Abstract
When a momentary pressure pulsation takes place at a turbine, and its frequency coincides with the natural frequency of a steel penstock, there occurs resonance thus resulting in a noticeable vibration. Lately, there is a tendency to increase the rotation frequency of turbines, on the other hand, the plate thickness of penstocks becomes thiner compared with before. As a result of the tendency, the oval vibration was found at some hydraulic power stations. Considering that rules on prevention of vibration are not necessarily corresponding to the oval vibration, we gathered information about existing penstocks constructed within the last 30 years. Based on the information, formulae to calculate the natural frequencies were proposed including the case with circumferential stiffener rings.
