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Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 12:51 am
by AllenWallace
I have see several references that you should not let a turbo pump spin down without venting first. They say to vent during the spin down. Why should this be the case? Are they worried that a decompression might stress the blades? and why should this stress be any different stationary vs spinning?

Re: Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 5:32 am
by Jerry Biehler
Whoa, you got it backwards. Spin down then vent. You can vent somewhere around 20% speed. Venting at full speed will trash the pump.

The only real reason to vent while it is running is to speed up cycle times. Remember, a whole lot of these pumps go into semiconductor manufacturing equipment where time is megabucks.

Re: Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 5:45 am
by Tyler Christensen
When spinning down small turbos at work, I usually close the foreline valve and then open a valve on the high vac side causing a bleed rate on the foreline meter of about 0.5 micron/second increase. This has it down to a low enough speed to vent to atmospheric about 3 times faster than if I just leave all the valves closed (about 6-8 minutes instead of 20).

I sure wish small turbo controllers typically had magnetic braking.

Re: Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 7:49 am
by Chris Bradley
My turbo has a 'leak' port on it for the purpose of decelerating the turbo - I though most did, actually.

But I just valve it off high vac side for a minute (so there is max vacuum infront of the turbo) then valve off the backing pump and let it spin down on its own, i.e. the opposite of what you are suggesting, I allow it to spin for as long as it likes. Is that bad?

It takes 11 mins 40 sec (+-10 sec) to come to a dead stop. I know that because I occasionally time it, as a means to monitor any bearing degradation, on the assumption that if they were degrading then the spin down time would become longer.

One thing I have wondered, though, and I don't suppose anyone can ever really know this - do you generate more wear on the bearings by turning it off then back on a few hours later, or just to leave it running?

The reason it might be worse in the former case is because the rotor has to cover the whole of its rotating speed range, and there will be resonances in there somewhere, which might cause more bearing degradation as the rotor passes those resonances compared with just leaving it running. I tend to dismiss this argument because if there were some sort of 'heavy load' on the bearings due to resonance, then the rotor would just slow down quicker at that point and drop out of the resonance - wouldn't it!?

If this is a genuine concern, I could then make use of the deceleration valve. But I tend to think this is there just for industrial purposes where they need to open up the chamber quick for the next 'load' of whatever they are processing. Does anyone have a 'definitive' answer on this point?

Re: Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 7:57 am
by Tyler Christensen
Most turbos at least have a location to put a leak system, however the leak isn't always actually present. On the turbos I've seen, the leak assembly and control is a separate purchase. I have never used this setup.

There is nothing wrong with letting a turbo spin down on its own with no bleed. This is actually the ideal way to let it turn off, as it gives everything minimal physical stresses. But a very small and controlled bleed is not going to take any significant life off of the pump (Think about the fact that turbo manufacturers even sell products to let you bleed gas in).

I would say there is more wear by turning it on and off versus leaving it on, I would guess particularly for the spin up as there is much more torque in that event than a spin-down. Most pumps do have an on-off cycle lifetime rating, however it is a high enough number that you shouldn't really worry about counting or limiting yourself. But in general I would leave it on rather than turn it off for no purpose. You also get to maintain a better base pressure by leaving it on.

As far as resonances, I have noticed this. One pump that I have been working with a lot over the last few months has a distinct point on the spin-down cycle where it makes a grinding noise for about one second. I can only imagine it is some resonance given the repeatability of the sound. I don't think that's any reason to feel a need to bleed though, it's not like the pump is going to rip itself apart being in some resonance for a second. After all, they are made to turn off, not just on.

Re: Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 1:15 pm
by Jim Kovalchick
Venting on the high (discharge) side at the point installed by the manufacturer will help take thrust off the bearing. Whenever you vent into a turbo pump system, no matter what side, you need to take care not to introduce foreign material. The system my son and I use has a screened centering ring on the KF16 vent port. We have a simple ball valve installed on it that we slowly open it as the pump spins down. The manual for the pump gives specific instructions on when and how to do this. Our Ebay purchased pump didn't come with a manual, but it wasn't too hard to find a pdf online.

