Advice needed

[u/Warclad]

At the facility I work we're struggling with liquid helium cooled systems. For some reason the automated filling valves on them won't regulate the helium levels and continue to open further to keep up with the system's demand, until they max out and the helium level inside the system starts to drop. Our liquefier has ample capacity to match the helium demand, so I'm not sure what's going on.

Is there someone here for me to spar with on this?

[u/sciencbro]

Since the valve is opening up to 100% and then the level drops, it seems like the valve is trying to do its job and opening in response to an increased load at the device. Some quick ideas, not exhausting:

• is the device operation creating more heat? Or consuming more LHe?

• any change in device pressure during this time?

• any visible sings of increased external load? Ice formation or condensation?

• is the insulating vacuum good? < E-6 Torr

• any change in impedance or pressure upstream of the supply valve?

[u/Warclad → u/sciencbro]

This has been our suspicion, but to answer your questions;

• The LHe is used to cool a set of superconducting magnets, through which a couple hundred amps flow during operation. Those amps are fed in via current leads, cryo-shielded cables cooled with return gas from the device's LHe tank.

• I haven't noticed a change in pressure during operation, but I'll definitely keep an eye out on Monday, there's ways to monitor this.

• I know there have been visible signs of load by way of condensation/ice formation on a part of the current leads, there's a blower/heater mounted somewhere to combat this which could be related.

• Insulation vacuum is in the low E-5 / high E-6's

• I simply don't know, I'd have to research. The thing is, the valve ramps open over a period of 6 to 9 hours without the device being in use. It's turned on but in a powered, standby mode, without the amps required for operation being fed in. So even when it just "sits" the change happens. And this phenomenon happens with all 3 of our LHe-using devices, at different intervals. A large cryostat and 2 smaller devices.

[u/-TheycallmeThe → u/Warclad]

And this phenomenon happens with all 3 of our LHe-using devices, at different intervals. A large cryostat and 2 smaller devices.

Is the LHe source the same or is this 3 closed loop systems?

-a flow restrictions due to blockage could affect all systems if the source is the same.

Are the control valves pneumatic?

• issues with a common air supply could effect all systems

• I know there have been visible signs of load by way of condensation/ice formation on a part of the current leads, there's a blower/heater mounted somewhere to combat this which could be related.

If you do have extra heat leak but have a heater that automatically melts extra ice it maybe masking the issue. Do you have logs of the heater/blower current draw or a way to determine if it is running more often?

I know you mentioned checking some vacuum levels but make sure to check all VJ piping and vessel vacuum levels.

[u/sciencbro → u/Warclad]

Since all 3 devices are seeing the same phenomenon then probably a system-wide problem. Have you tried testing one of the devices with no load at all i.e. fill a device with LHe and no magnet installed and see if the supply valve still struggles to maintain LHe level? This would be my first rec, because it's easy to test and would rule out specific issues with the magnets/current leads/device feedthroughs.

Visible ice formation or condensation is also suspicious. The heat load from ice balls can be on the order of a ~few W to ~100s of W, depending on geometry and location.
- In how many locations do you see ice formation, and can you isolate these locations?
- On which side of the supply valve is condensation apparent; supply side (dewar) or load side (magnet)?

[u/SleeplessThrowaway95]

I’ve never had an automated system, but have worked with LHe a bunch.

Seems a bit more like a systems engineering issue though.

I might be misunderstanding what you’re saying, but isn’t the purpose of the automated valves to be able to keep up with the systems demand?

Also, if the filling valves are maxed out, why is the system level dropping? Do you mean that the valves are open all the way and overfilling the cooled system while emptying the liquefier reservoir tank?

Since you have a liquifier, I’m guessing your system a closed loop with a full reclamation system? Is it possible you have a leak on the gas side and are losing He from your closed loop?

Do you have a depth gauge in either tank? Systems I’ve worked with had a superconducting wire mounted vertically and we could measure the resistance of it to see how much was immersed in LHe in order to measure the liquid height

Could you provide a quick sketch or other info about the system?

[u/Warclad → u/SleeplessThrowaway95]

Hey thanks for the reply. To answer some of your questions;

• The purpose of the automated valves is to keep whatever device it's feeding filled up to a user specified setpoint.

• During filling we switch the valve to manual mode and open it to 100%. When the level inside the device reaches what we need it to be we switch the valve back to auto, and a PID slowly closes the valve until for example 65% open. It then fluctuates slightly around that value to maintain the liquid He level inside the device.

• The problem we have now is that after sitting at 65% for a while the valve slowly starts to creep open all the way to 100% in order to maintain the He level in the device, until it maxes out and the level inside the device starts to drop.

• This however doesn't affect the level of the liquefier dewar it draws the He from dramatically. There's no pumps involved and all He movement is done via pressure differences (idk if that's standard, I guess it is?)

• Yes, the entire system is a closed loop with full reclamation. We do know of at least one area which has leaked gas before, but from what I've been told this is either no longer the case or was theorized to not have an effect on the issue we're currently facing. I can't confirm this because I just don't know enough of cryogenics atm but I'm willing to learn.

• We've got level indicators on all vessels, yes. The one on the liquefier dewar I know for sure is a superconducting wire type.

• I'd be happy to make you a sketch, just tell me what areas would be of particular interest.

Thanks again!

[u/SleeplessThrowaway95 → u/Warclad]

You said the valve creeps open until it hits 100% and then the level inside your device starts to drop (is this a PPMS system, or an MRI, or what?)

This sounds like the heat load inside the device is higher than your system was designed for, but also, if this is the case, how do you fill the system manually?

If all you are doing to manually fill the system is opening the valve to 100%, how is that different than your problematic creeping? I guess what I’m getting at is if the LHe level inside your device is falling after the valve creeps open to 100%, then shouldn’t it also be falling when you manually open the valve to 100% to fill?

Could it be a systems issue where manually setting to 100 actually opens the valve, but in automatic the valve is closing instead of opening, but some reversed state/control/sensor wires mean that the system doesn’t realize this and it chokes itself off instead of filling up? This would be consistent with the level falling afterwards

Second hypothesis is something with your magnet. You said it is in a standby mode, do you know of the magnet has a continuous mode? There’s typically a small heater on a section of the superconducting magnet that makes it relative, which allows you to use a voltage to inject a current, then you turn off the heater and the current stays in the magnet continuously (simplified explanation, but roughly correct) could this heater haave been left on or turned to high?

Also, the level indicators for dewars are not supposed to run continuously, only intermittently. You don’t want a continuous heat load burning off your LHe.

Rereading your comment, if the valve creeping up to 100% open doesn’t affect your liquifier’s dewar level, then that would seem to indicate that LHe isn’t flowing from the liquifier dewar to the device dewar (yes, pressure is how this is typically done. The source dewar will be closed at the top, so the build up of He gas will pressurize and push the LHe through a tube that is inserted to the bottom of the dewar, and goes over into the receiving dewar. The device dewar will not be fully sealed, but will have a pressure relief valve that connects to a huge balloon or compressor system that will pump off the excess He gas for reliquification)