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Bolt Torque for Gasket Contact Stress per. ASME PCC-1
Bolt Torque on Assembly for Gasket Contact Stress per. ASME PCC-1 Equations and Calculator
It is recommended that bolt assembly stresses be established with due consideration of the following joint integrity issues.
(a) Sufficient Gasket Stress to Seal the Joint. The assembly bolt stress should provide sufficient gasket stress to seat the gasket and sufficient gasket stress during operation to maintain a seal.
(b) Damage to the Gasket. The assembly bolt stress should not be high enough to cause over-compression (physical damage) of the gasket or excessive flange rotation of the flange, which can also lead to localized gasket over-compression.
(c) Damage to the Bolts. The specified bolt stress should be below the bolt yield point, such that bolt failure does not occur. In addition, the life of the bolt can be extended by specifying an even lower load.
(d) Damage to the Flange. The assembly bolt stress should be selected such that permanent deformation of the flange does not occur. If the flange is deformed during assembly, then it is likely that it will leak during operation or that successive assemblies will not be able to seal due to excessive flange rotation. Leakage due to flange rotation may be due to concentration of the gasket stress on the gasket outer diameter causing damage or additional relaxation. Another potential issue is the flange face outer diameter touching, which reduces the effective gasket stress.
Assembly bolt stress across contact surfaces, it is recommended that, as a minimum, the target gasket stress, SgT, for a given gasket type be considered. Further integrity issues, as outlined in the following section on the joint component approach, may also be considered, as deemed necessary.
The appropriate bolt stress for a range of typical joint configurations may be determined via eq. 1
Sbsel = SgT · Ag / ( nb · Ab )
The average bolt stress across the joints considered may then be selected and this value can be converted into a torque table using eq. (2M) for metric units or eq. (2) for U.S. Customary units.
Tb = Sbsel · K · Ab · Φb / 12
Tb = Sbsel · K · Ab · Φb / 1,000
An example of the type of table produced using this method is given in Table 1, which was constructed using a bolt stress of 50 ksi and a nut factor, K, of 0.20. If another bolt stress or nut factor is required, then the table may be converted to the new values using eq. (3), where Sb'sel, T'b, and K′ are the original values.
Tb = ( K / K′ ) · Sbsel / ( Sb'sel · T'b )
Sbsel = selected assembly bolt stress, MPa (psi)
SgT = target assembly gasket stress, MPa (psi)
Ag = gasket area, mm2 (in.2)1
nb = number of bolts
Ab = bolt root area or [ (Minor Dia / 2)2 π ], mm2 (in.2)
Tb = assembly bolt torque, N·m (ft-lb)
K = nut factor (for bolt material and temperature) 0.20 and 0.16 for noncoated and coated bolts, respectively.
Φb = bolt diameter, mm (in.)
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