Sealing Hellium at Higher Temperatures: Possible Solutions?

[Pkruse]

Hellium leaks through all normal seals. I've used Indium seals to get leak rates down as low as 0.0000000001 grams/sec, but that only works for normal ambient temperatures. (Customer requirements were no greater than ten to the minus six grams/sec. The Indium seals were orders of magnitued below that.) The stuff has a low melting point, which prohibits its use in my current application.

Has anyone successfully sealed Hellium at higher temperatures? I'm talking about a number of different temperature ranges, some of them up to 1800 degrees F. It is possible that some future applications will be higher than that. I'm looking for any ideas for temperatures starting as low as 400 degrees F, since some of the appliacations are not so high.

I understand that I will have to accept some leakage at rates higher than customers have found acceptable in previous applications. We just need to keep the leak rates as low as reasonable.

 

[Pkruse]

Nobody has any good ideas?

I could not find any, either.

Looks like I might be reduced to cooling the joints and using Indium. Maybe I could find a way to use Indium as a brazing material to install the copper cooling tube, and then find a really clever means of using the soldier surface as a sealing surface.

Anyone ever braze or soldier with Indium?

 

[f95toli]

CF flanges can be used up to at least 450C (around 800 F) with standard copper gaskets and the leak rate should not be a problem.
But I have no idea what you would use for temperatures above that.

 

[Pkruse]

Thanks much, f95toli. I had not given them a thought because while they are awesome for vacuum systems, the suppliers of CF seals all have warnings in their literature against using them for pressure systems. But if I modify the flange design a little and test appropriately, I can probably sell the concept.

On a side note, many years ago I worked on a high pressure Helium system that had a near zero leak design requirement. On the flare fittings, we simply used a modified CF gasket on a standard flare fitting, and got leak rates that we needed some very expensive equipment to measure any leak rates at all. The atoms must have almost been leaking out one at a time. That also was a pressure system rather than vacuum.

 

[Danger]

Would there be any advantage to stacking multiple seals (like an airlock sort of setup)?
I know nothing of the subject; just thinking out loud.

 

[Mike_In_Plano]

We used to weld the seams. But then again, I don't know of anyone going that high. You'd better have great materials, or you'll have helium wiggling through defects in the microstructure.

 

[Pkruse]

CF seals do something of that effect on vacuum systems. If I use that concept for a high pressure system, I'll redesign the interface to make it more so. Certainly the Indium seals for lower temperature Helium do that extremely well. Essentially, we are talking about making a labrinth along the leak path, and packing that labrinth with either Indium for low temperature applications, or copper at higher temperatures. Clamping the flange together provides the pressure to force either the Indium or the Copper to flow through that labrinth leak path, thereby blocking it.

In one potential application, the Helium may be contaminated such that we must keep our leak rate down very low as a matter in Industrial Safety.