Influence of mechanical draft tube fish barrier on the hydraulic thrust of small Francis turbines

Abstract

Environmental regulations have required from hydroelectric power plants more effective means of restricting the access of fish into the draft tube of hydraulic turbines. The decline of fish populations on rivers used for electricity generation has motivated the installation of mechanical gates as fish barriers on the tailrace pipe of hydroelectric power plants as a technical solution to preserve aquatic wild life. However, variations in the flow conditions through the turbine runner may modify the performance characteristics of turbines, thus affecting their capacity for electricity production. This note presents an experimental study of the influence of draft tube gates on the hydraulic forces acting on Francis turbines. A Francis turbine test bench was specially designed to allow this investigation. Measurements of the hydraulic thrust in relation to the level of flow restriction at the draft tube exit indicate that the mechanical gates excessively increase the axial load on the runner shaft, which may shorten the hydrogenerator unit life.