It’s a fresh air vent to purge and argon environmentI used to design valves for a living only low pressure ones though up to 15000 PSI, boring i know right
What sort of pressure are you looking at? What is the medium running through the valve?
Cheap Chinese valves will have Delrin or PTFE sets which is fine for most low pressure applications, if your looking at a high pressure application then you would normally switch over to a PEEK seated valve.
If you want to buy a valve again not sure where you are in the country but we use a company called Orseal for all our nipples and valves that we buy at my current place
As for sealing on a BSP threaded connection i would always recommend PTFE as its easier to make and break the seal, however if you need the valve in a specific orientation and access is a problem a strong thread sealant is the way to go.
Not after the last fewPotato cannon i bet.