Nosal O. Vibration diagnosis of damage to structural elements of a steam turbine during its operation and repair

The dissertation is devoted to the study of the influence of damage to the structural elements of a steam turbine on the vibration parameters that arise during its operation and repair. Many elements of power equipment, in particular turbine blades and rotors, in real operating conditions are subjected to joint or separate action of mechanical load (static and dynamic), temperature and aggressive environment. The consequence of such loading is the gradual accumulation of scattered fatigue damage, which leads to the appearance of fatigue and thermal fatigue cracks, corrosion cracking and other types of damage. When a crack reaches a critical size, there is a danger of destruction of a particular structural element with catastrophic consequences for the entire turbine. Therefore, the development of methods that allow timely diagnosis of such a crack is an urgent practical problem. Detection of damage in the structural elements of a steam turbine is a complex task, the solution of which is accompanied by significant theoretical and practical difficulties. The use of local non-destructive methods of damage diagnosis is possible only during repair work and requires significant time. Vibration diagnostic methods for detecting damage in rotating shafts of steam turbines are practically the only way to prevent a technical catastrophe. Vibration diagnostics of damage, which is based on the fact that the dynamic characteristics of an undamaged body and a body with broken integrity differ significantly, has an advantage over local diagnostic methods, since it is integral, i.e. characterizes the state of the object as a whole and therefore requires significantly less time to obtain the result of the diagnosis of dimensional structures (turbine rotors) or numerous structural elements (turbine blades). The difference in the dynamic characteristics of the body, due to a change in its stiffness, can be used to diagnose damage (for example, fatigue cracks) and determine its parameters (size and location). The dissertation consists of an introduction, four sections, which set out the main scientific results of the work, general conclusions, a list of sources used and appendices. The introduction substantiates the current state of the problem considered in the dissertation, its relevance and significance, formulates the goal and objectives, object, subject and methods of scientific research. The first section is devoted to an overview of the state of the problem and justification of the direction of research of the dissertation. An analysis of literary sources is carried out and the current state of research related to the topic of the dissertation is revealed. In the second section of the dissertation, a numerical-analytical calculation model of a turbine blade with a crack is proposed, which allows predicting the influence of the parameters of the edge surface crack and the geometric parameters of the blade on its natural oscillation frequencies, damping characteristics and oscillation spectrum. The adequacy of the model is confirmed by comparison with the results of experiments. Based on analytical studies performed using the proposed model, a sufficiently high level of sensitivity of the considered vibration diagnostics methods for detecting relatively small cracks, the size of which does not pose a threat to the integrity of the blade, has been demonstrated. A comparative analysis of the sensitivity of the vibration characteristics of turbine blade damage, based on the relative change in its natural frequencies of oscillations, damping characteristics and the spectrum of oscillations at the main resonance, has been performed. The influence of the crack parameters, namely its size and location along the blade, as well as the blade stiffness, which changed due to a change in its length, on the vibration characteristics under study has been shown. In the third section of the dissertation work, a numerical-analytical calculation model is proposed to determine the change in the natural frequencies of transverse oscillations of steam turbine rotors. Based on analytical studies performed using the proposed model, the results of modeling transverse oscillations of rotors and turbine shafting with an edge transverse crack are presented. Based on these studies, methods for vibration diagnostics of turbine rotor damage at the stage of their repair and turbine shafting during operation have been developed. The fourth section of the dissertation presents a methodology for assessing the effectiveness of the proposed methods for vibration diagnostics of turbine blade and rotor damage, which is based on a pre-specified value of the change in the information characteristic of the damage, which must exceed the test error and random factors of the experimental method.