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Compressor oil in the air lines is not good because it indicates that you have a developing problem with your air compressor pump, and it also isn’t good to have compressor oil in the air lines because the oil that lubricates the air compressor pump is not a suitable lubricating oil for other compressed air equipment, including your air tools.
The compressor oil in the air lines will migrate down the lines to all your air using equipment. Depending on what the parts inside the air tools are made of the air compressor oil may interact with them, making seals brittle, or swelling them to the point where the tool may not work properly.
Having compressor oil in the air lines is even more of a problem if the compressed air is flowing through an air filter or lubricator and then on to a high-cycling air valve, many of which may contain plastic components and all of which will likely react negatively to the additives in the compressor lubricating oil.
Caution: Compressor lubricating oil may react with plastic filter bowls
The additives that are in the compressor lubricating oil attack some plastics. Over time, the oil can affect the elasticity of the plastic, increasing brittleness of the filter bowl or lubricator bowl, and causing crazing, cracking, and possibly contributing to bowl failure.
Having a plastic filter or air line lubricator bowl explode under 120 PSI of pressure is not a pleasant thing to be around.
If you have compressor oil in the air lines – and it’s hard to think that any oil lubricated air compressor won’t send some oil into the air – and you have a filter or lubricator with a plastic bowl, we would always recommend installing a metal shroud over the bowl. The metal covering has slots to allow you to see into the bowl without removing the cover, and in the event of a bowl failure, the covering will help to trap plastic debris if the bowl shatters under pressure.
Compressor oil in the air lines and compressor pump problems
Industrial air compressors like rotary screw and vane types have oil separators as part of their standard equipment. Their purpose is to strip any oil used in the compression process and removing that lubricating oil from the air stream before it gets into the lines.
The small workshop air compressor is quite often a reciprocating type with one or more pistons inside of cylinders. The pistons move up and down (reciprocate) inside the cylinders as the compressor runs, alternatively pulling air into the compression chamber and then driving that air into another cylinder or down into the tank.
Many of the lower cost (under $350 or so) oil-lubricated air compressors are lubricated via the splash method. Essentially, moving parts and vibration of the pump splash lubricating oil from the oil sump onto the moving pump parts, including up under the piston and onto the cylinder walls.
What keeps that lubricating oil from getting into the compressed air compression chamber is the piston seal. This wraps the piston, typically moves with it, and as the piston reciprocates inside the cylinder, that piston seal wipes the walls of the cylinder, driving the oil back down into the sump as the piston cycles, keeping the bulk of the oil from getting past the piston and into the compression chamber.
When it is new, that piston seal is very capable of wiping the cylinder wall and keeping almost all of the oil in the sump. When (and that is when, not if) the piston seal wears due to air compressor use, more oil can wick by and into the compression chamber. As the seal wear continues the amount of oil getting into the air increases, and you end up with an increasing amount compressor oil in the air lines, with the concerns noted earlier.
This also generates the need to closely monitor the sump oil level to ensure the pump is adequately lubricated as more and more oil leaves the sump and enters the air going into the compressor tank.
Resolving the problem
Wicking past the piston seals is pretty much the only way that compressor lubricating oil can get into the compressor tank, at least on reciprocating type air compressors.
The upshot of that is the only remedial action is to tear down the pump and replace the seals.
The tendency is to try and get OEM parts which is almost impossible for an air compressor that has been purchased from a discount or big box store.
One source for piston seals is the compressor repair company that your store would send a compressor for repair under warranty, if they do. Check with all the stores that sell compressors and ask where they would get the compressor fixed under warranty. That shop may be able to help getting seals.
There are companies that specialize in providing seals as well, and some research on line will locate those companies. I suspect you will need to be able to provide details of the seal you are trying to replace, so the downside is that your compressor will be pieces while you are searching for a seal replacement, if you haven’t been able to find a seal kit before tearing down the compressor.
One additional note; we would suspect that if the piston seal is worn, then your valves have been well used as well, and since you have the pump apart anyway, we would recommend that your replace the valve plate and the gaskets at the same time.