Abstract The electricity market liberalisation of 1998 and the growing impact of renewable energies have led to changes in the operating behaviour of European hydroelectric power plants by increasing the start-up and shut-down procedures of machine and plant components. Hydroelectric power plants that were built before 1998 had been construed solely under a static point of view, plus extra safety calculations to withstand dynamic loads. Nowadays, these static calculations need to be rechecked because of the increased start-up and shut-down procedures. Thereby, it is of significant importance to determine the validity for present and future exposure frequency and to evaluate the possible occurrence of fatigue. Power plant operators want to keep the old plant components - regardless of the changed operating behaviour. Still, they do not want to risk unplanned outages and consequently, this calculation is of (economic) importance to them. This thesis analyses those machine components of spherical valves as substantial parts of hydroelectric power plants that might be susceptible to fatigue. Usually, the rotor in the shut-off elements is pivotally mounted at an angle of 90 degrees in order to open or close the supply canal of a pipe. When fully open or closed, all the components of the spherical valve are subject to mechanical forces and pressure fluctuations. Additionally, during the opening and closing process, these components are subject tomechanical torques. Depending on the frequency of use of the closing gates and depending on the stress intensity, total failure of the spherical valve might occur. This thesis focuses on the description of mechanical stresses in both the analytical and numerical way. In order to fulfill this task, a broad knowledge of hydraulic flow conditions and pressure fluctuations in spherical valves is needed. Finally, an appropriate standard norm for the calculation of fatigue strength for each main component of a spherical valve has to be chosen.