Hi everybody,
Thanks to everyone who responded. I've learned a lot from this
experience, and from all the replies.
One thing I learned is that many AMMRLers go on vacation this time of
year. I received about as many out-of-office replies as normal
responses. Note to self: try not to have major problems during the month
of August :)
Everyone agreed with me that my problem was caused by an ice blockage of
the nitrogen port. People offered various possible reasons why an ice
blockage might occur. One thing I hadn't really been aware of is that
ice sometimes forms in the nitrogen ports during helium fills. The
pressure in the nitrogen dewar can become very low while helium is
filling, and air can enter. And in fact, I did do a helium fill the week
before. So in my case, that's probably what happened. One person
suggested filling nitrogen during the helium fill to prevent this. I'm
not wild about that idea, but I may start closing off the nitrogen vent
port during the helium fill to prevent air from getting in. Another
possible solution is to always fill nitrogen just before filling helium
(which I normally do, but did NOT do the most recent time I filled
helium - another indication that that He fill caused the problem).
Also, Doug Lyons from Bruker said that the internal welds on Magnex
magnets result in very narrow spaces in the nitrogen ports. This makes
ice blockages more likely than on other types of magnets. He also asked
if the magnet's nitrogen dewar was empty or very low. He said if you
fill nitrogen at the normal rate when the magnet's dewar is very low,
you can build very high pressure in the dewar. In my case, I don't think
that was the reason, but it's something to keep in mind.
Several people suggested that my nitrogen wasn't pure (or that it wasn't
nitrogen). I don't think that was the case - we have a large nitrogen
storage tank that we use to fill our dewars. If there was something
wrong with the bulk nitrogen, I'm sure other people would have had problems.
There were also several theories as to what caused the strange cycling
behavior. I'd love to know what caused it, but I'm afraid I'll have to
live with uncertainty there.
Several people wondered about the vacuum seals and how well the magnet
has been maintained. I think everything is OK there. The behavior of the
magnet (hold times and flow rates) has not changed at all in the 20
years since it was installed. This is the first unusual incident I've
ever seen.
Many people cautioned me to carefully watch the boiloff rates. It's only
been a week, but so far the boiloff and everything else is completely
normal. The nitrogen fill this week went smoothly. Hopefully nothing was
damaged and this will be an isolated incident.
Thanks again to everyone.
-Robert
-------- Forwarded Message --------
Subject: Fwd: very scary nitrogen fill
Date: Wed, 10 Aug 2016 09:16:58 -0700
From: Robert Peterson <peterson_at_mbi.ucla.edu>
To: AMMRL <ammrl_at_ammrl.org>
Dear group,
OK, I think I mostly know what happened. There was actually a partial
blockage on the vent side. At the time that I wrote, it had just
happened and the radiator that I have on the vent side was still frozen
from the fill, so I hadn't actually checked whether that side was
blocked. And nitrogen had been shooting out that side very hard all
through the fill so I assumed it was clear.
But now it's clear that it was partly blocked during the fill. That must
have led to pressure building up inside the nitrogen dewar. I cleared it
and refilled nitrogen this morning. Everything behaved completely
normally, so hopefully it will be OK.
Two things I don't understand though: the first is the cycling of the
pressure when I removed the filling hose from the fill port yesterday.
It was truly amazing to see. It cycled between almost no flow at all to
erupting like a geyser over a period of about 20 seconds, and continued
for 6-8 cycles before it started to wind down.
The other thing is the change in sound that I heard during the fill
yesterday. In retrospect it really seems like that could have been the
blockage forming. Is that possible?
Anyway, I will pay closer attention to sound changes in the future.
Thanks in advance for any information.
-Robert
-------- Forwarded Message --------
Subject: very scary nitrogen fill
Date: Tue, 09 Aug 2016 15:56:48 -0700
From: Robert Peterson <peterson_at_mbi.ucla.edu>
To: AMMRL <ammrl_at_ammrl.org>
Dear group,
I just had a very disturbing experience just after filling nitrogen. The
system is a Bruker 600 - Magnex magnet, installed in 1996.
I filled nitrogen, and it took much longer than usual. It took about 90
minutes, while it usually takes ~40. Also, during the fill there was a
change in the sound the fill made - it suddenly became louder and
higher. The fill did finish eventually (liquid came out). After it
finished, there was a very strong outflow from the vent port - almost
like I was still filling. After it had gone on like this for a few
minutes, I double checked to make sure the tank was closed (it was).
(The magnet has 3 nitrogen ports. One is occupied by the nitrogen
sensor, and the other two are used for filling and venting during the fill.)
After a few minutes (while it was still venting like crazy), I removed
the filling hose from the magnet: nitrogen came blasting out harder and
harder until _liquid_ nitrogen was blasting out so hard it was hitting
the ceiling 5 feet above and splashing down on me. Then it gradually
calmed down until only gas was coming out relatively slowly. Then the
pressure got higher and higher until again liquid was shooting out. The
full cycle took maybe 20 seconds. This continued for 2-3 minutes. After
maybe 6 or 8 of these cycles it started getting less intense and a few
minutes later it had stopped. 10 minutes later it seemed completely normal.
Both of my open nitrogen ports are clear. Nothing was different with my
nitrogen dewar (low pressure - 22psi). The magnet is still a magnet, and
the lock was stable even while the nitrogen was blasting out.
Does anyone know what could cause this? I will post a summary or an
explanation.
thanks,
-Robert
--
-----------------------------------------------
Robert Peterson, Ph.D.
Facility Manager - NMR Technology Center
UCLA-DOE Institute for Genomics and Proteomics
UCLA Dept. of Chemistry and Biochemistry
phone: (310)825-1816
fax: (310)825-0982
peterson_at_mbi.ucla.edu
-----------------------------------------------
Received on Fri Aug 19 2016 - 12:13:29 MST