Failure to vent a rotary vane vacuum pump?

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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AllenWallace
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Failure to vent a rotary vane vacuum pump?

Post by AllenWallace » Sun Dec 06, 2015 3:55 am

I know that the pump oil will eventually suck back up into the vacuum chamber if you fail to vent the system. I've seen a turbo pump with oil in the outlet port.
The question I have is: how quick does this happen? Minutes? Hours? Days?
if oil floods the vacuum system and you turn on the pump, will this damage the pump? After all, it's designed as a vacuum pump and not a liquid pump!
What happens with a flooded pump and the chamber is already under vacuum? Will the pump clear itself? or will the back pressure prevent the oil from pumping and the oil cavitate?

Jerry Biehler
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Re: Failure to vent a rotary vane vacuum pump?

Post by Jerry Biehler » Sun Dec 06, 2015 9:17 am

Most pumps have anti-suckback valve and they do work 99.999% of the time. The issue is what happens that .001% of the time. Most pumps can sit with vacuum on the inlet for years and never have anything go back up the line. Chances are what you saw in the pump inlet was from oil vapor back streaming, not actual oil filling the roughing line.

A couple things you can do, vent the line. This can be done automatically with a small N.O. solenoid valve on the inlet. You see this method on SEMs. Shut the machine down and the valves close on the SEM and the backing line gets vented.

Another option is an isolation valve. Varian makes one that works off of atmospheric pressure to actuate it. It is connected to the vacuum pump's power and closes the valve when the pump looses power, I have installed them on systems and they work well. http://www.ebay.com/itm/like/1516438444 ... ps&lpid=82 These are different than standard electric vacuum valves where they will close when they loose power, most electric solenoid style vacuum valves open on power loss. You can use a pneumatic one if you have compressed air available.

If a pump does backfill a turbo you are pretty screwed. If it has standard greased bearings they will probably have to be replaced the grease in the bearings will be washed out and the bearings contaminated. It is very, very hard to clean bearings to new specs. If it is a maglev turbo, I dont know. It might be able to be cleaned out and the landing bearing might have to be replaced.

If it backfills into a diff pump you just clean it out and fill with new oil.

Don't try to pump oil back through the inlet of the roughing pump.

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