This paper deals with the valve disc vibrations occurring during safety valve opening transient. These vibrations always induce pressure fluctuations that are not damped if pressure is not sufficiently increased. The tests, performed on three different safety valves, showed heavy pressure fluctuations in the valve inlet connecting pipe. For connecting pipe lengths over 0.7 m, excessive pressure fluctuations were measured even in the protected device. The analysis of vibration inception and development is presented, showing that the phenomenon is driven by an harmonic self-exited motion of the valve disc coupled to a standing wave system in the connecting pipe. Vibration frequencies always strongly exceeded the spring-disc natural frequency, meaning that the disc movement occurred between a fixed position of the stem head and the disc seat on the nozzle. No impact on this latter surface was ever detected. On the contrary, significant damages of the stem-disc coupling were observed. Finally, the influence of plant and valve parameters that affect the pressure fluctuation amplitude is presented and discussed.
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