On the Effects of a Shaft Crack Propagation on the Dynamic Behavior of a Steam Turbine

The timely detection of the propagation of a shaft crack in rotating machines is very important to avoid a catastrophic damage and severe secondary faults. However, the symptoms of a shaft crack propagation that can be detected in the vibration data collected by condition monitoring systems are often not very evident, especially when the crack depth is not very high. As the shaft vibrations of many rotating machines can be only measured at cross-sections that are close to the supports, the sensitivity of the machine dynamic behaviour to the excitations caused by the propagation of a shaft crack, which commonly occurs rather far from the measurement points, is often fairly low. Therefore, it is very important to improve the capabilities of diagnostic techniques aimed at the early detection of shaft cracks.

The rotors of high-pressure (HP) steam turbines are more exposed to the risk of the propagation of circumferential cracks owing to the considerable effects of thermal fatigue phenomena that can be generated in the area close to the first HP stage, during the machine life.

The most important changes in the shaft vibration caused by transverse cracks are due to two main phenomena. The first one is the periodic breathing mechanism, that is, the periodic opening and closure of the crack caused by the gravity force, in horizontal rotors, during a complete revolution. The second phenomenon is the non-periodic breathing of the crack caused by changes in the machine thermal state, like those due to the load rises carried out after the start-ups of power units.

In this paper, the dynamic behaviour of the cracked shaft of a HP-IP steam turbine is shown. The vibration data that have been analysed in this investigation have been collected during experimental tests carried out in a balance bunker of the turbine manufacturer, which was provided with a suitable equipment for causing a significant heating of the rotor. Additional vibration probes were installed at a shaft cross-section very close to the location of the transverse crack. Comparisons of the 1X and 2X vibrations measured at the turbine supports and at the cracked section, during runups and coast-downs, are shown in the paper. This analysis has been carried out for different thermal states of the shaft, in order to point out the effects of the non-periodic opening of the crack caused by a rotor cooling. Some unconventional techniques have been applied for the analysis of the vibration data in order to bring out additional symptoms of the fault that could be taken into account by diagnostic methods.

The results of this investigation have shown the opportunity of mounting unconventional vibration sensors in the area of steam turbines near the first HP stage, where the probability of a circumferential crack caused by fatigue stress is higher.