Experimental data from the EPRI Piping and Fitting Dynamic Reliability Program are evaluated. High-amplitude, seismic time history loads were applied to 32 piping components in a cantilevered configuration. Components included elbows, tees, reducers, a reinforced fabricated tee, a nozzle, and lugs. The test levels were about 2 to 8 times Level D. The seismic capability of elbows is shown to be remarkable. The seismic performance of lugs is judged to be poor. The tests demonstrate that collapse is a potential failure mode, particularly in low-frequency systems. The tests also demonstrate that a fatigue failure in a single, high-level, seismic event is possible. Of the 15 nonelbow tests that failed by fatigue, six (40 percent) failed during the first high-level seismic test. Design and fabrication details are crucial to seismic performance. Trends in the data need to be assessed to determine appropriate Section III stress limits for seismic loads. In particular, the impact of component frequency on seismic capability has to be quantified.
Much of the piping in a nuclear plant is designed to the ASME Boiler and Pressure Vessel Code, Section III, Class 2 or 3 requirements. The Code rules are given in NC/ND-3600 of Section III. These rules were developed from the B31.1 rules, and incorporated into Section III 1971 for nuclear components. However, the Class 2/3 piping Code requirements have changed significantly since that time. There is not a criteria document available from ASME that describes the Class 2/3 piping rules. The purpose of this paper is to provide background information and a discussion of the Code stress limits so that the piping analyst can properly interpret the meaning of the Code requirements.
The ASME Section III Design-by-Analysis rules for pressure-retaining components include a detailed fatigue evaluation based on elastically predicted primary, secondary, and peak stresses. A prerequisite for the fatigue analysis is that the primary-plus-secondary stress range does not exceed 3Sm. If this limit is exceeded, the code provides “Simplified Elastic-Plastic Analysis” rules for the fatigue evaluation. A Ke penalty factor is applied to the elastically predicted alternating stress. The maximum value of Ke (3.3 or 5) is a severe design limitation. Test data indicate that the code specified maximum value of Ke is very conservative. The simplified elastic-plastic rules were originally developed for piping and published in B31.7. When the piping rules were incorporated into Section III in 1971, the B31.7 procedure was replaced by a less complex procedure. The development of the simplified elastic-plastic analysis approach is reviewed to establish the technical basis for the present code rules. The concepts of fatigue, shakedown to elastic action, thermal bending, elastic follow-up, notch factor, and strain redistribution are discussed. Recommendations for changes to the plastic strain correction factor are provided.
The ASME Section III Design-by-Analysis rules for pressure-retaining components include a detailed fatigue evaluation based on elastically-predicted primary, secondary, and peak stresses. A pre-requisite for the fatigue analysis is that the primary-plus-secondary stress range does not exceed 3Sm. If this limit is exceeded, the code provides “Simplified Elastic-Plastic Analysis” rules for the fatigue evaluation. A Ke penalty factor is applied to the elastically-predicting alternating stress. The maximum value of Ke (3.3 or 5) is a severe design limitation. Test data indicate that the code specified value of Ke is very conservative. The simplified elastic-plastic rules were originally developed for piping and published in B31.7. When the piping rules were incorporated into Section III in 1971, the B31.7 procedure was replaced by a less complex procedure. The development of the simplified elastic-plastic analysis approach is reviewed to establish the technical basis for the present code rules. The concepts of fatigue, shakedown to elastic action, thermal bending, elastic follow-up, notch factor, and strain redistribution are discussed. Recommendations for changes to the plastic strain correction factor are provided.
A summary of The 2006 Forum on Seismic Design of Piping Systems for the Year 2010 is provided. This forum session is the tenth in a series that was started in 1992. Previously, the title was Appropriate Criteria and Methods for Seismic Design of Nuclear Piping. In this 2006 forum, the main topics of discussion were: “SSE only” design, a comparison of the Level B (OBE) and Level D (SSE) design requirements, and inspection after an earthquake.
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