The scavenge line (also called the oil return line) removes collected oil from the bottom of the separator element and returns it to the compressor. If this line is blocked, oil accumulates in the separator and eventually gets carried out with the compressed air.

What you'll see

Oil in the compressed air even though the separator element is relatively new. The scavenge system works like a vacuum cleaner -- it uses the pressure differential between the separator vessel and the screw element inlet to suck oil from the bottom of the separator back into the compression cycle. If the scavenge line is blocked (carbon buildup, kinked tube, clogged orifice), this recirculation stops and oil accumulates until it overflows into the air stream.
Before you assume this is the problem

If the separator element is old or has high hours, replace the separator first. A clogged separator causes oil carry-over regardless of the scavenge line condition. See: Dirty Separator Element.

See all causes of oil in compressed air →

How to diagnose

  1. Locate and inspect the scavenge line

    The scavenge line is a small-diameter tube or pipe that runs from the bottom of the separator element to the screw element inlet (or just before it). It may include a small orifice (restrictor) and a check valve or sight glass. Look for kinks, damage, or disconnections in the line. The orifice is a very small hole (typically 1-2mm) that can clog with carbon or debris.
    Result: Kinked/damaged line = replace. Need to check orifice for blockage.

  2. Check the scavenge orifice

    Disconnect the scavenge line and blow through it. If blocked, clear the orifice with a thin wire or needle. If the line has a sight glass, observe it during running -- you should see a mixture of oil and air being drawn through. If nothing moves, the line is blocked.
    Result: Flow through line = OK. No flow = blocked.

How to fix it

  1. Clear or replace the scavenge line

    Clean the orifice with a thin wire. Blow the line clear with compressed air. If the line is kinked or damaged, replace it. Check that all connections are tight. After clearing, verify operation by watching the sight glass (if fitted) during running -- you should see oil being drawn through.

Common mistakes

Do not enlarge the scavenge orifice thinking it will help. The orifice size is calibrated for the pressure differential and oil flow rate. Too large an orifice allows too much air to bypass through the scavenge line, reducing compressor efficiency. Clean it, do not enlarge it.

Parts & tools

Thin wire or needle for clearing the orifice. Replacement scavenge tube if damaged. Compressed air for blowing clear. Basic hand tools.

Review safety precautions before starting →

Safety

The scavenge line connects a high-pressure zone (separator) to a lower-pressure zone (element inlet). Depressurize the compressor before disconnecting.