Page 38 - Valve Selection Handbook

Fundamentals 25

rigid construction materials, which would not perform as well in com-
pression packings. On the debit side, the sealing action of lip-type pack-
ings is in one direction only.
Most lip-type packings for valve stems are made of virgin or filled
PTFE. However, fabric-reinforced rubber and leather are also used,
mainly for hydraulic applications. Most lip-type packings for valve stems
are V shaped, because they accommodate themselves conveniently in
narrow packing spaces.
The rings of V-packings made of PTFE and reinforced rubber are
designed to touch each other on small areas near the tips of their lips, and
large areas are separated by a gap that permits the fluid pressure to act
freely on the lips. Leather V-packing rings lack the rigidity of those made
of PTFE and reinforced rubber, and are therefore designed to fully sup-
port each other.
V-packings made of PTFE and reinforced rubber are commonly pro-
vided with flared lips that automatically preload the restraining lateral
faces. In this case, only slight initial tightening of the packing is neces-
sary to achieve a fluid seal. V-packing rings made of leather have straight
walls and require a slightly higher axial preload. If a low packing friction
is important, as in automatic control valves, the packing is frequently
loaded from the bottom by a spring of predetermined strength to prevent
manual overloading of the packing.

Squeeze-Type Packings

The name squeeze-type packing applies to O-ring packings and the
like. Such packings are installed with lateral squeeze, and rely on the
elastic strain of the packing material for the maintenance of the lateral
preload. When the fluid pressure enters the packing housing from the
bottom, the packing moves towards the gap between the valve stem and
the back-up support and thereby plugs the leakage path. When the pack-
ing housing is depressurized again, the packing regains its original con-
figuration. Because elastomers display the high-yield strain necessary for
this mode of action, most squeeze packings are made of these materials.
Extrusion of the packing is controlled by the width of the clearance gap
between the stem and the packing back-up support, and by the rigidity of
the elastomer as expressed by the modulus of elasticity. Manufacturers
express the rigidity of elastomers conventionally in terms of Durometer
hardness, although Durometer hardness may express different moduli of
elasticity for different classes of compounds. Very small clearance gaps are