Non-linear analysis of the defects of a vertical turbine

Abstract

Vertical turbines installed in hydroelectric dams and guided by hydrodynamic cylindrical journal bearings may suffer from sub-synchronous vibrations. This is especially true when the rotor, the bearings and the seals are perfectly aligned. When present, the sub-synchronous vibrations have large amplitudes and generally limit the rotation speed of the machine to values lower than its operating regime. Nevertheless, machining and assembly defects lead to the misalignment of the rotor and its bearings and seals therefore strongly affecting its vibration signature. The present work is based on a numerical non-linear model of a vertical turbine able to take into account all its components and the most often encountered defects. The rotor is modeled by using 1-D Timoshenko beam finite elements and the presence of a rigid coupling is taken into account. The non-linear forces in the tilting pad thrust bearing and the journal bearings are obtained from the solution of Reynolds equation. The numerical results show how the radial misalignment of the journal bearings eliminates the sub-synchronous vibrations.