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.
Willyr, I have a GMC AC24L compressor. Recently it took a tumble out of the boot of my car and cracked the outlet manifold. I picked up a replacement with all new fittings on ebay and replaced the damaged one. I needed a heat gun to remove the old manifold due to use of locktite I believe. So, new switch, new gauges, new PRV, etc., but when I turn the compressor on the PRV almost immediately starts to release air. The compressor DOES build up to around 9 bar and then shuts off but the PRV is releasing air continuously and the compressor soon starts up again. If I turn the compressor off, all the air escapes from the PRV. Any ideas of what could be wrong … or what I could have done wrong?
If the PRV has been damaged it may remain open. With the compressor off, however, air should not exit the tank to escape through the PRV. Sounds like you have a failed tank check valve, and a broken PRV. Try rinsing the PRV in solvent, letting it dry, manually cycle it a few times, then reinstall to see if that makes any difference in the cracking pressure.
Willyr, the PRV is brand new. it came with the replacement manifold. I have two other PRVs – one, the old one of the cracked manifold and another off a new compressor that I bought when this one failed. All three leak air. What is the tank check valve and where would I find it?
It’s description, function and typical location are found here: http://fix-my-compressor.com/compressor-tank-check-valve/
Willyr, I removed the check valve and it looked as clean as a whistle, replaced it but the problem still exists. Anything else I could look for?
To recap then, when the compressor is running air is escaping the PRV regardless of the tank pressure, and the the compressor stops, all the air bleeds out of the tank through the PRV, is that correct? Are we, for sure, talking about a PRV… or is it possible that we are talking about the unloader valve? Regardless, if air is escaping from either the unloader valve or the Pressure Relief Valve when the compressor is stopped, the only place that air can be coming from is the tank, and that means that the tank check valve must be failing.
My compressor, when it shuts down, does not release the pressure off the high side and goes into a stall like situation when it tries to kick on. It acts like it is under pressure. If I drain the tank it runs with no problem. Do I need to replace the the pop off or check the back pressure valve? Any suggestions?
Sure. I doubt it’s the PRV that is the problem. I suspect it is the unloader valve that is not working. Please see: http://fix-my-compressor.com/the-air-compressor-unloader-valve/ for more information on them.
I have a Rol Air compressor with a GP series 63 pump. It is powered by a Honda 13hp engine. The compressor runs fine, but runs the pressure to 120 and pops the safety valve. I tried turning the regulator knob to no avail. Is it the unloader?
Al, the regulator is there so you can dial down the air pressure to the hose and to your air tools. It really has bearing on the PRV. Is it the unloader? In a gas powered air compressor the pressure switch and the unloader work together to “unload” the compressing of air when the tank pressure reaches cut out, while leaving the motor running at a lower RPM. If your air compressor continues building air past the normal throttle down and reaches the pressure level where the PRV has to crack open to vent over pressure, then yes, we too think that your unloader valve needs adjustment. Start by giving it a good cleaning and lubricating where you can. Sometimes that’s all it takes to correct this type of compressor problem.
DOES YORK AIR COOLED SCREW COMPRESSOR MODEL MTS45SBAD46/240 HAS A PRESSURE RELIEF VALVE……IF YES WHERE CAN I FIND IT ON THE COMPRESSOR AND HOW CAN I TEST IT IF IT IS WORKING PROPERLY OR NOT….PLEASE HELP