Sciencemadness Discussion Board - Keeping the vacuum pump clean

Keeping the vacuum pump clean

Jacob - 18-12-2019 at 11:44

Well, this is rarely if ever discussed.

How do you keep solvents and volatiles away from your vacuum pump?

The oil in my rotary vane pump turns into milk every couple of vacuum distillations.

I have a glass cold trap. Maybe filling it with mineral oil and putting it in ice will work.

12thealchemist - 18-12-2019 at 12:13

Quote: Originally posted by Jacob

I have a glass cold trap. Maybe filling it with mineral oil and putting it in ice will work.

I have no experience of using a vacuum pump outside of university (where I use them daily - Schlenk line work), so I can't help too much with the hobby side of things.

That being said, a trap is key to protecting the pump and prolonging its life. I don't know how cold one needs to go; I've always used liquid nitrogen, but I hear dry ice/acetone also works. If you can't access dry ice (I can't outside uni), ice and salt will reach -18°C at best, and ice and calcium chloride hexahydrate will reach -40°C at best. I imagine this latter is good enough to catch the vast majority of your volatiles. I don't know how good the other options are.

Jacob - 18-12-2019 at 12:30

Quote: Originally posted by 12thealchemist

Quote: Originally posted by Jacob

I have a glass cold trap. Maybe filling it with mineral oil and putting it in ice will work.

My bad. What I have is a gas washing bottle not a cold trap. Not that it makes a difference since ice and salt is the best cooling available right now. I'll try activated carbon or mineral oil.

unionised - 18-12-2019 at 12:52

Do you know what a gas ballast valve is?

wg48temp9 - 18-12-2019 at 13:42

In case the above comment was lost on you, below is some info about using the air ballast on a vacuum pump.

"It is possible to throughput some of these contaminants from your system and through the pump and that are where the gas ballast valve comes into play. If you do not require high vacuum less than say 300 micron for instance, then you can open up the gas ballast valve during the evacuation procedure and get some of these bad molecules out of the pump through the exhaust port of the pump. Remember also if they are coming out the exhaust they potentially are entering your workplace. This may pose a hazard either health based or fire based. If you are looking for better vacuum pressures, the valve is adjustable. This feature is not the total answer; it helps, but should not be considered the solution in problem applications with lots of contaminates.

If and once the pump is contaminated you can again use the gas ballast valve to assist in purging some of these contaminants from the vacuum pump oil in which it now resides. Close the pump inlet port off to full vacuum and allow pump to run/actuate with gas ballast valve in open position. This again will help purge the pump. Take a long lunch or let the pump actuate overnight with the valve open. Care again should be considered to the exhaust stream. If the pump is "smoking" or exiting oil mist in good quantity, you may come in the next morning to a room full of oil mist. Piping to a hood or using a coalescing filter like the HyVac Capture filters can help eliminate this problem area."

Bezaleel - 20-12-2019 at 02:58

wg48temp9, when purging the pump oil, do you recommend to close the vacuum inlet and use the gas ballast in order to avoid excessive oil mist? Usually, immediately before I switch of my pump, I let it pump air through the inlet for half a minute or so in order to clean the inlet tubes. This also cleans the pump's vacuum chambers.

My personal experience is that using the gas ballast is of essential value to preserve your pump. Even pumping on just water will corrode the interior of your pump over time. Let alone if you need to pump on filtrates containing HCl and the like.

The idea behind using the gas ballast is that you allow the second vacuum chamber of your pump (the one with the really low pressure) to receive a little air. This prevents contaminants from condensing and mixing with the pump oil, because they will remain mixed (dissolved) in the air which came in through the gas ballast valve.

And I can confirm that the HyVac Capture filters are a must also. Since I use them my work space retains a clear sky.

DavidJR - 20-12-2019 at 08:32

I use a chemical resistant diaphragm vacuum pump precisely to avoid this. You don't get as good a vacuum as a rotary vane pump, but for my purposes it has been fine, even for Schlenk line work.

wg48temp9 - 20-12-2019 at 09:50

Quote: Originally posted by Bezaleel

wg48temp9, when purging the pump oil, do you recommend to close the vacuum inlet and use the gas ballast in order to avoid excessive oil mist?

Yes that is the recommended procedure as described in my previous post. Ideally it is done for a few hours with the pump hot.

What I used to do after evacuating a large chamber (100l) I would connected to the outlet of the high vacuum pump to a fridge compressor operating as a vacuum pump. The high vacuum pump would have its inlet isolated. The high vacuum pump was the type that have the exhaust port connect to the space above the oil chamber. So the fridge compressor could pull a vacuum on the high vacuum pump oil and remove any water vapour or volatile contaminate very quickly compared to running the high vacuum pump. It was possible to observe the pump oil bubbling thru the oil level sight glass and use the observation to determine when the bubbling had stopped and the process was complete.

Bezaleel - 23-12-2019 at 03:41

That's a clever idea, wg48temp9. Thanks for posting it here!

My rotary vane pump has exactly that configuration (I think most RV pumps do) - the exhaust chamber is right above the oil level, only separated by two plates, in order to avoid oil droplets from getting into the exhaust chamber directly. And to allow any oil collecting on the surfaces of the exhaust chamber to flow back into the oil bath.