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 the design of reinforced openings, two types of stress are the most important: primary stresses, including both primary membrane stress and primary bending stress; and peak stresses for fatigue evaluation. Although both UG-22 of VIII-1 and Article 188.8.131.52 of VIII-2 require that both types of stresses be considered when evaluating loadings applied to nozzles and openings, in VIII-1 rules are given only for calculating primary stresses due to internal pressure and external pressure. Some higher stresses located at points of stress concentrations, such as corners, are partially controlled by weld size and shape. VIII-2 contains design by rule requirements with equations which are almost the same as the rules of VIII-1 but also cover external forces and moments from applied loadings such as from piping. In addition, however, VIII-2 contains rules for design by analysis which includes methods and rules for fatigue analysis when needed for nozzles and openings as well as other nozzle/opening design rules (see Chapter 8 for more details).