Validation of Effectiveness of Swirl Brakes and Fluid Destabilization Force Prediction Method by Excitation Test During Load Operation of Large Steam Turbine

Abstract In turbomachinery such as steam turbines, sealing technology has been advanced to improve performance. On the contrary, the fluid destabilization force generated in the blade and labyrinth seal has increased, and the risk of unstable vibration called steam whirl has become apparent. Although swirl brakes are widely used as a stabilizing device against the fluid destabilization force, there is no case in which the stabilizing effect has been quantitatively validated in actual steam turbines. Therefore, in this study, the damping ratio of rotor system under load operation was evaluated under 2 conditions with and without swirl brakes for a large steam turbine of 100MW class. In order to evaluate the damping ratio accurately, a hydraulic exciter was installed at the bearing casing, and the turbine rotor was excited during load operation, and the response of the first natural frequency mode was measured. Furthermore, the relation between the load and the damping ratio was confirmed by carrying out excitation tests with the turbine load as a parameter and estimating the damping ratio of rotor system. The validity of the prediction method of fluid destabilization force based on CFD (Computational Fluid Dynamic) analysis was also confirmed by comparing the measurement results of the damping ratio under load change with the prediction analysis results. Regarding stability evaluation, CFD analysis results showed that the shroud corner roundness (fillet) can have a large effect on the fluid destabilization force, and it was confirmed that the prediction results became closer to the trend of the measured damping ratio by taking into account the shroud corner roundness.