Cold-Pumping: our experience

Alan Olson (awo@helix.nih.gov)
Thu, 1 May 1997 08:39:29 -0400

Hello all -
Ken Fishbein's comments (below) on the difficulty of pumping a cold
dewar are well considered. In our experience, when the LHe boil-off rate
doubled and tripled on our 2T/45cm Oxford magnet several years ago, we had
moderate success in cold-pumping the magnet with the proviso that the turbo
pump must be no closer than the 50-gauss line to avoid eddy current forces
from overloading it. To reduce the flow resistance, if possible make the
stainless line from magnet to pump much larger in diameter than the magnet
vacuum valve. In the case of a moderately slow leak, this procedure can
easily extend the life of the dewar to a time more convenient and
economical for you to bring down the field and warm it up for proper repair.
In the way of an integrating He boil-off meter, on that same magnet when
the LHe level sensor was in doubt, we used a gas volume mechanical meter
(Wolgroth AG Zurich, 0.04 to 6 m3/hour) similar to a household natural gas
meter. However, I found that simply the use of this meter on the magnet
(increased back-pressure?) increased the helium consumption over not using
the meter. That's the observation: anyone have an explanation? We have
since resurrected the LHe level sensor and use a non-linear flow meter.
The gas meter is sitting here unused. Anyone like to try it?

- Alan Olson

> Dear AMMRL Members, Weds Apr 30, 97
>
> In response to Rich's suggestion to pump a failing magnet while
>cold, I would like to point out that there are safety valves to protect
>against accidental loss of vacuum if the power fails during the cold
>pump-out. These are simply solenoid-activated gate valves which are
>normally closed. They are wired to the pump power mains so that if there is
>a loss of electric power, the gate will fall, closing the valve. These
>valves are available from manfacturers of high vacuum equipment like
>Alcatel, Leybold, and Varian.
> In my experience, cold-pumping a magnet is usually not very
>effective. The magnet, while filled with liquid helium, is an excellent
>cryopump, exceeding the pumping speed of just about any commercially
>available turbopump system. Also, it is not possible to bring a turbopump
>very close to a large NMR magnet without having serious eddy current
>problems. Thus, there usually has to be at least a one meter metal hose
>between the magnet and the pump, and this can seriously degrade pumping
>efficiency. In particular, it is very hard to pump helium gas out of a cold
>magnet cryostat (or a warm one, for that matter). Since the presence of
>traces of helium gas in the cryostat (due, for example, to an O-ring seal
>frozen during a helium fill) is often the cause for degraded magnet
>cryogenic performance, removing this helium will usually be necessary to
>restore the magnet to specifications.
> Provided that proper safety devices and practices are used, pumping
>a cold magnet doesn't present a great hazard and it's not a very expensive
>procedure, but we haven't seen much benefit from cold pumping our magnets.
>In particular, pumping our 400/104 superwidebore magnet had no benefit at
>all in restoring its liquid helium hold time to specifications, and pumping
>our 1.9T/30 cm horizontal magnet provided only a few days of reduced
>boiloff. Ultimately, the magnet continued to degrade, and a complete
>overhaul similar to the one Rich described was necessary.
>
> Regards,
>
> Ken Fishbein
> Facility Manager, NMR Unit
> NIH/NIA/GRC

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From Alan Olson, National Institutes of Health
Building 10 Room B1D 125
10 CENTER DR MSC 1060
Bethesda, Maryland 20892-1060
Phone: 301 496 8139, FAX: 301 402 0119
e-mail: awo@helix.nih.gov, olsonc@cvn.net
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