The minimum pressure valve (MPV) opens too early or doesn't seal. Without enough internal pressure, the separator can't do its job properly.
What you'll see
The MPV is usually the last thing to check for oil carry-over. First rule out: separator element (most common), scavenge line, oil level, temperature, and oil type. If all of those are fine and you still have oil in the air, check the MPV. See: Separator Element Worn, Scavenge Line Blocked.
Could also be:
How to diagnose
-
Understand what the MPV does
The minimum pressure valve sits on top of the separator vessel. It has two jobs: (1) prevent compressed air from flowing back into the compressor when it stops (non-return function), and (2) maintain minimum internal pressure of about 2.5-3 bar. This minimum pressure is essential -- it keeps oil flowing through the cooling circuit, and it ensures the separator element has enough pressure differential to work properly. If the MPV spring is broken or weak, it opens too early at too low a pressure. -
Check the MPV function
With the compressor running, close the outlet valve slowly. The internal pressure should build up and the compressor should unload normally. If the MPV is leaking or opening at very low pressure, you might notice the sump pressure never builds up properly during startup or unloaded running. Also check: does compressed air flow backward into the compressor when it stops? (Listen for hissing at the MPV after shutdown -- there shouldn't be any.)Result: MPV opens at low pressure or leaks backward = rebuild or replace.
How to fix it
-
Re-kit the minimum pressure valve
Remove the MPV from the separator vessel. Disassemble it, clean all parts, replace all O-rings and seals. Check the spring -- if it's broken or significantly weakened, replace it. MPV overhaul kits from the manufacturer are inexpensive. The MPV is on top of the separator vessel on about 99% of all screw compressors -- you can't miss it.
-
Replace the complete MPV if re-kit fails
If the valve body or seat is scored or damaged, a re-kit won't fix it. Replace the complete valve. Make sure the replacement has the correct cracking pressure for your machine.
Don't overlook the MPV when you can't figure out where the oil carry-over is coming from. It's the least obvious cause and often the last one checked. The standard troubleshooting checklist for oil in air should always include: (1) separator, (2) scavenge line, (3) oil level, (4) temperature, (5) oil type, (6) MPV.
MPV overhaul kit (O-rings, seals, possibly spring). Or complete replacement MPV. The valve is located on top of the separator vessel.
This issue can also cause
- Compressor Won't Load Compressor runs but won't build pressure—the inlet valve isn't opening. Usually a control air or solenoid valv...
- Safety Valve Blows / Too High Pressure Safety valve opens or pressure exceeds setpoint, the compressor isn't unloading properly or there's a restrict...