Reliability Assessment of a Turbogenerator Coil Retaining Ring Based on Low Cycle Fatigue Data

Abstract

Turbogenerator coil retaining rings are shrunk-fitted onto the rotor over the coils, in order to restrain them against the centrifugal force. They are typically subjected to low cycle fatigue, with a cycle being completed at every machine switch-on and switch-off. The subject of this paper consists in the determination of the failure probability of a coil retaining ring. The failure mode of the ring cracking, when it swells in tension, due to the centrifugal force is here considered. The reliability assessment is preceded by the study of the input variables affecting the low-cycle fatigue load and of their stochastic distributions. This question is tackled by the experimental determination of the static, cyclic and fatigue curves of the involved material and by the application of a statistical model to compute related parameters and their standard deviations. Upon the determination of variable distributions, the probability of failure is estimated in the form of a cumulative distribution function by a computationally efficient methodology, based on the Advanced Mean Value approach. The obtained results account for the material response and the local stressstrain states at the most loaded coil retaining ring region. The determined probability at the end of the machine life, in the order of 10-12, is compatible with reference values for structures under fatigue in the mechanical and aeronautical fields.

Improvement of the Mechanical Characteristics, Hydrogen Crack Resistance and Durability of Turbine Rotor Steels Welded Joints

“…There are precipitates of the cementite type The calculated and experimental data characterizing the regions of the formation of the corresponding structures were compared with the data of the thermos-kinetic diagram of the steels 25H2NMFA and 20H3NMFA. The comparison, as well as taking into account the known results [22][23][24][25][26][27][28][29][30][31], made it possible to improve the mathematical model of welding heating of the manufactured rotor and to refine the numerical data characterizing the thermal cycle.…”

“…The operating time of most of the NPP power units has already exceeded its park resource [25][26][27][28][29][30][31][32][33][34]. Thus, the priority direction of research is to increase the reliability and extend the service life of the operation of power units, including the components of power units.…”

“…The increase in operational requirements for the rotors of high-power steam turbines at nuclear power plants necessitates improving the quality characteristics of the initial metal structure of their welded joints, which is characterized by the presence of a certain heterogeneity. It is advisable, by using the optimal welding heating of the manufactured joints, to reduce the level of the initial structural heterogeneity, which increases with an increase in their operating time [25][26][27][28][29][30][31][32][33][34].…”

“…The operation of many structures-particularly, steam and hydrogen turbine rotors, cooling infrastructures of powerful semiconductor devices and industrial singlecrystal growth applications [3][4][5][6][7][8][9][25][26][27][28][29][30][31][32][33][34]45,[55][56][57][58][59][79][80][81][82][83][84][85][86][87][88][89][90][91]110]-is accompanied by their cyclic loads, which often exceed the yield strength of the material [52,[54][55][56], based on understanding the hydrogen embrittlement of materials from the atomistic level to the continuum [100][101][102][103][123][124][125]…”

“…1-rotor shaft; 2-current supply slot; 3-ventilation slot; 4-slot wedges; 5-current supply tire; 6-shaft fracture areas due to fretting (wedge joining areas) (e). Design slot wedges rotor tail after application of the antifretting measures system: 1-rotor shaft; 2-current supply slot; 3-ventilation slot; 4-current supply wedge; 5-current supply tire; 6-ventilation slot wedge; 7-easy shifting seal (f) [2,6,[23][24][25][26][27][28][29][30][31][32][33][34].…”