What do you need to do to run a vacuum system unattended

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ian_krase
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What do you need to do to run a vacuum system unattended

Post by ian_krase » Sat Nov 11, 2017 6:43 pm

What do you consider the minimum requirements to run a vacuum system unattended? The most obvious problem is that a power failure will kill the forepump, and then one is in a race between the cooling or slowing of the high vacuum pump and the rise in pressure, both in the foreline and in the chamber, whether or not one can automatically valve ones system.

Dan Knapp
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Re: What do you need to do to run a vacuum system unattended

Post by Dan Knapp » Sat Nov 11, 2017 9:32 pm

I routinely leave vacuum systems running with no provision for handling a power failure (I've been lucky not to have a problem). I recently had a forevacuum pump motor overheat and activate the thermal cutoff, but I was standing nearby and responded. This may indicate I have used up my luck and need to better provide for a failure.
Presumably, an unattended system does not have a gas flow running and you're just pumping against a closed system. A good forevacuum pump will have an antibackflow valve that would prevent suckback of pump oil, but depending upon it working perfectly is putting a lot of faith in it. Minimum good practice would be to have a solenoid valve in the forevacuum line that would close upon power failure. A reasonably tight system would hold enough vacuum for the time needed for a turbo to spin down or a diffusion pump to cool below a damage threshold temperature. A bit more sophisticated system would have a buffer volume on the high vacuum side of the valve. You would also want to provide for the power to the pumps to drop out in a failure so it doesn't try to restart upon return of the power. A more sophisticated system would include a forevacuum gauge wired to a system that would restart the high vacuum pumps only after the required forevacuum level is reached. For the amateur system, I would recommend if possible installing a normally closed solenoid valve in the forevacuum line that closes upon power failure and a drop out relay for the pump power that requires a manual restart upon power restoration. This arrangement would protect your system from suckback of forevacuum pump oil and damage to a turbo or diffusion pump. I do not normally leave my high vacuum gauges burning when the system is left unattended. Most hot filament gauges automatically shut off when the pressure gets too high, but again this is putting faith in the system to work.

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Dennis P Brown
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Re: What do you need to do to run a vacuum system unattended

Post by Dennis P Brown » Sat Nov 11, 2017 9:56 pm

I pump out my fusor (sometimes weekly, very often only monthly) and leave it both connected to the fore-line pump and under vacuum with no valve between. My anti-back flow inline device appears to handle all issues. I, as I just posted, fired up my fusor after nearly a year and after 15 minutes of plasma burn (and I had no issue getting to low 10^-5 torr for initial purge) my fusor ran rock steady.

This tends to indicate that I have no issues from the fore line pump oil back-streaming into my DP pump. Never really considered running the fore line pump more than just when the fusor is going to be used. I do keep the main gate valve closed but that means the DP and half the fusor body are exposed to the fore line trap. So, I guess those devices work.

Also, I opened the fusor to air and cleaned/scrubbed the main window with acetone before this run, too. Still, the system ran pretty flawlessly. I guess if I didn't have the trap, I'd just back fill the fusor with dry air. That is far cheaper than paying the electric bill for running the fore pump 24/7/365.

I NEVER run a DP or my turbo unattended; those things are either a pain to clean or costly to replace. If I have to leave, I simply shut the system down. An issue for me, my DP is water cooled so that is a very big issue that needs to be watched. One very nice feature of my DP, it allows the oil to be cooled. I have a valve for that so I can get my DP oil to RmT in under fifteen minutes.

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Bob Reite
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Re: What do you need to do to run a vacuum system unattended

Post by Bob Reite » Sat Nov 11, 2017 11:19 pm

I run my turbo and roughing pump unattended with no special precautions and have not had a problem. The turbo is run in "standby mode" (reduced RPM) to save some wear on it, yet keep the chamber under a good vacuum so I don't have to go through a bakeout like I would if I let it go to atmosphere. I do shut off the ion gauge, convection gauge and exhaust valve controller when not actually conducting an experiment however.
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Re: What do you need to do to run a vacuum system unattended

Post by ian_krase » Sun Nov 12, 2017 5:27 am

Hmmm.

My rotary vane pump has an anti-suckback valve but it leaks -- it holds long enough for a conscious human to close the foreline valve but over hours will allow bulk oil suckback -- *not* good.

I see that there are some specialized angle valves meant for this purpose -- for some reason usually hexagonal. And expensive. By solenoid valve you mean valves actually actuated by a big solenoid, right?

What is going to fail first in a leak-tight-ish system with a valved off foreline?

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Dennis P Brown
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Re: What do you need to do to run a vacuum system unattended

Post by Dennis P Brown » Sun Nov 12, 2017 9:14 am

Just out of curiosity, why do you need to run your fore line 24/7? A fusor is not a high vacuum system and generally does not need that level of cleanliness.

My high vacuum system has both a fore line valve and an anti-back flow adapter. But that requires low end of 10^-6 torr. I also flush it with dry air before opening it for repairs. But that, unlike my fusor, is a far more demanding vacuum application.

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Re: What do you need to do to run a vacuum system unattended

Post by Richard Hull » Sun Nov 12, 2017 6:17 pm

I have a KF25 connection to my fore pump hose. Once I close the Diff pump isolation valve, I just turn off the forepump and immediately remove the clamp to the KF25 coupling, tilt it, letting air into the pump, and then close it up and re-clamp. This has worked since 2004

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