Design rules for Safety Related Class 2 and 3 piping systems were first incorporated in the ASME Boiler and Pressure Vessel Code, Section III, Division 1 in the 1971 Edition. Since that time, the rules have evolved. The most significant changes were (a) implementation of the use of B’s stress indices for primary stresses, (b) reduction in the factor of safety for base pressure design from 4 to 3.5, and (c) implementation of new criteria for reversing dynamic loads. Each of these changes is discussed and the sequential and cumulative effect of changes on design margin is presented. In addition, the general history of the evolution of Class 2/3 designs in Section III of the ASME Boiler and Pressure Vessel Code is provided.

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