Diagnostics of Shaft Trains of Large Turbine Sets for Integrity Using Torsional Vibration Measurements

Abstract

A promising solution to the problem of early detection for the growth of fatigue cracks in shaft trains of large turbine units at nuclear and thermal power stations based on the results of torsional vibration monitoring by the method of time intervals is considered. It has been demonstrated in the laboratory that this method enables us to detect the growth of fatigue cracks in the shafts of rotor systems from a decrease in the value on natural frequencies of torsional oscillations. However, attempts to use it in the industry were not so successful due to a slight influence of fatigue cracks on the values of the natural frequencies of torsional oscillations as compared with the effect of other factors. Nevertheless, the data obtained in the study of torsional oscillations of the shaft train in a turbine installation of an operating 300-MW power unit using a high-precision optoelectronic monitoring system enabled the establishment and consideration of the effect that the turbine operating conditions has on the natural frequencies of torsional oscillations for the first time. A modal finite-element analysis of the turbine installation shaft train’s most dangerous annular fatigue crack, which cannot be detected at an early stage of development using standard or special vibration-monitoring systems during steady state operation of the turbine installation, has been performed. The analysis yielded the dependence of the torsional natural frequencies on the cross-sectional area and location of the fatigue crack. Monitoring of fatigue crack development has been demonstrated to be performed by measuring the natural frequencies of torsional oscillations (or vibrations) during steady state operation of the turbine set considering the effect of its operating conditions.