Severe sustained vibration of a closed hydraulic-turbine penstock valve was associated with accidentally inadequate inflation of the valve service seal. The vibration stopped when the valve bypass was opened or full seal pressure was applied. Theory indicates that intermittent valve leakage and corresponding oscillating penstock pressure can be self-sustaining in some instances, and that opening a bypass around the leaking valve can suppress them. Field tests were performed to confirm the general theory and prove the reliability of utilizing the bypass to prevent future vibration.

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