Double Riser Not Applied — Oil Stagnation at Partial Load

Question

Accepted Answer

Single large-diameter riser only — low velocity at low load causes oil stagnation. Check: Measure suction line velocity at inverter compressor minimum capacity (10–30%) — below 2 m/s means double riser needed; check for compressor oil hammer noise after morning startup What to do: Install double riser: large pipe (4–8 m/s at full load) + small pipe (3.5 m/s at minimum load) in parallel; U-trap at base of large pipe (seals large pipe at low load, oil circulates through small pipe); U-trap at base of small pipe Oil stagnation followed by sudden large inrush — compressor oil hammer. Check: After night low-load operation, check for compressor noise, vibration, suction line vibration at morning startup; check for sudden oil level rise in compressor What to do: Stop immediately → install oil separator to improve oil return rate (70%+) → install double riser for root cause fix; routinize morning oil level checks

Double Riser Not Applied — Oil Stagnation at Partial Load

Likely causes

Single large-diameter riser only — low velocity at low load causes oil stagnation

Oil stagnation followed by sudden large inrush — compressor oil hammer

Related cases