TL;DR

High oil level in the separator means oil is not returning to the compressor. Check the oil return line and float trap, then verify the evaporators are not oil-logged.

What you might see

oil separator level high alarm
oil carryover to refrigerant system
oil return line temperature lower than expected
evaporator performance declining

Likely causes

Tap to mark as ruled out

Oil return float trap stuck closed preventing oil from draining back to the sump
Oil return line partially blocked by a foreign particle or valve restriction
Oil viscosity too low at the operating temperature allowing excessive carryover
Compressor running at reduced capacity sending less gas flow through the separator

Required tools

Refrigeration gauges
IR thermometer (for oil return line check)
LOTO kit
PPE: refrigerant-rated gloves and face shield

Safety first

Ammonia refrigerant is toxic and flammable. Work on the refrigerant-side oil return system requires a qualified refrigeration mechanic and appropriate PPE including supplied-air respirator for high concentrations.
The oil separator and compressor discharge are at elevated pressure and temperature. Confirm system pressures before disconnecting any oil line.

Procedure

7 steps · ~30 min

1
Check the oil separator level sight glass or electronic level reading. Confirm the alarm is genuine.[1]
2
Locate the oil return line from the separator to the compressor suction or sump. Check the line temperature. A cold return line indicates flow is blocked.[1]
3
Inspect the oil return float trap. Manually operate the test bypass valve to see if oil flows. A stuck-closed trap is the most common cause.
4
Check the oil return line for any closed isolation valves.
5
Verify the compressor oil injection temperature is normal. Low oil temperature increases viscosity and can prevent return flow at the float valve orifice.
6
If the evaporators have been running with elevated oil level for an extended period, plan an oil drain from the evaporator drain connections.
7
After clearing the oil return path, monitor the separator level for at least one hour to confirm oil is returning.

Sources

Frick (Johnson Controls) Frick RWB / RXF screw Industrial Refrigeration / Blast Freezer general technical documentation, Frick (Johnson Controls)
Frick RWB / RXF screw compressor general oil separator and oil return system procedures (general)

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