The History of Allowable Damping Values for U.S. Nuclear Plant Piping

Prior to 1982, pipe damping values in nuclear plants were prescribed by Regulatory Guide 1.61 and Appendix N to Section III of the ASME Boiler and Pressure Vessel Code. In the early 1980s, it became clear that piping design was too conservative resulting in systems that had far too many supports, particularly snubbers. These supports were costly to design, install, and inspect; contributed to increased worker radiation exposure; and since snubbers sometimes lock when unloaded causing higher fatigue usage in piping, the safety margin of the systems was reduced. A series of steps was undertaken by the Pressure Vessel Research Committee (PVRC) to propose new damping limits, which culminated in alternate damping allowable values, called PVRC damping. This damping was later adopted as Code Case N-411 to the ASME Code. Code Case N-411 has enabled several utilities to make significant reductions in the number of snubbers on their plants, resulting in lower maintenance costs, lower worker radiation exposure, and greater reliability (since the consequences of snubber malfunction are reduced). More recently, the Electric Power Research Institute sponsored a project by Bechtel to review the damping data, perform a regression analysis, and recommend a permanent change to the ASME Code to replace Code Case N-411.