Summary: Clearly there is a touch between the nitrogen and helium cans.
The best evidence for this is the nitrogen boiloff goes to zero as the
helium boiloff goes through the roof. Likely causes seem to be a slight
expansion of the nitrogen can due to pressure or tilting of the nitrogen
can due to differential thermal expansion of the supporting system.
Certainly this does not bode well for a permanent touch occuring down
the road.
Thanks everyone for the generous input. Original email and responses
follow.
Ted Burkey
Dept. of Chemistry
Campus Box 526060
The University of Memphis
Memphis, TN 38152-6060
tburkey_at_memphis.edu
fax 901-678-3447
voice 901 678-2634
http://www.chem.memphis.edu/burkey/default.htm
For four months subsequent to energizing a refurbished 500 MHz magnet
the helium boiloff was normal (0.6%/day). Due to various delays (not
Varian's fault) the magnet is only now being shimmed to specs. When the
installation engineer recently installed the ice inhibiting heat sinks,
flow meters, and a relief valve to the nitrogen exhaust the helium
boiloff eventually pegged the flow meter and the helium dewar lost
10%/day. Boiloff returns to normal when only the nitrogen relief valve
is removed. The high boiloff returned when a new relieve valve was used
or when engineer tried every permutation of switching the nitrogen
detector and heat sinks on the three nitrogen exhaust ports. (The port
with the nitrogen level detector was cooler than the other ports but no
obvious leak was detected). I also confirmed that the relief valve did
indeed release at 0.5 psi. Varian and Oxford don't seem to be able to
explain the problem. Varian has offered to take back the magnet (at
their cost of about $20k) and refurbish it which would delay the
installation for 2 more months and the faculty has manuscripts and
grants on the line. First, does anyone have an explanation for the high
boiloff rate? Second, how risky is it to accept the magnet as is and use
it without the nitrogen boiloff relief valve. Third, if we do accept
the magnet as is what guarantees should we request that the magnet be
refurbished free of charge should it spontaneously quench or launch into
a high boiloff rate. I was thinking of an additional 3-4 year guarantee
on the magnet and returning the system back to specs.
Markus Voehler [m.voehler_at_vanderbilt.edu]
Sounds like you might have offset the He boil off with N2 evaporation
while not having the relieve valve installed (was that excessive?). What
is the magnet drift? Do you have a bad cryoshim or a touch of the He
can? 10% He / day is definitively too much and I would not accept the
magnet under any circumstances.
Hope you get it resolved well.
James Gladden [gladden_at_chem.washington.edu]
I think it would be very risky to keep the magnet as is; something is
clearly wrong. The only thing I can guess is that the slight mechanical
distortion of the nitrogen can that occurs when it is pressurized is
causing a "touch", a thermal short to the helium can. This would
suggest that, either due to damage or improper installation, the innards
are not properly aligned. If the situation is this marginal there is no
guarantee that a slight settling, or small accumulation of frozen gas in
the vacuum space, won't make the short permanent.
You might try to pressure Varian into coming up with a quicker solution.
Rokosz, Michael (M.J.) [mrokosz_at_ford.com]
That's a thermal contact between two of the radiation shields. If you
change the nitrogen
pressure, the geometry of the nitrogen vessel changes and that
apparently causesthe contact. I strongly advertise to exchange the
magnet or at least - if possible readjust the radiation shields. The
same effect might occur later without any clearlyvisible reason.
Rokosz, Michael (M.J.) [mrokosz_at_ford.com]
My only "guess" is based on the assumption that the nitrogen can in your
magnet is slightly flexible (as most are) and even slight pressurization
is causing a dimensional change that brings it close to, or in contact
with the helium vessel. Good Luck
Chris Rithner [cdr_at_colostate.edu]
We have an Oxford 300 magnet that behaved in a similar way, although
when this magnet was installed we didn't have nearly the same level of
cryogen control installed (or needed).
Try changing the fill ports and the boil-off ports if you haven't done
this already - here's why:
We have several 300s. The cryostat in question has three N2 fill ports.
