In this paper the theoretical investigation of five plants, and the results of measurements obtained during testing, are presented to explain and describe pressure oscillations in Francis turbines of the critical frequency fs = nrzr/60, where nr is the runner speed (rev/min) and zr is the number of runner vanes.
The oscillations, although causing no damage to the machinery or its foundations, are intolerable owing to the intense sound produced. This is an interference phenomenon, caused by the interaction of guide vanes, runner vanes, and the pressure propagating velocity in water, and may be avoided by careful construction.
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