Air compressors with one cylinder typically have only one pressure relief valve (PRV), so why are there two pressure relief valves on my air compressor?
Air compressors with twin cylinders, those with two pumping cylinders, will typically have two PRV’s.
It is common to have a PRV located near the pressure switch, one that is plumbed into the line coming from the tank to the pressure switch. This PRV monitors the pressure in the tank. It will typically look something like the PRV in the image below.
Should the compressor not shut off when it is supposed to for any reason, the pressure in the tank could rise to catastrophic levels. Long before a tank might burst due to over pressurization, the PRV will react to the too high pressure, open itself up, and vent tank air pressure, even while the compressor pump still runs.
Multi-cylinder compressor pumps
On a two cylinder (or those with more than two) two stage air compressor, air is compressed from the first cylinder through a line into the second cylinder. That second cylinder then compresses the air from the first cylinder even further, and drives it down into the compressor tank.
Both cylinders have intake valves and pressure valves. This to help keep the flow of air in a uni-direction, from the outside into and through the first cylinder, into the second, and then into the tank.
What if the air cannot flow to the second cylinder
Let us surmise for a second that something negative happens to the valve system in the second cylinder. Suddenly, for some reason, air can be pumped into that second cylinder from the first, but the failure means that the air cannot be pumped further along and into the tank.
The first cylinder is still pumping away, driving compressed air towards the second cylinder.
The second cylinder has become, effectively, a small tank.
The first cylinder continues to pump, and the pressure of the compressed air in the line between the two cylinders can quickly exceed safe pressure levels.
To ensure that an air pressure crisis does not occur if the secondary cylinder fails to allow air past it, there will typically be a PRV in the line somewhere between the two cylinders. That’s the second PRV.
That PRV will blow off once the pressure in the line between the two cylinders reaches the pressure setting of that pressure relief valve.
A potentially dangerous situation will have been averted.
The same scenario occurs if there are more than two cylinders, each pumping air from the preceding cylinder. All should be protected by PRV’s in the event the air flow path gets blocked for any reason.
Test your PRVs
Test your compressor PRVs regularly to help ensure that they will work when they are supposed to.
Many PRVs have a ring on the end. While the line or tank has pressure – and wearing a work glove – pull out on the ring to release air, and then push it back in. That helps ensure that the PRV has not become contaminated with airborne crud to the point where it won’t open when it should.