Circumferential Crack Growth in Steam Turbine Shafting Because of Torsional Vibrations

The origination and growth of fatigue cracks in steam turbines structural elements in most cases is the result of long-term operation under conditions of cyclic mechanical and thermal loading in corrosive environment. Thus, the investigations into the causes of catastrophic destructions of steam turbines made it possible to conclude that the main reason of that destructions was long-term accumulation of fatigue damage in the form of transverse (edge and circumferential) and longitudinal cracks in turbines shafting because of intense vibrations. At the same time, torsional vibrations were recognized as one of the most dangerous and least controlled types of vibration, the sources of which are practically all abnormal operating regimes of turbogenerators. In the work, a study on the intensity of a circumferential crack growth in a steam turbine shafting because of multiple connection of turbogenerator to a power network with an inaccurate synchronization was performed. The algorithm of crack growth intensity determination included the calculation of torsional vibrations of turbine shafting at different levels of synchronization inaccuracy, as well as the experimental dependence of the circumferential crack growth rate on the range of the stress intensity factor for the rotor steel. Critical levels of synchronization inaccuracy have been identified, the exceeding of which creates conditions for intensive crack growth.