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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.


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2 years ago

A couple of years have gone by. Both of you have forgotten what a vacuum pump LOOKs like. Have any goodies for a beginner?

I have 3 vacuum pumps. 2 industrial 26"Hg 1960's variety and a 2 stage Chinese reverse engineered. I too wish to simply play with them.

The 2 stage pump heats up the oil quite a bit. It is supposed to use [HFV 32?] a synthetic oil. So, I remove the oil from that and pump it through a gasoline filter using dry CO2 gas. Any large spots of water and dark grey stuff is left behind.

Next, I make sure it is heated around 155 F and place the filtered oil in a glass jar with a hose barb in the lid to a 1/4 OD hose back to the old pump. The water in the oil starts boiling. I run for a few hours keeping the fluid at 155 F until there are no bubbles in the oil. Then I pour this oil into another cleaned [spaghetti sauce] jar and seal it with a lid. It looks clear and doesn't have an emulsified look.

So, I was thinking this could be done simultaneously with heating/vacuuming that apple. If I place a one way valve after the food vacuum tank I believe that should stay fresh? (Turn off pump by first opening air intake behind food chamber. Air rushes into chamber then through vacuum hoses then into pump.)

Next I'm thinking of using the outgas from the vacuum pump to push some oil from the pump into a vacuum oil cleaning chamber. I guess I only want the oil line to the chamber and back to open when the pump has enough pressure to push some oil. Prior to opening that line, the vacuum draw line from the vacuum oil cleaning chamber should be closed.

I would guess there are simple ways to do this any ideas? I have 3d printers.


Reply 2 years ago

I like your thinking :)
Although I don't use as much anymore I had some more quality time with my pump since writing the above.
Most people will use a vacuum pump for the purpose of evacuating something, be it the freezer pipes or a vacuum chamber to get air free casting resin.
And here only water and dust or particles are your enemy.

Oil will get used no matter how good we clean it.
The moving parts contaminate it and it also acts a medium for heat transfer.
A lot of stress...
I noticed that after several cleaning rounds due to fixing a flooded airconditioner without being smart enough to put a dryer in the line the pump got warmer than what it should.
Another indicator was the struggle to reach the max vacuum the pump was able to reach when new.
Using the spoiled oil after one more cleaning round to clean out the pump was a good use.
The flushing with hot oil (about 60°C) got a lot of fine debris and still some water out.
With new oil filled in the pump was back to normal operating temps.

A while later I noticed the pump struggled to hold a high vacuum.
Sometimes it would get too hot and at other times it stared to fill the oil filter on the outlet with oil drops instead of fumes.
Couldn't find any listed spares for my china import so I took it all apart.
The slots for the vanes still had some stuff accumulated and stuck the walls.
Cleaning them properly allowed for a smooth movement of the vanes, which also needed some TLC.
Got courious and decided to order two slightly different sets of replacement vanes.
All same thickness and lenght but different "heights".
To my utter surprise even the oversized one that I though would never fit did fit and rotated fine in the housing!
The smaller set still had far more material than my used ones, so I used those first.
Pump reached a higher vacuum and faster than what it ever did...

Only weeks after this great improvement the pump started to struggle again, this time with not reaching full vacuum and creating a lot of air inside the pump.
The seals started to fail, so I replaced both seals and bearings....

As for your re-using idea:
Try what I did for a while...
Make a nice particle filter for the intake side.
If in doubt a tube filled with fine cotton wool will do.
I prefer rock wool is I can just dry it out by flame or in the oven if I need to.
You might be surprised to see what can be sucked out of clean air ;)
Another good addition is a long dryer rod.
A thick walled perspex tube is prefered but anthing with some vieport at the end will do.
Filled with silica gel beads mixed with some indicating ones a lot of water will be absorbed.
Once the viewport or end shows discolartion is time to dry the beads in the oven.
For the dreaded fumes and fine oil mist coming out I found that some backpressure can be really helpful.
Especially during the start the pump throws out a lot.
I used a vinyl tube to connect to a little catch can filled with stainless steel wool.
As I had no clue what would work best I placed a needle valve as the outlet for the catch can.
Starting from fully open bor both the valve and the pump intake I closed the valve slowly until the pump moise changed slightly.
A test with a jar showed the time to reach max vacuum was basically the same.
Let it run for half an hour and opened the catch can.
Definately some oily residue at first sight.
Closed the valve a bit more and then the pump showed a noticable slower start speed under no load conditions.
Not much but you can hear it.
From there on the amount of oil ending up in the can was reduced to an amount I was happy to consider waste.
After 10 oil changes I only had about 5ml of oil that I could salvage from the can.

A combined heating and vacuum treatment certainly works but it also brings the risk of too much boiling and foaming.
You would need a quite large vacuum container for this to prevent oil foam from getting into the pump stream.
I found that a funnel seperator and some spare oil work almost as good as all heavy duty cleaning.
Leave the oil in the seperator and forget about it for a while.
You end up with clean oil and ina perfect worls a filthy layer at the bottom.
But even with another fine layer on top the rest is the same.
Once there is a fully clean and clear oil layer drain the crap and the drain the clean oil into a suitable container, stop before the possible top layer runs through.
With about 220ml drained off the pump I end up with the water plus less then 5ml of undefined something as real waste.
A laser can help too.
If the beam stays "clean" the oil is seperated.
If it diverts into a "plasma beam" it means there is still fine debris or fine water in the oil.
Just compare with a sample of new oil ;)


4 years 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.