Forces in centrifugal pumps

 Axial thrust 

The axial forces of thrust generated in a centrifugal pump results from the internal 

pressures acting on the exposed areas of the rotating element. 

It may appear as simple as a product of the net of discharge and suction pressure and the 

exposed area of the impeller. Though this is truly the basis, there are many uncertainties 

that are not covered by this simple approach. 

The other variables that may affect the evaluation of axial loads are: 

• Location of the impeller relative to the casing wall 

• Impeller shroud symmetry 

• Surface roughness of the walls 

• Wearing ring clearance 

• Geometry of the balancing holes. 

Many of these variables are indeterminate and hence the axial forces calculated are at 

best approximate values. 

In critical applications and during the testing of prototypes or to trouble-shoot repeated 

thrust bearing failures, it maybe essential to arrive at accurate thrust values. In such cases, thrust-measuring devices are installed. The internal pressures may also be recorded to get an idea of the hydraulic forces. 

If the results are found unacceptable then design modifications maybe necessary. 

The approximate method of axial thrust calculations is explained below. The following assumptions are made: 

• Impeller profile is symmetrical. 

• Impeller centerlin coincides reasonably with the volute centerline (within 0.8 mm).

• Pressure on the front cover and back shroud is equal and constant from the impeller outside diameter to the wearing ring diameter. 

• The pressure reduction, which is parabolic in nature, is ignored and average pressure 

acting on the shrouds is considered to be 3/4th of the pump discharge pressure. 

4.1.1 Axial thrust in single-stage overhung impeller pumps Axial thrust calculation in these simple pumps is made complex due to many variants in the design of the impellers and mating components. Axial thrust in some of the typical construction is discussed below. 

Consider perfect impeller symmetry (Figure 4.1), the axial forces in the region above the diameter D1 is balanced. The pressure on either side is assumed to be the same.