Hi,
all Oxford magnets but also Magnex/Varian/Agilent magnets indeed use a
"flame deflector" and it is discouraged to refill helium without it (even more
so with Oxford magnets):
The magnet have a funnel at the bottom of the helium refill turret (in
Magnex/Varian/Agilent this funnel is only on the "left" turret containing the
wiring where refilling should be done only in emergency case because of all
the cables going down there). This funnel has a long tube attached going all
the way to the bottom plate of the helium dewar (for better cooldown efficiency).
With a straight transfer line exit this funnel may push any helium gas (in
case the refill dewar is empty or a vacuum issue in the transfer line) down
towards the bottom of the magnet, which then bubbles up along the magnet,
possibly "warming" up exposed supercon wires like those going to the switches.
This can lead to a quench… The minimum is to never insert the transfer
line all the way but this is a bit hard to do exact and it may also slide down
over time during the refill.
There may be a bit higher helium boiloff at the beginning simply because the
liquid helium sprays upwards, where it can touch "warmer" parts of the magnet.
For safety reasons I'd rather keep using the deflector…
Cheers
Bert
> Am 20.06.2025 um 21:53 schrieb Kim, Yong Wah via groups.io:
>
> Hi All,
> Yes, it has been an informative thread.
> In our older magnets, we use a deflector tip on the transfer line. Bruker
> transfer line does not have a deflector tip. Which may speed up liquid
> Helium transfer. While inserting the transfer leg into the magnet, it could
> drag the warmer helium gas into the magnet. The deflector tip will probably
> pull helium into the magnet from the other ‘legs’! Possibly
> contributing to a larger initial outflow of Helium gas. I would like to
> know whether we should use a deflector tip.
> We have a separate line for the regular boiloff gas from all the magnets
> feeding through a single mechanical back pressure controller. During liquid
> helium transfer, the magnet exhaust opens directly to the 2” return
> line, without any in line check valve. The pressure in the other magnets
> is not affected. The valve should be open for a few minutes before inserting
> the transfer line to bleed off the pressure in the magnet. This should reduce
> the amount of helium vented and reduce the freezing of the port and o-ring
> when inserting the transfer line.
> There should not be any moving parts and a minimum of joints near the exhaust
> of the magnet. The ball valve is situated after the heat exchanger. It does
> not have any ice frosting during helium transfer. However, slight frosting
> is observed at the T-joint where the heat exchanger meets the 2” return
> line because the gas flow is mostly laminal and the central core of gas flow
> is colder than the outer rim.
> We do not have a bag. The helium gas is stored in 3 X 1000-Liter tanks.
> I bench tested the mechanical back pressure controller. It maintained the
> set pressure even when the exhaust side is a few tens of Torr. However,
> higher pressure than the set pressure on the exhaust side is showing up on
> the intake side. A 0.5 psi pop off valve is inserted in the body of the back
> pressure controller to prevent the pressure buildup in the system.
> Samrat, I like your pressure displays. Could you please tell me where I can
> get this equipment. Thanks
>
> Best Regards
>
>
> Yong-Wah Kim, PhD.
> Director of NMR Facility
> The University of Toledo
>
>
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Received on Mon Jun 23 2025 - 08:56:18 MST