The purpose of the packing in a
valve is to prevent the leakage of the media in the valve to the exterior atmosphere past the
stem of the valve. The valve packing is contained in the
valve bonnet. Of course this only applies if the valve has a stem, so stemless valves like for example
check valves do not have packing.
The biggest threat to valve packing is either the rotation of the stem (in ball valves) or the linear motion of the stem as in globe valves. In either case, the motion of the metal parts wears, over time, the elastomers used to maintain the seal and eventually develops a leak.
The form of the packing can vary significantly from a simple Buna O-ring to elaborate double packed, spring loaded configurations with an intermediate monitoring port. The port, you may think is overkill but let me assure you it is highly desirable when the gas going through the valve is phosgene or cyanide.
The elastomeric material is compressed from the top by a gland follower or compression ring. The elastomer needs to be replaced when it has worn out which is the biggest cause for valve repairs. Some industrial plants have statisticly analyzed how long valve packing lasts to the point they have incorporated valve maintenance into their predictive maintenance programs.
If the utmost in valve packing reliability is required, globe valves with bellows seal bonnets are used. The bellows is welded to the body and stem but, like a spring, can be stretched so it allows the movement of the stem. Because of the welds, there is no elastomers involved and leaks can only be caused by corrosion or fracture.