Axial thrust arises in centrifugal pumps due to their asymmetry. The clearances between casing cover and impeller back shroud, casing and impeller front shroud are filled with fluid at delivery pressure. This pressure acts on the impeller shrouds.
As the back shroud has a larger surface area than the front shroud, a net thrust acts on the impeller in the direction opposite to that of the incoming flow. The force contributed by the change in the momentum of incoming flow is also to be considered.
Mechanical axial thrust balancing
The absorption of the axial thrust by a rolling element bearing is the most efficient, cost-effective solution. However, in the absence of special balancing equipment, complex thrust bearings are required as substitutes. These bearings have long term benefits in terms of efficiency and savings in cost.
Design-based axial thrust balancing
In case of an impeller arrangement in a pipeline pump with four stages, each featuring a 2 x 2 back-to-back arrangement, a maximum of twice the normal axial thrust per stage can occur. This happens if that system causes cavitation in two stages.
If, however, a more complex, parallel-coupled back-to-back impeller arrangement is chosen, only the normal axial thrust per stage occurs.
Axial thrust balancing at the impeller
This is the oldest method for balancing axial thrusts and involves reducing the pressure in a chamber equipped with a throttling gap. This pressure is usually as low as the pressure level encountered at the impeller inlet. The pressure is balanced via balancing holes in the impeller. These balancing holes may lead to variations in axial thrust balancing as a result of varying inlet conditions.