Page 36 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 1
Introduction to Water Systems
EXAMPLE 1.5 ESTIMATION OF THE SIZE OF A WATER CONDUIT
Estimate roughly the size of a supply conduit leading to an adequate distributing reservoir serving (i) relatively small residential
community of 10,000 people and (ii) a relatively large industrial community of 400,000 people.
The following are the design conditions specifically for a North American region:
a. The average daily water consumption for small communities with populations of 10,000 or less = 100 gpcd (378.5Lpcd).
b. Average daily water consumption for communities with populations of greater than 10,000 = 150 gpcd (567.8 Lpcd).
c. Maximum daily water consumption is about 50% greater than average daily water consumption.
d. The design water velocity in the circular conduit when flowing full = 4ft∕s = 1.22 m∕s.
Solution 1 (US Customary System):
1. Average daily water consumption at (a) 100 gpcd and (b) 150 gpcd for the 10,000 people community and the 400,000 people
community, respectively:
(i) 10,000 × 100∕1,000,000 = 1.0MGD.
(ii) 400,000 × 150∕1,000,000 = 60 MGD.
2. Maximum daily water consumption is 50% greater than the average:
3
(i) 1.0 × 1.5 = 1.5MGD = 1.5 × 1,000,000∕(7.5 × 24 × 60 × 60) = 2.32 ft ∕s.
3
(ii) 60 × 1.5 = 90 MGD = 90 × 1,000,000∕(7.5 × 24 × 60 × 60) = 139 ft ∕s.
3. Diameter of circular conduit flowing at 4 ft/s:
2
2
(i) Cross-sectional area A = Q∕v = 2.32∕4 = D ∕4 = 0.785 D .
Diameter D = 0.833 ft = 10 in. for the small 10,000 people community.
2
2
(ii) Cross-sectional area A = Q∕v = 139∕4 = D ∕4 = 0.785 D .
Diameter D = 6.667 ft = 80 in. for the large 400,000 people community.
Solution 2 (SI System):
1. Average daily water consumption = 378.5 Lpcd for the 10,000 people community and average daily water consumption
= 567.8 Lpcd for the 400,000 people community.
(i) 10,000 × 378.5∕1,000,000 = 3.785 MLD.
(ii) 400,000 × 567.8∕1,000,000 = 227.1MLD.
2. Maximum daily water consumption is 50% greater than the average:
3 3 3
(i) (3.785 MLD) × 1.5 = 5.6775 MLD = 5677.5m ∕d = (5677.5m )∕(1,440 × 60) s = 0.066 m ∕s = 66 L∕s.
3 3 3
(ii) (227.1MLD) × 1.5 = 340.65 MLD = 340,650 m ∕d = (340,650 m )∕(1,440 × 60)s = 3.94 m ∕s.
3. Diameter of circular conduit flowing at 1.22 m/s:
3 2 2
(i) Cross-sectional area A = Q∕v = (0.066 m ∕s)∕(1.22 m∕s) = 0.054 m = 0.785 D .
Diameter D = 0.26 m = 260 mm for the small 10,000 people community.
3 2 2
(ii) Cross-sectional area A = Q∕v = (3.94 m ∕s)∕(1.22 m∕s) = 3.23 m = 0.785 D .
Diameter D = 2.03 m = 2,030 mm for the large 400,000 people community.
1.9.1 High and Low Services 1.9.2 Fire Supplies
Sections of the community too high to be supplied directly Before the days of high-capacity, high-pressure, motorized
from the principal, or low-service, works are generally incor- fire engines, conflagrations in the congested central, or high-
porated into separate distribution systems with indepen- value, district of some large cities were fought through inde-
dent piping and service storage. The resulting high ser- pendent high-pressure systems of pipes and hydrants. Tak-
vices are normally fed by pumps that take water from the ing water from the public supply and boosting its pressure
main supply and boost its pressure as required. Areas vary- by pumps in power stations whenever an alarm was rung
ing widely in elevation may be formed into intermediate in, these systems performed well. For extreme emergencies,
districts or zones. Gated connections between the differ- rigorously protected connections usually led to independent
ent systems are opened by hand during emergencies or go sources of water: rivers, lakes, or tidal estuaries. Large indus-
into operation automatically by means of pressure-regulating trial establishments, with heavy investments in plant, equip-
valves. ment, raw materials, and finished products, concentrated in