Page 278 - Water Engineering Hydraulics, Distribution and Treatment

256
Pumping, Storage, and Dual Water Systems
Chapter 8
Solution 2 (SI System):
3
Average daily consumption = 7.57 MLD = 0.088 m ∕s = 88 L∕s.
3
Maximum daily consumption = 12.87 MLD = 0.150 m ∕s = 150 L∕s.
3
Peak hourly demand = 18.17 MLD = 0.210 m ∕s.
With sufficient water storage capacity available in the future, two pumps of equal capacity should be operating in parallel to
3
supply the maximum daily consumption of = 12.87 MLD = 0.150 m ∕s = 150 L∕s.
3
3
Each pump is to supply 0.077 m /s, and both pumps 0.153 m /s.
3
3
Head loss, at 12.87 MLD or 0.150 m /s, H = (12.19 m)(0.150 m ∕s∕0.101 m ∕s) = 26.5m.
The effective head of each pump should be 26.5 m (87 ft) less the difference in water surface elevations.
Difference in water surface elevation = 152.4m − 149.3m = 3.1m. 3 2
Effective head of each pump, H = 26.5m − 3.1m = 23.4m.
Water power (WMP) = 9.8066(QH)
= 9.8066(0.077)(23.5) = 17.7kW.
Brake power (BMP) = power input to each pump = WMP∕E
pump
= (17.7)∕0.8 = 22.2kW.
Motor power (MMP) = power input to motor = BMP∕E
motor
= 22.2∕0.9 = 24.6kW.
The sketch of the selected centrifugal pump will have the pump characteristic curves shown in Fig. 8.12.
Head Efficiency
Head (ft) 80 BHP
1250
Flow (gpm)
Figure 8.12 Pump characteristic curves for Example 8.10. Conversion factors: 1 ft = 0.3048 m; 1 gpm = 3.785 L∕min =
3
0.0000631 m ∕s = 0.0631 L∕s.
The engineer’s recommendations are as follows:
a. Install three pump units, each with a capacity of 1,250 gpm (79 L/s) at a total developed head of 80 ft (24.38 m) at full capacity.
Two of the pumps should have standard 60-cycle, AC induction-motor drives, 1,750 rpm. The third pump to be held in reserve
in case of electric power failure or shutdown of one electric-drive pump for repairs may be driven with a gasoline engine,
in case an emergency power generator is not available. If an emergency power generator is available, the third pump can be
identical to the selected two pumps.
3
b. The storage tank capacity (550,000 gal or 2,082 m ) is normally sufficient to meet the maximum hourly demand and
fluctuation during maximum daily demand. However, the existing storage tank capacity is insufficient according to the
latest edition (2007) of the Ten-States Standards for water works. Expansion of the water storage tank is recommended if
construction funds are available. The quantity of water in the water storage tank should be controlled by a floating control
system. If the float drops below a specified level, the control system actuates the starting mechanism of the second pump
to put it in parallel with the other pump. The pump station efficiency at average daily consumption will be approximately
50–60%.
c. Each pump must be driven by a 35 hp (26.1 kW), 1,750 rpm motor.