SEDIMENT EROSION OF HYDRODYNAMIC MACHINES

Abstract

Sediment erosion is the material loss from the wetted surface of the hydrodynamic machines due to friction and impact of hard abrasive particles. The change in the profile of blade/bucket due to erosion leads to flow instability, efficiency loss and vibrations, all of which are responsible for the final breakdown of the machines. The problem of sediment erosion is prevalent in run-of-river hydropower plants of the Himalayan region due to the flow of large quantity sediment. Understanding the erosion wear behaviour of the machine components is a fundamental need from the design perspective to enhance the service life of machines subjected to sediment erosion. Several attempts were made in the past to study the effect and cause of sediment erosion in hydraulic machines. However, the problem needs more attention as it has not been resolved satisfactorily. The rotating part or rotors are majorly responsible for the performance of the hydrodynamic machines. Therefore, it is essential to understand the erosion wear phenomenon of the rotor to develop a more effective anti-wear design. Erosion accelerates the efficiency drop of hydraulic machines, pumps or turbines. It is a complex phenomenon that causes major challenges in the planning, design, operation and maintenance of hydropower plants. Based on the literature survey, it is found that the knowledge of parameters affecting the wear behaviour of hydrodynamic machines is still inconclusive. The unavailability of any correlation based on the wear behaviour of the base material of hydraulic machines may limit numerical modelling tools for a reasonable estimation of erosion wear. Further, there is also a need to develop a numerical methodology that can effectively predict the wear characteristics of the hydrodynamic machines while working in the sand-laden flow. Moreover, to understand the effect and cause of sediment erosion on the performance and operation of hydroturbines, the measurement of the actual operating turbine after one complete operational life is also needed. In view of the gaps mentioned above, the present study is aimed to model and simulate the sediment erosion along the flow passage of hydraulic machines handling the sand-laden flow. An experimental and numerical investigation has been performed to study the sediment erosion of hydrodynamic machines. The work has started with the selection and identification of the parameters which influence the erosion wear behaviour of hydrodynamic machines. An investigation on parametric dependence of the erosion wear of different ductile type target materials has been carried out using a high-speed slurry pot tester. Four different ductile materials namely AA-6063 (Aluminium alloy), Brass, AISI304 stainless steel (SS) and CA6NM (ASTM 743) turbine steel, have been selected as target materials. The solid-liquid vii mixture is prepared using the Indian standard sand and tap water for the experiments. Experiments have been performed under the impingement velocity range of 13 to 32 m/s for the particle size range of 90.5 - 362.5 μm and solid concentration range of 500 - 4000 ppm at different orientation angles from 15◦ to 90◦. Two stainless steels, CA6NM and AISI304 SS depict the maximum erosion rate at around 32°, whereas it is observed at 30° and 25° for AA 6063 and Brass, respectively. The erosion wear behaviour of the two steels, CA6NM and AISI304 SS, is compared under the hydro-abrasive condition of hydropower plants. The CA6NM is found to be more erosion resistive steel compared to the AISI3004 SS. However, at normal impingement angle conditions, the erosion wear rate of CA6NM and AISI 304 stainless steel appeared to be dominantly dependent on the flow velocity and sediment particle size but weakly on solid particle concentration. The maximum erosion rate is about 3.36 and 2.98 times higher than that of the 90° impact angle for the AISI 304 and CA6NM stainless steel, respectively. The erosion rate of AISI 304 steel at a low angle (30°) impinging condition is around 1.11 to 1.3 times that of the CA6NM steel. However, the factor reduces to 1.06 to 1.25 at a higher angle (90°) impingement condition.