PREDICTION OF A CRACK PROPAGATION IN A STEAM TURBINE ROTOR

The early detection of shaft cracks in rotating machines is essential to avoid catastrophic damage, long outages and high maintenance costs. An increasing number of transverse and circumferential cracks caused by thermal fatigue have been detected in steam turbines, in recent years. Because of usual long-time intervals between two subsequent planned inspections of the rotors, it is very important to identify the presence of a shaft crack by the online analysis of the harmonic content of the machine vibrations measured by Condition Monitoring Systems. As the symptoms caused by shaft cracks in the system response are often masked by the effects induced by other common malfunctions, it is very useful to study the sensitivity of the dynamic behavior of turbine shafts to the propagation of transverse cracks and to predict the lowest value of the crack depth that can cause detectable symptoms in the machine vibration. This paper shows the results provided by a numerical approach, aimed at identifying the propagation of shaft cracks, which has been applied to the analysis of a real intermediate-pressure steam turbine in which a first crack initiation had been detected.