Application of thermally sprayed coating in hydrokinetic turbines to reduce the slurry-based erosion: A review

Slurry-based erosion in the hydro kinetic turbines (HKTs) is a major problem in sediment-laden rivers particularly in the northern and northeast Himalayan region of India. Severe erosion of HKT blade often leads to degradation in the performance of the turbine. Hydro power plants in these regions are generally shut down when the slurry content reaches in the range of 3000 to 5000 ppm. The erodent particles mostly composed of quartz which is primarily responsible for the blade erosion and could exceeds more than 95% in monsoon season in the Himalayan rivers. In this aspect, the present article provides a comprehensive insight into the various factors that effects the erosion process and the implication of thermal spray coasting (TSC) to reduce the erosion. It is noted that an average hardness of about 7 Mohr's value is enough to cause the surface erosion. Some of the major factors that affect the erosion rate includes suspended sediment concentration (SSC), size and shape, mineralogy, velocity and impact angle, operation time, and the flow characteristics. The erosion is comparatively high at lower impact angle (less than 45°) for ductile and higher at large angle for brittle base material. The study suggests that the thermal spray coating (TSC) can provide favourable resistance against slurry erosion. Unlike other anti-slurry-erosion methods, like nitriding, boronizing cold spraying, laser cladding, microwave cladding, etc., TSC offers versatile and improved erosion resistant (both wear and corrosion) coating with better base material protection and thermal insulation that provide cost effective solution for slurry erosion. In particular, the high velocity oxy fuel that uses ceramic and cermet coatings subtracts shows better performance against slurry erosion.