We placed the supplied pressure relief valve on one port (the N2 fill
port for us) and left the other two ports open but with our own homemade
version of heat sinks with hoses attached on the other two ports (to
eliminate ice buildup). We cork the end of the hoses and then cut a
slice into the hose above the cork to allow N2 gas to escape. The N2
dewar is slightly pressurized with this setup. Actually works pretty
well - very low tech. Probably not appropriate at 500 MHz, but you get
the idea.
So what happened? On this particular cryostat, we noticed that 1) the
probe was ice cold (sub zero) when it was removed; 2) the helium
boil-off was high - apparently an internal touch. The engineer screwed
around and was about take the whole thing back down, warm up, take apart
- you get the drill.
However, we did an N2 fill during all of this and before the final
decision was made and in the process changed the fill ports and the
exhaust ports (sloppiness, not planned). Believe it or not, the helium
boil-off dropped almost immediately (over night) and when we pulled the
probe the next morning - it was room temperature. Put the fill port
back where it was and the "touch" returned so this was reversible.
We left it this way with a promise from the engineer that they would
"fix it" if it proved unstable. That was fifteen years ago.
What's going on? The hypothesis (from the mother ship) is that we have
cans that are right on the edge for clearance. The helium can hangs
from the N2 ports (that's what I was told). The "hangers" (or whatever)
shorten up and lengthen depending upon thermal stresses and apparently
there is enough difference there to pull the helium can slightly to one
side or the other. Pulled one way, it touches an outer structure and
this is bad. Pulled the other way, this concern is relieved.
Dr. W. L. Jarrett [William.Jarrett_at_usm.edu]
I've used Bruker wide-bore magnets for years without a N2 relief valve
with no problems (one port had a relief valve, the other two had
heatsinks and ~3' latex tubes venting to atmosphere). Just make sure
you keep the system filled. As for your problem, could the partial
pressure cause enough mechanical stress to distort the dewar (i.e. make
one side come in close proximity to a "warm spot"?). As for your idea
of extended warranted, I say go for it, but Varian will probably nix
the idea.
Neal J Stolowich [stolowich_at_louisville.edu]
I can respond to question 2: I'm embarrassed to say that our older of
the two Oxford 500 magnets, has never had nitrogen heat sinks nor a
relief value (unlike the newer system) and only has short curled pieces
of tygon on the ports. As far as I know it has been this way since
installation- about 15 years - and the magnet seals have yet to been
rebuilt! I'm sure this is the exception, and not the rule....and I do
occassionally (every few years) do pick up an ice plug - which reminds
me that I should check them.
Charles G. Fry [cgfry_at_wisc.edu]
Have a lot of empathy with your problem, but have no idea what might be
happening. It is important that the magnet perform properly. I
recommend you take the time to do things right. You'll have to live
with the results for 14 years or so, at a minimum. Perhaps the faculty
can find an alternative site for use for a couple months.
Ezell, Edward L. [elezell_at_UTMB.EDU]
I think the heat is removed from the cryostat by H-vap of LN2 and
anything that retards that process retains heat in the system. Boiling
LN2 is cheap enough that it would be tempting in the short run to settle
for a high rate to conserve precious He. Do you have boil-off and drift
rate data for the magnet prior to its refurbishment? I guess if the
dewar vacuum was insufficient it might present itself in this way, but I
don't think mechanical pumping on a cold dewar will help. I would have
to recommend swallowing the time hickey here at the end of the year and
let Oxford or Varian warm it up and start over.
Alan Kenwright [a.m.kenwright_at_durham.ac.uk]
It's not possible to be definite about anything from thousands of miles
away, but what happens when you put in the non-return valves is that you
increase the pressure in the nitrogen can to 0.5 psi. This is probably
what is triggering the increase in helium boil-off. I don't know the
mechanism, but it isn't good whatever it is. Possibilities include a
slight change in the dimensions of the nitrogen can are causing thermal
contact to the helium can somewhere, or are causing a leak into the
vacuum space. I would not recommend accepting the magnet because
whatever the problem is it may get worse. But as far as running without
the non-return valve in place as a temporary measure, no problem. Just
put a long length of tygon tubing on the outflow from the heat sink
stacks and it will be OK.
