Page 176 -

........................................ Special Pumps

Although the NPSH required rises quickly with increased flow rates,
this is rarely a problem when pumping from an open sump. However,
the matter of submergence must always be considered as it is
independent of the NPSHR. The submergence is the static elevation
difference between the free surface of the liquid and the centerlinc of
the first stage impeller. It is interesting to note that the submergence
required by all vertical pumps can be represented on the pump curve by
a horizontal straight line - but only up to a point. Beyond that point
the Submergence required increases dramatically, causing a vortexing
problem where air will be drawn into the impeller.

9.2.5 Vertical pump sump requirements
Vertical pump performance is also dependent on sump design as this
dctermincs the flow of liquid to thc pump. Unfortunately this is much
less precise than the inlet piping of a horizontal pump and frequently
results in performance problems. The layout of the sump and the
relative positioning of the pump(s) within that sump arc of such
importance to the pump reliability as to warrant model testing prior to
the design and installation of the sump. The objective is to provide a
sump in which there is minimal turbulence and no negative influence
from the incoming lines.
Figure 9.13 shows a possible installation with some very general
guidelines that would apply to the particular layout shown. Somc of thc
key elements include the velocity of the approaching water which
should usually be about 1.0 ft/sec., and the location of each pump to

I i

o

_ 2o J_ ,o

Suction Bell Diameter = D
Clearanoe under Bell = 0.5 x D
Figure 9.13: Vertical pump sump layout

155 m