How the scavenge line keeps your compressed air oil free

This is the first post in the ‘part of the day’ series. In each post I will discuss another air compressor part. I will explain what they do, how they work and how to troubleshoot when there are problems.

The part of today is:

the scavenge line.

The scavenge line is found on all bigger oil injected rotary screw compressors and plays a crucial part in the removal of oil from the compressed air. Most people know that the oil separator filter is the part that separates the oil from the compressed air. But that is only part of the story.

The oil separation process consists of three steps. I have added a picture with some beautiful hand-drawn extras to explain the process. 🙂

1) Centrifugal separation.

Oil separation on rotary screw compressors
Oil separation on rotary screw compressors

The air/oil mixture from the screw element outlet enters the separator vessel at an angle. The air/oil mixture flows in circles around the circular inner surface of the vessel (think of an tornado). The air/oil mixture enters the vessel at a pretty high velocity, but will lose momentum quickly.

To facilitate this ‘tornado action’ and to protect the oil separator from direct contact with the incoming air/oil mixture, there is an oil screen or splash screen installed.  See the green arrows in the picture where the oil screen is located.

When the air/oil mixtures loses velocity, the oil falls down into the oil sump, while the compressed air finds it ways to the exit. This process removes already 95% of the oil from the air. (the blue dots is oil 😉

2) Oil separator filter.

The compressed air that finds it way up reaches the oil separator element. The 5% oil that is left is in the form of tiny little droplets (but lot’s of ’em).

The oil separator is made of a special fabric that makes the oil droplets stick together to form bigger drops.

These drops become so big that they separate from the air flow and drip down to the bottom of the separator filter. In time a pool of oil will collect on the bottom of the separator.

3) The scavenge line

The scavenge line sucks up this oil from the bottom of the separator filter. The oil is sucked back to the air compressor element.

The scavenge line actually consists of two parts: the scavenge line and the scavenge pipe. The scavenge line is the flexible hose coming form the vacuum side of the compressor element to the top of the separator vessel. Inside the separator vessel it continues as the scavenge pipe.

The scavenge pipe hovers about a centimeter above the bottom of the oil separator.

Sometimes there is a filter and/or sight glass installed in the scavenge line. Often there is also a check valve installed where the scavenge line connects to the separator vessel (see ‘CV’ in image).

When things go wrong..

There can be a lot of sources for the common ‘oil in compressed air’ problem, but since this post is about the scavenge line, I will focus on this part.

Common problems:

  • The scavenge line or scavenge pipe is blocked or plugged with dirt.
  • The scavenge pipe is bent so it doesn’t reach the bottom
  • The scavenge pipe is too long, it touches the bottom.
  • The scavenge line filter is dirty

All of these causes has the same result: the scavenge line stops sucking up oil from the bottom of the oil separator.

Result: the oil level in the oil separator rises. At some point you will see lot’s of oil carry over to the compressed air system.

  Do you have oil-in-air problems?

While I really appreciate any comments on my blog, for specific help with your air compressor problems, please go to my air compressor troubleshooting section.



My name is Cas. I’ve worked in the compressed air business for years and saw the same mistakes over and over again. This website is my answer. The best way to get started is to download my free e-book “Hidden Compressor Problems”.

Articles in 'Stationary Air Compressors'

18 thoughts on “How the scavenge line keeps your compressed air oil free”


  2. Hi, Cas
    I read your post How the scavenge line keeps your compressed air oil free on Air Compressor Guide. Today there is a need of oil water separation techniques to save our environment from pollution. Thanks for sharing this valuable information. Keep writing i want to read your other posts also.

    1. That’s correct. In most/many countries, you’ll need an oil-water separator to remove any oil from the condensate, before discharging the condensate in the sewers.


  3. Dear sir
    I have a problem with a rotary screw compressor, when the power goes off I have a lot of oil coming out of the air intake. Also the rotor does a few turns reverse.

    1. If your compressor model has a check-valve below the compressor element, check that.

      Never stop your compressor while it’s running loaded… Always switch to ‘unload’ before stopping, to prevent problems like this.


  4. Okay Mr Case. I understand that part. But my question is what causes the scavenge line to stop suck when the line and pipe is not block?..

    1. Sometimes the scavenge pipe inside the separator vessel is too long (touches bottom of separator) or too short, bend, etc. Please check that also.


  5. Please i facing problems withe the compressor its saving the water withe oil and the oil level increases in the compressor the suprator filter keep the water inside the compressor the water cameing from the humidity weather what i can do is i know should d water go out with the air

  6. So if I have a scavenge line that is 100% clogged (as in not even air is getting through it) and that has been in it’s state for 4-6 months with no attention to it, and now I am having to add a couple of gallons of oil to it every few days to keep it running, then wouldn’t that point to the Separator probably being ruined at this point as well? Great post btw!

  7. Hi!

    Rigth now I’m currently upgrading a compressed air system, and I’m having issues regarding the condensate water/oil separators, I’ve been told that the amount of oil in my compressors blowdown its to high (168 ppm) it’s there any parameter that I can have on the correct amount of oil on the blowdown discharge?

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