Page 381 - Centrifugal Pumps 2E

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 re-
ducing 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 in-
creased, 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.