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Cleaning the oil for a vacuum pump Answered

Was too lazy to do an Instructable about it and think a lot of pics or even videos won't help much if you know what I mean ;)

Some of us use rotary vane pumps not for the purpose of evacuating refrigeration systems but for all sorts of fun and experiments.
This means quite soon or often we face the problem of the oil taking in a lot of water or even worse particles and solvent fumes.
I don't know about you but I was getting sick and tired of replacing the expensive oil every few weeks or sometimes even days if something got too wrong.

There are many different blends of compressor oil out there that will work very well in our rotary vane pumps.
The main difference is whyt the oil is designed for.
Some are perfect for aircon systems, others for the work with solvent fumes and there are even those special oils that bind moisture.
Unless you really need to evacuted special gases or solvents basically any low viscosity oil will do us just fine!
So instead of paying 20 bucks for special compressor or even vacuum pump oil we can select the cheap everyday oil.
Well, not exactly...
We also want to be able to recycle our oil to save even more money.
That means we don't want an oil that binds to water to keep it out of harms way.
We also don't want any oil that has special coating abilities for example these oils claiming to reduce wear and tear on your engine.
But any other low viscosity mineral oil or if you prefer synthetic oil will do - just stay away from silicone based oils!!

If you have not used your pump for a few days you can often see a slude at the bottom of the viewing glass.
If the rest of the oil is clear you can simply drain this worst bit and top up with fresh oil if required.
This simple procedure saves you a lot of oil already, at least if your pump has some rest every now and then.
Once your oil looks wasted it is time for the recycling and cleaning:

Release the oil into a high glass jar or these facy spaghetti glasses.
Fill with fresh oil and give it a short run.
Release this oil as well and wait for it to properly drain.
You now have the inside of your pump nice and clean again, time to fill one last time with fresh oil to keep using the pump.
The filthy oil we now have in our jar should be covered with some fine cloth or filter paper and placed somewhere warm.
After a week or two the oil, filth and water will have seperated and you pump, drain or siphon out the now clean oil for further use.
Don't be too exact here trying to get all the oil out, just remove what you can without risking to suck in the filth from the bottom.
Simply leave the rest in the jar and add the next oil change to it for the same recycling process.

In some cases we will work with solvents and that means the oil might bind to them.
Acetone for example is quite nasty here and can change the oil itself by breaking down certain components.
In most cases it means the viscosity will be reduced, which is not really too bad for us.
What is bad however is the fact that these solvents often refuse to fully seperate or evaporate.
Once the oil looks clean do a smell test and if it smells like solvent then for sure there is solvent in it.
Next step to confirm is to do a lube test.
Simply place on some smooth metal or glass and smear it with your finger - a drop is enough here.
If it feels sticky on the surface, gives you a rubber like feeling when sliding over the surface or is far less "slippery" than the fresh oil you also have a problem.
I found that filling this contaminated oil into a proper container and applying a strong vacuum will remove all solvent residue in a very short time.
Downside is that the oil in the pump is contaminated again, so it pays off to collect solvent contaminated oil seperately and once you got enough for several refills use the pump to get rid of the solvent.
When done do another smear and smell test, if still smelly repeat if no longer smelly but still the same bad feeling on the surface: Discard as the oil might be broken down by the solvent.



1 year ago

Be careful with using non-vacuum oils in pumps. Vacuum pump oil is a specific type of oil, a low vapor pressure oil.


Regular oils are formulated for operating at regular atmospheric pressures, but might outgas, or release vapor when the pressure drops too low. Vacuum pump oils are formulated to not outgas even at extremely low pressures. While conventional oils may "work" in vacuum pumps you may find it will not reach as high of a vacuum as it could with real low vapor pressure oil. Conventional oils can also become more flammable at low pressures, and can combust. In terms of cleaning water out of the oil, you may be able to get away with pouring the oil over a desiccant like silica gel, the silica gel will trap the moisture, and possibly you may have cleaner oil, I gues depending on what the operating temperature of the oil is, you could try to heat it a little and evaporate the water, but use this as a last resort, and don't boil the oil. Hot oil is dangerous to be around.