Failure of Safety Valves Due to Flow-Induced Vibration

Flow-induced sonic vibration in boiler safety valve nozzles led to premature valve wear and failure. The valves were mounted just downstream of a 1 1/2D pipe elbow, in contravention of guidelines suggesting an 8 to 10 diameter separation to avoid sonic vibrations. Initial modifications proved unsuccessful. A consultant then recommended replacing the cylindrical valve nozzles with reducers, which stopped the vibration. A review of flow-induced cavity vibrations is presented. In the case of the safety valves the vibration is believed due to fluid-dynamic instability of the cavity shear layer, enhanced and controlled by the resonant characteristics of the adjacent cavity. The precise feedback, or coupling, mechanism that sustains the oscillation is unknown. Possible reasons for the success of the tapered shape reducer are discussed. The limited design guidance available for safety valve placement is reviewed. Results of a safety valve vibration experience survey are presented and discussed. A two-parameter guideline for safety valve placement is suggested, involving steam velocity in addition to valve location. Possible future work on safety vibration is outlined.