For un-insulated flanged joints, the bolts are assumed to be a different temperature from the flanges, causing differential thermal expansion across the bolted joint. For systems operating above installation temperatures, this differential will tend to tighten the joint. For systems operating below the installation temperature this differential will tend to open the joints.
Additionally if the flange joint will be subjected to high temperatures, the initial calculated bolt load shall be increased by the amount necessary to ensure an adequate working life for the joint, considering loss of bolt load due to the effect of "creep and relaxation" on the bolts. Gasket relaxation should be incorporated as well.
The hydrostatic test condition shall be also considered.
For mechanical equipment stresses are compared against the lowest minimum yield stress determined
for bolting materials found in the ASME VIII, Div-1
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