Study on the influence of pressure-balance pipe diameter on the volumetric efficiency of pump turbine

Hydraulic machinery experiences volume loss due to flow leakage from the clearance between rotating parts and the shell. For pump turbines, fluid leakage from high-pressure to low-pressure areas through the clearance between the upper crown and lower band sealing parts causes volume loss and reduces hydraulic efficiency. The pressure-balance pipe is crucial for balancing axial hydraulic thrust, and its starting end typically lies on the inner side of the upper crown clearance. In this study, we analyze a pump turbine with three flow rates (0.9Q, 0.75Q, and 0.5Q) to illustrate the impact of clearance flow on volumetric efficiency. We consider the effect of varying pressure-balance pipe diameters on volumetric efficiency and analyze the influence of diameter changes on clearance flow. Our findings indicate that changing pressure-balance pipe diameter affects volumetric efficiency and pressure distribution in the upper clearance, with different mechanisms for high and low flow conditions. This study has important implications for pressure-balance pipe design. While ensuring that axial hydraulic thrust balancing is maintained, resizing the pressure-balance pipe diameter can improve unit operation efficiency.