Blade vibration measurements and excitation force evaluation

Abstract

Tip timing blade vibration measuring systems have become nowadays common for monitoring blade vibrations, due to rather easy and non-intrusive installation of sensors. The data collected by the sensors must be heavily processed in order to get the vibration time histories of all blades at rated speed or during a run up or run down transient. In the paper a real case of tip timing measurement for a steam turbine is shown. Resonant/natural frequencies, amplitude and phase of each blade vibration with respect to the synchronous reference, are identified. The first measurement results shown are related to asynchronous vibrations of a steam turbine last stage blade row due to an instability. The second results are instead transient synchronous vibrations when passing a blade row resonance during a run up of the steam turbine. It is further shown how synchronous vibrations, although non stationary, allowed to identify damping and excitation amplitudes by means of an original approach based on modal analysis.