Page 129 - Water and wastewater engineering

3-26 WATER AND WASTEWATER ENGINEERING

Solution:
a. Calculate H stat for the maximum and minimum drawdown in the wet well.
Using the sketch elevations, the maximum static head is

.
H stat 190 9 m 172 9 m 18 0 m
.
.
and the minimum static head is
.
.
.
H stat 190 9 m 175 6 m 153 m
b. Calculate the friction losses using Equation 3-5 for a 400 mm diameter ductile iron pipe
(DIP) at three flow rates.
3

From Appendix C, select C 100 for DIP. At the maximum flow rate of 12,000 m /d
3
or 0.139 m /s, for the 300 m long pipe the total friction headloss is
.
⎛ 0139 m /s 185 ⎛ 300 m ⎞
3 ⎞
.
h 10 7 ⎜ ⎟
.
fd ⎜ 4 487 ⎟
⎝ 100 ⎠ ( ⎝ 04 m) ⎠
.
.
.
144 m
Similarly for the average and low flow rates
h fd 0 399 or 0 4 m at average flow
.
.
.
m
h fd 011 m at low flow
c. Compared to the values calculated in (a) and (b), the losses for valves and fittings are
2
negligible and are neglected here. Likewise, the velocity headloss ( v /2 g ) is very small
and is neglected here.
d. The plotting points for the system curves for the maximum drawdown are
3
18 0 m 1 44 m 19 44 m at Q 12 000 m /d
.
.
.
,
3
.
m
,
18 0 m 0 4 . m 18 4 . m at Q 6 000 m /d
3
.
,
m
18 0 . m 0 11 m 18 11 at Q 3 000 m /d
.
For the minimum drawdown the plotting points are
3
153 m + 144 m 1674 m at Q 12 000 m /d
.
.
,
.
3
153 m + 04 . m 15 7 . m at Q 6 000 m /d
.
m
,
3
153 . m + 011 m 15 4 . at Q 3 000 m /d
.
,
m
.
These points are plotted in Figure 3-16
Pump Selection. Multiple pumps and/or variable speed pumps are selected to cover the range
of conditions described by the system head curve. It is important to select a pump that will have
its best efficiency within the operating range of the system and preferably at the operating point
(head and discharge) where the pump will operate most often.