Page 34 - Valve Selection Handbook

Fundamentals 21

fiber or foil. Other packing materials include vegetable fibers such as
cotton, flax, and ramie (frequently lubricated with PTFE), and twisted
and folded metal ribbons.
The types of fibrous packing constructions in order of mechanical
strength are loose fill, twisted yarn, braid over twisted core, square-plait
braid, and interbraid constructions. The covers of the latter three types of
packing constructions often contain metal wire within the strands to
increase the mechanical strength of the packing for high fluid pressure
and high temperature applications.
Reference 9 offers advice on selection and application of compression
packings. Standards on packings may be found in Appendix C.

Sealing action. The sealing action of compression packings is due to
their ability to expand laterally against the stem and stuffing box walls
when stressed by tightening of the gland.
The stress exerted on the lateral faces of a confined elastic solid by an
applied axial stress depends on Poisson's ratio for the material, as
expressed by:

where
GI = lateral stress
a a = axial stress
\Ji = Poisson's ratio
= ratio of lateral expansion to axial compression of an elastic solid
compressed between two faces

Thus, the lateral stress equals the axial stress only if |H = 0.5, in which
case the material is incompressible in bulk.
A material with a Poisson's ratio nearly equal to 0.5 is soft rubber, and
it is known that soft rubber transmits pressure in much the same way as a
liquid. 10 Solid PTFE has a Poisson's ratio of 0.46 at 23°C (73°F) and
n
0.36 at 100°C (212°F). A solid PTFE packing is capable of transmitting
85% and 56% of the axial stress to the lateral faces at the respective tem-
peratures. Other packing materials, however, are much more compress-
ible in bulk, so Poisson's ratio, if it can be defined for these materials, is
considerably less than 0.5.