Companion Guide to the ASME Boiler & Pressure Vessel Code, Volume 2, Second Edition: Criteria and Commentary on Select Aspects of the Boiler & Pressure Vessel and Piping Codes
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In Chapter 38, Everett C. Rodabaugh discusses stress intensification factors, stress indices, and flexibility factors with the aid of equations, references, and tables. Everett, a well-recognized authority on the aforementioned topics, notes that piping systems tend to be rather complex structures that include straight pipe and a variety of complex components, such as elbows and tees. A typical piping system might include about 50 components along with many intervening lengths of straight pipe. Each component is subjected to a complex set of loadings. The evaluation of any one component by the detailed analysis methods prescribed in Subsection NB-3200 is an onerous task. The complexity of analyses of piping components and the “standard” aspect of piping components has led to use of stress intensification factors (also called i-factors), stress indices, and flexibility factors for evaluations of piping systems. In this chapter, the general concepts behind the development of i-factors, stress indices, and flexibility factors are discussed briefly, with references to details of developments.
Everett discusses strain control and stress control as design considerations. Chapter 38 is replete with scholastic discussions and references to substantiate the use of indices. The discussions are provided for the nominal design margin and tests to support these. Stress intensification factors are discussed for girth butt welds; C and K stress indices are discussed for internal-pressure loading, elbows, and butt-welding tees; moment loading; and thermal-gradient loadings (including branch connections). Fatigue evaluations, ASME Class 2 or 3 piping, and Class 1 Codes for straight pipe, elbows, and seismic analysis are also addressed. Everett provides examples to cover piping systems; moments; Code equations; girth butt welds; elbows; branch connections for Class 2 or 3 piping with branches; best estimates for Class 1 piping; and ASME B31.1 and B31.3 Codes. These examples illustrate how i-factors and stress indices are used to check Code compliance and, for a branch connection, to illustrate the quantitative significance of flexibility factors.