Guidebook for the Design of ASME Section VIII Pressure Vessels, Third Edition
Openings through the pressure boundary of a vessel require extra care to keep loadings and stresses at an acceptable level. Loads may be generated from both internal and external pressure and from applied external loadings. An examination of the pressure boundary may indicate that extra material is needed near the opening to keep stresses from loadings at an acceptable level. This may be provided by increasing the wall thickness of the shell or nozzle or by adding a reinforcement plate around the opening. At some openings, there may be a nozzle to which is attached external piping generating external forces and moments from dead loads or thermal expansion. At other openings only a blind flange or flat cover with little or no available reinforcement may exist.
In designing openings, two types of stresses are important: primary stresses, including both primary membrane stress and primary bending stress; and peak stresses for fatigue evaluation. Although UG-22 of VIII-1 and AD-110 of VIII-2 require that both types be considered when evaluating loadings, in VII-1 rules are given only for calculating the primary membrane stresses.