Sediment erosion in low specific speed francis turbines: A case study on effects and causes

Abstract

Hydraulic turbines experience severe operational and maintenance challenges when operated in sediment-laden water. The combined effect of erosive and abrasive wear in turbine components deteriorates their life and efficiency. The quantity and pattern of sediment erosion depends on the nature of the flow and the amount of hard minerals contained in water. Localized erosion patterns are observed mostly in guide vanes, runner blades and facing plates of Francis turbines due to different natures of fluid flow in those regions. Accelerating flow around the guide vanes and its shaft causes abrasive and erosive wear in its surface, which causes increase in the size of the clearance gap between the facing plates and the guide vanes. Flow leaving the clearance gap forms a vortex filament due to the leakage from high pressure side to the low-pressure side of the guide vane, which eventually strikes the rotating runner blades. This paper presents a case study of a power plant in India with low specific speed Francis turbines, which is severely affected by sediment erosion problems. A numerical analysis of the flow is conducted inside the turbine to study causes of various erosion patterns in the turbine components. The results from CFD are compared with the actual erosion in turbines. Erosion in guide vanes and runner blades are taken into consideration in this paper, due to the complex flow phenomena around these regions. It is found that the leakage flow through clearance gaps of guide vanes is the primary cause of erosion at the inlet of the runner blades. Furthermore, the effects of size and shape of quartz particles are studied which shows that erosion is directly proportional to these parameters.