Re: Turbo pump spin down and venting

Posted: Sat Aug 27, 2011 3:41 pm
by Doug Coulter
The only reason to vent is to get it spun down quicker (or perhaps make sure that the gas you let into the system isn't dirty shop air - I sometimes use argon or He). Always vent from the high pressure side if you do it, and yes, don't let in dirt! If you're willing to wait, there's no harm just letting it spin down gradually under no gas load. That can require some patience on a big pump.

I would suspect you need to be more careful with a straight turbo than with a turbo-drag design. If you hear noises, you went too fast, and the blade tips are making sonic booms in there and getting very hot, which is can melt them with the energy stored in the rotor doing that.

Tyler has a good point -- the turbo pump station controller I'm designing (for the cheap Pfeiffer turbos we got off ebay) will indeed have an electromagnetic brake in it - it's just silly they didn't do that themselves and it doesn't cost diddly to add the feature. In my case I'm generally spinning the thing down to run at fusor pressures, down to say 25% of the full speed, and I let in just a little D to get that to happen quicker, but that is risky if you don't have very good inlet control, and does waste gas that flows back into the fusor once it's spun down - and it still takes some minutes. It can also pressure the foreline to the point of making troubles there. These things have quite a compression ratio when spinning fast.

Re: Turbo pump spin down and venting

Posted: Sun Aug 28, 2011 3:59 am
by Jerry Biehler
The little pfeiffer on my portable RGA has an auto vent function. Though it does not work right. Might be a parameter that must be set in the head unit I don't have.

Re: Turbo pump spin down and venting

Posted: Sun Aug 28, 2011 4:16 pm
by Doug Coulter
The function in the controllers I have requires an external solenoid valve and I bought one - not cheap (a few hundred $). It's a little tricky to set up to do what you want (the Austrian version of instructions translated from Chinese by someone who didn't understand the device), and in any case that valve has such a slow flow, I'm considering using it for a controlled D inlet. Their rep told me it was meant for controlled shutdown taking an hour or so, and even that seemed optimistic when I tried it. I replaced it with a manual tiny needle valve I got at McMaster, which is more the right speed and has a handy nipple so I can use some bottled gas instead of shop air. That works very nicely. The loosen-able screw/knob the pumps came with also works well if you just want to let in shop air. The inlet seems to be in the middle of the "drag" section of the turbo-drag on the ones I have. At any rate, you can spin 'er down pretty quick with no untoward noises or rotor/bearing damage. This is what I use on both my active systems to let them up to air pressure before opening the doors. If I'm going to want to go right back to vacuum -- like when I'm just making some small change in the tank I can do fast, I use argon or helium to let the system up, and it gets back to base pressure a heck of a lot faster if the door wasn't open long (to let in wet shop air).

But if I'm going to be fiddling in there half an hour or more - I've realized that the pure gas is a waste of money, since it will exchange with the shop air anyway with the door open, and it's still going to take a long time to pump back down all the way.

Re: Turbo pump spin down and venting

Posted: Mon Aug 29, 2011 12:02 pm
by Jeroen Vriesman
The reason for "not letting a turbo pump spin down without venting" might be back flow from the fore vacuum pump.

When the turbo pump is off, oil from the fore vacuum pump might flow into the (colder) turbo pump and form a thin layer of oil on the blades.

I let my turbo spin down without venting, but first I close the valve between my fore vacuum pump and the turbo pump.

If you do want to vent, it's possible to slowly open the venting valve on the turbo manually, letting in something will slow the turbo down, do this very carefully.

The way the controller would vent the turbo pump is spinning it down until it's spinning slow enough and then open the venting valve, opening the venting valve at high speed would destroy the pump, opening it at low speed would allow the back flow from the fore vacuum to pollute the turbo pump.

A good automatic venting valve is closed in no-power condition, so a power failure would not destroy your pump.

Having a valve between the fore-vacuum pump and the turbo pump is the best I think.