Page 41 - Design Criteria.qxd

42 ® Welcome to the Next Generation of Sealing Technology
PVRC METHOD
Current gasket design calculations for bolted
joints such as ASME VIII, DIN 2505, etc., have
many shortcomings surrounding the expected
tightness and optimum operating stress levels to
ensure against joint leakage. In general, current
design methods only ensure that the optimum
bolt load is available to seat the gasket and
accommodate the hydraulic loads created by the
internal pressure. Little information is given
regarding the tightness of the joint in service or
the optimum level of gasket stress to fulfill the
legislative, environmental and company emission
requirements at the source of application.
Flexitallic financially supports, and is
actively involved in the research efforts of the
ASME's Pressure Vessel Research Council
(PVRC) to review and update current gasket
design methodology. The PVRC has, through
many years of research and development (involv-
ing hundreds of actual gasket tests), conceived a
new philosophy that addresses the mechanisms of
sealing that will benefit gasket manufacturers, vessel designers and the operators of process equipment in general. The result is a pack-
age that recommends minimum levels of gasket assembly stress to fulfill the operational requirements of the user. The new procedure
is similar to the existing ASME Section VIII calculation, except it incorporates new gasket factors (to replace the traditional m & Y
gasket factors) that have been determined through an extensive test program.
The new gasket factors are (Gb), (a), and (Gs).
(Gb) and (a) represent the initial gasket compression characteristics and relate to the initial installation, while (Gs) represents
the unloading characteristics typically associated with the operating behavior.
The PVRC method has been developed over the years using the following parameters for bolted joint designs and determining gasket
constants:
1. Determine the tightness class 'Tc' that corresponds to the acceptable leak rate for the application (legislative, environmental, or
company emission legislation).
T2: Standard; represents a mass leak rate per unit diameter of 0.002 mg/sec/mm-dia.
T3: Tight; represents a mass leak rate per unit diameter of 0.00002 mg/sec/mm-dia.
2. Select the tightness constant that corresponds to the chosen tightness class
C = 1.0 for tightness class T2 (Standard).
C = 10.0 for tightness class T3 (Tight).
3. Select the appropriate gasket constants (Gb), a, and (Gs) for the gasket style and material, (see table, page 43).
4. Determine gasket parameters (N), (b o ), (b), and (G) as per table (page 40).
5. Gasket seating area, Ag = 0.7854(OD 2 -ID 2 ).
6. Hydraulic area, Ai = 0.7854G 2
7. Minimum required tightness, Tpmin = 0.1243 x C x P d , P d = Design Pressure
8. Assembly Tightness T pa = 0.1243 x C x P t , P t = Test Pressure (Typically 1.5 x P d )
9. Tightness Parameter Ratio, Tr = Log(Tpa)/Log(Tpmin)
10. Gasket Operating Stress, Sm1 = Gs[G b /Gs x Tpa a ] 1/Tr
10000
Tightness Parameter Tp
1000 100 10
Gs
Gb
Gasket
Stress
a
Part A
Part B
Cycles
Tp min Tpn
Sgmin > P
Sa
Idealization of Stress vs. Tightness showing the basis
for the gasket constants Gb, a and Gs