Page 38 - Design Criteria.qxd

39 ® Welcome to the Next Generation of Sealing Technology
ASME Boiler and Pressure Vessel Code Calculations
Gasket Seating Stress "Y"
Defined as the applied stress required to seat the gasket upon the flange faces. The actual required seating stress is a function
of flange surface finish, gasket material, density, thickness, fluid to be sealed and allowable leak rate.
Gasket Factor "m"
Appendix II, Section VIII, of the Boiler Code makes the statement the "m" factor is a function of the gasket material and con-
struction. We do not agree entirely with this interpretation of "m". Actually, the gasket does not create any forces and can
only react to external forces. We believe a more realistic interpretation of "m" would be “the residual compressive force exert-
ed against the gasket contact area must be greater than internal pressure when the compressive force has been relieved by the
hydrostatic end force”. It is the ratio of residual gasket contact pressure to internal pressure and must be greater than unity oth-
erwise leakage would occur. It follows then, the use of a higher value for "m" would result in a closure design with a greater
factor of safety. Experience has indicated a value of 3 for “m” is satisfactory for flanged designs utilizing Spiral Wound gas-
kets regardless of the materials of construction. In order to maintain a satisfactory ratio of gasket contact pressure to internal
pressure, two points must be considered. First, the flanges must be sufficiently rigid to prevent unloading the gasket due to
flange rotation when internal pressure is introduced. Secondly, the bolts must be adequately prestressed. The Boiler Code rec-
ognizes the importance of pre-stressing bolts sufficiently to withstand hydrostatic test pressure. Appendix S, in the Code, dis-
cusses this problem in detail.
Notations
A b = Actual total cross sectional root area of bolts or section of least diameter under stress; square inches
Am = Total required cross sectional area of bolts, taken as greater of Am1 or Am2; square inches
Am1 = Total required cross sectional area of bolts required for operating conditions; square inches
Am2 = Total required cross sectional area of bolts required for gasket seating; square inches
b = Effective sealing width; inches
bo = Basic gasket seating width; inches
2b = Joint-contact-surface pressure width; inches
G = Diameter of location of gasket load reaction; inches
m = Gasket factor
N = Radial flange width of spiral wound component
P = Design pressure; psi
Sa = Allowable bolt stress at atmospheric temperature; psi
Sb = Allowable bolt stress at design temperature; psi
W = Flange design bolt load; pounds
Wm1 = Minimum required bolt load for operating conditions; pounds force
Wm2 = Minimum required bolt load for gasket seating; pounds force
y = Minimum gasket seating stress; psi
Sg = Actual unit stress at gasket bearing surface; psi
do = Outside diameter of gasket; inches
di = Inside diameter of gasket; inches
The ASME boiler and pressure vessel code is currently under review by the Pressure Vessel Research Council. Details of
these proposed improvements, including the effects on gasket design procedures are highlighted on page 42.