The topic of this thesis is damage of hydraulic machinery due to sand erosion. This is a very broad topic in which all the aspects of design of hydraulic machinery namely material selection; mechanics of material and hydraulics are involved. The issue of sand erosion is equally important for operation and maintenance of hydropower plants. The erosion damage of turbines of hydropower plants in Himalayan Rivers, in particular in Nepal, due to high sediment contents in river is a motivating factor for this research. The findings of this study are believed to help to reduce erosion of turbine components, increase their life time and improve maintenance procedure. The objectives of this study are to: 1. Study the erosion process on different materials and compare lifetime of turbine material with hard surface coating. 2. Study the flow in the turbine and identify the region of highest velocity and acceleration where most serious sand erosion occurs. 3. Study erosion rate of turbine as a function of operating parameters (head and speed), particle characteristics and turbine material. 4. Assess loss of efficiency and to analyze repair method and repair interval that gives optimal economy of the hydropower plants. The goals laid out in the objectives are achieved by understanding the theory of erosion of material through literature, investigating the nature of turbine damage from field observation and finally by experiments to study effect of variables of erosion rate and investigation of particle separation process in rotational flow.