You don't supply much details, but I assume we're talking about a rotary screw compressor here, because what you describe is a common rotary screw compressor problem.

This happens when the compressor stops. Pressure inside the compressor tries to find its way out.. and it does: back through the inlet valve and inlet filter.

It depends on the type and model of compressor what the problem is.

• The inlet valve. The inlet valve is design that it closes when air (and oil!) tries to flow back, it acts like a check valve.

It could be stuck or broken.

• Bypass valve in inlet valve is stuck or missing.

Some inlet valves have a bypass valve (for unload operation). Inside the bypass is a check valve, to prevent oil flowing back when the machine stops.

This valve is sometimes stuck, or completely missing.

• Oil stop valve.

On some compressors, there is an oil stop valve installed under the compressor element. This valve is a check valve that prevents oil flow back to the element when the compressor stops.

It could be stuck or dirty.

• Wrong usage.

When you stop the machine abruptly, for example by the emergency stop when the compressor is running full-load, you will get the same problem.

Good luck! Let me know if you need any more info!

Cas

I have the same problem on a quincy qgb-30. I've replaced the blow down valve, min pressure valve and both shuttle valves and oil still comes out the inlet. I cleaned the solenoid valve but didn't have a spare to try. Any other ideas

Hi Eric,

Maybe your inlet valve ('unloader valve') is stuck or closes too slow.

(Please be carefull dismanteling inlet valves, they (can) contain powerfull springs).

Depending on the design, it could also be that a vent-hole is plugged/dirty, which makes the inlet valves close slowly.

Good luck. Let me know how it works out.

I took the inlet apart to inspect the diaphragm and it looked ok (no holes or tears). I don't think it's the solenoid valve because the machine modulates when it reaches full pressure and no oil comes out. It only happens when the unit stops. I was looking at control diagram and saw a check valve inline with the scavenge orifice. It's a possility that when the machine blows down air is looping back through the sump and pushing oil back through the inlet. I'm going to check that next.

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