High Specific Speed Francis Turbine Operating At Low Load Stability

In the last few years, hydrodynamic performance of modern hydro turbines was farther improved along with the advances of computer technique and CFD technology; the efficiency of grand Francis turbines has nearly got a high point. As the increasing of single unit capacity (size) and parameters, the stability problem of Francis turbine is emerging. Many Francis turbines operated in domestic and abroad suffered severe vibration or crack problems. Moreover, operation stability of the turbine has a momentous influence on the normal production of units and benefit of power plant. Accordingly, in-depth research is of theoretically and practically significance.The basic information and operation states of many large hydropower plants installed Francis turbine in domestic and abroad were collected statistically, and based on the outcomes and methods of former researchers, a theoretical analysis on the characteristics and low load operation stability of Francis turbine, especially high special speed turbine, was taken in these papers, concentrating on the theoretical analysis of structure and shape traits of high special speed Francis turbine, the relationship among clash angle of runner inlet, annulose quantity of outlet, pressure grads and operation conditions, and the cause of the formation of part load vortex rope and its pressure fluctuation characteristics in frequency and amplitude. Moreover, based on the analysis of vibration characteristics of hydropower unit and methods of data processing of stability test, stability study of a large prototype Francis turbine was made, with operation idiosyncrasy and head influence on it in different conditions investigated. Then, improving measurements in design and operation for the low partial load was discussed, including a method of the turbine's rated head selection based on partial load optimization.Flow states in inlet and outlet of Francis turbine runner is variable in different conditions, thus the stability of turbine is of distinctness. Besides the well-know vortex rope vibration district, swirl of blades path in a lower load may cause biggish pressure pulsation or vibration. Prototype test investigation indicates that, operation head has great influence on the stability of turbine, the stable district moving towards higher load region when head increasing, and becoming narrow. For high special speed turbine, the part load operation stability problem may more severe in large head variability or far away from rating conditions. The low partial load instability and vibration of Francis turbine is inherently decided by un-adjustment of its blades, improvement may gained by optimization of flow pass components including the blades, cone and draft tube in a certain extent. For a certain turbine, there must be some unstable operation district; it can be restricted by filling air or installing flow pin . In the other hand, optimization of the turbine work parameters (rated head) selection and operation arrangement of units is of importance and significance.