Page 142 - Centrifugal Pumps Design and Application

120 Centrifugal Pumps: Design and Application

Figure 9-7. Vertical solid shaft motor (courtesy U.S. Electrical Motors, Division
of Emerson Electric Co.).

Barrel-Mounted or Can-Mounted Pumps

This pump type, as depicted in Figure 9-8, is mounted in a suction bar-
rel or can that is filled with liquid from the suction source. In this type of
closed system, the NPSH available is not related to the atmospheric pres-
sure. It is a function of the absolute pressure (above absolute vacuum) at
the centerline of the suction flange, plus the liquid head from the center
line of the suction to the first-stage impeller, less barrel losses, and less
the vapor pressure of the pumped liquid. The available NPSH can there-
fore be increased by lowering the elevation of the first-stage impeller and
extending the suction barrel until the available NPSH meets or exceeds
the required NPSH. This feature is in fact frequently the reason for se-
lecting a vertical, barrel-mounted pump. To achieve the same result with
a horizontal pump would require lowering the entire unit at great ex-