How to eliminate the axial force in horizontal multi-stage pumps

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I. Axial force of multi-stage pumps: 1. Causes of axial force: The unequal pressures at the suction and discharge ports result in unequal liquid pressures on the two sides of the impeller, which is not completely symmetrical, thereby generating an axial force. If we ignore the cross-sectional area of the shaft and also disregard the effect of the impeller’s rotation on the pressure distribution, then the force acting on the impeller is equal to the difference between the force exerted on the disc and that exerted on the shroud. In mathematical terms, this is expressed as the product of the difference between the outlet and inlet pressures and the area of the impeller shroud. Since the outlet pressure is always greater than the inlet pressure, when a multi-stage pump begins to rotate, there must be a force acting along the shaft in the direction toward the inlet, which acts on the rotor. 2. Problems caused by axial forces: Unbalanced axial forces increase the workload on the thrust bearings, which is detrimental to them. Additionally, axial forces cause the pump rotor to move towards the suction inlet, resulting in vibration; friction at the impeller’s mouth ring may also lead to damage to the pump body. II. Proper elimination of the axial force in multi-stage pumps: In multi-stage pumps, the outlet pressure is generally much higher than the inlet pressure; therefore, it is particularly important to use balance discs to eliminate the axial force. So how can this axial force be eliminated? 1. Multi-stage pumps generally employ the symmetrical installation of balance discs, balance drums, and impellers, while single-stage pumps usually have balance holes made in the impeller; of course, balance blades can also be installed on the impeller to counteract axial forces. 2. Although what we aim for is to eliminate axial forces, completely eliminating them would cause instability in the rotation of the rotor. Therefore, during design, 25–30% of such forces are intended to be absorbed by the bearings. This is why the bearings on the non-driving end of multi-stage pumps are usually angular contact bearings, as they are capable of withstanding large axial forces. This ensures that the rotor components of the multi-stage pump remain in dynamic equilibrium at all times. 3. Multi-stage pumps can also achieve dynamic balance of axial forces by adjusting the clearance between stages. This method has gradually been adopted in the field of multi-stage pumps.