connectors for superfluid He pipes

[ringwraith]

Hi all,

i'm new to the forum, so please bear with me if i'm asking something stupid.

i am involved in a science project which is going to use superfluid helium (at ~1.8K and 20bar), and i am wondering if there are any standard connectors/flanges available on the market which can be used to connect pieces of pipes together, such that He leakage into the insulation vacuum of the device is below, say, 10^-8 mbar*l/s.

This whole "superfluid" thing gets me a bit nervous when thinking about tiny leaks/gaps...but i'd really prefer not having to weld all the ducts together. So i'd be thankful to anyone who could offer some first-hand experience and tips on this issue.

 

[ringwraith]

Sorry, i forgot to mention that those pipes will have diameters somewhere between 20 and 40mm.

 

[iainuts]

I do a lot of work with liquid helium, but not superfluid LHe. Nevertheless, I think the VJ bayonets used for LHe would work just fine with superfluid LHe. Are you familiar with them? A conventional bayonet works by creating a very long conduction path between the inner and outer line. The male half of the bayonet slides inside a female half, typically 6 to 18 inches in length. The longer the connection, the less heat leak you'll get. Between the male and female bayonets is a very thin air gap, a few thousandths of an inch across. Both the male and female parts are VJ. There are a few examples of them on these web sites:

http://www.cryocomp.com/products/b3000_series_bayonets.htm

http://www.cpc-cryolab.com/PdfFiles_Old/34n35n36n37.pdf#search="bayonet vacuum"

Note that this air gap is at LHe temp where it contacts the inner line fluid, and at ambient temperature at the flange where the gasket is, typically an O-ring.

Note also, these bayonets should always be oriented so the male end points down or at least at a downward angle. Horizontal and upward pointing male bayonets may not work well because the more dense liquid will be drawn downward by gravity in the space between the two halves, where it will reach the warm flange, flash off and go back into the liquid flow stream carrying that heat, and the process will start all over again.

I would think these would work well with superfluid LHe, because although it may be superfluid at the inner line, there will be a layer of 'normal' LHe above that inside this air gap, and then gas above that. The thermal gradient will still be set up, so the superfluid LHe can't simply flow all the way up to the warm flange and flash off.

 

[ringwraith]

Thanks for your help so far. I think get the point about how these connectors work. However, in our case the conditions seem to be a bit different from what you were pointing at:

The whole apparatus including the piping is at 1.8K, and the only "ambient" temperature it sees is the 5K from a heat+radiation shield which encloses everything. Moreover, the whole assembly (including the radiation shield) is going to sit in a large vacuum vessel at 10^-7 torr or so. The connectors i am asking about are not going to be at any place near the He supply from outside. They are rather intended to allow to disassemble the internal piping without having to cut and re-weld it every time we need to pull the apparatus apart (which will definitely occur). They do not need to be easy to assemble - some kind of screw flange or fitting would be OK.

Because of the negligible heat intake over the pipes (at least at the point of interest), i am not worried about heat leaks at all, only about gas leaks into the isolation vacuum. Especially because under our conditions the He might not become normal-fluid before leaking out.

 

[iainuts]

Hi ringwraith. Ok, I get it now. Yes, the threat of a leak is very serious. Here's a few suggestions/questions:

Perhaps you could contact the author of a research paper that had to do testing using superfluid LHe. See if they can tell you what fittings they used or how they did it.

Have you looked into metal to metal vacuum connections? The electronics industry uses a lot of VCR fittings (from Swagelok) which might work. They're intended for high vacuum systems as well as high pressure, and being all stainless and nickel, they should work well under cryogenic conditions. We use various metal to metal fittings in cryogenic systems (down to 4 K) and they seem to work quite well (albeit, at positive pressure ~6000 psi). Copper and nickel gaskets work nicely also. Here's a web site for the VCR fittings (note: only go to 1 inch diameter tube)

http://www.swagelok.com/search/find...CR®+Metal+Gasket+Face+Seal/id-10000216/type-1

There are quite a few companies that offer similar products. Have you done a search in Thomas Register for high vacuum fittings? If there is some way of testing a few different connectors, you might evaluate a handful of different products to see if any of them might work well for your situation before making an entire system from them.
Thomas Register:

http://www.thomasnet.com/index.html

Note also that if you have a vacuum pump connected to your process, it might not be so bad to have a very tiny leak. In this case, a leak from the LHe side will only increase heat leak as the pressure builds up as opposed to a leak from the ambient side in which the warm air must solidify against the cold surfaces, which carries considerable heat with it. If you can maintain a 1 micron vacuum even with a small leak, it shouldn't noticably increase heat leak if it's only an inner line leak. You could determine the size of the leak that could be tolerated from the pumping speed of your vacuum pump with a 1 micron suction pressure.

 

[unclesyd]

You might want to take a look at the QDS system.
Helicoflex used to have have a very good and responsive tech group. I have had any dealings with them since Garlock grabbed them.

http://www.helicoflex.com/products/

 

[eric2718]

We have used standard compression fittings by Swagelok succesfully with hydrogen, which leaks as easily as helium. We have thouroughly tested the fittings for leaks with hydrogen up to 200 psi for ¼”, ½”, 1”, and 1-1/4” tubing. I have been told that VCR fittings by Swagelok, along with other styles of vacuum fittings, are suppose to be the best for stopping even the smallest of leaks. However, I believe all of the vacuum fittings require welding.

 

[ringwraith]

Yeah, by now I have also found one or two publications which actually mentioned VCR fittings in relation to He II. However, none of them specified what size of pump they had to use to maintain their isolation vacuum.

I have also seen a paper where somebody put together a viewport for cryo-applications with He II, which was sealed between a flange and a cup using two indium rings, and which supposedly withstood at least 5 cooling cycles. So it seems to me that a proper VCR fitting with a 316 SS gasket should not only do the job but stay sealed for much more than a few cycles. And getting them welded to the pipes is not a problem here.