Page 386 - Centrifugal Pumps 2E

Shaft Design and Axial Thrust 351

Figure 16-10. Increasing number of bleedoff lines reduces thrust.

rangement. This aids axial balance and subjects the inboard and outboard
mechanical seal to only suction pressure. It is essential that the bleed-off
line be adequately sized to transfer leakage back to suction without per-
mitting a pressure build-up in the stuffing box. Any such build-up of
pressure will have an adverse effect on axial thrust. If pressure build-up
is detected, or suspected, the bleed-off line should be increased in size or
additional lines added. This principle also applies to overhung process
pumps with balance holes, where the back pressure can be reduced by
increasing the size of the holes. The effect on thrust of a two-stage pump
with one and two balance lines is illustrated in Figure 16-10. Note, that
the thrust level and direction of thrust is not constant across the capacity
range of the pump. This tendency is typical of most pumps.

Notation

Shaft Design

TI Shear stress (psi)
r Torsional stress (psi)