ANALYSIS AND OPTIMUM DESIGN OF FOREBAY TANK

Abstract

With the increase in population, the demand of energy in the world is increasing day by day. Small hydropower (SHP) one of the proven, mature and reliable renewable sources is more enticing because of absence of fuel cost, low maintenance and clean source. In the most of the mountainous countries, the natural streams originate from the mountains where the catchrnents are steep and vulnerable to high soil erosion. The sediments entering into the canal is somehow removed by desilting devices. 1-lowever, the forebay tank a small reservoir at the head of the penstock that carries water to the turbine; is the last free water surface of a SHP to arrest undesirable material. The primary functions of forebay is to regulate water, ensure steady water flow and reduce water level fluctuation to avoid any vortex formation. It also prevents .4 any floating debris and undesirable sediments to pass through penstock. It has the provision to spill the design discharge in the event of lower demand and sudden closure of the machine. There are some basic requirements such as smooth water flow, low head loss, no flow separation and vortex phenomenon which are required when generating units are under normal working condition. Under some unsteady working conditions such as load rejection or load acceptance, the rise and fall of water level in the forebay tank should not be large and need to be accommodated in the intake. Any backflow, insufficient submerged water depth and air-breathing vortex are prevented which otherwise will affect the safe operation of generating units. Minimum submergence depth, geometrical design, transient analysis and optimization design of forebay tank in the run-of-river SHP projects have been analysed and presented. Based on the analysis of data of 21 different existing SHP stations for computation of minimum submergence depth, among many others, the Gordon empirical formula is recommended. Capacity of forebay tank provided at projects was found to be larger in comparison to the capacity computed from the hydraulic transient analysis. The capacity and depth was finally optimized with MS Excel solver tool to achieve the economic design of the forebay tanks.