ASSESSMENT OF ROTOR STABILITY FOR STEAM TURBINE CONSIDERING LABYRINTH SEAL CHARACTERISTICS OF FLUID DESTABILIZATION FORCE AND VIBRATIONAL FREQUENCY EFFECT OF BEARING COEEFICIENTS

Accurate evaluation of the rotor stability is important for increasing the performance of the Steam Turbines. This paper discusses the important factors (such as destabilization force, bearing coefficient) for the evaluation of rotor stability. The destabilization force which varies with the type of seal suggests that seal shape plays an important role. In the past, several researchers have studied the fluid destabilization force both numerically and experimentally and prediction of the same can be done fairly accurately by applying CFD techniques. The characteristics of the fluid destabilization force can be accurately evaluated by investigating the sensitivity of parameters such as clearance and swirl velocity for each seal type using CFD. In the case of partial admission operation in which the bearing load changes (such as control stage of steam turbines), the frequency ratio effect on bearing coefficients is higher in the case of light-load (Sommerfeld number is large) than in the case of high-load (Sommerfeld number is small). In order to estimate the frequency ratio effects due to varying load accurately, an experimental study and analytical study were carried out. As a result of comparison of the test results to analytical results, the test results are in good agreement with thermo-elastic-hydrodynamic-lubrication (TEHL) analysis which considers deformation of pad obtained by 3D-FEM. The evaluation of rotor stability at each bearing load by partial admission (example: Governing Valve test) is in agreement with the field data of steam turbine. For new designs and modification designs, this assessment considering the characteristics of each parameter is effective for improving the quality of rotor design.