Decoupling procedure for seismic analysis of piping systems.

Cost minimization in the design and installation of seismically qualified piping systems requires a decoupling procedure that is less conservative than those presently available in the literature. Existing criteria entail satisfying some relations beween modal mass and/or frequency ratio for acceptability in decoupling. Such criteria must be checked for all combinations of model mass pairs between the primary and secondary systems in each of the criteria, even some of these mode pairs fail the criteria, even though little influence from the secondary system is manifested in the primary system response. In the present work, the decoupling criteria proposed in the publications, namely, ASME 78-PVP-27 and NRC Standard Review Plan Section 3.7.2 Rev 1, are extended by the introduction of five additional checks. Presentation of this six step procedure is in terms of the modal participation factor, eigenvectors and primary system response spectra. The proposed method has been found useful and acceptable for design of many nuclear power plant piping systems. (A