Page 28 - Valve Selection Handbook

Fundamentals 15

If the friction factor were zero, the gasket pressure could not exceed
twice the yield shear stress. Thus, a high friction factor improves the
load-bearing capacity of the gasket.
Lok has also shown that a friction factor lower than 0.5, but not less
than 0.2, diminishes the load-bearing capacity of the gasket only by a
small amount. Fortunately, the friction factor of finely machined flange
faces is higher than 0.2. But the friction factor for normal aluminum gas-
kets in contact with lapped flange faces has been found to be only 0.05.
The degree to which surface irregularities are filled in this case is very
low. Polishing the flange face, as is sometimes done for important joints,
is therefore not recommended.
Lok considers spiral grooves with an apex angle of 90° and a depth of
O.lmm (125 grooves per inch) representative for flange face finishes in
the steam class, and a depth of 0.01mm (1250 grooves per inch) repre-
sentative in the atom class. To achieve the desired degree of filling of
these grooves, Lok proposes the following dimensional and pressure-
stress relationships.

Compressed Asbestos Fiber Gaskets

Compressed asbestos fiber is designed to combine the properties of
rubber and asbestos. Rubber has the ability to follow readily the surface
irregularities of the flange face, but it cannot support high loads in plain
strain or withstand higher temperatures. To increase the load-carrying
capacity and temperature resistance of rubber, but still retain some of its
original property to accommodate itself to the mating face, the rubber is
reinforced with asbestos fiber. Binders, fillers, and colors are added to
these materials.
This composition contains fine capillaries that are large enough to per-
mit the passage of gas. The numbers and sizes of the capillaries vary for
product grades, and tend to increase with decreasing rubber content.
Reinforcing wire, which is sometimes provided in compressed asbestos
fiber gaskets, tends to increase the permeability of the gasket to gas.
Consequently, an optimum seal against gas will result when not only the