Measures for balancing the axial force of multi-stage pumps

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The axial force on the rotor of a multi-stage pump is very large, especially in multi-stage pumps. To reduce the axial load and friction on the shaft, in addition to small single-stage pumps and ball bearings that bear axial forces, hydraulic or mechanical balancing measures are generally employed. 1. A double-suction impeller is used. Since the pressure on both sides of the double-suction impeller is equal due to its symmetry, the axial force can be balanced. However, due to casting variations and different wear levels of the rings on both sides, a residual unbalanced axial force remains, which must be borne by the bearings. A double-suction impeller is used in operation not only to balance the axial force but also to increase the flow rate as a whole. 2. Creating balance holes or installing balance tubes: Several balance holes are made along the circumference at the corresponding positions on the rear cover plate of the impeller and the inlet, allowing the external fluid to flow into the impeller’s inlet and thus achieving equilibrium between the liquids on both sides of the impeller. At the same time, a ring is installed between the rear cover plate and the pump casing; its diameter is the same as that of the ring on the front cover plate. There is a pressure drop as the fluid flows through the balance holes, and it is not possible to completely eliminate the pressure differences between the front and rear cover plates. As a result, 10%–15% of the axial force remains unbalanced, and bearings are used to balance this axial force. Furthermore, with this method, since the direction of the liquid leaking back into the suction inlet is opposite to the direction of the liquid being sucked in, the uniformity of the sucked-in liquid is disrupted, thereby reducing the efficiency of the pump. The advantage of this method is its simple structure, but it increases internal leakage. When drilling the balance holes, they should be placed as close as possible to the inlet ring, as this yields better results. The total area of these balance holes should be 4 to 5 times greater than the flow area of the gap between the rings. 3. Utilization of balance vanes: Several radial ribs are installed on the back side of the impeller’s rear cover. As the impeller rotates, these ribs force the fluid on the back side of the impeller to rotate faster, increasing the centrifugal force and thereby causing a significant drop in the pressure of the fluid on that side. This results in pressure equilibrium on both sides of the impeller. 4. Use thrust bearings or the existing bearings to bear the axial force. This is the mechanical balance method; for small pumps, rolling ball bearings can be used to withstand axial forces.