Evaluation and Analysis of High Frequency Vibration on the Cavitating Venturi Downstream Piping

During hot functional test (HFT) of the auxiliary feedwater (AFW) system of a nuclear power plant, high vibration and noise occurred in the downstream of the cavitating venturi (CV), which caused severe damages, such as the disengagement of internal parts, failure of threaded connection to the temperature gauge located nearby downstream piping, relaxation of fastening nuts for pipe clamp and damaged socket weld connection. The objective of this paper is to evaluate the venturi downstream piping vibration in accordance with the ASME OM 3 requirements and identify the characteristics of the high frequency piping vibration. The vibration test was performed for the steam generator-operating pressure range to confirm whether the amplitudes of the vibration are within the acceptable limits to assure that no piping failures would occur due to cyclic loads. In addition to the vibration evaluation from the fatigue standpoint, the root cause of piping vibration is investigated. From the test results, the characteristics of vibration are acoustically coupled with vortex-shedding and cavitations, which results in a frequency range from 2,000 to 5,000 Hz. During the test, the vibration and noise were severer at the steam generator pressure of 40 kg/cm² rather than at the atmospheric pressure, or even at 82 kg/cm². It was also detected that degrees of cavitation and locations where the collapse occurred varied depending on operating conditions. Due to the high frequency characteristics, the shell mode vibration was more dominant than the lateral beam mode vibration.