What a npt valve is in a water tank pressure system?
npt, national pipe thread, is a standard for threads on a screw.
I (not a plumber) don't believe "npt valve" makes sense , but
probably refers to a valve which has npt threading.
Yes, a NPT Valve would be a valve with pipe threads as opposed
to a sweat valve which has solder connections. There are several
other type of piping connections too.