STUDY ON THE CIRCUMFERENTIAL CRACK GROWTH PROCESS DURING TORSIONAL VIBRATIONS OF A STEAM TURBINE SHAFTING

Abstract

During operation, a steam turbine shaft is subjected to a wide range of thermomechanical and thermochemical loading. Long-term operation of steam turbines has one of its consequences the formation of fatigue cracks of various types in their structural elements. Evidence of this is a number of accidents and catastrophic destruction of steam turbines. Potential reasons of damage in turbine shafts are all technological operations used in the process of manufacture (forging, turning, and milling, heat treatment), since they are accompanied with plastic deformation of material. It accumulates during long-term cyclic deformation and turns into local damage of a fatigue crack type. In addition, cracking in turbine shafts is caused by the presence of stress concentrators. One of the reasons for the long-term accumulation of fatigue damage in steam turbines elements is torsional vibrations of the shafting caused by many reasons. Among these reasons are a short circuit on the turbogenerator, the connection of the turbogenerator to an electrical network with inaccurate synchronization, as well as the dynamic instability of the system turbogenerator-electrical network. A short circuit in a turbine generator is a rare phenomenon that under certain conditions can cause serious damage or even complete destruction of the turbine but does not cause the accumulation of fatigue damage in the material. At the same time, the source of multiple excitations of torsional vibrations of turbine shafting, which operates throughout their service life and can cause fatigue damage the material, is the connection of the turbogenerator to an electrical network. A methodology is proposed to study the process of circumferential crack growth in the turbine shafting because of many such connections. The methodology is based on the use of a 3D finite element model of the steam turbine K-200-130 shafting and a fracture mechanics approach. There was demonstrated that the inaccuracy of turbogenerator connection to an electric network is the most influential factor affecting the crack growth and the conditions for the crack to reach a critical size has been evaluated. In the problem being considered damping capacity of the rotor steel influences the intensity of crack growth insignificantly.