Erosion in Francis turbines due to geometrical positioning of runner and guide vanes

Abstract

Hydro-abrasive erosion in turbine components is one of the prominent operational challenges for hydropower plants located across basins of regions such as Himalayas in Asia, the Andes in South America, and Pacific coast. Large sediment concentration with higher percentage of hard minerals such as quartz and feldspar affect exposed hydromechanical components which can cause significant erosion resulting reduced efficiency over time, huge operation, and maintenance cost along with undesired shutdown causing loss of valuable energy generation. In case of Francis turbines, erosion is mostly observed around stay vanes, guide vanes, facing plates and runner blades. Quantity and pattern of erosion depends upon the operating conditions, amount, and size of sand particles in the flow along with type of flow phenomena in particular regions. This study has conducted case study on the sediment erosion problems in one of the hydropower plants located in India that has collaboration with the project. Field reports from the power plant showed abundance of quartz particles contained in the water. A numerical analysis is conducted for turbine components in order to study the causes of various erosion patterns in the turbine components. The results from the CFD are compared with the actual erosion in the turbines. Leakage flow through the clearance gaps of guide vanes is found to be the primary cause of erosion at the inlet of the runner blades. This work comprehends a systematic investigation of spatial temporal progression of leakage vortex (LV) for varying operating conditions. The progression of LV at hub area is roughly classified into three forms i.e., elongation stage, disintegration stage and dissolving stage. Whereas few LV at the shroud area is found to pass through outlet of the runner, majority of it dissolves before reaching the mid-stream of the blade. Study was also conducted to find the optimal runner design with a trade-off between performance and erosion behavior by modifying blade angle distribution from inlet to outlet. Varying blade angle distribution changes the blade profile which also seemed to yield a better reduction in erosion. This study has identified research gap as the investigation of flow pattern in sidewall gaps in Francis turbines which refers to the clearance region between the stationary and rotary components. For hydro-turbines operating in sediment laden water, sediment will pass through these gaps and erode the parent material. Due to continuous effects of abrasion and erosion, these side wall clearance region increases and consequently the leakages through these gaps. The work focusses on the study of flow behavior in those regions with an experimental setup. Experiments were conducted in Rotating Disc Apparatus (RDA), which utilizes the method of rotating test specimen in a mixture of calculated amount of sand and water. It has a base plate which mounts test specimens that is attached to the shaft connected to the motor. First set of test specimens constitute three different slot width and another set has six different slot height. All experiments were carried out at Turbine Testing Laboratory, Kathmandu University, Nepal. Results for different slot width show its positive correlation with the rotational speed and erosion loss. It shows that as rotational speed increases, the shift of erosion wear is more towards the higher width slot. Another set of results including specimens with different height shows the increase in erosion rate with the corresponding increase in slot height. It is observed that, fluid flowing from the outlet of the guide vanes hits the side wall of the rotor component normally at positive slot height. Some particles recirculate in the slot width and again impinges the opposite side walls. Whereas in the case of negative slot, the direct impingement of sediment particle to the opposite face does not happen. However, during the flow re-circulation, the sediment again hits back to the guide vane face. For the specimen with height difference, weight loss in nearly constant at lowest rotational speeds. Erosion rate is observed increasing linearly up to zero height difference, but it increases significantly at the positive height difference. Sidewall gaps and flow in such regions is inevitable in hydraulic machineries. Further investigation on the shape of slot and erosion around labyrinth rings should be done to get a complete picture of erosion phenomenon. Flow phenomenon associated with sidewall gaps should be well considered and alalyzed during the preliminary phase of machine design.