Page 132 - Water and wastewater engineering

INTAKE STRUCTURES 3-29

3
e. From Figure 3-16 , the maximum TDH at 250 m /h is 18.4 m.
The specific speed is

.
3 0 5
.
(1750 rpm )(0 0694 m /s ) 4 461 0
,
.
n .
s
.
(18 4 . m ) 075 888
5192
.
f. For specific speeds less than 87, the POR is from 70 to 120 percent of the BEP flow rate or
3
3
)
.
(070 )(250 m /h 175 m /h
to
3
3
.
)
(12 )(250 m /h 300 m /h
g. The pump curve is plotted on the system curve in Figure 3-18 .
h. Using Equation 3-3, the motor brake power is
3 3
.
(9 807 kN/m )(0 0694 m /s )(18 m )
.
P
078
.
15 7 . kW
i. The depth of the wet well is determined by the hydraulics of the conduit, the depth of the
conduit carrying water to the wet well, and the minimum submergence required by
the pump. The following sketch shows the relationships using the results of calculations
in Example 3-3, the NPSH R for the selected pump, and the NPSH A assuming an altitude

of 500 m and a temperature of 5 C.
Pump motor
Operating deck
Pump room floor
Maximum lake
elevation

Minimum lake Maximum NPSH R = 1.6 m NPSH A 9.6 m
surface evevation Elev. = 175.6 m
Maximum drawdown
Elev. = 172.9
Minimum pump
submergence
Intake conduit
0.5D
D

Comment. The selected pump is not “ideal” for the system curves. A pump with fewer stages
and a flatter head-discharge curve would be a better choice.