Brochure

2020
The axial bearing absorbs the entire axial thrust and is therefore exposed to
the forces aecting the impeller.
The impeller must be axially balanced if it is not possible to absorb the entire
axial thrust in the axial bearing. There are several possibilities of reducing
the thrust on the shaft and thereby balance the axial bearing. All axial
balancing methods result in hydraulic losses.
One approach to balance the axial forces is to make small holes in the hub
plate, see figure 1.16. The leak flow through the holes influences the flow
in the cavities above the impeller and thereby reduces the axial force but it
results in leakage.
Another approach to reduce the axial thrust is to combine balancing holes
with an impeller seal on the hub plate, see figure 1.17. This reduces the pres-
sure in the cavity between the shaft and the impeller seal and a better bal-
ance can be achieved. The impeller seal causes extra friction but smaller
leak flow through the balancing holes compared to the solution without the
impeller seal.
A third method of balancing the axial forces is to mount blades on the back
of the impeller, see figure 1.18. Like the two previous solutions, this method
changes the velocities in the flow at the hub plate whereby the pressure
distribution is changed proportionally. However, the additional blades use
power without contributing to the pump performance. The construction
will therefore reduce the eciency.
Atmospheric pressure Outlet pressure
Figure 1.16: Axial thrust reduction using
balancing holes.
Figure 1.17: Axial thrust reduction using impel-
ler seal and balancing holes.
Figure 1.15: Pressure forces which cause
axial thrust.
1. Introduction to Centrifugal Pumps
Axial thrust
Outlet pressure
Inlet pressure
Axial balancing holeImpeller seal
Axial balancing hole