Postponed Packing Gland

Hi,

Maybe it relates to valves known as "zero leakage", that's valves with a metal bellow seal (or a diaphragm, but you've talking about bellows) to reduce the leakage through the stem (usually when required less than 10^-6 scc/sec). Most of this valve type have a standard packing above the bellow.

Just google "zero leakage valves" or have a look at Emerson Control Valve Handbook (you can find it free on the net)

Kind regards