Testing of the rotary slide valve (RSV) model prototype has revealed adverse effects which do not allow RSV to operate normally within a specified range of fluid parameters. These adverse effects include an occurrence of a significant force that acts on the servo actuator for the valve, thus preventing RSV from opening when water-steam mixture is flowing, and violent vibrations leading to RSV damage when superheated steam is flowing. The possible causes of these effects have been analyzed based on 3D calculations of hydrodynamic properties of RSV flow passage using the STAR-CD package and 3D calculations of thermal mechanical behavior of RSV structure using the ANSYS Mechanical package. The temperature and stress-strained state calculations have shown that for the existing valve clearances the structure deformation under temperature gradients cannot cause an increase in the friction forces which hamper the slide movement for opening RSV. Results of special-purpose experiments have demonstrated that it is hydrodynamic forces that prevent RSV from opening and the value of these forces is determined by the fluid parameters and degree of RSV opening. The hydrodynamic calculations have demonstrated improvement in the RSV vibration behavior under reverse fluid flow.