On-line Monitoring of Rotor-stator Rub in Steam Turbines

Abstract

In recent years, the operation of steam turbines has been associated with the occurrence of rotor-stator rub. This is because the clearences in the turbine flow paths decrease in an effort to increase the turbine efficiency. At present, detailed rubbing diagnostics is implemented as off-line analysis of measured data. The detection of the rotor-stator rub is based on analysis of vibration signals in time domain when a machine operator monitors the overall level of vibration or the vector of rotation frequency contained in the measured vibration signal. This paper deals with design and implementation of the system for online monitoring of rotor-stator rub implemented as the Rub Advanced Monitoring System (RAMS), which, in addition to detection phase, provides also localization of the place of contact occurrence, based on additional knowledge of the rotor geometry. Except the contact on the shaft, the contact between the blade tips and the stator seal can also be detected and localized. The contact is in that case accompanied by periodic impacts whose frequency for example corresponds to the product of the number of swirl brakes above blade tips and the rotation frequency. With this information obtained from standard rotor vibration signals, the rotor operation can be diagnosed in more detail and, if necessary, the turbine operation can be adapted to the current situation.