STUDY OF VORTEX TYPE SEDIMENT EXTRACTOR FOR JALDHAKA HYDROELECTRIC PROJECT

Abstract

Sediment transport by natural stream flow constitutes one of the major problems in the design/operation of river valley projects (hydroelectric /irrigation scheme). The sediment load reaching the power plant passes through the turbine and damages the turbine runner blades/buckets where as in case of irrigation channel, the sediment load tends to deposit in it thus causing silting of the canal that results in a reduction of discharge carrying capacity of the channel itself. Thus for successful functioning of a hydroelectric project, the removal of unwanted sediment is the most important factor which requires great attention and that systematic control of sediments is to be done at the proper time and proper place with suitable device. The measures for control of sediment entry into a canal system can be applied either at the canal head works/intake structure or in the canal downstream; in exceptional cases at both these locations the measures are provided. The method/devices adopted for this purpose at canal head works/intake structures are known as excluding devices where as the devices used in the canal itself are known as sediment ejectors or sediment extractors. Various ejectors such as tunnel type, vortex tube sand traps, settling basins, vortex chamber type extractor are being generally used in practice. Vortex chamber type sediment extractor has been found to have large efficiencies (of the order of 90 percent or above) for removal of sediment at small water abstraction ratio (about 3 to 10 percent) to be used for flushing of settled sediment. To remove sediment a vortex chamber type sediment extractor utilizes the secondary flow generated by the circulatory flow induced in a circular basin. The secondary flow moves the fluid layers near the basin floor toward the orifice of the flushing pipe installed at the center of the basin. Sediment particle heavier than the fluid are thus moved along spiral paths from the basin periphery toward the orifice. The design of vortex chamber type extractor includes the determination of the basin dimensions and sediment removal efficiency when the flow related parameters are known. Since there is no general relationship available from analytical background for designing a vortex chamber type extractor, empirical equations as proposed by different investigators can be adopted as a guide lines for designing a vortex chamber type extractor. To study the effects of variation of different components on sediment removal efficiency of a vortex chamber type extractor, a hydraulic model study was carried out in the River Engineering Laboratory of WRD&M, IIT, Roorkee. For study purposes, necessary data have been adopted from Jaldhaka hydroelectric project, WBSEB. Experimental investigation has been done on a geometrically similar model of 1/10 scale. In the model, the sediment size was simulated by "Froude's Law" and "Criterion of fall velocity". This dissertation report deals with the findings of the model investigation of proposed vortex chamber type of extractor.