Page 27 - Valve Selection Handbook

14 Valve Selection Handbook

Heat damage to the soft seating element is combated in globe valves
by a heat sink resembling a metallic button with a large heat-absorbing
surface, which is located ahead of the soft seating element. In the case of
oxygen service, this design measure may not be enough to prevent the
soft seating element from bursting into flames. To prevent such failure,
the valve inlet passage may have to be extended beyond the seat passage,
so that the end of the inlet passage forms a pocket in which the high tem-
perature gas can accumulate away from the seatings.
In designing soft seatings, the main consideration is to prevent the soft
seating element from being displaced or extruded by the fluid pressure.

GASKETS

Flat Metallic Gaskets

Flat metallic gaskets adapt to the irregularities of the flange face by
elastic and plastic deformation. To inhibit plastic deformation of the
flange face, the yield shear strength of the gasket material must be con-
siderably lower than that of the flange material.
The free lateral expansion of the gasket due to yielding is resisted by
the roughness of the flange face. This resistance to lateral expansion
causes the yield zone to enter the gasket from its lateral boundaries,
while the remainder of the gasket deforms elastically initially. If the
flange face is rough enough to prevent slippage of the gasket altogeth-
er—in which case the friction factor is 0.5—the gasket will not expand
5
until the yield zones have met in the center of the gasket.
For gaskets of a non-strain-hardening material mounted between per-
1
fectly rough flange faces, the mean gasket pressure is, according to Lok,
approximately:

where
P m = mean gasket pressure
k = yield shear stress of gasket material
w = gasket width
t = gasket thickness