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284 Chapter 8 Pumping, Storage, and Dual Water Systems

H 1 head loss at flow of Q 1 , ft (m)
H 2 head loss at flow of Q 2 , ft (m).
Solution 1 (U.S. Customary System):
Head loss at average daily water demand of 2 MGD (or 1,388 gpm):
2
H 2 (40)(1,388>1,600) 30 ft
Head loss at maximum daily water demand of 3.4 MGD (or 2,360 gpm):
2
H 2 (40)(2,360>1,600) 87 ft
Head loss at maximum hourly water demand of 4.8 MGD (or 3,331 gpm):
2
H 2 (40)(3,331>1,600) 173 ft
Solution 2 (SI System):
3
Head loss at average daily water demand of 7.6 MLD (or 0.088 m /s, or 88 L/s):
2
H 2 (12.2)(0.088>0.101) 9.36 m
3
Head loss at maximum daily water demand of 12.9 MLD (or 0.150 m /s, or 150 L/s):
2
H 2 (12.2)(0.150>0.101) 27.00 m
3
Head loss at maximum hourly water demand of 18.2 MLD (or 0.210 m /s, or 210 L/s):
2
H 2 (12.2)(0.210>0.101) 53.80 m.
The system will not be economically feasible if the maximum hourly water demand is used for
design of a pump station due to its related high head loss (173 ft or 52.73 m). Normally the maxi-
mum daily water demand is used for designing a pump station. The peak hourly water demand
of the city will be provided by the elevated water storage tank.
According to the latest edition of the Ten-States Standards, the minimum water storage capac-
ity for water systems not providing fire protection shall be equal to the average daily consumption,
3
3
or 2 MG (7,570 m ). A new water storage tank (2 MG or 7,570 m fire demand) is needed
3
because the existing water storage tank (550,000 gal or 2,082 m ) is not big enough.

EXAMPLE 8.10 DESIGN OF A PUMPING STATION
Design a pump station for the water system examined in Examples 8.8 and 8.9. Double-suction,
horizontal centrifugal pumps driven by AC electric motors are considered the best application for
this project. Select pumps having rated speeds of 1,750, 1,150, or 870 rpm for this pump station
using assumed pump characteristics.
Recommend the pump capacity, the number of pumps, and pumping mode (parallel operation or
series operation). Calculate the effective head of the selected pump, its water horsepower, brake
horsepower (horsepower input to each pump), and motor horsepower assuming the pump efficiency is
80% and the motor efficiency is 90%. Show the assumed capacity-efficiency curve, the head-capacity
curve, and the BHP-capacity curve (or the BMP-capacity curve) on a sketch, and then indicate on the
sketch the rated points of the pumps. Write brief engineering conclusions stating the following: (a) the
number of installed pump units and the type of motor driving each pump unit; (b) the method, if any,
of capacity control that an engineer would propose and the probable overall pump station efficiency at
the average daily water consumption; and (c) the probable horsepower of the motor driving each unit.