Steam Turbine Vibration Caused by Oil Coking Rubs: Case Studies and Numerical Simulation

Rotor-stator rub interactions play an important role in the operation of high performance turbomachinery such as steam turbines, compressors, motors, and generators with small clearances between the rotating and stationary components. It is difficult to diagnose the problem because the vibration characteristics are widely varied and often the same as those resulting from other common causes. Also, rubs can cause secondary effects which can be misdiagnosed as the primary cause of the vibration excursion. The paper investigates one of the rub phenomena — the bearing lubricant (oil) coking in the close clearance regions while the other operational parameters are unchanged. The amplitude of vibration typically increases suddenly and then decreases after a few minutes. An FFT analysis of the vibration spectrum result indicates that most of the vibration energy is distributed to synchronous components or super-synchronous components. Two industrial case studies are presented and the possible factors are reviewed. Simulation results indicate that the model can describe the behavior of the steam turbine under oil coking rotor-stator rub conditions well and help locate the axial position of the oil coking rub. The analysis results of the paper are very useful for transient vibration fault diagnosis.