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.
On 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.