STUDY OF CAVITATION EFFECT ON HYDRO TURBINE MATERIALS

Abstract

Energy is one of the key factors that influence the development of a nation providing economic and social benefits to its people. This is more important in developing countries like India, for economic development at micro level necessitating the need for the availability of secure and sustainable energy. Continuous demand of electrical energy in India has necessitated a smooth work of various power plants mainly in the Himalayan region. Hydro power is considered to be most promising source among other renewable energy sources. In underwater components of hydro power plants, the main problem encountered is the erosion. The erosion is mainly caused due to cavitation and due to the presence of silt in the flowing water. ' Cavitation erosion is a usual damage phenomenon in flow handling parts of hydraulic turbines. It is considered as one of the most ubiquitous problems in hydraulic machines since it causes changes in flow kinematics, drop in machine efficiency, noise and probably the most important erosion of solid surfaces. In spite of numerous theoretical and experimental investigations the mechanism of cavitation erosion has not yet been completely clarified. Further, In order to overcome the problems of cavitation erosion, there is a need to develop cavitation erosion resistant materials for the fabrication of the underwater parts of hydro turbines. _ _ The 13Cr-4Ni Martensitic stainless steel (termed as 13/4 MSS or CA6NM) is currently being used for fabrication of underwater parts in hydro turbines. There are several maintenance problems associated with the use of this steel. It is therefore, there is a need for the development of erosion resistant materials. The material removal during erosion is dependent on many interrelated factors that include metallurgical as well as operating parameters. The basic objective of the present study is to experimentally investigate the cavitation erosion effect on hydro turbine materials such as 13/4 martensitic stainless steel, 304 and 316L austenitic stainless steels. The test samples were prepared by cutting the steel bars of required standard dimensions. The samples of these steels iii were well polished and the microstructures are observed. It was seen that the 13/4 martensitic stainless steel consists of martensitic laths i.e needle like structure and in the case of austenite steel grains are observed. The steel specimens were put to test under ultrasonic vibrator for cavitation erosion with different time periods. The weight loss of these steels at different time periods were measured accurately and results obtained are discussed. The eroded surfaces obtained for different duration of cavitation were analyzed by Scanning Electron Microscopy. Roughness of the eroded surfaces was calculated using surface profilometer and microhardness of the tested materials was also measured to indicate work-hardening ability. Attempt has been made to develop a correlation for the cavitation erosion rate which depends on mechanical properties of the material, erosion time at particular amplitude. Based on the study it has been found that the cavitation erosion resistance of austenite 316L stainless steel is higher than the other two materials, which is mainly attributed due to its work-hardening ability. iv