TRASH RACK ARRANGEMENT IN INTAKES FOR HYDRO-POWER DEVELOPMENT

Abstract

Trash racks for the intake of a hydro-electric development is an important item. The function of the trash rack is to keep out debris and other materials including fish carried by water which are likely to damage or otherwise render inoperative the control equip-ments or the turbines. Design criteria for trash rack structure and trash rack along with its operation and maintenance have been detailed in this dissertation. For overall economy of the design, various aspects like velocity of water, spacing and shape of bars, their vibration, head losses, cleaning arrangement are to be considered. Determination of clear spacing between rack bars is important. This depends on the characteristics of the turbine to be protected and the nature of trash. As the spacing becomes closer the head losses increases. Hence the maxi-mum allowable clear spacing should be determined. In the of case reaction turbines for finding out the clear spacing of bars a co-axial correlation curve based on experience data of 11+ projects has been drawn. In case of Impulse turbines the clear spacing of rack bars are comparatively smaller and will depend on the jet opening. Allowable loss of head through trash rack will depend on gross energy produced. Necessary changes in the design should be made so as to reduce the loss of head considering overall economy. Determination of loss of head as per older formulae such as Kirschmer etc. gave lower 1 values than the actuals. Reason for this has been found to be due to not considering certain parameters which influence loss of head. An example using different formulae given by various authorities has been worked out to determine the best formula applicable. For right angle flow Berezinski's formula and for angular flow Ideltchik's formula have been found to be more accurate. Methods of minimising loss of head considering economy have been given in detail. In the structural design the effect of vibration is also considered, as otherwise bars may fail due to fatigue. Shapes of bar should be selected considering head loss, vibration and economy. Streamlined section which givesminimum loss of head is quite unfavourable as far as vibration and economy is concerned. With rectangular sections which is comparatively economical and favourable considering problem of vibration, the loss of head will be more.,