Minimum Stress Design of Nozzles in Pressure Vessel Heads

In the early design stage of pressure vessels the configuration of the piping systems is not yet established; hence forces transmitted by the piping systems to the nozzles in the pressure vessels cannot be determined. This often leads to the design of nozzles in pressure vessels guided by consideration of pressure loadings such as the area-replacement method. However, it is true that in many cases the stresses due to external loads can be more critical than those due to the internal pressure. Therefore, engineers often redesign the piping system several times by adding more pipe bends or special restraints for a hot piping system to reduce the reactions at a previously designed nozzle so that the resulting stresses at the nozzle are within the acceptable limit. This paper introduces a rational mechanism whereby the stresses due to the unforeseen external loads can be minimized in the early design stage of the nozzle. An appropriate analysis is discussed which is based on the classical thin shell theory. Analyses using this method allow one to obtain the minimum stresses at a nozzle in a pressure vessel head or a spherical vessel for moment and thrust loadings.