Investigation on the combined effect of sediment erosion and cavitation in Hydraulic turbines. Case study of Giciye l and Gihira Hydropower plants in Rwanda

Abstract

Normally, a proper hydraulic design of turbine components should results in cavitation free geometry. However, a cavitation-free design is demonstrated to be subjected to cavitation when the surface integrity changes because of sediments erosion. The reason for this is that both occurences are more likely to happen in areas with high water velocity. Some of the deteriorated turbines show signs of sediment erosion as well as cavitation. It is difficult to study and analyse the synergetic effect of sediments erosion and cavitation. The effects of individual sediments erosion or cavitation have been studied in a number of experments and theoretical researches. However, there has been relatively little research on the synergestic effects of cavitation and sediments erosion. For completing the research gap, an invistigation on combined effects of sediments erosion and cavitation was conducted. Two hydropower plants on high sediments loaded rivers were selected as a case study. The selected hydropower plants are Giciye I on Giciye river and Gihira on Sebeya river in Rwanda. In this work, different approaches have been used such as data collection, laboratory testing, data recording and observation test. The collected data and results were analysed and discussed.The results showed that both sebeya and Giciye river have sediments. The particles that reach the turbines at Giciye I hydropower plant are sand and fine, with 97% sand and 3% fine, and 84% sand and 16% fine at Gihira hydropower plant. It was observed that high vibration signals were found and corresponding decrease of efficiency resulting from the increase in sediment erosions and cavitation. These resulted in repetitive turbines breakdown.