Hi Luke,
If you can send some pictures of the connection between the recovery system
and the manifold that might help us provide useful input. There is one
detail that I noticed--not sure if it's because of some misunderstanding on
my side or if I missed some useful detail.
I spent some time administering a cryomech liquefier. (Worked like a charm.
Admittedly all the bugs had probably been ironed out by the time I arrived, but
it was a good system). On that system there were two options for helium
exhaust--the thin flexible hose that was used for normal exhaust, and some larger
accordion hoses that were connected for the gas to exhaust during helium fills.
Since the exhaust during a fill is much higher than during normal use, the 1/4"ID
(or something like that--I forget the size of tygon tubing that was used) didn't
have sufficient capacity for the helium fills. During the fill, a ball valve
was opened and allowed the helium to exhaust through a much bigger hose instead
which could handle the extra flow.
What I interpret from your email below is that there is a thin hose connecting
the output of the magnet to the liquefier manifold but I don't see that there's a
way to set up for the increased flow during a fill. Like I said--if you can
send some pics so that we know what you're working with, that might be helpful.
I found a decent picture of what I'm describing at Helium Recovery | UD NMR Center.
You can see that there's a piece of clear tubing connected to the output of the magnet
that goes through the flow gauge on the magnet and then is plumbed into the manifold on
the wall. You can also see the accordion hose leading to the wall and the heat
exchangers that would be used during the helium fill.
If I'm way off, sorry for wasting the precious electrons.
Cheers,Mike
On Friday, April 18, 2025 at 02:22:14 PM MDT, Fulton, Luke via groups.io wrote:
Hi all,
I’m asking about the nitty gritty details of filling procedures while
connected to helium recovery. We made some troubling observations during our
most recent fill and want to identify/minimize risks for next time. It’s
a looong detailed story to read through. If anyone makes it to the end I’d
appreciate any opinions on what to change or if you agree with our conclusions.
My lab recently completed installing a full Quantum Technologies helium recovery
system. Each magnet’s helium exhaust port is connected to a copper header
line via manifold and long flexible whip. The header has a maximum pressure of
0.4 psi (before triggering relief valve), and each magnet has its own one-way
helium check valve, 15 mbar or ~0.2 psi. So at maximum I expect around 0.6 psi
back pressure while filling.
Historically we applied ~1.2-1.5 psi on the transfer dewar without any issue. Fills
went smooth, magnets stayed happy. We employed that same procedure for our very
first fill test during the install after connecting to the header. At the end
of the fill, helium liquid came shooting out from the fillport upon removing the
transfer line. Not fun. This happened on both an Oxford 600 and Bruker Ultrashield
300. We attributed it to the newly increased backpressure, and having to relearn
when to stop the fill.
Hoping to prevent it from happening again, this week we opted to lower the
pressure on our transfer dewar. Our hypothesis being the magnet was accidentally
over pressurized, and our historic pressures were no longer appropriate given
the new header connection. So we used 0.9 psi on the Ultrashield 300 this week,
and planned to lower it steadily near the end of the fill to end more gently.
Things went terribly.
Our helium level sensor usually “freezes” in the high 90’s for a
few minutes after putting in the transfer line stinger. As the fill progresses
the sensor unfreezes and provides a reliable impression of fill progress. This
time it never unfroze and we ended up flying blind from start to finish. After
eventually accepting that we had to operate without the sensor we decided to stop
the fill early. Better to regain our bearings than risk overfilling. During this
discussion, the flexible whip started to shake, which happens when liquid enters
the line from the magnet and rapidly converts to gas. This surprised us, by our
estimates the magnet shouldn’t have finished filling yet. We stopped the fill
and removed the transfer line from the magnet. Liquid came shooting out the fill port.
Again, not fun. The helium level unfroze shortly afterwards and came in at a mere 86%.
The helium level sensor freezes at filling start because the stinger coats
the thermocouple in liquid. It should unfreeze as the fill proceeds because that
initial liquid drains from the helium stack into the internal dewar. It shouldn’t
remain frozen unless liquid is trapped there, by say an ice block, or unless liquid
reenters the stack, preferably because full. Liquid can’t eject out the exhaust
port without climbing the helium stack, again preferably because full. But our sensor
did remain frozen, and liquid did come out the exhaust port. And at 86% the magnet
is certainly not full.
We concluded that the dewar pressure was too low. It couldn’t decisively
overpower the back pressure, and the tussle between the two pressure sources
destabilized liquid contents within the magnet. We think the liquid was sloshing
dangerously. Perhaps there are niche details when working on lower field superconductors
with relatively small helium volumes, we’re not sure. That all happened a
couple days ago and not one user complaint since. The magnet appears to be fine and
we’re thankful we didn’t quench. Next time we’re returning to our higher historical
pressures and may revisit the original plan to lower dewar pressure near
the end for a gentle finish.
For those who read to the end you’re a saint. I’ll happily
accept any pointers and will attempt to answer clarifying questions. The situation
spiked my adrenaline to the roof, and I could do without that becoming routine.
Happy magnet happy manager.
Kind regards,
Luke
Luke Fulton, PhD
CHEM BLDG R003
NMR Core Facility Director
Unit 3060
COR2E & Department of Chemistry
55 N Eagleville Road
University of Connecticut
Storrs, CT 06279
email: fko24003_at_uconn.edu
mobile: (603) 953-5275
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Received on Fri Apr 18 2025 - 13:50:42 MST