The impeller in a centrifugal pump experiences axial force due to the different pressures at the suction and discharge ports. As the liquid pressure on the two sides of the impeller is not completely symmetrical, it gives rise to the force. The calculation of this force depends on the difference between the force on the disc and the force on the wheel cover and is proportional to the product of the outlet pressure and the inlet pressure deviation, along with the area of the impeller wheel cover. The inlet and outlet pressure difference generated as the pump rotates creates a force that acts on the rotor along the axis pointing towards the inlet. It is essential to consider the shaft's cross-sectional area and the impeller rotation's impact on the pressure distribution while calculating the axial force generated in a centrifugal pump.
When there is an unbalanced axial force in a centrifugal pump, it can lead to excessive working load on the thrust bearing, which can ultimately damage it. Moreover, the axial force can also cause the pump rotor to move towards the suction port, leading to increased vibration and friction in the pump body. Hence, balance is crucial, particularly in multi-stage pumps, where the outlet pressure is much higher than the inlet pressure.
To address the problem of axial force, plastic-lined magnetic pumps typically use a symmetrical installation of balance discs and impellers, while single-stage pumps often have balance holes on the impeller. Alternatively, one can install balance blades on the impeller disc to counteract the axial force and maintain balance in the pump.
In any case, achieving proper balance is key to ensuring the smooth and efficient operation of a centrifugal pump, while reducing the risk of damage to its components. So, it is important to take necessary measures to eliminate the axial force and maintain balance in the pump.
In order to eliminate the axial force, it is crucial to adequately manage it rather than completely eliminate it. If the axial force is completely eliminated, it will lead to the instability of the rotating rotor. Therefore, during the design process, it is necessary to allocate approximately 30% of the design capacity to the bearing in order to counterbalance the axial force. This is why angular contact bearings are commonly used for the non-driving end bearings in multi-stage pumps as they have the capability to withstand significant axial forces.