Applicable Systems
QSI/QGV Series
Symptoms
Minimum pressure valve (MPV) is leaking or stuck open, causing sump pressure to drop below 4.0 bar (58 psi) during loaded operation. The compressor cannot deliver rated flow to the plant.
Possible Causes
- MPV spring fatigued or broken, failing to hold the poppet closed against blow-back
- MPV seat scored by debris or corroded by acidic condensate
- MPV piston stuck in the open position by oil varnish or rust in the bore
Troubleshooting Steps
- Step 1: Isolate the compressor and run a load test while monitoring sump pressure
If sump pressure cannot reach 4.5 bar within 45 seconds, remove and inspect the MPV - Step 2: Disassemble the MPV and inspect the poppet, seat, and spring for wear or damage
Replace the spring if free length is <95% of spec; lap the seat if scoring is minor - Step 3: Inspect the MPV bore for scoring, rust, or varnish buildup
Hone the bore to original dimensions or replace the complete MPV cartridge if wear exceeds tolerance - Step 4: Check the oil condition - acidic or varnished oil accelerates MPV wear
Change oil and separator if oil condition is poor, and establish a shorter oil change interval
How to Reset
Reset: After MPV repair or replacement, run a load test and verify sump pressure reaches 4.5 bar within 30 seconds. The alarm auto-clears when pressure is maintained for 60 seconds.
Common Replacement Parts
- 2023-4020-00: Minimum pressure valve kit, QSI 200-300 (Spring, poppet, seat, O-rings)
- 2023-4021-00: MPV cartridge assembly (Drop-in replacement, pre-tested)
- 2023-4022-00: MPV seat lapping compound (Fine diamond paste, 3-micron, 5g tube)
Frequently Asked Questions
- Q: What pressure should the MPV hold closed?
The MPV must remain fully closed until sump pressure reaches 3.5-4.5 bar. Below that, any leakage indicates a worn seat or weak spring.
- Q: Can a bad MPV cause high oil consumption?
Yes. Blow-back through a leaking MPV forces oil-laden air back through the separator and out the breather, increasing oil consumption by 50-200%.
Source: Quincy QSI Series Operation and Maintenance Manual, 2023-OM-100
Category: Compressor