Excessive vibrations experienced in a Kaplan turbine at speed no load

Abstract

A Kaplan turbine unit from a new powerplant experienced high levels of vibrations and noise during the acceleration of the unit to nominal speed. These levels remained very high at speed no load. A first analysis seemed to indicate a possible blade resonance phenomenon as the highest vibrations were measured on the head cover. Nevertheless, after some modifications on the root of the blades, the vibrations remained high and the commissioning of the unit was delayed several months. To determine the origin of these vibrations, an array of sensors was installed in the machine. Different types of tests were performed. Experimental modal analyses of the runner head cover were conducted. Furthermore, experimental tests to determine the transmission of vibrations from the runner blades to the rest of the unit were carried out. Finally, the unit was tested during the acceleration phase to speed no load. Thanks to these tests, the origin of the high amplitude vibrations and loud noise perceived close to the head cover of the turbine was found. A solution to mitigate this problem was implemented by modifying the diameters of the air admission openings and their distribution, according to the up-to-date design rules. Both vibrations and noise amplitudes were drastically decreased. After this, the subsequent steps of the commissioning tests were performed and the unit started the commercial operation without major issues.