Experimental investigation on the effect of silt erosion on hydro turbine runner materials and different surface hard coatings

Abstract

newlineHydropower is the major source of renewable energy globally and presently supplying 71% newlineof all renewable electricity. Hydropower contributes 16.4% of the world s electricity newlinegeneration with an installed capacity of 1,064 GW in 2016. India stands fifth in hydro newlineelectricity generation in the world with total production of 120 TWH. Hydropower plants newlineare broadly classified into two types i.e. Small Hydropower Plants (SHP) and Large newlineHydropower Plants (LHP). Hydropower projects with capacities up to 25 MW are Small newlineHydropower projects. The SHP plants face severe erosion problem as the sediment newlineparticles flowing in the river water cause extensive damage to the hydro turbine parts and newlinethe parts through which the water is flowing. The sediments are generally quartz particles, newlinewhich have a hardness of 7 in Moh scale. These silt particles when strike the hydro newlineturbine, damage it causing the lowering of performance and efficiency of the turbine. newlineMany investigators have studied the effect of silt erosion, the mechanism involved in silt newlineerosion and measures to be taken for preventing the effect of erosion on different parts of a newlinehydro turbine. Different models related to wear loss as a function various silt parameters newlineand operating parameters have been developed by researchers. Many investigators have newlinerecently carried out experiments on surface hard coatings and have analyzed the effect of newlinesilt parameters and operating parameters on these coatings. newlineFrom the review of literature it is found that fewer efforts have been made regarding newlinethe erosive behaviour of HVOF sprayed NiCrBSi-Al2O3 coating. In the present newlineinvestigation grade 420 ss is coated with NiCrBSi powder with different proportions of newlinealumina i.e. 5%Al2O3, 10%Al2O3 and 15%Al2O3 using High Velocity Oxy Fuel thermal newlinespraying process. Experimental investigation regarding solid particle erosion on Grade 420 newlinestainless steel is very few. Also it is necessary to further investigate the effect of different newlineparameters, i.e., angle of impingement, velocity of flow and erodent size on erosion of newlinecoated and uncoated specimens with a view to predicting erosion more accurately. newlineThe objectives of the study include newline1. Study of the physical characteristics of coating powder, coated materials and newlinesubstrate material. newline(vi) newline2. Study of performance of the coatings and bare steel when subjected to silt impact newlineunder dry conditions and wet conditions. newline3. Study of the mechanisms involved in the erosion of the coated and uncoated newlinesurfaces. newline4. Study of the effect of Al2O3 present in different proportions in NiCrBSi-Al2O3 newlinecoating. newline5. Study of effect of parameters such as impact velocity and impingement angle on the newlinecomposite coatings and bare steel. newline6. Establishment of correlations for erosion wear as a function of parameters such as newlineimpact velocity and impingement angle. newlineThe coated samples are subjected to dry erosion tests and slurry erosion tests in an air jet newlineerosion tester and slurry jet erosion tester respectively. The coatings are characterized by newlineoptical microscope, XRD and SEM. The micro hardness tests of the coatings and bare steel newlinereveal that the coatings exhibited better hardness than the bare steel. The effects of newlineparameters like impact angle and impact velocity on the wear behaviour of the as sprayed newlinecoated and uncoated samples are analyzed. It is observed from the erosion results of both newlineair jet tester and slurry jet tester of all the coated samples, minimum erosion has taken place newlineat 300 impact angle and maximum erosion has taken place at 900 impact angle. In case of newlinethe uncoated bare steel due to its ductile behaviour the erosion rate was minimum at 900 newlineand maximum at 300 impact angle. newlineIn air jet erosion the damage due to erosion gradually increases as the impact velocity of newlinethe erodent increases (33m/s, 68m/s and 100m/s). The SEM morphology reveals that the newlinemechanism responsible for erosion in all the as sprayed coated samples is quite similar in newlineall the cases. Fracturing of splats, development of craters, micro cutting and ploughing newlineaction are visible on the surface. In case of the bare steel it is observed that ploughing, newlinepitting action, craters formation are the erosion mechanisms. newlineIn slurry jet erosion the SEM morphology reveals that the mechanisms involved in the as newlinesprayed coated samples are removal of splats, lip formation; which are dominant factors, newlinewhereas ploughing action and micro cutting are also observed. In case of bare steel, the newlinemechanism of material removal is ploughing, micro cutting and lip formation along with newlinepores and craters. newline(vii) newlineThe sprayed coatings in all the cases have exhibited better erosion resistance in comparison newlineto the uncoated bare steel, both in dry erosion and slurry erosion. NiCrBSi-15%Al2O3 newlinecoated samples have exhibited better resistance to silt erosion of turbine components than newlineNiCrBSi-5Al2O3 and NiCrBSi-10Al2O3 samples. Erosion prediction model has been newlinedeveloped for different coatings and bare steel (420ss) as given below: