My compressor slowly overpressurizes

I've got a Quincy 325 turned by a gas engine. When it reaches 125 psi the unloader works both the intake valves and the engine drops to idle. A while ago the pressure started to creep up after reaching 125 and I need to stop it. It's gotten slowly worse over the last year. Took the valves and unloaders apart and cleaned them up with no change in the problem. I run this compressor for hours on end for breathing air and don't want to operate it if it's not working right. Anyone solve a similar problem?

Reply to
ogyver
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On 21 Oct 2005 23:32:45 -0700, with neither quill nor qualm, snipped-for-privacy@gmail.com quickly quoth:

Since I've never seen the unit, here are two SWAGs:

1) Have you tried adjusting your pressure switch? Knock it down to 100 and see if it still creeps. 2)Are the intakes opening enough to prevent ALL compression, and is the unloader adjustable for that? It may have worn a bit and isn't opening them enough.

What does Quincy say about it, BTW?

Reply to
Larry Jaques

Is the tank located near a heat source?

Wes

Reply to
clutch

Unloaders aren't opening far enough. Probably the unloader pistons are sticky, or maybe the pins are worn down short so that the valves don't lift off the seats much. Or the governor is acting up. I was 12 years as foreman in a truck/equipment air brake machine shop. We rebuilt 17,000 compressors in that time. I would estimate that half of them were there because of governor or unloader problems, both cheap to fix, but the mechanics tended to replace the compressor, stick the old governor on it, and then complain that we built crap. Kind of like alternators and regulators; almost always the regulator is at fault when the alternator won't charge, but the electrical rebuilders make lots of money rebuilding alternators that are perfectly fine.

Dan

Reply to
Dan_Thomas_nospam

I refinished the brass unloader pistons, I thought sticking pistons might have been the problem, but it wasn't. The brass unloader pistons stick up about 1/8" pass the top of the tower where the steel caps sit on them so it's only being plunged about that 1/8". I thought that was pretty marginal but that's where the o-ring of the piston tops out on the seat on the tower. There's about a 1/4" of play on the pins that depress the washer seal so it seems there might be something up with that, but I don't think the brass unloader piston has been pounded shorter. I did have to shave some length off the steel cylinder that the brass unloader sits on in order to not have to use 3 aluminum gaskets under the tower. It seems the unloader pistons or the pins ( they look about the same length as new ones) would have to be shorter. I don't have a new unloader piston to compare. Which have been the most problem for you?

Reply to
ogyver

Adjusting the pressure doesn't change the problem.

The 2nd stage unloader vents fine. The first stage unloader doesn't vent at all. I thought the first stage pressure cycle was sucked in by the second stage intake and it vented there. I had both unloader vents plumbed into the first stage intake tap in the head to keep mud dobbers from building nests in the tower holes. Is the first stage not venting a normal thing or is it giving me the problem?

Reply to
ogyver

It's over the gas engine, but it always has been.

Reply to
ogyver

The unloader pins should be just long enough that the intake valves don't touch them while the valves are at work and the unloaders are retracted. The unloaders should be able to lift the valves far enough to hold them tight against their upper stops. Bad governors would either not release the unloaders all the way, holding the pins in contact with the valves, or would apply the unloaders slowly. Either way, the pins would get pounded shorter and the pistons would dance in their bores and wear out the o-rings. The leaking o-rings would cause a pressure drop in the governor when it unloaded the compressor, causing it to think that the tank pressure had fallen and so would re-engage the compressor. It would cycle on and off every two or three seconds. In extreme cases, the unloaders would get busted to pieces and the bits would get sucked into the cylinder; end of compressor. Sometimes crud would accumulate in the bottom of the unloader piston bores and keep them from lowering all the way. If the pressure is creeping up, the valves are able to get onto or very near their seats while unloaded. You will have to try to figure out why they aren't being held al the way open. Dan

Reply to
Dan_Thomas_nospam

Thanks for the help, I'm going to dig into it further this week. The governor I haven't looked at but I've had problems with everyone of them, now that you bring it up. Even put a petcock at its low point to keep the moisture drained from the thing.

Reply to
ogyver

Replaced the valves (washers), springs and adjusted the unloader piston to where it put a little pressure on the pins. I guess there was just enough wear on the pins and piston to cause the problem. Problem fixed. Thanks for the help.

Reply to
ogyver

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