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D-34
Calculating Load Requirements
Two formulas that defne the minimum load re-
quired to effect a seal on a particular gasket are Wm1
and Wm2. When these formulas have been calculated,
the larger load of the two is the load necessary to effect
a seal.
Let:
π
= 3.14
P = Maximum internal pressure
M = Gasket factor "M" defned on page D-33.
(M = 3 for spiral wound gaskets)
Y = Seating stress "Y" defned on page D-33.
(Y = 10,000 psi for spiral wound gaskets)
N = Basic width of a gasket per chart on page D-32.
(For raised face fanges see diagram 1a)
B
0
= Basic seating width of a gasket per chart
on page D-32. (For raised face fanges, B
0
= N/2)
B
1
= Effective seating width of a gasket; must
be determined.
ID = Inside diameter of gasket
OD = Outside diameter of gasket
For gaskets where the raised face is smaller
than the OD of the gasket face, the OD is
equal to the outer diameter of the raised face.
Find:
ID = _ ______
OD = _ ______
Given the ID and OD, fnd the value of N. Then
defne B
0
in terms of N (See page D-32):
N = _ ______
B
0
= _ ______
Determine if B
0
is greater or less than 1/4", then
fnd B
1
:
If B
0
≤ 1/4", then B
1
= B
0
;
If B
0
> 1/4", then B
1
= (√ B
0
)/ 2
B
1
= _ ______
Using B
1
, determine G:
G = OD - [(B
1
)(2)]
Now, insert these values in the fnal equations to
determine minimum required load:
Wm1 =
[
π
(P)(G
2
)/4] + [2(B
1
)(
π
)(G)(M)(P)]
Wm2 _=
π
(B
1
)(G)(Y)
When Wm1 and Wm2 have been calculated, the
larger of the two numbers is the minimum load required
to seat a gasket. In most cases the available bolt load
in a connection is greater than the minimum load on
the gasket. If not, higher bolt stresses or changes in the
gasket design are required for an effective seal.
Note:
Flange design code suggestions for low pressure
applications calling for minimum seating stress (Y
value) are sometimes inadequate to seat the gasket
because the bolting and fange rigidity are insuffcient
to effect a proper seal. Care should be taken to en-
sure that fange conditions provide a suitable seating
surface. For internal pressures to be contained, fange
rotation and suffcient residual loads must also be con-
sidered in the fange design.
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