Analysis of the vibrations of a complete french pwr power plant piping system

This paper deals with a numerical and experimental study of the vibrations of a complete industrial piping system. Its goal is to determine the reasons of fatigue failures of the nozzles, to check out the stress levels in the main pipes and to find out efficient ways to reduce the vibrations levels. The piping system under study is a safety one of the French PWR power plants built in the 80's. It includes two pumps and more than 100 meters of pipes. Several nozzles failed by vibrational fatigue, though their natural frequencies range over 50 Hz, higher than any usual excitation frequency of such pipes. A study of the complete piping system was performed: the numerical model was built with the CIRCUS code using 700 different elements (such as straight pipes, elbows, supports...) and an acoustic and mechanical computation of the vibrations was made. In the same time, sensors were arranged on 70 locations during periodic tests. Matching calculations and measurement showed an abnormal behavior of the pump, which generates vibrations in the 50-100 Hz range throughout the whole piping system by mechanical transmission and fluid-structure interaction. Parametric investigation proves that most nozzles could be damaged when their natural frequency is identical with a maximum of the main pipe vibration spectrum. Further investigation is required to study and reduce the vibrations of the pump.