Civil Engineering Reference
In-Depth Information
VARIABLE-SPEED PUMPING
Chapter 30, ''Energy Management for Water Treatment Facilities,'' describes variable-
speed drives. Variable-speed drives for pumping systems can be applied either to cen-
trifugal pumps or positive-displacement pumps, like the progressing cavity pump. The
intent of variable-speed pumping is to match the flow rate to the water-demand rate.
Oftentimes, the variation in flow that can be attained with multiple pumps provides
the necessary flexibility. Where more flexibility is needed, variable-speed drives on
pumping systems are used.
Where there are several pumping units, the question arises as to how many pumps
need to be provided with variable output. Oftentimes the availability of one pumping
unit fitted with a variable-output capability is enough to gain the needed flexibility. In
those cases, the criticality of the variable output needs to be assessed to determine if
a standby pump or variable drive is necessary.
OTHER ISSUES
Axial flow pumps and higher-specific-speed mixed flow pumps have an unstable pump-
ing condition at higher heads. Unlike radial flow pumps that can be essentially operated
throughout the pump curve to shutoff, high-specific-speed pumps have a characteristic
hump in their curve whereby there are three flow rates at a specific head (see Fig. 29-
26). If the pumping head reaches this zone on the pump curve, the flow will become
unstable as it moves from point to point on the pumping curve. On applications for
pumping units with this characteristic, it is important to select a pump that matches
the system curve to avoid this zone. Also, for pumps with variable-speed drives, limit
the speed reduction to avoid operating above the critical zone on the pump curve.
Radial flow pumps can be operated from shutoff throughout the pumping range.
Particularly for booster pumping applications, where the outflow may fall to zero, it
is necessary to provide a bleed-off on the discharge piping. If the pump is operating
at shutoff, the pumping efficiency loss generates heat in the pump, and eventually the
water temperature in the pump will reach boiling. The amount of bleed-off water needs
to exceed the amount needed to take the heat buildup away. The bleed-off water may
be reconnected to the upstream distribution system to avoid the loss of this treated
water.
PUMP LAYOUT
In making pump layouts, the designer should keep in mind a number of considerations:
Provide at least a 3-ft (0.9-m) clearance between obstacles to permit wheeled
hand truck movement on the pumping station floor.
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Arrange suction piping to avoid high points where air or gas may collect. Re-
ducers, where they are required in horizontal piping, should be eccentric.
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Discharge piping also should avoid high points. Dissolved gas will have less
tendency to come out of solution on the discharge side than on the suction side;
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