Rotary screw compressor has oil in exhaust air

by Tony


I have a single rotary screw compressor. At the beginning it worked just fine without any problem.

Now i noticed that when i drain the reservoir tank from water, there is lots of oil mixed with the water.

After further investigation i noticed that when the compressor is loading, the oil from the sight glass disappears into the system, but when it is unloading, the oil returns to the sight glass. Also the oil has air bubbles when it returns. I am adding 1 liter of oil to the system every 24 hours of use.

I was able to find a broken one way valve that feeds oil to the unloading valve. I replaced the broken valve but same symptoms still continue.

Any advice?

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Apr 16, 2015
by: Carl

When was the last time the seperator filter replaced? This filter should be replaced every 4000 running hours or every 2 years. I would replace it then see if there is a difference.

Im a little confused about where you say the scavenge tube is? You say it is before where the air is compressed?

The scavenge tube goes inside the seperator filter and sucks up (scavenges) any oil that may creep through the seperator filter. There is also a tiny filter in the scavenge line which should be checked and cleaned, if it is blocked then you will have oil go through into your system.

But ideally you should have your seperator filter replaced

Apr 14, 2015
Hi Carl and Doug
by: Anonymous

Well the thing is that i actually have a reference another exactly the same compressor. Only difference is that in the broken one the oil viscosity is bit higher. And in this other compressor the oil level remains in the sight glass as it is loading and no bubble so be seen.

I already opened the oil separator and it looks perfectly fine to me. I dont know what exactly to look for, but i couldnt see any physical damage in it.

Also i want to make a correction to my previous post, the broken valve was located in the scavenge tube just before where the oil mixes with oil.

Model of the compressor is Landward OGLC 11 kW/13 Bar, but its only being operated in 7,5 Bar.

Will also double check the scavenge tube for any abnormalities.

Thanks in advance

Apr 13, 2015
This is normal
by: Carl

Hi Toni,
Where the oil is in your compressor, that is the same place as where your air goes when it is compressed, so bubbles in the oil is perfectly normal, the same as when the oil disappears when the compressor is running, ie when compressed air is in the seperator tank (where the oil is) it pushes the oil down so looks like there is no oil, again, very normal whilst in use.

With screw compressors you should check the oil when it hasn't been used for a while (first thing in the morning)

What isn't normal is that you have oil in the receiver and that you have to top the oil up regularly. This could be caused by a problem with the separator filter.

Like I said previously, the air is compressed in the oil tank, it then goes through the separator filter where the oil is separated from the air, then the air goes to your air system. If your separator is either old or has a hole in it then oil will get into your system or go into your air receiver.

So I'd advise you to have your compressor serviced including your separator filter being replaced.

Also have your scavenge tube checked out.

Hope that helps

Apr 13, 2015
oil/water separator . . .
by: Doug in

Screw compressors usually have one.

You got a model number, or at least make?

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