Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

346 Centrifugal Pumps: Design and Application
Figure 16-5. Axial thrust on single-suction closed impellers with balance holes.
Closed Impeller with front ring and back balancing ribs. Where it is
not desirable to design the impeller with a back ring, axial thrust can be
balanced by designing integrally cast ribs on the back of the impeller
shroud. A gap between rib and cover of 0.015 in. maximum, permits the
liquid to rotate at approximately full impeller angular velocity, thus reducing
the pressure on the impeller back shroud over the area of the rib.
This method of balance is not very accurate or positive. If the gap is increased,
the pressure acting on the back shroud increases, resulting in
higher axial loads. Experience has shown that this type of impeller is less
efficient than one with a back ring. Axial thrust can be estimated as
shown in Figure 16-7.