Booster Trips on Low Oil Pressure After Every Restart
A 300-hp ammonia booster compressor serving a -40°F blast freezer trips on low oil pressure about 20 minutes after every restart. The freezer is being pulled down after a defrost. Logged at the trip: suction pressure 1.5 psig, discharge 175 psig, suction superheat at the compressor 1–2°F, oil level fallen from mid-glass to the bottom of the sight glass, and the oil foamy at 110°F although the OEM requires a minimum 130°F oil pot temperature. The recirculator runs a 3:1 overfeed and its liquid level controller was replaced last week; the evaporator fans were rebalanced at the same time. The oil pot heater is energized and its thermostat checks good. The booster has already been restarted three times in two hours.
1. Give the most likely mechanism that ties all of these symptoms into one chain, and name the two measurements you would take to confirm it before changing anything.
Read the worked answer
Mechanism: wet return causing refrigerant dilution of the oil.
Build the chain from the data:
- 1–2°F superheat at the compressor suction is essentially saturated vapor - the machine is being fed wet gas, not superheated gas.
- Wet gas carries liquid ammonia into the crankcase, where it dissolves in the oil. Ammonia-saturated oil loses viscosity and load-carrying ability, and it foams as it off-gasses in the crankcase.
- Foamy, diluted oil means the oil pump cannot build a stable pressure differential, so the oil pressure safety trips - roughly the 20-minute interval seen after each restart.
- The oil pot at 110°F instead of 130°F confirms the oil is refrigerant-loaded: the heater is doing its job, but it cannot boil off that much dissolved refrigerant while the machine keeps being re-wetted.
- The recent work is the likely cause: a replacement level controller set too high, miscalibrated, or wired with the wrong action will overfill the recirculator, and rebalanced fans raise the evaporator duty and carryover at the same time.
Two confirming measurements:
- Superheat measured at the evaporator outlet and at the compressor suction with a calibrated thermometer cross-checked against the saturation temperature for the measured pressure (they should agree within a couple of degrees if the separator is working).
- An oil sample drawn from the bottom of the crankcase and allowed to flash at atmospheric pressure - bubbling/fizzing and a strong ammonia odor confirm dissolved refrigerant; also compare the oil level in the sight glass with the oil charged.
Also read the recirculator level directly on the sight glass rather than trusting the new controller's display.
2. List your immediate actions in priority order for the next 30 minutes. Justify the ordering.
Read the worked answer
Order is driven by 'stop the damage first, diagnose second, restore third.'
- Stop the booster and pump it down; stop restarting it. Every restart with no oil pressure grinds bearings and crank surfaces. This is the only irreversible damage path on the list, so it goes first.
- Lock out the starter and verify oil pressure differential with a second, independent gauge. Confirms the trip is real and not a failed switch or a plugged sensing line.
- Drain the diluted oil, change oil and filters, and inspect the drained oil for sludge and metal. Dirty oil must not be circulated back through bearings.
- Check the oil strainer, oil pump suction, and oil pot heater/thermostat. A 110°F pot with the heater energized is a symptom, not the cause - verify the thermostat setting and that the heater element is actually drawing current.
- Verify the recirculator level against the sight glass and check the new level controller's setpoint, calibration, and fail action. Correct the setpoint to the OEM value before any restart.
- Restart only under supervision with the freezer unloaded, watching suction superheat, oil level, oil pressure differential, and oil temperature for the first 30 minutes.
Do not restart to 'see if it clears up' - the third restart already told you it will not.
3. What permanent correction do you make, and what acceptance criteria tell you the fix worked?
Read the worked answer
Permanent correction - fix the wet return at its source, not at the symptom.
- Recalibrate or re-range the recirculator level controller and set the liquid level to maintain the vapor space the low-pressure receiver needs for separation. If the controller is the wrong type or has the wrong fail action (e.g., fails open on loss of signal), replace it with one that fails safe.
- Verify the overfeed ratio is actually at the OEM value at the evaporator, not just at the pump - a correct pump ratio with a flooded separator still produces carryover.
- Inspect the separator's mist eliminator/screen for damage or blockage; a fouled mesh passes droplets.
- Reset the oil pot thermostat so oil temperature holds 130–140°F during operation.
- Re-check evaporator airflow after the fan rebalance; if airflow was raised, the load and carryover both rose and the separator must be re-verified at the new duty.
Acceptance criteria for the restart:
- Suction superheat at the compressor steady in the 8–12°F range (or the OEM value) for at least one full hour at rated load.
- Oil level stable in the sight glass and oil free of foaming.
- Oil pressure differential comfortably above the trip point and flat, not drifting.
- Recirculator level holding at setpoint with the pump running, and no liquid visible returning to the separator beyond normal.
If superheat stays low after the level is corrected, the separator is undersized or damaged for the new load and that becomes the next engineering issue.