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Re: FAQ: Vacuum line diagram v2.0

Posted: Tue Jun 18, 2013 6:55 pm
by Chris Bradley
Rich Feldman wrote:I was taught not to leave mechanical pumps turned off with inlet under vacuum, even if valved off from the foreline.
The pump chambers will slowly fill with oil, via the clearances provided for lubrication.
OK. Thanks. I can see that is an argument that could apply to some pumps. I guess all the pumps I've ever come across have an oil distribution valve to prevent suck-back, and thought all pumps have them. :oops:

I reverse my comment - I guess if you don't know if your pump has an anti-suckback distribution valve then best assume it doesn't, and vent the intake port!!!

Just a reminder; if it's ever necessary to vent the intake port (whether for this reason, or otherwise), make sure the intake port is vented to air while the pump is running, then turn it off.

Re: FAQ: Vacuum line diagram v2.0

Posted: Tue Jun 18, 2013 7:40 pm
by Rich Feldman
Something I learned today: apparently even some fancy expensive pumps have suck back issues.

Here is an "auto-Off Safety Vent Valve", with KF flanges for inline plumbing, and third port open to atmosphere.
In the event of electrical power failure (which I suppose can happen in amateur fusor labs),
it isolates the evacuated system and vents the mechanical pump inlet.

http://www.lesker.com/newweb/valves/ven ... cfm?pgid=0
The KJL flyer says:
-Eliminate time consuming and costly cleanup of dirty vacuum lines, caused by oil backstreaming from the pump
-Small orifice in the shut-off valves vents the mechanical pump to atmospheric pressure for easy restart when power resumes
-Even when the mechanical pump is equipped with an anti-suckback valve, these valves are recommended because the pump’s integral valve will not vent the pump

In amateur systems that depend on manual sequencing of valves and switches,
it might be a good idea to plan in advance (and write down) any actions to take when power fails.

Re: FAQ: Vacuum line diagram v2.0

Posted: Tue Jun 18, 2013 9:55 pm
by John Futter
From my 35 + years of experience relying on an automatic suck back valve is asking for a diff or turbo pump filled with backing pump oil.
I have had to clean out many after this feature failed / or had a minor leak that ends up giving an expensive messy problem.

If the foreline is at atmospheric pressure the oil will stay in the backing pump.
Purists would backfill the backing line with dry N2 or similar to keep the oil clean and water free

Re: FAQ: Vacuum line diagram v2.0

Posted: Tue Jun 18, 2013 10:20 pm
by Rich Feldman
When a mechanical pump is not right-side-up, then running the motor for even a second can discharge a full load of oil through the exhaust port.
Does anybody here know how they get high vacuum for experiments on the International Space Station?
Hair_clippers_with_vacuum_device_on_the_ISS.jpg
from http://spaceinimages.esa.int/Images/200 ... on_the_ISS

Re: FAQ: Vacuum line diagram v2.0

Posted: Tue Jun 18, 2013 11:27 pm
by Chris Bradley
Rich Feldman wrote:Does anybody here know how they get high vacuum for experiments on the International Space Station?
Is that meant to be a joke?

Re: FAQ: Vacuum line diagram v2.0

Posted: Tue Jun 18, 2013 11:38 pm
by Chris Bradley
John Futter wrote:From my 35 + years of experience relying on an automatic suck back valve is asking for a diff or turbo pump filled with backing pump oil.
I have had to clean out many after this feature failed / or had a minor leak that ends up giving an expensive messy problem.
I'd never rely on the pump's suck-back valve. That'd be the job of a foreline valve.

Thing is, I have an alumina bead trap that sits right on top of my mech pump, then the foreline valve on top of that. By relying on the mech pump suck-back valve, I just leave the trap under vacuum all the time, keeps it clean. If oil were to get sucked into the intake port, or even into the trap, I can't see it'd be a Big Deal, even if it hydralocks. Open the ballast valve, wait a few minutes?

Maybe it's even happened and I've never noticed!?!? Funnily enough, the 5A fuse in the plug started blowing out every time I started the pump up, a year or so ago. After running out of 5A fuses, I put in a 13A fuse. No problem after that!! :-?

I'm asking, just because it has never really dawned on me before that it'd ever be a problem. If it could be, I'm open to hearing about any 'risks'. Not sure there are any big-ones I can see. I guess, in that sense, Dan's method to use the ballast valve, once the pump stops, is actually the most direct way to even out the in-out port pressures of the mech pump.

Re: FAQ: Vacuum line diagram v2.0

Posted: Wed Jun 19, 2013 12:47 am
by Rich Feldman
>> Is that meant to be a joke?

My question about the ISS was perfectly serious.

We know that various microgravity experiment units, for example the Material Science Laboratory, are designed to bolt into a standard rack on board ISS.
" The crew then connects all required ISS resources, and once the solid state power control module (SSPCM) receives power, the master controller initiates an automatic startup of rack systems. "
from http://www.nasa.gov/mission_pages/stati ... operations

We can bet that the facility resources include electric power, some kind of cooling system connection, and a computer network.
How about compressed air (used widely in industrial motion control systems), or dry nitrogen ?
How about various kinds of vacuum -- must each experiment module be self-contained?
The question came up because most terrestrial HV systems include orientation-sensitive pumps. An exception might be a turbo-drag pump backed by a diaphragm pump.

Some laboratory "rooms" on ISS might have spigots for roughing vacuum, connected by lightweight pipes to "the outside", but I would not bet on it. Today I learned that the static pressure outside is usually in the 1e-8 to 1e-9 torr range. To bring that into an experimental chamber, you'd need a fat pipe and big hole in the wall right there, built to code and installed by a licensed plumber. :-)

[edit] I just found the answers myself, in this ISS Users' Guide: http://www.spaceref.com/iss/ops/ISS.User.Guide.R2.pdf
Page 20 has general specs for the Vacuum Exhaust System (Waste Gas), with 1 inch pipes in each module. It can vent a 100 liter chamber to 1e-3 torr in less than 2 hours, if nobody else is using it. Selected locations also have independent Vacuum Resource lines, guaranteed to deliver 1e-3 torr.

Re: FAQ: Vacuum line diagram v2.0

Posted: Wed Jun 19, 2013 6:25 am
by Chris Bradley
Rich Feldman wrote: I just found the answers ... Page 20 has general specs for the Vacuum Exhaust System (Waste Gas), with 1 inch pipes in each module.
That' why I thought you were joking, with the biggest, deepest vacuum reservoir being just a few feet and a trivial bit of plumbing away.

Re: FAQ: Vacuum line diagram v2.0

Posted: Wed Jun 19, 2013 7:26 pm
by Tom McCarthy
Rich, I completely agree. Sorry I forget to do it earlier, but I had the pdf bookmarked all the same.http://microlab.berkeley.edu/labmanual/chap6/vacuum.pdf That's the pdf where you'll find the diagram I put up of the vacuum line. It's easy enough to see. There's variations of the drawing over the next 2-3 pages that give a good explanation of what valves to open/close when operating each pump.

Tom

Re: FAQ: Vacuum line diagram v2.0

Posted: Mon Jul 28, 2014 8:48 am
by Aditya Somasundaram
Hello.
Does the valve between roughing pump and diffusion pump need to be ball valve? Or can I use other valves such as right angle valve?
Thank you!
-Adi