Protection of runner blades of hydroturbines used in Himalayan terrain against erosion by polymeric coatings

Runner blades of hydro-turbines used in Himalayan terrain suffer from high erosion rates due to heavy silt, specially during monsoon. Conventional method of repair/rebuilding is hard facing, which needs careful control of various parameters, which are difficult to achieve at the site. Moreover, in such process the substrate gets heat affected and becomes weaker. The reasons for high erosion rates are (1) Presence of highly abrasive silt in large quantity (2) Probable occurrence of cavitation (3) Stress due to vibration caused by loss of materials (unbalance & changed natural frequencies of oscillation). (4) Stress corrosion cracking The desirable characteristics of a protective coating under heavy silt condition are (1) Abrasion resistance (2) Good adhesion to the substrate without weakening it. (3) Easy applicability at site (4) Cavitation resistance (5) Vibration damping Looking into these requirements Electrical Research & Development Association, Baroda, India has developed a three element coating system which has shown good results both in laboratory and at three power stations. The first coat is a metal filled epoxy which covers up the eroded areas. The second coat is a polyurethane which offers cavitation resistance and vibration damping. The third coat is a silica filled epoxy which offers abrasion resistance. The three coat system is based on the hypothesis that cavitation do not cause much damage if the cavitating bubbles could be kept away from the hard surface.