For a reliable seal between the stem and the bonnet, a gasket is needed. This is called a Packing, and it is fitted with e.g. the following components:
Gland follower, a sleeve which compresses the packing, by a gland into the so called stuffing box.
Gland, a kind of bushing, which compressed de packing into the stuffing box.
Stuffing box, a chamber in which the packing is compressed.
Packing, available in several materials, like Teflon®, elastomeric material, fibrous material etc..
A backseat is a seating arrangement inside the bonnet. It provides a seal between the stem and bonnet and prevents system pressure from building against the valve pakking, when the valve is fully open. Back seats are often applied in gate and globe valves.
An important aspect of the life time of a valve is the sealing assembly. Almost all valves, like standard Ball, Globe, Gate, Plug and Butterfly valves have their sealing assembly based upon shear force, friction and tearing.
Therefore valve packaging must be properly happen, to prevent damage to the stem and fluid or gas loss. When a packing is too loose, the valve will leak. If the packing is too tight, it will affect the movement and possible damage to the stem.
TYPICAL SEALING ASSEMBLY
1. Gland Follover 2. Gland 3. Stuffing Box with Packing 4. Back Seat