> Also, some control valving used on the flammable cryogenic storage tanks use pressurized Helium because it introduces no electrical spark issues and will not have problem turning into liquid because it has a lower temperature.
Not only that (IIRC; IOW might be wrong) but small amounts of liquid helium (etc) can be used to replace the functionality normally represented by rubber (etc) seals on valves altogether. (rubber itself can’t be used since the temperature of liquid reactants is too cold.) the small leakage between the metal to metal contact is tolerated since helium is inert and won’t oxidize anywhere, nor will small amounts of leaked helium decrease anything such as thrust significantly. iirc this might have originally been a technique used by the russians. anyways, cool stuff (if accurate at all, i have no expertise).
It would have to be extremely low pressure seals. Helium is so light it is incredibly difficult to store. When running some of our tests, I watched helium literally flow through the metal of a non-stainless steel connection on top of the LHe Dewar.
We ran some tests on pressure transducers at -450 f. Used a Cryostat to vacuum pull Liquid helium out of the Dewar and into our Cryostat. It was amazing figuring out that the pure Gaseous Nitrogen we used in our pressure lines was liquifying and then freezing SOLID when we pulled a vacuum on our pressure lines. We ended up having to use Helium as our pressurized gas in the pressure lines so that we could obtain accurate pressure readings.