Influence of initial nozzle opening on the maximum pressure head at the impulse turbine inlet and set of needle closing law

Abstract

The maximum pressure head at the impulse turbine inlet is one of the most important parameters for operation of the impulse turbine. In order to determine the most critical operating condition for the maximum pressure head rise at the impulse turbine inlet, the formula which demonstrates the relation between relative pressure head and the initial opening is obtained through theoretical derivation. It shows a good agreement with the numerical results by the method of characteristics. A parabolic shaped flow characteristic of the nozzle is utilized in the derivation. Besides, the fact that the maximum pressure often occurs in the moment of the first wave reflection for the hydropower plants with impulse turbines is considered. The formula indicates that the maximum of pressure head rise increases monotonously with the initial opening decreasing. Therefore, the most critical condition for the maximum pressure head at the impulse turbine inlet is the condition of load rejection from small opening rather than the load rejection condition from full opening in the rated or maximum head. When adopting linear or two-speed closing law, the derived formula can be used to calculate the closing speed and the opening of break point, realizing the optimization of closure.