LeRoi/Dresser Rand compressor second stage prv pops off at 70lbs

by Michael
(Washington State)

LeRoi/Dresser Rand compressor

LeRoi/Dresser Rand compressor

LeRoi/Dresser Rand compressor
LeRoi/Dresser Rand compressor
LeRoi/Dresser Rand compressor


I have a LeRoi/Dresser Rand compressor with a 20hp 3 phase motor with a 200 gallon tank, model 770A. I have replaced the o-rings and felts on the valves and resealed the caps. I removed the air lines and found the secondary leaking so I resealed all of them. While running it will pump up to 70 lbs and then the PRV will pop on the high pressure secondary stage cylinder. Can you explain to me why this might be happening and what is needed to do? I also see that the PRV is mounted on the intercooler off the high pressure head if that matters.



Comments for LeRoi/Dresser Rand compressor second stage prv pops off at 70lbs

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Jan 14, 2017
Interstate Relief Valve Lifting.
by: LarryK

If your interstage relief valve (70 psi) is lifting it indicates that you either have a leaking HP Suction valve or a leaking unloader plunger.

Most likely the Suction valve since you renewed the plunger felts and o-rings.

The unloader plunger only gets air pressure if the air control valve on the rear of the compressor is 1) leaking by or 2) is sensing a low oil pressure. It will also get air from the constant run unloader pilot valve when the compressor gets to the pilot valve set pressure.

LeRoi Compressors don't usually have tank check valves.

Jan 07, 2014
check valve
by: Doug in s.d.ca

I realised we did not say much/anything about the check valve to the tank. Is it closing?
If not, that could be the problem, possibly.

Jan 05, 2014
by: Doug in s.d.ca

OK. I'm just gonna have to think out loud here, as not having any luck finding a (free) diagram of this thing.

If I'm reading this right, you can run and get 70psi into the tank, then the prv trips.

If that's right, then that means the intake and exhaust valves are not completely stuck closed.

It also means that at 70psi the intake secondary is either stuck open, or the exhaust is failing to open, leaving the secondary pressurized, which makes the prv open.

If the exhaust were failing to open, then you would not be getting 70psi in the tank.

So, that means the secondary intake is probably not closing.

That's all I got, sorry.

Jan 03, 2014
by: Michael

Doug, all I did was replace the felts and o-rings in the suction valve assemblies. When I took the air hoses off of the suction valve caps, I felt air coming out of the secondary showing I had a leak and that's when I replaced the felts/0-rings. I bought this compressor in an auction and so far I've had it wired in, plumbed and ran to 70 lbs. I have not changed anything else except for a new diaphragm in the unloader. Could a suction valve or discharge valve be causing my issue? Right now i'm running my air tools and lifts when needed off of my Craftsman 3hp 50 gallon compressor but it needs to be retired. Any help is appreciated.

Jan 01, 2014
by: Doug in s.d.ca

Anything is possible, but I'd double check whatever you did to the second stage to stop leaks - perhaps you got something sealed that's not supposed to be? If gasket(s) were involved, maybe on backward? Sometimes they'll seem to fit, but be blocking something inappropriately.

Dec 31, 2013
by: Michael

I've researched my PRV and find it is set to go at 70 psi so it is working correctly. Where do I go to test for a blockage? Could the intercooler cause this problem or is it more likely a valve issue?

Dec 29, 2013
by: Doug in s.d.ca

For starters, I'd look to the prv - you can test it with another source of air and a regulator to see if it goes off at the correct pressure.

If that's ok, you likely have a blockage into the secondary.

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