Page 33 - Valve Selection Handbook

20 Valve Selection Handbook

the gasket should be not less than five times its thickness to prevent
blowout of the gasket without prior leakage warning.

VALVE STEM SEALS

Compression Packings

Construction. Compression packings are the sealing elements in stuffing
boxes (see Figures 3-17 through 3-19). They consist of a soft material
that is stuffed into the stuffing box and compressed by a gland to form a
seal around the valve stem.
The packings may have to withstand extremes of temperature, be
resistant to aggressive media, display a low friction factor and adequate
structural strength, and be impervious to the fluid to be sealed. To meet
this wide range of requirements, and at the same time offer economy of
use, innumerable types of packing constructions have evolved.
Constructions of compression packings for valve stems were, in the
past, based largely on asbestos fiber because of its suitability for a wide
range of applications. Asbestos is suitable for extremes of temperatures,
is resistant to a wide range of aggressive media, and does not change its
properties over time. On the debit side, asbestos has poor lubricating
properties. Therefore, a lubricant must be added—one which does not
interfere with the properties of asbestos, such as flake graphite or mica.
This combination is still permeable to fluids, and a liquid lubricant is
added to fill the voids. Again, the lubricant must not interfere with the
properties of the construction. This is often very difficult, and in response
to this challenge, thousands of variations of packings based on asbestos
have been produced.
The types of lubricants used for this purpose are oils and greases when
water and aqueous solutions are to be sealed, and soaps and insoluble sub-
stances when fluids like oil or gasoline are to be sealed. Unfortunately,
liquid lubricants tend to migrate under pressure, particularly at higher
temperatures, causing the packing to shrink and harden. Such packings
must, therefore, be retightened from time to time to make up for loss of
packing volume. To keep this loss to a minimum, the liquid content of
valve stem packings is normally held to 10% of the weight of the packing.
With the advent of PTFE, a solid lubricant became available that can
be used in fibrous packings without the addition of a liquid lubricant.
Asbestos is now avoided in packings where possible, replaced by
polymer filament yarns, such as PTFE and aramid, and by pure graphite