engleski

Reliability assessment of Francis turbines damaged by cracks

The thesis presents theoretical and experimental analysis of the problems related to the occurrence and propagation of cracks on the Francis water turbine rotor runners. Special attention has been paid to the causes which affect the cracks initialization and fast propagation in connection with material properties and the action of external and internal forces, as well as water as the working medium. The thesis combines the results of non destructive (NDT) method test and the results of material experimental testing, accompanied by applicable theoretical methods. In the experimental part of the thesis, complex testing of microstructure and mechanical properties of material on samples taken from practice were performed, determining also the fracture toughness (K1c). From the application of experimental results, selected characteristic cracks and known stresses, the calculated critical and allowed length of the cracks were calculated by using LEFA and NLFA fracture mechanics. Based on all the performed analyses of turbine component materials and calculation results by using fracture mechanics methods, the causes of cracks initialization and their propagation in working conditions have been determined and actions have been recommended for systematic monitoring of the cracks and turbine rotor runner.

corrosive-resistant steel casting; microstructure; fracture toughness; stress corrosion; cracks progression; critical cracks

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