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

[Chris Westerfeldt]

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 peaksŠ. tank gas components. Another scan ½
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. It¹s 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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