over your floor!
Nevertheless, the PRV in the hot water tank will ensure that the tank connot over-pressurize - a critical safety feature.
Air compressors are supplied equipped with a pressure relief valve on the tank or between cylinders in mutli-cylinder air compressor models. Always be sure that the receiver, air tank or reservoir into which any compressor pumps air, has a working PRV. Please see the video below.
When your air system calls for air, your compressor cuts in. The compressor pump gulps in free air through an intake filter and scrunches that air into the compressor tank. It continues this process until the pressure inside the tank reaches the cut out pressure level. At this time, your compressor should stop.
Compressed air is explosive when it is released suddenly. Being around if a compressed air tank should disintegrate due to unsafe high pressure levels inside would be unhealthy, with the tank opening up like a bomb!
To help ensure that this does not happen the compressed air receiver has a PRV installed that has access to the air pressure inside the tank. The PRV might be part of the check valve, pressure switch and PRV assembly, where the air line from the compressor head enters the air tank.
Many compressor PRV 's look a bit like this.
There is a ring on the end of the PRV which, when pulled, will activate the PRV and allow air from the receiver to vent to atmosphere.
Some PRV's rely on "sticktion"; the friction between its parts, to generate enough internal resistance to keep the PRV from cracking open to vent air. Other PRV's have an internal spring performing the same task.
In both cases when the pressure inside the tank reaches the PRV set point, the air pressure acting on the piston inside the PRV overcomes the "sticktion" or the spring pressure, forcing the piston out, and opening the flow path from the air tank to atmosphere, venting over pressure.
While I have never had a PRV on a receiver open due to over-pressure inside the tank, having the PRV there provides a critical and mandatory additional line of defense against over-pressurization.
The PRV is a cheap insurance. Having a working PRV in an air circuit backs up the pressure switch and, in extreme cases, will prevent a catastrophic over pressure and failure of a compressor tank.