IMPROVEMENT OF STRUCTURAL DESIGN OF PENSTOCK BIFURCATION BY STRESS ANALYSIS USING FINITE ELEMENT METHOD

Abstract

The present study is concerned with the improvement of structural design of penstock bifurcation. The Penstock bifurcation is most complicated structural part of a water conductor system in a hydroelectric electric project. Due to its complex geometry, it is very difficult for carrying out structural analysis by analytical method. Also analytical method does not give exact solution because of complex geometry formation by combination of cylindrical shell / conical shell and internally reinforced sickle plate: In this study the structural design of symmetric penstock bifurcation has been carried out by conventional design approach. The design has been validated by conducting structural model studies by fabricating geometrically similar physical model and by applying strain gauge technology. The design of penstock bifurcation has also been compared by mathematical model studies using Finite Element Method. Finally hydrostatic test has been conducted on full scale prototype. After comparing results of various studies it was found that design of penstock bifurcation by conventional methods is very approximate because of oversimplification of behaviour of various structural components. The conventional design does not consider the limitation of weld efficiency. Also physical model, studies gives better idea of design deficiency by conventional approach but it also does not take into account weld defects and is also time consuming. Further stress analysis has been carried out by conducting mathematical model studies by applying Finite Element Technique. Upon comparing various studies it was found that mathematical model studies give better pattern of stress results but it also results some approximations as compared to actual behaviour of penstock bifurcation. The design can be improved very fast by mathematical studies by making changes in the size various stiffeners. After conducting the hydrostatic test it was found that Mathematical model studies gives best results as compared to other studies. The nature or patterns of stresses at various locations fairly match in various studies. In the present study it was found that during hydrostatic test the prototype can withstand only 50 kg / cm2 internal pressure as compared to actual design pressure of the order of 70 kg / cm2. Finally it was concluded that further refinement is necessary in• the design of penstock bifurcations. The mathematical model studies should be carried out for design improvement. The hydrostatic test on prototype should be carried out to assess weld efficiency and actual pressure resisting capacity.