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On December 31, 1973, the American Society of Mechanical Engineers (ASME) published Subsection NF  of the ASME Boiler and Pressure Vessel Code Section III, Division 1 (hereafter addressed as Subsection NF) as part of the winter 1973 addenda to the 1971 Code edition . This was a historic publication; prior to it, supports (also called pipe hangers and restraints) were not addressed as part of ASME Section III. Existing nuclear plants and plants under construction during that time were using ANSI B31.1 , ANSI B31.7 , MSS-SP58 , and the AISC Manual of Steel Construction  to design supports. ASME Section III, Subsection NF, provided a stabilizing position for future nuclear plant support design by designating a single source of rules for the design, construction, fabrication, and examination of supports.
This criteria-and-commentary chapter provides information on the origins and evolution of design rules and is intended to allow designers, engineers, and fabricators to make better use of Subsection NF of the ASME Code. Topics of greatest interest are discussed and addressed from both a technical and historical viewpoint. It is not the intent, however, to address every detail or anticipate every question associated with the use of the subsection. However, there will be situations when engineering judgment and special considerations will be used in conjunction with Subsection NF to qualify supports.
ASME Boiler and Pressure Vessel Code Section III, Division 1, Subsection NF, was developed in an attempt to provide rules for the estimated 10,000 piping and component supports existing in a typical nuclear power plant. These rules have evolved so dramatically that the existing support rules seldom resemble the original rules of 1973. This document follows the evolution of Subsection NF as the industry attempted to apply the subsection's rules. Commentary is provided to explain how the criteria are used, the source and technical basis for the equations and the rationale, and the reasons for change. It is anticipated that readers will develop a better understanding of Subsection NF to appreciate its complexities and usefulness.