Suspended sediments which are present in the turbine water of hydroelectric power plants, specifically those with medium and high-heads pose financial and operational drawbacks owing to hydro-abrasion at turbine parts. For this reason, desanding facilities are located between the intake zone and the headwater leading to the turbines in the power house. The purpose of the facilities is to reduce the sediment mass and average particle size in the turbine water.
Sediment laden water is diverted into a basin with a large flow area, consequently reducing the flow velocity. This allows for settling of sediments within the basin which can then be continually or intermittently flushed downstream. The `clear’ water is then conveyed to the turbines.
However, recent studies have indicated insufficient flow conditions as well as low particle trapping efficiencies. This is irrespective of the current facilities meeting current design specifications. Sediment trapping efficiency of the desanding facilities is related to hydro-abrasion of the turbines. Therefore, the performance of desanding facilities represents an important component of the economic hydroelectric power plants operations.
Christopher Paschmann and colleagues from the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich in Switzerland presented a study featuring measurement instrumentation, specific measurement approaches and specifications, and the composition of the modular as well as flexible setup for field analyses. Above all, their paper delineated the experiences gained with the combination of instrumentation used to analyze flow and sediment attributes with high temporal resolutions applied in desanding facilities. Their research work is published in Flow Measurement and Instrumentation.
A desanding facility is composed of an intake structure, an intake channel for directing water into the basin, a transition zone designed with an increasing flow area, a basin, and an outlet structure. The authors developed a measurement concept with an aim of describing flow as well as sediment fluxes in desanding facilities. The concept was to obtain high resolution measurement data of flow velocity, turbidity, density, and temperature of the water-sediment mixture in a grid throughout the basin, and other analyses of flow velocity and turbidity in the inlet and outlet structures.
Implementing acoustic Doppler velocimeters allowed for recording 3D flow velocities at high spatial and temporal resolution. The authors took water samples to establish particle size distribution implementing laser particle size analyzer, and suspended sediment concentration by weighing oven-dried residue. By combining turbidity measurement and sound water specimen analyses allowed the researchers to define reliable correlations to approximate suspended sediment concentration based on turbidity.
Density measurement allowed the authors correlate density and suspended sediment concentration particularly at high concentration values in a bid to supplement the correlation of turbidity and suspended sediment concentration. At low suspended sediment concentration values compared to the uncertainty range of the employed Coriolis flow- and density meter, no correlations for suspended sediment concentration and the measured density was found.
The developed experimental setup appeared to be robust and reliable despite being exposed to sundry stresses, for example, transport, weather conditions, hydrodynamic forces, and continuous operation. In just six measurement days, the authors were able to realize 1500 flow velocity and 375 turbidity measurement points. Approximately 75 water specimens as well as 6 deposition samples were taken.
“In the meanwhile, all measurement data were processed and successfully utilized in numerical 3D desanding facility simulations of flow and sediment transport” says Christopher Paschmann. He added: “Findings of these comprehensive numerical studies will be published in a PhD thesis at the beginning of next year.”
Paschmann, J.N. Fernandes, D.F. Vetsch, R.M. Boes. Experimental setup for flow and sediment flux characterization at desanding facilities. Flow Measurement and Instrumentation, volume 54 (2017), pages 197–204.
Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, Zurich, Switzerland.Go To Flow Measurement and Instrumentation