Page 277 - Water Engineering Hydraulics, Distribution and Treatment

Head loss at maximum hourly water demand of 4.8 MGD (or 3,331 gpm):
H = (40)(3,331∕1,600) = 173 ft.
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Solution 2 (SI System):
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Head loss at average daily water demand of 7.6 MLD (or 0.088 m /s, or 88 L/s):
H = (12.2)(0.088∕0.101) = 9.36 m.
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Head loss at maximum daily water demand of 12.9 MLD (or 0.150 m /s, or 150 L/s):
H = (12.2)(0.150∕0.101) = 27.00 m.
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Head loss at maximum hourly water demand of 18.2 MLD (or 0.210 m /s, or 210 L/s):
2 2 2 2 8.6 Types of Distributing Reservoirs 255
H = (12.2)(0.210∕0.101) = 53.80 m.
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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 maximum 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 capacity for water systems not
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providing fire protection shall be equal to the average daily consumption, or 2 MG (7,570 m ). A new water storage tank
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(2 MG or 7,570 m + fire demand) is needed 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.
Solution 1 (US Customary System):
Average daily consumption = 2MGD = 1,388 gpm.
Maximum daily consumption = 3.4MGD = 2,360 gpm.
Peak hourly demand = 4.8MGD = 3,331 gpm.
With sufficient water storage capacity available in the future, two pumps of equal capacity should be operating in parallel to
supply the maximum daily consumption of 2,360 gpm. Each pump is to supply 1,215 gpm, and both pumps 2,430 gpm.
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Head loss at 2,360 gpm, H = 40 (2,360∕1,600) = 87 ft.
The effective head of each pump should be 87 ft less the difference in water surface elevations.

Difference in water surface elevation = 500 ft − 490 ft = 10 ft.
Effective head of each pump, H = 87 ft − 10 ft = 77 ft.
Water horsepower (WHP) = (QH)∕3,957
= (1,215)(77)∕3,957 = 23.6hp.
Brake horsepower (BHP) = horsepower input to each pump = WHP∕E
pump
= 23.6∕0.8 = 29.5hp.
Motor horsepower (MHP) = horsepower input to motor = BHP∕E motor
= 29.5hp∕0.9 = 32.8hp.