34 Perspective on Cyclic, Impact, and Impulse Loads
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John D. Stevenson initially authored Chapter 34 for the first and second editions of this publication. This update of this chapter is covered by Michael A. Porter.
In the previous editions John Stevenson dealt with perspectives on cyclic, impact, and impulse dynamic loads. John notes that dynamic loads applied to the design of mechanical systems and components are of three basic types: cyclic, impulse, and impact. In addition, there is a fourth potential cyclic-type load in the vibratory motion category. Although vibratory motion is not usually considered in the original design basis, it may be observed during steady-state or transient operations to cause premature fatigue or ratchet failure of metal components. The original Section III definition of a plant's operating life includes design-basis normal, abnormal, emergency, and faulted plant- or system- operating conditions, as defined in the Design Specification. John notes that these operating conditions should not be confused with Service Levels A, B, C, and D currently defined in the Code for design purposes. It is possible to have different Service Level design conditions for the same operating condition, depending on the required response of a component.
John covered in-depth Nonmandatory Appendix N (Dynamic Analysis Methods) of Section III. For completeness, he covers other types of dynamic loads not addressed explicitly by Appendix N that the designer of pressure-retaining nuclear components must consider. He also discusses the ASCE Standard used for defining earthquake motions to a building foundation and for supporting the mechanical system or component; these two references deal primarily with earthquake cyclic—type dynamic loads. In addition, commentary is provided on the guidelines used for dynamic impulse and impact loadings provided in the ANS, ASME Appendix B, and B31.1 Code Standards. A discussion is provided about the ASME Operation and Maintenance (O&M) Standard, used for determining the effect of operational vibratory motion independent of the cause of vibration.
In past editions of the handbook, this chapter has primarily addressed issues as they pertained to Section III of the ASME B&PV Code. In particular, it has addressed issues concerning the seismic response of nuclear facilities. Currently, many other facilities covered by the ASME B&PV Code have had to address these same issues, often with little guidance from the appropriate Code section. Modern LNG terminals, for example, have had to undergo extensive seismic reviews. These facilities contain equipment covered by Section VIII and B31.3 of the Code. Neither of these Code section give any guidance to the designer concerning seismic analysis, other than to require that seismic loads be addressed. In this revision of the chapter by Michael A. Porter, some of these issues will be addressed. In addition, a new section discussing the use of computer software for analysis has been included. This section will address some of the issues associated with different computer codes used for different parts of a plant. In addition, the Code references have been revised to reflect the current (2007) Code provisions.