Page 287 - Water Engineering Hydraulics, Distribution and Treatment

265
Problems/Questions
% of daily demand
Period
transmission line between the reservoir and the pumping station,
and 10,000 ft (3,048 m) of 10 in. (250 mm) pressure transmission
Midnight to 3 a.m.
5
line between the pumping station and the water storage tank. The
7
3 a.m. to 6 a.m.
water at the water storage tank site is treated by UV for disinfection
13
6 a.m. to 9 a.m.
and then discharged to the water distribution system.
9 a.m. to noon
20
Noon to 3 p.m.
20
3 p.m. to 6 p.m.
15
10
Storage
6 p.m. to 9 p.m.
Top water surface
10
9 p.m. to midnight
200 ft
The average daily water consumption is 106 gpcd (400 Lpcd)
Water supply
Pumping station
and the maximum daily consumption is 125% of the daily average. elevation of 200 ft (60.96 m) at point C, 500 ft (152.4 m) of suction
100 ft
3
Determine the storage volume (in m ) necessary to balance supply 80 ft 10 in.
and demand in each of the following cases: 36 in.
(a) If water is supplied into the reservoir at a constant rate over the A B C
24-h period.
(b) If water is supplied into the reservoir at a constant rate, but only Figure 8.16 Water supply system for Problem 8.22. Conversion
for a period of 12 h from 6 a.m. to 6 p.m. factors: 1 in. = 25.4 mm; 1 ft = 0.3048 m.
8.20 A residential complex consists of 60 duplex villas (two
housing units in each). Each housing unit can accommodate a max-
A proposal has been made to replace the current pumps with
imum of 10 persons. The expected maximum daily water consump-
two centrifugal pumps, construct 10,000 ft (3,048 m) of 16 in.
tion is 106 gal/capita (400 L/capita). Water is supplied from the
(400 mm) transmission main to parallel the current 10 in. (250 mm)
municipal network at a uniform rate throughout the day and is col-
water main and to provide additional water storage at an elevation
lected in a ground storage reservoir. Water is then pumped to an
of 200 ft (60.96 m) at the current water storage site. The following
elevated reservoir at a constant rate but only for a period of 9 h from
conditions are assumed: (a) C factor for all pipes = 120; (b) fire
6 a.m. to 3 p.m. Water then flows from the elevated reservoir to the
flow requirements in the city = 2,000 gpm (126.2 L/s) for 10 h;
various villas by gravity to satisfy the following daily demands:
(c) average daily demand = 3MGD = 131 L/s; (d) maximum daily
demand = 5MGD = 219 L/s; (e) peak hourly demand = 10 MGD =
Period % of daily demand 438 L/s; (f) pumping station head losses = 500 ft of 16 in. pipe (or
152.4 m of 400 mm pipe) equivalent; and (g) the field-measured
Midnight to 3 a.m. 5 total dynamic head delivered by the pump from the raw water supply
3 a.m. to 6 a.m. 5 reservoir to the pump station, then to the water storage tank = 150 ft
6 a.m. to 9 a.m. 25 (45.72 m).
9 a.m. to noon 15
(a) What should the pump rating (gpm or L/min) be for each of the
Noon to 3 p.m. 15
two new pumps?
3 p.m. to 6 p.m. 10
(b) What should the brake horsepower (BHP or BMP) be for pump
6 p.m. to 9 p.m. 15
selection, and what should the motor horsepower (MHP or
9 p.m. to midnight 10
MMP) be for motor selection?
(c) Will the two selected pumps be connected in parallel or in
In order to balance supply and demand, determine series?
(a) Required volume of the ground reservoir. 8.23 Using the same given data in Problem 8.22, explain why the
(b) Required volume of the elevated reservoir. pipe line is designed based on the maximum daily water demand,
and determine the total storage capacity of a new water storage tank.
8.21 A small water system in a rural area has a population of 800,
and its daily per capita usage is estimated to be 100 gpcd (379 Lpcd). 8.24 Design a concrete raw water intake structure with two
The required fire flow determined by the village engineer is 500 gpm equal-size square gates under the following assumed engineering
(32 L/s) for a duration of 2 h. There is no particular hazard to conditions:
the water supply works. Determine the required water storage to (a) Elevation of the intake structure top (maximum) = 285.43 ft
be provided for the village using the Recommended Standards for (msl = mean sea level).
Water Works, 2007 Edition. Visit the website or contact the local (b) Elevation of maximum reservoir water level = 269.03 ft (msl).
health department for the latest edition of these Ten-States Standards
(c) Elevation of normal reservoir water level = 265.75 ft (msl).
for water works.
(d) Elevation of minimum reservoir water level = 262.47 ft (msl).
8.22 A city is planning to improve its water supply system
(Fig. 8.16). At present the city has a surface water supply reservoir (e) Elevation of the intake structure bottom (planned) = 196.85 ft
at an elevation of 100 ft (30.48 m) at point A, a pumping station at (msl).
an elevation of 80 ft (24.38 m) at point B, water storage tank at an (f) Design flow (maximum daily demand) = 10 MGD.