Effect of impeller cutting on axial force

This post was last edited by 3983596_FPPZ on 2018-10-8 at 15:07. After cutting the impeller, the head decreases, and the residual axial force should decrease as well. The balance hole is a typical structure for balancing the axial forces in centrifugal pumps. It is used in conjunction with the sealing ring to reduce the pressure in the cavity between the rear sealing ring and the rear hub of the impeller, thereby balancing the pressures across the inlet area of the impeller. The size of the balance holes is determined based on the total area of multiple balance holes arranged circumferentially; the resulting value is rounded to give the diameter of each balance hole. The number of balance holes is equal to the number of blades, and this total area is generally recommended to be 5–7 times the area of the gap ring at the rear seal. Theoretically, larger balance holes yield a better effect in balancing axial forces, but excessively large holes lead to an increased flow rate of the fluid returning to the inlet of the impeller, reducing the pump’s efficiency and causing disturbances in the fluid flow at the impeller inlet, as well as poor cavitation performance. Therefore, a ratio of 5–7 is usually adopted. Balance holes typically result in a flow loss of 3–5%. In pumps with a balance hole structure, the axial force on the rotor may not be zero even after being balanced, due to reasons such as calculation errors, casting errors, machining errors, and deviations from operating conditions; the remaining small amount of residual axial force is borne by the rolling bearings. Generally, the size of the balance hole in high-flow, high-specific-speed pumps is set to be larger, while it is smaller in low-flow, low-specific-speed pumps. These contents are covered in the chapters on axial force balance in numerous pump design manuals, which can be consulted on your own.