Appendice
Piping - Heat transmission

Thermal stresses due to temperature differences in the metal were first taken into account by the ASME Code - Section III - Chapter NB (relating to Code Class I piping). In all other cases, these stresses are not taken into account, which implies that they must be low, and therefore temperature variations fairly slow.

However, there are cases where more or less rapid temperature variation is unavoidable (reheating from a cold state, thermal shocks on bypass circuits and, in general, on any piping system with a normally closed control valve that can open suddenly to let a hot fluid through). In all these cases, it may be necessary to check whether or not thermal gradients are permissible.