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No U-Trap on Discharge Line — Oil Backflow at Shutdown and Liquid Slugging on Start
Likely causes
No U-trap on vertical descending discharge section — oil flows back to compressor at shutdown
Check: Check if condenser is lower than compressor; immediately after shutdown, check for oil foaming or noise in compressor; verify discharge line check valve is installed and seals properly
What to do: Install U-trap at base of vertical descending discharge pipe (same diameter, height 300+ mm); install or replace check valve (no leakage); verify crankcase heater operation
Start after long shutdown — oil and liquid refrigerant accumulation creates liquid slugging risk
Check: Verify crankcase heater pre-heat before startup (4+ hours if ambient below 10°C); listen for abnormal compressor noise or vibration during startup
What to do: If liquid slugging suspected, stop immediately; pre-heat crankcase heater 4–8 hours before restart; replace oil if heavily diluted
Discharge line U-trap height must be at least pipe diameter × 2 — minimum standard to prevent oil siphoning
All 565 cases work offline in R-Pro, with error codes for 62 brands and P-T data for 108 refrigerants.
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