Influence of Different Operating Patterns on the Life of Francis Turbine Runners

The deregulation of the Norwegian energy market at the beginning of the 1990s has led to an increased load and stress on hydro power components due to peak load production and more market-driven operating strategies. Changed operating conditions such as frequent and faster start-ups and shut-downs can affect the life of a hydraulic turbine runner. Thus, there is a need to assess the influence of different operating strategies on the turbine runner life. The paper presents an approximate method for fatigue life assessment of hydraulic turbine runners. The life of a runner mainly depends on the statistical distribution of material defects. Assuming that defects of a given size are present, Paris’ law provides a useful model for crack growth simulation, and the time to failure can be estimated. As a precondition for such a simulation, the stress response for different operating conditions has to be known. The stress responses for various turbine output levels and load changes are obtained by means of strain gauge measurements on a full-scale Francis turbine runner. The measurements have shown that special operating conditions can cause high stresses in the runner. Based on these measurements, a stress spectrum for relevant operating conditions is established. Such load spectra form the basis for the simulation of fatigue crack growth by means of the Paris-law model. In order to generate a stress spectrum, which represents turbine operation in a realistic manner, operating profiles are established based on measurements taken from real power stations under different operating conditions. A procedure is proposed to generate such operating profiles based on data recorded in the Supervisory Control and Data Acquisition (SCADA) system. Finally, fatigue crack growth analyses can be carried out for various operating profiles. Thus, the mean time to failure of the turbine runner can be calculated and the influence of different operating patterns on the turbine runner life can be evaluated. Examples of real operating profiles and results of fatigue crack growth analyses are presented.