WRC 529

The traditional area-replacement method has been used in ASME pressure vessel and piping codes for many years. The area-replacement concept requires that the metal removed to make an opening be replaced by an equal area of reinforcement within a prescribed region around the opening. This concept is still used in the ASME B&PV Code, Section VIII, Division 1, paragraph UG-37 for the design of reinforcement at openings in shells and formed heads. However, a substantial amount of information accumulated in recent years indicates that the area-replacement method may lead to excessive conservatism in some applications [1]. In order to overcome the over conservatism of the area replacement method, the pressure-area method was recently introduced in the ASME B&PV Code, Section VIII, Division 2, Part 4, paragraph 4.5. The pressure-area method is well known and widely used in Europe for many years [2, 3]. This concept is based on ensuring that the resistive internal force provided by the material is greater than or equal to the reactive load from the applied internal pressure.

In this WRC Bulletin the basic philosophy and theory behind the pressure-area method that is incorporated in the ASME B&PV Code, Section VIII, Division 2 is presented. The nozzle rules of ASME B&PV Code, Section VIII, Division 2, Part 4, paragraph 4.5 along with a commentary providing background and insight to the rules is provided. Recommendations for modifying the current nozzles rules, those published in ASME B&PV Code, Section VIII, Division 2, 2010 Edition, is also given based on continuing research and development efforts. A comparison between experimental results, results derived from detailed finite element analysis (FEA), the current rules and newly proposed rules is provided in terms of a design margin and permissible maximum allowable working pressure (MAWP) computed with the design rules. The results of this comparison indicate that there is not a significant difference between the current and newly proposed calculation methods. However, implementation of the new rules is still recommended as continuing research and development is being undertaken and it is desirable to analytically define rules without empirical assumptions.