Turbo pump spin down and venting

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Chris Bradley
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Re: Turbo pump spin down and venting

Post by Chris Bradley » Mon Aug 29, 2011 12:32 pm

I think you should want to leave the backing pump running, valve open, while operating the vent valve, else the intake gas ends up on the high-side of the turbo rather than taken out into the backing pump!

I suggest the sensible MOs are either;
- [without vent valve] isolate the turbo from the chamber, then the turbo from the backing pump, finally power off the turbo, sping down in its own time
- [with vent valve] isolate the turbo from the chamber, power off the turbo, open the vent valve, turbo spin down, close the vent valve, isolate turbo from backing, power off turbo.

Are there other alternative ways folks do this?

I seem to recall the vent valve on my V60 was open when unpowered, which was why I removed it and blanked the vent hole (else I'd have to occupy yet another power supply in its operation), so the flow rate was never high enough through the valve from 1 bar to damage the turbo. I could be wrong on that, but I can't recall why I'd have taken it off, if otherwise.

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Doug Coulter
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Re: Turbo pump spin down and venting

Post by Doug Coulter » Mon Aug 29, 2011 3:23 pm

The Pfeiffer system opens the vent valve on power fail. It's a slow controlled leak, so no danger of wrecking the pump. Yes, it's good to avoid backstreaming if you have an oil type forepump and want an oil-free tank, but IMO if you want an oil free system, the way to get there is not to have oil in it anywhere in the first place, as is easy to do with turbo-drag pumps that don't need much in the way of a forepump so can use diaphragm or piston (teflon rings) pumps for that. That's what I'm using at present here, but will be forced back into some oil forepumps to use the cheap turbo-only pumps we've been getting surplus - those need a lot lower foreline pressure to work well than affordable oil free forepumps can reach.

Getting to pressure in the foreline fairly quick (at least above the oil vapor pressure) while the turbo is still spinning is probably the best way to cut backstreaming in a power fail event..at least the amount, I kinda doubt you ever fully eliminate it without some fancy cryo trap.

Fusors, luckily, seem not to mind a bit of oil...other things care a lot more about it. If a fusor decomposes a little oil onto the grid into carbon, there's really no ill (or lasting) effect.
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