Your air compressor may have just one compressor PRV, or it may have more than one.
The compressor PRV is the compressor Pressure Relief Valve. It does exactly what the name suggests. It’s purpose is to relieve pressure.
Why does the typical home, workshop or industrial air compressor need a compressor PRV?
We will have a look at how they work first, and then we will discuss the scenario where a compressor PRV comes into play.
In the photo above the pressure relief valve has a split ring installed on one end. The other end, the threaded end, is turned into a port in the plumbing of the air compressor. Once installed the PRV can “read” the pressure of the plumbing being protected. That plumbing could be the compressed air in the tank, air in the line from the tank to the pressure switch and regulator, and the air line on down to the discharge coupler on the compressor.
Once the pressure relief valve is threaded into a compressors plumbing, it is ready to react should the pressure in that plumbing exceed the “shift point” or “set point” of that particular PRV.
Compressor PRV Settings
The set points of a PRV are factory established. The compressor manufacturer has the PRV they use sized to provide the protection that the manufacturer of the air compressor wants to establish for their compressor air systems.
Therefore, not all PRV’s operate at the same set point. If you need to replace the pressure relief valve on your air compressor, you must ensure that the new valve settings are as close to the original settings as possible, to ensure that the margin of safety the PRV will provide for the compressor is comparable to the old one.
PRV pressure settings are not field adjustable. If your PRV is opening prematurely, that is, at a pressure below where it blew off originally, the PRV has failed and should be replaced before the compressor is used again.
What is inside a Compressor PRV
Inside the PRV will be some sort of piston – a surface area against which the pressure inside the compressor plumbing will push.
As long as the pressure inside the plumbing is below the force needed to move the PRV internal piston, the PRV stays shut, and the compressed air stays inside the compressor plumbing and compressor tank.
The piston area inside the PRV might be held in place against the plumbing pressure by simple friction, or there may be some sort of spring involved to keep the PRV closed as long as the pressure in the plumbing is low enough.
Where is the Compressor PRV on the compressor
Well that can certainly vary. Typically though, the PRV is threaded into a port under or near the pressure switch as shown in the photo following.
Quite a number of DIY or small workshop air compressors are sold with the pump, motor, and much of the plumbing hidden behind a plastic or metal cover. In that case, the PRV normally protrudes through the cover and the pull ring is easily accessible.
Compressor PRV Test
The PRV has a ring on one end deliberately. It is designed to be pulled. When the ring is pulled the pressure relief valve is forced open and compressed air from the air tank is vented to atmoshphere. Of course, if there is no air in the tank, pulling on the PRV ring will not vent air, but it will move the internal components, and that is a good thing.
Over time your compressor PRV can become bunged up. Crud gets into its workings, and may contribute to jamming. If that happens, the PRV will not open at the air pressure it should, and that could be dangerous.
Get in the habit of, periodically, pulling on the ring when there is air in the tank. That way you can tell if the PRV is actually opened, as when you pull on the ring compressed air will vent. Push back in on the stud, and the air will stop venting, and the PRV will be closed. Push / pull the PRV ring a few times every month or so to ensure that the internal piston moves freely.
Warn others before you pull the ring, and prepared for the venting noise when you do. I can be quite loud. You might consider wearing work gloves too.
When will the Compressor PRV be needed?
There are two safety systems on the typical air compressor to keep the tank and plumbing from becoming over pressured, with possibly catastrophic results.
The first one is the compressor pressure switch the purpose of which is to turn the compressor on when the tank pressure is low, and shut the air compressor off when the tank pressure reaches the correct pressure.
Should the pressure switch fail, the compressor will not shut off at the high pressure cut out, and the tank pressure will continue to rise, possible to the tank burst point.
Before the pressure reaches that level, the PRV should blow open and vent the air, even though the air compressor will still be running, to atmosphere. This ensures that the pressure in the tank will not reach a dangerous level even though the compressor may then continue running until it overheats and stops.
A PRV that has let go is a signal that you need to shut down the air compressor until you resolve the issue as to why the air compressor failed to stop at the normal high pressure cut out. The first check is to see if the compressor pressure switch is working.
So, the first safety system on the typical air compressor is the pressure switch and the second is the PRV.
Never run your air compressor unless both of these are working properly.
Why more than one Compressor PRV?
Some air compressors have more than one PRV. Typically, that will be a two-stage air compressor that has more than one cylinder in the pump. In a two stage air compressor, air is pre-compressed in the first cylinder. pumped into the second cylinder, and then compressed air as it is forced into the tank.
The air line between the two cylinders is, in effect, a miniature tank.
If for some reason the air flowing from the first cylinder cannot get into the second cylinder, or get through the second cylinder to the tank, the pressure in the line between the two cylinders can quickly rise to dangerous levels.
That is why it is the norm for multiple-cylinder, multiple-stage air compressors compressors to have more than one PRV to protect the system.