Sediment Erosion in Hydro turbines
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Sediment erosion is caused by the dynamic action of sediment flowing along with water impacting against a solid surface. Hydraulic turbine components operating in sediment-laden water are subject to abrasive and erosive wear. This wear not only reduces the efficiency and the life of the turbine but also causes problems in operation and maintenance, which ultimately leads to economic losses. This is a global operation and maintenance problem of hydropower plants. The high sediment concentration combined with high percentage of quartz content in water causes severe damage to hydraulic turbine components. Withdrawal of clean water from the river for power production is expensive due to design, construction and operation of sediment settling basins. Even with the settling basins, 100 % removal of fine sediments is impossible and uneconomical. A number of factors can influence the process of sediment erosion damage in hydro turbine components. The erosion intensity depends on the sediment type and its characteristics (shape, size, hardness, concentration etc.), hydraulic design and operating conditions of turbine (flow rate, head, rotational speed, velocity, acceleration, turbulence, impingement angle etc.), and material used for the turbine components. All these factors are needed to be considered for predicting the erosion. Therefore, dealing with sediment erosion problems requires a multidisciplinary approach. More research and development is needed to investigate the relationship between the particle movement and erosion inside a turbine and to establish the operating strategy for the turbine operating in sediment-laden water. In order to achieve the main objective of this PhD study, the overall research methodology adopted for this work ‘sediment erosion in hydro turbines’ include; experimental studies, numerical simulation, and field studies. This research work is based on result from laboratory experiment, and numerical simulation. A previously made test rig (Thapa, 2004), was reviewed and modified to create a strong swirl flow in curved path. This flow was found similar to the flow between the guide vane outlet and the runner inlet of a Francis turbine. The flow in the guide vane cascade was simulated in order to verify the particle separation process and to investigate the relation of the velocity and the drag coefficient with different shape and size of the particle. There was a provision to introduce particles, with sizes ranging from 1 to 10 mm, and to observe the motion of the particles from Plexiglas windows located on the cover of the tank using a high-speed digital camera. When a particle is flowing in swirl flow, drag force and centrifugal force are two major forces influencing the particle equilibrium. The equilibrium of these two forces provides a critical diameter of the particle. While, a particle larger than the critical diameter move away from the centre and hit the wall, a particle smaller than the critical diameter flows along with the water, and ultimately sinks. For critical diameter, the particle continues to rotate in the turbine. Different shapes and sizes of particles were tested with the same operating conditions and found that triangularly shaped particles were more likely to hit the suction side of the guide vane cascade. Furthermore, this study supports the concept of separation of particles from streamlines inside the test rig, which led to the development of an operating strategy for a Francis turbine processing sediment-laden water. This study also permitted experimental verification of the size and the shape of a particle as it orbits in the turbine, until either the velocity components are changed or the particle became smaller. The steady state numerical simulations were carried out on the Cahua power plant Francis turbine design, mainly at two operating conditions with varying particle size, shape, and concentration using ANSYS CFX. The predictions of erosion, based on the Lagrangian calculation of particle paths in a viscous flow, are described for stay vanes, guide vanes, and runner vanes of a Francis turbine, for which the results of the field tests have been available for verification. The flow simulation was obtained through use of a commercially available computational fluids dynamics (CFD) code, namely ANSYS CFX. The code utilizes a finite-volume, multi-block approach to solve the governing equations of fluid motion numerically on a user-defined computational grid. The flow solution procedure first generates the computational grid. A pre-processor is available in the software to perform this task. Second, the solution option such as inlet and boundary conditions, turbulence model, and discretization scheme, are specified. The final step is running the flow solver to generate the actual flow field simulation. Sediment erosion analysis of a Francis turbine gives an indication of relative erosion intensity and critical zones of erosion damage of the turbine components. The most realistic numerical prediction of erosion is found on a turbine blade. The highest velocities and accelerations occurred at outlet of the runner blade and more erosion was predicted especially at the pressure side of the blade outlet and at the lower cover. Furthermore, unexpected sediment erosion was found at the suction side of the guide vane where concept of critical diameter can be utilized. It has been concluded that if the particle size in the water is more than critical particle sizes, the turbine should not be operated at low guide vane opening. The numerically obtained erosion pattern and the field test observation and inspection at Cahua Francis turbine components are in good qualitative agreement. The encouraging agreement shows that, for this application, numerical simulation really can be used in a predictive manner. This information may serve as an input in an early stage of turbine design process to identify the regions where special surface treatment is necessary in order to increase the lifetime of the components for new hydropower projects involving risks of sediment erosion. The size of a particle is inversely proportional to the velocity of the particle, and it was determined that spherically shaped particles had higher settling velocities than particles with other shapes. However, non-spherical shape of the particles will tend to have lower settling velocities because both decreases in spheroid and increases in angularity tend to decrease velocities. Moreover, larger cross-sectional areas tend to be directed perpendicular to the transport path. As a result, higher coefficient of drag, higher rotational motion and more separation of flow are likely to occur and hence more erosion rate was predicted. The roles played by the shape of the particle significantly affect erosion rate prediction inside the Francis turbine components. Furthermore, it has been found that the erosion process is strongly dependent on the particle size, shape, concentration, and operating conditions of the turbine. The reduction of the erosion is not only linked to the reduction of particle velocity but also is linked to the reduction of separation of flow, which further depends on shape, size, and concentration of the particle. The significant reduction of erosion rate can be achieved by operating turbine at best efficiency point. The full load operation reduced efficiency, increased turbulence, and increased relative velocity of flow at outlet of the blades. The present knowledge and findings, although may not be enough to deal with this problem completely, can be utilised to achieve one major step forward in sediment erosion prediction and prevention.