SILT EROSION IN INDIAN HYDROELECTRIC PROJECTS - LABORATORY STUDIES OF THERMAL SPRAY COATINGS OVER HYDRO TURBINE COMPONENTS

摘要

Silt erosion damage in under water parts of hydro electric projects and minimisation of these problems (generally termed as ‘Silt Menace') have been constantly engaging the attention of hydro engineers and manufacturers of hydro power plant equipment world wide. The Indian sub-continent is one of the most vulnerable zones in the world for silt erosion and sedimentation. The rivers of most of the power plants in northern India, China, Bhutan and Nepal originate in the Himalayan region which has relatively weak and fragile geology. During the monsoon days of April-September, the streams carry huge silt load, predominantly comprising of quartz particles, which are very hard in nature and irregular in shape. This heavily silt laden water containing up to 15,000 ppm silt or even more during rainy season, find entry into the water conductor system and impinges on the surface of the turbine, underwater components such as runner assembly, guide vanes, labyrinth sealing rings and surface liners of turbine top cover and bottom ring causing severe metal loss due to erosion. Hydro turbine components are manufactured from 13%Cr-4%Ni stainless steel. The erosion due to silt is so rapid that within one year of operation for 4000-5000 hours, the worn out components need extensive repairs by way of rebuilding by welding followed by thermal spray metallic coatings or by polymeric coatings. The various generation utilities in the central and state sectors have been incurring huge expenditure in India for the reconstruction of the damaged parts. Investigations made by conducting experimental studies of silt laden water erosion of 13/4 stainless steel, over laid as well as subjected to thermal spray by Plasma, HVOF, D-Gun coatings are presented in this paper. The silt characteristics of Indian rivers are also discussed. The studies presented here were conducted under actual hydraulic conditions simulated in a unique "Silt-Water Impingement Erosion Test Rig". In this unique test rig which has been indigenously designed and developed and happens to be the only one such rig in India, the conditions encountered in hydro turbines can be simulated by varying and controlling the various operating parameters. Details of extensive experimental investigations carried out in this test rig are reported and discussed in this paper.