Sediment erosion is one of the significant problems in operation of hydro power plants. The high presence of solid particles harder than the parent turbine material has led to significant reduction in efficiency and loss in economy of the country. The rise in concentration of sediment particles in river waters during monsoon season reach as high as 25000 PPM and on average 6000 PPM in most of the hydro power plants. Similarly, the mineral composition analysis of the sediment particles shows high concentration hard materials like Quartz, Feldspar etc. These minerals cause erosion in the hydraulic turbines by erosive or abrasive behavior when they pass through them. As many measures have been taken into action and have brought some reduction in the erosion tendency, a general method applicable all types of hydraulic turbines has still been lacking.
Hydro cyclones are one of the efficient devices for solid liquid separation and have been in use for centuries. They have been known to be used for separation of particles as small as 5 µm. But they have not been used or rarely been studied to be applied for preventing sediment particles from entering the hydraulic turbines and reduce sediment erosion. So, this study has been focused on numerical and experimental analysis of these devices to stretch forward the feasibility of their application to actually tackle the erosion problem.Thedesign of the hydro cyclones is quite simple and straight forward. The model developed by Bradley and Reitema have been in use in the market as many manufacturers have adopted their designs for commercial purpose. Amongst these designs, the one developed by Bradley is more famous and highly efficient. Bradley hydro cyclone gives a best compromise between separation efficiency, recovery discharge and pressure drop. So, for this study too, Bradley design for hydro cyclone separator has been adopted.
This study has been focused on numerical as well as experimental study of the hydro cyclone for hydraulic turbines, specifically for the application in micro hydro. Micro hydro is efficient power generating units and their impact to environment is significantly less as compared to other sources of energy. Another important fact about micro hydro plants is that they provide power to remote and isolated regions. But unfortunately, these plants are also vulnerable to sediment erosion and the remoteness of the sitesthatthey are installed in, make it even more complicated. In the first stage of the study, the numerical analysis of the hydro cyclone has been accomplished. The results of the analysis showed excellent performance of hydro cyclones in separating sediment particles of different sizes and concentration. Under relatively low pressure drop across the hydro cyclone, itshowedseparation efficiency of about100 % for critical particle size of 125 µm and above87 % for the particle size of 50 µm. The loss of water through underflow is also less than 5% of the total discharge. The experimental setup has been designed with hydro cyclone separator in-line with the Francis turbine. The main objective of the setup design was to test the performance of the separator at different elevation downstream from the intake. From this analysis, a proper elevation of the separator can be determined with minimum pressure drop and high separation efficiency. Consequently, the separator was operated at elevation of 4.5m, 5 m, 5.5m and 8m, from which 5m elevation for separator operation was found to be most efficient. At 5m elevation, the pressure drop obtained, i.e. 2.7 m at designed load condition of Q 1.0, was the minimum as compared to others. With the drop in pressure, the consequent drop in performance of the turbine was also measured. The drop in mechanical power of the turbine was 145 W however, there was no significant drop in efficiency. The efficiency of the turbine was found to be fairly constant at 45%, while operated with and without separator in-line with the separator. After the determination of the proper separator location, the sediment separation analysis was commenced. The sediment particle sample of 3.5 kg, within the size range of 45 µm to 300 µm, was acquired from Kathmandu University, Nepal. As the particle sample amount was limited only two cases for experimental analysis were done. The concentration of sediment particle used in the analysis was 500 ppm. From the analysis, the separation efficiency of the critical particle size i.e. 125 µm was found to be 92% and for particle with 55 µm diameter was 82% at the designed load condition, Q1.0. The analysis of separator at load condition of Q1.1 showed slightly higher efficiency due to increase in pressure drop.
So, from the numerical and experimental analysis, it can be justified that hydro cyclone separators can be used as a tool to reduce losses due to sediment erosion in hydraulic turbines. As the operation of the separator can be limited to monsoon season (2 months), when the sediment concentration is significantly high, proper compromise in technical and economic aspect can be achieved.