I asked last week about Bruker Prodigy lN2 dewar exhaust port clogs
(which prevent refilling of the dewar).
I received over 20 email responses. Clearly, many users have
experienced the same problem. There is general agreement that the clog
is actually ice buildup.
Possible causes:
"wet" nitrogen gas: N2 from a separator can still be wet or have O2 in
it. House N2 could be contaminated.
not using the Bruker-provided plastic tube with the porous rock on the
end on the exhaust port: we do use it, and given the flow of gas out
during a fill, I am not sure anything could get back in
(Note that Bruker calls it a noise-suppressor, a metal filter, cold trap)
A leak somewhere: o-rings, dewar neck, the orange tubes
each of these can be checked
One person said to break the seal lock on the top of the dewar and
re-tighten the nut for that seal
leak at the probe where the hose plugs into the probe
bad vacuum line to the probe
a screw-on attachment to the bottom of the fill pipe- this has been
known to come off (or be taken off)
Most likely cause:
Frost on the intake and exhaust ports. If they are still icy or frosted
when the black caps are put on, they could leak a little after warming
up. (The caps should be hand-tight, the metal thread ones will possibly
strip the threads if tightened too much, the plastic thread ones will
deform.) They can't leak very much or the dewar won't pressurize, but
even a small leak could allow ice to build up over time. (The inlet has
highish pressure liquid nitrogen flowing through it, that would clear
out any frost or ice on the sides.)
Prevention:
Getting the ice out of the port after it is frozen solid takes time and
effort. Many facilities do some sort of preventative action
before/during the dewar refill.
These include:
check all seals
flush the transfer hose before connection
use the Bruker exhaust hose on the exhaust port
purge the Dewar with dry nitrogen before re-filling (run the gas into
the exhaust port and vent via the fill port)
In addition, most facilities that responded ream out the exhaust port
periodically (possibly every fill). This prevents the ice from building
up and blocking the port. Of course, the ice just falls to the bottom
of the dewar. Periodically, the dewar should be warmed to room
temperature, emptied, and dried.
suggestions for clearing the exhaust port include:
copper wire (room temp or heated)
stainless steel spring stock (
https://www.mcmaster.com/#9665K39 )
cable tie
stiff hose
stiff hose with N2 flowing
stiff hose with N2 flowing, attached to a metal pipe that can be heated
so that the N2 is warmed
Note that if using gas, be careful not to push the hose end into the
actual liquid.
There is no baffle on the port, anything inserted should go in far
(technically to the bottom of the dewar, but don't go that far!)
Indications of blockage:
orange hoses frosting up more than about a foot
higher than typical fill pressure
Fixes:
take everything apart, warm up the dewar (and probe and everything),
make sure the dewar is dry and the ports are clear, refill, and cool down
take off just the top clamp and transfer arm of the dewar
gently heat with a heat gun
check for blockage removal with a hose or wire
We use house nitrogen which is lN2 boiloff, and we use the Bruker
exhaust hose. New Mexico is generally dry, but we are going to start
reaming out the exhaust port before every fill.
Thanks again to all who responded,
kas
--
Karen Ann Smith Director, NMR Facility
Group Leader, Analytical Chemistry Services
Department of Chemistry and Chemical Biology
Clark Hall MSC03 2060
1 University of New Mexico Albuquerque, NM 87131-0001
karenann_at_unm.edu 505.277.4031 http://nmr.unm.edu
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"Curse your sudden but inevitable betrayal!" --- Wash (Firefly),
while playing with his dinosaurs.
Received on Fri Feb 23 2018 - 06:25:26 MST