Vortex Shedding at pipe junctions can create pressure pulsations and flow-induced vibrations. The flow through one pipe may result in a shear layer at the junction with a second pipe. Instabilities such as vortex shedding in the shear layer can then excite acoustic modes in the second pipe, especially when the flow in the secondary pipe is stagnant or weak. The effect is the excitation of a pipe organ mode, which under certain conditions, may result in unacceptable noise and/or vibration levels. Within the nuclear industry this phenomenon has been most frequently observed in boiling water reactors (BWRs), resulting in vortex-induced, main steam line associated stand pipe acoustic resonances.
This phenomenon has not been typically observed in pressurized water reactors (PWRs), especially in primary coolant loops due to the lengths of pipe needed to support acoustic resonances in water systems relevant to driving lower order structural piping modes. However, if certain conditions exist, PWRs do contain large sections of piping which can be susceptible to such flow-induced adverse noise and vibration effects. This paper describes the evaluation and mitigation of structural vibrations due to a vortex-induced excitation of an acoustic mode of a large side branch pipe in a high-energy, water-filled, PWR piping system. Specifically, an acoustic resonance was observed and structurally significant resultant vibration levels were measured on a safety related piping system directly connected to a PWR primary reactor coolant system (RCS) between the reactor and a steam generator.
A rapidly employed evaluation program was implemented, which included significant in-situ structural vibration measurements that informed a combination of acoustic, structural, and fluid-domain numerical modeling evaluations. These evaluations were performed in concert to provide both root cause insights and candidate mitigation strategies. Candidate mitigation strategies were then evaluated prior to inplant implementation via further modeling evaluations and a model-scale testing program. This paper describes the primary vibration characteristics of interest of the affected piping system, the data analyses and modeling methods used to successfully identify the vibro-acoustic phenomena, the developed mitigation strategies, and verification of the final mitigation strategy via model-scale with final demonstration occurring in the plant prior to fuel load.