Valve Packing:

Most valves use a few form of packing to avoid leakage from the space among the stem and the bonnet. Packing is generally a fibrous material (such as flax) or other elements (such as teflon) which forms a seal among the internal elements of a valve and the outside whereas the stem extends by the body.

Valve packing must be properly compressed to avoid fluid loss and damage to the valve's stem. The valve will leak, that is a safety hazard if a valve's packing is too loose. If the packing is too strong, it will impair the movement and probably damage the stem.