Traces of oil at the end of the hoses, paint that craters, an oil level dropping faster than usual: oil is getting into the compressed air. In an oil-injected screw compressor this is a signal to take seriously, but the cause is usually easy to find.
Why a screw compressor contains oil
In an oil-injected screw compressor, oil is injected into the screw element to lubricate, seal and cool. It comes out mixed with the air, then the air/oil separator holds back almost all of it and returns it to the circuit. A tiny amount of oil, as very fine droplets, always remains in the air: that is what precision filters are for. The full working principle is explained in how a screw compressor works.
The most frequent causes
The air/oil separator is at the end of its life
This is the number one cause. A saturated cartridge lets oil through; a clogged cartridge makes the machine strain. It is replaced during services: see air/oil separator: role and replacement.
The oil level is too high
Over-generous topping up floods the separator, which can no longer do its job. The level is read on the sight glass, using the method in the manual.
It is the wrong oil
An oil not designed for screw compressors, or a mix of two different oils, can foam and pass into the air. See which oil for a compressor.
The oil return is not working
The oil collected at the bottom of the separator cartridge must return to the circuit through a small line. If it is blocked, oil builds up and leaves with the air. This is a check for a technician.
The machine runs too little or too cold
A compressor running very short cycles never reaches its operating temperature: water mixes with the oil, which turns milky and loses its properties.
Outlet filtration is missing
Even a compressor in perfect condition leaves a little oil in the air. For painting, food or blowing clean parts, you need suitable precision filters, changed on time. See fine filters and air quality to ISO 8573-1.
What to do
- Check the oil level in the sight glass, machine stopped, as described in the manual.
- Look at the hour meter: is the service including the separator overdue? Our service kits group the parts by model.
- Check the outlet filters: clogging indicator, date of last change, drain.
- Drain the network at the low points to remove oil already present.
- If oil keeps passing, stop the machine: a compressor losing its oil risks overheating and damaging the screw element. Make a request from the warranty and service page.
Other symptoms are covered in our compressor troubleshooting guide.