I would recommend letting the LN2 evaporate and allowing the He boil off to
raise from that and purging the LN2 space with N2 gas to prevent
condensation and maybe even icing seems very wise. Allowing the He to
reach a fairly low level before starting this would also seem
helpful. Since you do not want the magnet, any possible damage to the coil
is not an issue and having the least amount of cryogens at the time of the
actual quench just seems prudent.
One issue that I do not think I have seen mentioned in this thread is that
older cryostats may not have the pressure release disc on the vacuum
space. If your O-rings have leaked and allowed N2 and other gases to
freeze into the vacuum space there is a chance that the dewar space may
pressurize as it warms up. As I understand (but only by second hand
information) there have been a few dramatic cases of this where the bore
tube failed under pressure and lots of super insulation was blow out of the
system. Newer systems have a drop plate on them and for a while it was
common practice to loosen the flange bolts after the system was at field
and under vacuum (only on vertical bore magnets as best as I know) to allow
pressure to release at that flange if necessary.
I have heard of people allowing a "burb" of Helium gas into the vacuum
space to elevate the boil-off. You can repeat as necessary after you give
it a chance to settle out and see what the impact is. Caution and patience
is helpful.
That said I have seen this done in a way that I would have been very
unhappy with the outcome. I have seen bits blow off the top of the magnet
and fly by someone in the room.
My final comment would be that this is not a spectator sport! Limit access
to the room to necessary, not interested personnel, lots of people may want
to watch when they hear you are going to quench it.
Good Luck
Joe
*******************************
Joe Dumais, Ph.D.
Associate Research Professor/Nuclear Magnetic Resonance Spectroscopist
Boise State University
College of Arts and Sciences
Department of Chemistry and Biochemistry
1910 University Drive, Boise, Idaho 83725-1520
(208) 426 4913
http://chemistry.boisestate.edu/
*******************************
Sent from my MS Windows PC
On Wed, Mar 7, 2018 at 9:38 AM, Hirschinger, Jerry D <jerryh56_at_purdue.edu>
wrote:
> Sorry Phil, but I beg to differ. Cracking the vacuum on a live magnet is
> NOT a good idea. Loss of vacuum results in masses of water condensing on
> the outside. Quenching will freeze that water, and can interfere with
> operation of the quench relief valves. Blocking the exhaust of a quenching
> magnet makes a bomb.
>
> To speed up a dry quench, you must blow out the cryogens through a siphon
> tube. Thick-walled polyethylene tubing can reach the bottom of the LN2
> can. The LHe transfer line, or the magnet L-tube can reach the bottom of
> the LHe can by way of the siphon cone. If they are sealed well, the
> boiloff pressure will push out the liquid, or you can use the appropriate
> pusher gas. Do NOT use N2 to push out LHe.
>
> After the magnet quenches, then the vacuum can be released, but beware
> this will make a great wet mess in the lab from water condensation. The
> mess can be avoided by warming the magnet, using the same siphon tubes to
> inject warm gas into the bottom of the cans. Again, do NOT use N2 to warm
> the LHe can until the magnet temperature is above 77K.
>
> Be SAFE out there! -Hirsch
> Jerry Hirschinger, NMR Instrumentation Specialist
> Purdue Interdepartmental NMR Facility
> 560 Oval Dr. West Lafayette, IN 47907-2084
> Office: Wetherill 365A
> Phone / Fax: (765) 494-5288 / 494-0239
> Cellular: (765) 427-3034
>
>
>
> -----Original Message-----
> From: Philip Dennison [mailto:dennison_at_uci.edu]
> Sent: Tuesday, March 6, 2018 10:06 PM
> To: Lingchao Zhu <lzhu_at_chem.wisc.edu>; ammrl <ammrl_at_ammrl.org>
> Subject: Re: AMMRL: quench a horizontal magnet
>
> On 20180306 11:41, Lingchao Zhu wrote:
> > Hello AMMRL,
> >
> > We have an old Thermo LTQ FT Ultra Mass spectrometer waiting for its
> > retirement. *(Model: Finnigan LTQ FT Serial No: SN06036F)* It has a
> > horizontal Oxford magnet*(Part No: AJH0280)* and we are not going to
> > reuse this magnet. We don't have any magnet book or charging rod of it.
> > According to the student in the lab, it has 7T magnetic field.
> >
> > Without too much information from the lab, we decide to dry-quench
> > this magnet, meaning we will let the cryogen boiloff itself to induce
> > a quench. I have following questions to ask and would appreciate any
> > input from AMMRL community:
> >
> > 1. Should we let the liquid nitrogen go dry completely first and then
> > waiting for all liquid helium boiloff? Or we need to keep filling
> > liquid nitrogen until the liquid helium level is low then stop filling
> > liquid nitrogen? Which way is safer?
> > 2. Please see the attached photo for top part of this magnet. It has
> > helium exhaust port built on top of it so when quench happens all the
> > helium gas will blow out through this big pipe. Anything we need to
> > check before the quench happens?
> > 3. Any other important notes before the quench? We will definitely
> > block the lab room when the cryogen is low.
> >
> > Thank you.
> >
> > Lingchao
> >
>
> Hi Lingchao,
>
> Do you have the ability to open the vacuum space of the magnet? If you can
> do that when the cryogens get low, a quench will happen fairly quickly.
> That way you can have some control over the quench timing.
>
> If you have anyone interested in re-using the magnet, then dry nitrogen
> gas is the best way, otherwise it won't matter.
>
> Regards,
>
> Phil.
>
>
> --
> Dr Phil Dennison
> NMR Facility Director (949)824-6010 (office)
> Department of Chemistry (949)824-5649 (lab)
> University of California (949)824-8571 (fax)
> Irvine, CA 92697-2025 dennison_at_uci.edu
> USA
>
>
Received on Wed Mar 07 2018 - 10:02:39 MST