Page 37 - Design Criteria.qxd

38 ® Welcome to the Next Generation of Sealing Technology
ASME Boiler and Pressure Vessel Code Calculations
Section VIII of the ASME Boiler & Pressure Vessel Code, establishes criteria for flange design and suggests values of "m" (gasket fac-
tor) and "y" (minimum gasket seating stress) as applied to gaskets. For the most part, the defined values have proven successful in actu-
al applications. However, much confusion exists regarding these values, primarily due to a misunderstanding of the definitions of the
terms and their significance in practical applications. Mandatory Appendix II, in Section VIII of the Boiler Code, requires in the design
of a bolted flange connection, complete calculations shall be made for two separate and independent sets of conditions.
Operating Conditions
Condition one (1) requires a minimum load be determined in accordance with the following equation:
This equation states the minimum required bolt load for operating conditions and is the sum of the hydrostatic end force, plus
a residual gasket load on the contact area of the gasket times a factor times internal pressure. Stated another way, this equation requires
the minimum bolt load be such that it will maintain a residual unit compressive load on the gasket area that is greater than internal pres-
sure when the total load is reduced by the hydrostatic end force.
Gasket Seating
Condition two (2) requires a minimum bolt load be determined to seat the gasket regardless of internal pressure and utilizes a formu-
la:
The "b" in these formulae is defined as the effective gasket width and "y" is defined as the minimum seating stress in psi. For
example, Section VIII of the Boiler Code suggests a minimum "y" value for a spiral wound gasket of 10,000 psi (Winter 1976
Addenda). These design values are suggested and not mandatory. The term "b" is defined as:
At this point, it is important to realize the gasket must be capable of carrying the entire compressive force applied by the bolts
when prestressed unless provisions are made to utilize a compression stop in the flange design or by the use of a compression gauge
ring. For this reason, FLEXITALLIC's standard practice is to assume W is equal to Ab Sa.
We are then able to determine the actual unit stress on the gasket bearing surface. This unit stress Sg is calculated as follows:
*Note: Based on 4.5mm (.175") thick spiral wound gasket. The “v” or Chevron shape on the gasket O.D. is not part of the effective
seating width, therefore .125” is subtracted from the actual gasket O.D.
Using the unit stress we can assign construction details which will lead to the fabrication of a gasket having sufficient density to carry
the entire bolt load.
b = bo when bo =1/4" b = 0.5 bo when bo > 1/4"
After Wm1, and Wm2 are determined, the minimum required bolt area Am is determined as follows:
Am = Wm1 where Sb is the allowable bolt stress at operating temperature, and
Am2 = where Sa is the allowable bolt stress at atmospheric temperature.
Then Am is equal to the greater of Am1 or Am2. Bolts are then selected so the actual bolt area, Ab, is
equal to or greater than Am.
Wm2
Sb
Sa
(1) Wm1 = + 2b 3.14GmP3.14G2P
4
(2) Wm2= 3.14bGy
(3) Sg (psi) = Ab Sa
.785 [(do - .125*)2 - (di)2]