Civil Engineering Reference
In-Depth Information
SUCTION CONDITIONS
Water should be introduced to the pump so the approach velocity is uniform and the
liquid is not rotating. If the velocity of approach is uneven, it will place an unbalanced
load on pump impeller, and exert more load on bearings and packing than they are
designed for. The Hydraulic Institute standards provide guidelines for arrangement of
sumps.
4
Some problems have been experienced using these guidelines. The Hydraulic
Institute subcommittee on sump design is in the process of revising many of its rec-
ommended sump layouts as a result of testing as of early 1996.
Intakes from lakes or rivers frequently use vertical turbine pumps mounted on a
platform and suspended in a wet sump. The ideal flow approach to the pump / pumps
needs to be uniform and evenly distributed to multiple pumps. Practically, the distur-
bance at the entrance, the direction changes at the pump suction, and difficulty in
avoiding eddies, swirls, and vortexing in the flow make this situation difficult to eval-
uate. Figure 29-16 shows baffling provided for a wet sump for vertical pumps. The
concept is to direct the flow to the pumps with orifices in a barrier wall and guiding
baffle walls that keep the flow moving straight to the pump without swirling. The
suction cone is to defeat a common vortex that forms under the pump suction.
Figure 29-17 shows the baffling for a can-pump-mounted vertical turbine pump.
The concept shown intends to deflect-baffle the flow downward into the can around
the pump column. A clever design is necessary to baffle-guide the flow while still
permitting easy removal of the pump from the can. Where the flow moves around the
pump suction bell, it passes through the grating and then turns and passes through the
grating to enter the pump suction.
Manufacturers' standard layouts for vertical end suction centrifugal pumps call for
a standard-radius elbow from the pump to the wetwell. During a hydraulic model test
in August 1996, however, column separation of the water was observed on the inner
radius of the elbow, as well as an unequal velocity distribution around the suction pipe
entrance to the pump. The observed phenomenon resulted in
Pumping
Unit
Baffle
Walls
Pumping
Units
Baffle Wall
=
fps
±
Flow
Flow
Orifices
in Inlet
Suction
Cone
Section View
Fig. 29-16.
Wet sump vertical turbine pump application
