Page 327 - Centrifugal Pumps 2E

292 Centrifugal Pumps: Design and Application

There are three basic types of back shroud configurations. One is a
full-open impeller where the back shroud is almost completely scalloped
out to reduce the area on which the hydraulic pressure can react thereby
almost eliminating axial thrust (Figure 15-9A). The second is a semi-
open impeller that has a partially scalloped back shroud (Figure 15-9B)
that has greater axial thrust than the full-open impeller but has better effi-
ciency and head characteristics. The third is the full back shroud (Figure
15-9C) that normally has about five points higher efficiency than the
scalloped impeller but has less head than the scalloped impeller because
of the regenerative action of the scallop. Most open-impeller designs are
of the scallop or full shroud variety. Full-open impellers are rarely used
in this industry because of low efficiency and the bending loads on the
vanes. If it is found that impellers with plain back surfaces produce inad-
equate bearing life due to excess axial thrust, then pump-out vanes are
usually employed on the back of the shroud to reduce the thrust. (Refer to
Chapter 18).

Figure 15-9A. Fully scalloped open impeller.