Sounds like you absorbed it pretty well. This handy temperature reference will help explain it’s uses:
Liquid Helium (LHe) has a temperature around -452F. Inert
Liquid Hydrogen (LH2) has a temperature around -423F. Extremely flammable.
Liquid Nitrogen (LN2) has a temperature around -320F. Inert
Liquid Oxygen (LOX) has a temperature around -297F. EXTREMELY Flammable. It is used as the oxidizer for other cryogenic fuels.
Liquid Methanes (there are different types) have a temperature near that of LOX.
System Purges are done with Gaseous Helium to help remove the the flammable fuels from the system. Helium is good to do that with because it’s liquid point is so low.
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 a subject-matter expert in the field but have had to use LHe turn run some tests on super-cooled devices, so we had to learn about it.
> 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).