[Sneap] Interesting Problem -- posted for Tom Miller at Purdue

Wed Oct 29 14:16:27 EDT 2008

Tom,

We see similar effects (i.e. changes in vacuum on both HE and LE side) when
venting or filling our EN. Those changes in vacuum are not very predictable,
sometimes it's only seen on HE, sometimes only on LE.
Our explanation - without any hard proof for it - is that the length change
of the accelerator vessel when pressurizing or venting the tank cause
movements of the column and hence movements of the acceleration tubes,
causing small "leaks" at the O-ring seals.
I monitored for example the vertical position of the stripper channel during
pressure changes and found out that it significantly sagged (if I remember
it correctly order of 1mm) when the tank is pressurized and lifted again,
when pumped out. 

Whether this is just coincidence or not, I'm not sure about, but when we
replaced our acceleration tubes some years ago, we replaced the O-rings
inside the tank with Buna O-rings instead of Viton O-rings and since then,
the effects were not as much pronounced as before. I tend to regard the Buna
as the improvement, but it is of course also possible that we were just
lucky and the tubes and bellows fit a bit better than before.

With regard to your CO2 question, does anybody know the
absorption/outgassing rate of CO2? 
At least I wonder wether the assumed reservoir on your HE side is the
acceleration tube itself. Considering how long it takes to pump down
acceleration tubes, once they were exposed to air, I think degassing would
be a possibility. Admittedly the long pump down times of acceleration tubes
are usually associated with water vapor. I don't know whether it is
difficult in general to get rid of absorbed CO2 or not, still I would
consider this an option.

Regards,
Klaus

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From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu] On
Behalf Of Chris Westerfeldt
Sent: 29. oktober 2008 16:56
To: SNEAP
Subject: [Sneap] Interesting Problem -- posted for Tom Miller at Purdue

Here is an interesting problem that I believe has been present since the
accelerator here was installed. When I came here in 1989 people just pumped
out the tank without watching the LE and HE vacuums. When filling the tank
they would watch both gauges for any pressure leaks. In 1991 I began
recording the vacuums when people pumped out. Lo and behold, at around 50-70
psig the HE vacuum would go from low 10-7 to low 10-6 range and then slowly
return to low 10-7 once the tank was pumped out. When tank was pressurized
all was okay. I mentioned this to several people that we need an RGA, as I
think there was a small tank gas leak that briefly opens when pumping out.
Nobody paid any attention to me.

Fast forward to this year. Lo and behold a used RGA appeared. I put a head
on the HE and waited for a tank pumpout, like yesterday. Bingo, while taking
scans as we pumped out suddenly at about 70 psig the HE vacuum went up to 1.
5 X 10-5 and began coming down again slowly. My scans showed a big N2, a
smaller CO2, and very small SF6 peaks…. tank gas components. Another scan
∏ hour showed no SF6, and the N2 and CO2 peaks were smaller while the
vacuum was in the low 10-6 range. Interestingly, by the time the tank was
vented the vacuum was still 2 X 10-6, but the N2 and CO2 peaks were about
the same size now. By this morning the vacuum was still 1.1 X 10-6, instead
of low 10-7, and the CO2 peak was 3 times as big as the N2 and H2O peaks. In
fact, valving off the cryopump causes the CO2 peak to grow really big! As
though there is a gaseous CO2 source somewhere in the vacuum system. Over a
long period of time, days-weeks, the vacuum will improve as the CO2 peak
slowly drops.

I suspect something briefly opens up at 70 psig, lets some tank gas in
briefly that fills up some reservoir within the HE part of accelerator
vacuum system and that leaks the N2 fairly quickly away but retains the CO2
over a much longer time frame as it very slowly gets pumped away over a
period of days-weeks, finally returning to a low 10-7 range.

Do not see this in the LE, hoping to install another RGA there soon. Helium
leak check in air of HE of accelerator found nothing. Testing of stripper
gas found only Argon and went away when leak valve closed off. It’s almost
like a big sponge in the vacuum system that sucks up a lot of the CO2 that
briefly leaks in and then slowly releases it over a very long period of
time. Obviously need to locate the source of the brief leak. I think this
has been happening since day 1.

Any thoughts, ideas???? If not, can you pass it on to the SNEAP community
for their thoughts?

Thanks a bunch. Cheers!!!!

Tom Miller

-- 

Chris R. Westerfeldt
Research Scientist / T.U.N.L. Radiation Safety Manager
Duke University Physics Department & 
Triangle Universities Nuclear Laboratory
Science Drive, Box 90308
Durham, NC  27708-0308
Tel: (919) 660-2600        
Fax: (919) 660-2634
Email: Cwest at Tunl.Duke.Edu

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