Page 284 - Water Engineering Hydraulics, Distribution and Treatment

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Pumping, Storage, and Dual Water Systems
Chapter 8
The weight of the intake structure must be greater than 2,079,500 kg in order to provide stability.
3
3
Weight of the side walls = [8.5 × 8.5 (87 – 60) – 6.5 × 6.5 (87 – 60)] m (2,308 kg/m ) = 1,869,580 kg.
Weight of foundation slab = (14 × 14 × 2.5) m (2,308 kg/m ) = 1,130,920 kg.
Total weight of the concrete intake structure = 1,869,480 + 1,130,920 = 3,000,400 kg.
The safety factor = (weight of intake structure)/(weight of water displaced) = 3,000,400 kg/ 2,079,500 kg = 1.44.
Discussion: Since the required safety factor is 1.5–2.0, it is concluded that the tentatively designed concrete intake structure
(safety factor = 1.44) should only be slightly corrected. The safety factor of this intake structure can be increased by adding additional
weight to the RC structure or by anchoring the concrete footing.
PROBLEMS/QUESTIONS 3 3 at a discharge of 1000 gpm (63 L/s). The pumping station has two
pumps, each of which possesses the following characteristics:
8.1 Determine the water horsepower, break horsepower, and
motor horsepower for a pump operating under the following con- Flow (gpm) Head (ft)
ditions: water flow of 490 gpm (31 L/s) is to be pumped against a
total head of 110 ft (33.53 m); the pump efficiency is 75%; and the 500 195
motor driving the pump has an efficiency of 85%. 1,000 180
1,500 150
8.2 A pump is to be located 6 ft (1.83 m) above a wet well and
2,000 110
must lift 600 gpm (38 L/s) of water another 52 ft (15.85 m) to
2,500 50
a storage reservoir through a piping system consisting of 1,250 ft
(381 m) of 6 in. (150 mm) DIP pipe (C = 110), two globe valves
(open), and two medium sweep elbows. Determine the total dynamic Flow (L/s) Head (m)
head (TDH) for this water pumping system.
8.3 Using the data from Problem 8.2 assume the following addi- 32 59.44
tional data: efficiency of pump = 80% efficiency of motor = 85%. 63 54.86
Determine (a) the motor horsepower in hp and kWh/day and (b) the 95 45.72
daily power cost if the unit power cost is $0.1 per kWh. 127 33.53
158 15.24
8.4 Determine the total power input if the electrical input to a
Shut-off head = 200 ft (60.96 m)
motor-pump system is 220 V and 36 amp.
8.5 Determine the wire-to-water efficiency (%) if Q =
What will be the discharge of the two pumps when operating
510 gpm (32 L∕s), TDH = 53.65 ft (16.35 m), V = 220 V, and
in parallel?
A = 36 amp.
8.10 The water supply system shown in Fig. 8.14 is designed to
8.6 Changing the speed of a centrifugal pump will change its
serve city ABCDEFA. Water is treated and collected in a water tank
operating characteristics, including the water flow. Determine the
in the treatment plant (WTP). A pump delivers the water through the
new flow rate or capacity, Q ,if main PR1 to an elevated reservoir (water level 2,700 ft, or 822.96 m)
2
Q = rated water flow = 620 gpm = 39 L∕s at the top of a hill. Pumping is done at a constant rate and only for
1
N = pump speed now = 1,320 rpm a period of 16 h per day from 4 a.m. to 8 p.m. Water flows from
2
N = rated pump speed = 1,650 rpm the elevated reservoir into the distribution main network ABCDEF
1
8.7 Changing the speed of a centrifugal pump will change its through a 24 in. main R A.
2
operating characteristics, including the head. Determine the new
head, if B C
H = rated head = 120 ft = 36.58 m
1
N = pump speed now = 1,320 rpm R 1 Elevated R 2 A D
2
N = rated pump speed = 1,650 rpm WTP P reservoir
1
8.8 Changing the speed of a centrifugal pump will change its
Pipe d (in.) L (ft) d (mm) L (m) F E
operating characteristics, including the power requirement. Deter-
mine the new power requirement, if PR 1 24 35,000 600 10,668
P = rated head = 16 hp = 12 kW R 2 A 24 15,000 600 4,572
1
14
350
8,000
2,438
AB
N = pump speed now = 1,320 rpm BC 12 10,000 300 3,048
2
N = rated pump speed = 1,650 rpm CD 8 5,000 200 1,524
1
8.9 A water pumping station is designed to raise water from a lake DE 8 5,000 200 1,524
EF 12 10,000 300 3,048
at an elevation of 50 ft (15.24 m) to a reservoir located at an elevation
AF 14 8,000 350 2,438
of 140 ft (42.67 m). Water is pumped through a 16 in. (40.64 cm)
cast-iron pipe that develops a head loss of 19 ft (5.79 m) of water Figure 8.14 Water system for Problem 8.10.