Robert Harker [rharker_at_chem.ufl.edu]
I have some ideas. In the old days people would simply run a length of
rubber tubing corked at the end with a razorblade slit at the bottom.
It worked well and kept the ice blocks out. I still see it sometimes.
Have you tried a lower pressure relief valve?
I've seen your problem before in perfectly good magnets. It is a form
of fluidic oscillation whose noise energy adds energy to the helium can.
Think of the N2 can as a big bell around the He can coupled mechanically
directly at the top and bottom. Put your ear on the magnet and you can
hear it sometimes. Sometimes the upper barrel oscillates too and that
you can see in the spectra as sidebands that don't change with spin.
You can feel that with your hand on the probe and on the upper barrel.
I've never seen that affect boiloff before though.
On one of our 300 Oxfords an Engineer from long past came up with a
clever solution to get rid of the radiation stacks because they were in
the way of a 100 sample changer. He simply installed the .5PSI relief
valve directly on one port and a machined aluminum port with 4 small
holes and a TO-3 size heater on top powered from the console. It
bypasses the N2 flowmeter which is really unnecessary if you have a
level meter and also prevents a clumsy TA from accidentally bending or
breaking the nitrogen necks (magnet killer). The main thing is there
is virtually no pressure drop and no oscillations. We also have a very
large ICR magnet that holds 150L N2 and Magnex installs no relief valve
on those with only one heat stack vented directly at the top through
small holes which is basically the same solution without the bother of a
heater. The heat stack does ice a small amount at the base. If you do
decide to go this route put a high flow relief valve on the open port.
Never two open ports.
One interesting and critical test would be to run a drift test during
high boiloff vs low boiloff. If the drift rate goes up then it's either
the magnet or how it was charged. Scan nt=1 a well shimmed lineshape
sample at dres < .25Hz, array D1 48 times 900 seconds 0 increment. Set
lockpower & gain =0, lock off disconnect lock cable. Leave Z0 where it
was. On ds(1) set cursor on peak nl rl, dss. You'll get ~12hour test
with 15 minute scans so you'll see whether it changes over the day.
james breeyear [james.breeyear_at_uvm.edu]
I dont know for sure but I would hazard a guess that maybe the pressure
built up in the nitrogen can may cause a touch between the nitrogen and
helium cans or associated supports. Just my $.02 . Mabye you should run
all you can with the heat sinks off .
Jeff Simpson [jeff.simpson_at_unh.edu]
As tempting as it might be to try to keep the magnet with some
additional guarantees, I think the wisest course of action is to let
Varian refurbish this magnet on their dime. In the long run, you will
be happier if the magnet issues are resolved properly.
John M. Wright [jmw_at_chem.ucsd.edu]
What was the exact configuration of your nitrogen ports before the
installation engineer installed the heat sinks and pressure relief?
All three venting to atmospheric pressure? Or two venting to atmospheric
pressure, LN2 level sensor installed on the third? Or?
Rearranging the configuration of the LN2 exhausts can change the
temperature of the LN2 neck tubes (I think the ones with N2 gas
exhausting through them are colder than those that are blocked off).
The change in the length of these tubes caused by this temperature
change can slightly affect the tilt of the LN2 can, and if the alignment
is marginal, can be enough to cause (or prevent) a touch between the
cans.
I have observed and dealt with this situation in two different magnets
over the years. The first time, I had switched the location of the vent
(the LN2 ports had one vent through a ~ two foot rubber hose, one LN2
level sensor, and one pressure relief cap). When I came in the next
day, there was ice around the top of the magnet, the LHe boiloff was
off-scale, and the LHe level was dropping at an alarming rate.
