Method for on-line detection of incipient cracks in turbine-generator rotors

A method for detecting incipient cracks in the rotor of a turbine-generator while the turbine-generator is in operation at running speed, comprising the steps of:

• (a) sensing mechanical vibrations of the rotor at at least one location thereof to produce at least one time-dependent vibration signal;
• (b) providing at least one enhanced background vibration signal prior to development of a crack in the rotor, said signal being obtained and processed by
• (b1) collecting an integral number of time segments of said vibration signal, each time segment corresponding to one revolution of the rotor;
• (b2) generating said enhanced background signal by summing together all such collected time segments of said vibration signal by superposing each on the other;
• (b3) storing said enhanced background signal;
• (c) providing at least one enhanced foreground signal by
• (c1) collecting an integral number of time segments of said vibration signal equal to the number collected in (b1), each time segment corresponding to one revolution of the rotor;
• (c2) generating said enhanced foreground signal by summing together all such collected time segments of said vibration signal by superposing each on the other;
• (d) providing a difference signal by subtracting said enhanced background signal from said enhanced foreground signal by superposing one on the other;
• and
• (e) determining the spectral content of said difference signal, the presence and relative amplitude of higher harmonics of the running speed being indicative of the presence and size of a rotor crack.