Compressor PRV Bleeds Air

When the compressor PRV bleeds air, it could signify a number of issues with your air compressor.

The PRV is the compressor Pressure Relief Valve. It’s purpose is to bleed air when the pressure in the compressor circuit reaches the cracking – or opening – pressure level of that PRV.

The image is of a typical air compressor pressure relief valve.

Air compressor pressure relief valveOn the left side of the PRV is the male thread that threads into the port on the air compressor. The ring on the right is connected to the piston that is inside the pressure switch. The ring allows the operator to pull on the piston, opening the PRV, and ensuring that it is able to move as necessary. You pull out on the ring to test the PRV, and then push the end of the pin that the ring is attached to back in towards the PRV to shut off the flow of air.

One reason why the compressor PRV bleeds air

If your PRV starts bleeding air, it could mean that the pressure in the compressor system has reached the cracking pressure setting of the PRV, and the PRV has done what is supposed to. The PRV has opened to bleed down pressure in the compressor circuit before that pressure becomes dangerous.

That the PRV bleeds air in this scenario is good.

Note the pressure on the tank pressure gauge. If the pressure level where the PRV opens is higher than the normal cut out pressure switch setting, it does indicate that the compressor switch, the device that controls the ON/OFF of the air compressor depending on its pressure set points, likely has failed. You should replace the pressure switch before using the compressor again.

PRV Bleeds Air before normal cut out pressure is reached

If that happens, it normally indicates a problem with the PRV itself. That is, unless the PRV is in the line between two air cylinders on one compressor. See the next section if that is the case.

The piston inside the PRV is usually held in place by friction. It’s often a metal-to-metal installation, and the friction between the metal pieces makes the PRV piston resistant to movement until the pressure on the end of the piston is high enough to blow the piston off seat. If you have oil or water in the PRV that may offer enough lubrication to allow the PRV piston move sooner than it is supposed to.

With the compressor off and the tank empty, remove the PRV and rinse it in a solvent. Let it dry thoroughly. Pull out and push in on the piston a few times, then push the piston all the way in, and reinstall the PRV. Fire up the compressor.

If the PRV does not bleed air and the compressor stops at the normal cut out pressure, that may have resolved the issue. To double check the PRV operation, pull out on the pin to bleed a little air, and then push the pin back in. If the PRV does not bleed air by itself you are good to go.

 PRV between two cylinders

If the PRV that is bleeding air before the normal cut out pressure of the air compressor is reached is in the line between two cylinders, that typically suggests that there is something blocking the air flow from the first to the second cylinder. It may not be a PRV issue. However, do the remove, wash and reinstall process as noted above. If the problem persists, then look to the intake or pressure valve on the second air cylinder to be the possible problem.

Spring operated PRV

The compressor may have a PRV that is held closed by a spring. If it does, when you pull out the PRV piston to test the PRV, the piston should retract by itself when the ring is released.

Follow the cleaning process as noted earlier. If that does not resolve the issue, it is quite likely that the spring may be failing, and cannot exert enough force to hold the piston closed. Replace the PRV if this is the case.

Comments

  1. I have a Harbor freight compressor. When I power it on, the compressor only builds to 40PSI and then the releif valve purges the air? the compressor is rated for 120. I do not hear air leaking from the releif valve while its filling so I am comfused why it can not get past 40 before it has a fit and starts letting the air out

    • I relocated your question to a more appropriate page. Sean, you don’t indicate the model of Central Pneumatic compressor you have, but would I be correct if I guessed it was oil lubricated? If so, I suspect oil from the sump has bypassed the piston seal in such quantity that it has contaminated the PRV, essentially lubricating it, and now it cracks far sooner than it should. Power off, tank empty, remove the PRV and rinse it in solvent, let it dry, and reinstall. Did that alter the cracking pressure? It is possible too that the PRV has a spring close and the spring has lost it’s strength, though typically lower cost compressors don’t have a very complex PRV.

  2. I do have a Husky compressor model # F2S20VWD, 947265. This morning I started up and suddenly the release pressure valve came on. after that the compressor doesn’t stop any more when it reaches 150 psi.

    Now if I need to use it, I need to keep an eye on it and shut it off before the release valve comes on.

    Thanks in advanced for your advice.

  3. skip guindon says:

    I have a Snap-on with 110 hours on it. The PRV bleeds air continuously and there appears to be moisture in the oil sight glass. Any ideas ?

    • Skip, the two issues may be unrelated, or there may be a connection, it’s hard to know for sure.

      If the PRV is bleeding at a pressure below the normal cracking pressure, then it’s time to service the PRV or replace it. With the compressor off and the tank empty, remove the PRV and immerse it in varsol or paint thinner for a while. Let it dry thoroughly and reinstall it. Does it work now? If not, replace it.

      Water vapor in the oil sump sight glass indicates the presence of water in the sump. That’s possibly caused by high water vapor content air bypassing the piston seal, possibly due to running it too hot or for too long. If you haven’t changed the oil in living memory (:-0), time to do so we would think.

  4. Doug Bailey says:

    I have an old SpeedAire compressor that recently has developed a problem that seems to be a combination of several listed here. After changing 3 of 4 tires on my van the compressor suddenly did not have any air in the tank. The pressure switch did not cut in and I exhausted all my supply. Eventually it cut back in and then would run for a very long time holding pressure at 40PSI. After a long period the PRV started to release pressure even thought the tank gauge was only reading 40PSI and it would blast off the pressure and the gauge would rocket up past 120, then blast more air and the gauge would drop. It would do this for some time sounding like a machine gun…brap brap brap pause brap brap brap. I replaced the pressure switch and all things seems to be OK for weeks. Next time I did another set of tires the same thing happened. The pressure died off without cutting in, even with the new switch, and then suddenly the PRV was blasting air again. Seems like the gauges and the PRV were not experiencing any pressure then suddenly the tank was overfull and blowing off. Pressure from nowhere??? I’ve replaced the pressure switch and unloader but not sure what to do next. Oh, and once this happens the compressor resumes normally for a while and then trips the 30amp breaker? HELP!!!

    • If the PRV is releasing, then either the PRV is responding to over pressure, or the PRV itself isn’t working properly. Test it by pulling and releasing the ring, and maybe even, with an empty air tank, remove it and give it a rinse in solvent.

      Air gauges fail, so we would suggest that you swap them around a bit to be sure they are working properly.

      You say “The pressure died off without cutting in, even with the new switch, and then suddenly the PRV was blasting air again.” Not quite sure how that could happen if the compressor is not running.

      What is the normal cut in and cut out pressure setting on the compressor, and is the new switch set to the same?

      It kind of sounds to us like the compressor oil supply has migrated into the air stream, and fouled some of the components. Is this a possibility? Was the compressor moved or tipped over before this started?

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