Switching things back to the original configuratioon brought everything
back to normal. Much later this magnet dewar went soft; in the process
of replacing all the O-rings before re-installing it, I discovered that
the LN2 bore tube was slightly bent. Eventually I had to resort to
placing a 1/16th inch aluminum plate under one of the legs to change the
tilt enough to avoid a touch.
But - from your description, it sounds like you have tried out every
permutation of arranging your LN2 ports. Do you have one with the
LN2 sensor, one with a pressure relief valve, and one vent (six possible
arrangements)? Or are the two that don't contain the LN2 sensor
connected together (only three possible arrangements)?
Do you see any cold spots appear while the LHe boiloff is excessive? You
might not notice them in the bore without removing the probe, RT shims
and top stack.
If it is just a marginal alignment problem, it should be correctable in
the field, by de-enrgizing, warming up, removing the bottom plate and
adjusting the alignment between the cans.
Redwine, David (OD) [ODRedwine_at_dow.com]
The relief valve changes the N2 boil-off flow which changes the
temperature of the pipes. This changes their length, or geometry, which
results in a touch to the Helium can.
The root cause is an incorrect alignment of the helium can, probably due
to a broken support. Exchange this magnet with a new one or have it
repaired.
Jerry Hirschinger [hirsch56_at_purdue.edu]
I'm a former field engineer for both Magnex and Cryomag. IMO, you need
to
have Varian rebuild the magnet. There is an alignment problem where the
LN2
bore tube and the LHe bore tubes are not mounted concentrically. When
you
increase the pressure in the LN2 can, it causes a touch between them.
If
you were monitoring the LN2 boiloff, I'll bet it drops to nearly 0 while
the
LHe boiloff goes through the roof. If LN2 boiloff does not drop to 0,
then
the touch could be between the shield and LHe can. Probable cause for
the
misalignment is likely poor assembly practices. This problem will never
go
away, and might get worse. I wonder if they knew about it earlier,
because
why did they wait until now to finish assembling the LN2 hardware? That
should be one of the first things to be completed when the dewar is
cooled.
To relieve LN2 boiloff pressure and prevent ice buildup around the LN2
ports, one can construct a simple Bunsen valve. Simply connect a length
of
1/2" ID Latex rubber tubing to one LN2 port (or whatever size fits
snugly
over the port). Clamp the end of the tubing closed and cut a 1/2"
lengthwise slot near the end. If that causes too much backpressure,
then
cut the slot longer as needed. Any ice formation will be at the end of
the
tubing, rather than on the magnet.
Roger Kautz [rakautz_at_neu.edu]
Sounds like the N2 boiloff is getting routed through the lHe tank under
0.5
psi pressure-- is the He boiloff helium? Will it float a dry cleaning
bag? Think of what combination of welds could be cracked inside that
wouldn't break the vacuum.
Rob Kleps [kleps_at_uic.edu]
We don't use a nitrogen relief valve, but simply vent thru a 1 meter
long piece of rubber tubing looped over the top of the magnet without a
problem.
petri.ingman_at_utu.fi
This is just a wild guess.
If I understood correctly your magnet has three nitrogen exhaust ports,
one closed with the meter, one closed with the relief valve and
only one is breathing to outside world through the heat sink. This may
mean that the breathing port is colder than the two others, which may
tilt the nitrogen dewar due to the thermal expansion. If the tilt is big
enough the nitrogen dewar can make a contact with helium dewar and
produce the excess in boiloff.
We saw the tilting when we were filled the nitrogen. We had a magnet
with two ports breathing out ( =cold) and one closed with
the relief valve (=warm). During the nitrogen fill we removed the valve,
which made also that port cool down. The tilt was seen in
the lock level. After the fill we replaced the valve, but it took hours
before the lock level recovered. Therefore, we decided not to
remove the valve for fills.
jhwalton_at_ucdavis.edu
The only thing I could possibly think of is that this magnet has a VERY
small clearance between the He can something warmer and SOMEHOW the
extra pressure is causing a small movement and thus a touch. I can not
conceive of how this could happen, but it is all I can think of that
would cause a boil off like that.
Received on Mon Dec 05 2005 - 15:00:10 MST