Rama Rao and Dutta presented a new on-condition monitoring approach of the GT’s rotor based on signal analysis of the casing vibration (outer casing instrumented with standard accelerometer). The response of the casing due to the excitation by the blade passing frequencies (BPF) of different stages is analyzed to diagnose the condition of the blades. During any load drop or transient regime of the turbine, the vibration amplitude peaks at the harmonic rotor frequencies are interacted with blades’ natural frequencies causing disturbance of the functional regime and inducing high vibration and noise in the entire engine. When a rotating blades vibrate, the amplitude of the BPF show significant variation. Therefore looking at the waterfall graph, any unusual side bands close to the BPF excitation peaks is indicating abnormal dynamic loading on the blade such axial loading by pulsating air and this could indicate crack in the blades. In some cases, significant unbalance of the machine component reveals also second side band on either sides of BPF. Investigations revealed that the proactive finding of the root cracks identified in five blades of rotor stage #2 (R2) of 210 MWe GT were based on appearance of side
band to the BPF of blades in rotor stage R2