The vibration of high-medium pressure rotor of a 300 MW unit turbine kept climbing during the operation

and the vibration was closely related to pressure rise and temperature drop of the main steam. By analyzing the abnormal Byrd curve during start-up and shutdown of the turbine and the highly sensitive characteristics of vibration and main steam pressure rising and temperature decreasing

it is diagnosed that the root cause of vibration climbing of the HP-IP rotor is the severe transverse cracks in the rotor. The inspection results of cylinder removal confirmed the diagnosis conclusion. It is found that

at the position of the stress relief groove after the adjustment of the HP-IP rotor

there existed a transverse crack with length of more than 3/4 of the circumference(the maximum of 194 mm). Deep peak regulation and flexible operation mode would affect the fatigue life of the turbine rotor

and even lead to rotor crack. It suggests that analysis on abnormal performance of the vibration characteristics should be strengthened

especially the abnormal Bode curve of the startup and shutdown process

so as to find out the existence of cracks and take corresponding measures.