Procedure to minimize rotor vibrations from flow-induced excitations in Kaplan turbines

Abstract

The flow pattern in the draft tube is commonly considered when investigating rotor vibration problems, especially for Francis turbines. However, phenomenon in the vaneless space for Kaplan turbines have been shown to induce rotor vibrations when operating in non-standard conditions such as speed no load (SNL). These flow disturbances develop in the vaneless space at small guide vane openings and high swirls. Depending on the number of flow disturbances and runner blades they can excite transversal, axial or torsional vibrations. In the case of transversal excitation they can in some cases cause resonance problems if the frequency is close to a natural frequency of the rotating structure. Recently it was observed that the excitation causes severe resonance in the rotating structure on a refurbished turbine when operated at SNL.The present paper presents frequency analysis of the flow-induced excitations coupled to the shaft vibrations on an old and a refurbished Kaplan turbine. To investigate the causes of the resonance problem, measurements have been performed on both the model and prototype turbine and on a twin unit with the old runner design. The measurements on the prototypes consist of pressure, shaft bending moment, and vibrations. Pressure and radial forces were measured on the model.The result shows the dependence of the runner blade opening on the transversal excitation and the frequency of the flow disturbances. The rotordynamic analysis showed the same phenomena with forward and backward precession. The results agree well between the model and prototype when measured data are normalized by the runner rotational frequency. It is proposed, when refurbishing a unit, to use measured pressures, shaft bending moment and vibrations on the prototype before the refurbishment and the new model turbine to identify critical excitation frequencies for the new prototype turbine, i.e., use the information from the tests to set rotordynamic requirements for the new prototype unit.