Page 271 - Water Engineering Hydraulics, Distribution and Treatment

EXAMPLE 8.5 DETERMINATION OF EQUALIZING STORAGE
Determine the equalizing, or operating, storage for the drafts of water shown in Table 8.2 (a) when inflow is uniform during 24 h and
(b) when flow is confined to 12 h from 6 a.m. to 6 p.m.
Observed drafts for Example 8.5
Table 8.2
Midnight
4 a.m.
(a) Time
3
(b) Draft MG (m )
0.506 (1,915)
0.818 (3,096)
0.484 (1,832) 0.874 (3,308) 1.216 (4,603) 1.102 (4,171)
3
(c) Cumulative draft MG (m ) 0.484 (1,832) 1.358 (5,140) 2.574 (9.743) 3.676 (13,914) 4.494 (17,010) 5.000 (18,925)
Conversion factors: 1 MG = 1 million gallons = 3.785 ML = 3,785 m .
Solution 1 (US Customary System): 8 a.m. 3 Noon 4 p.m. 8 p.m. 8.3 Service Storage 249
a. For steady supply during 24 h, the draft plotted in Fig. 8.6a exceeds the demand by 0.40 MG by 6 a.m. If this excess is stored,
it is used up by 11 a.m. In the afternoon, the demand exceeds the supply by 0.42 MG by 6 p.m. and must be drawn from
storage that is replenished by midnight. Hence, the required storage is the sum of the morning excess and afternoon deficiency,
or 0.82 MG. This storage volume as a percentage of total draft equals (0.82 MG∕5MG) × 100 = 16.4%.
b. For steady supply during the 12-h period from 6 a.m. to 6 p.m., the draft plotted in Fig. 8.6b exceeds the supply by 0.84 MG
between midnight and 6 a.m. and must be drawn from storage. In the afternoon, the supply exceeds the demand by 0.86 MG by
6 p.m., but this excess is required to furnish water from storage between 6 p.m. and midnight. Therefore, total storage equals
0.84 + 0.86 = 1.70 MG,or(1.7MG∕5MG) × 100 = 34% of the day’s consumption.
Solution 2 (SI System):
3
a. For steady supply during 24 h, the draft plotted in Fig. 8.6a exceeds the demand by 0.40 MG (1,514 m ) by 6 a.m. If
3
this excess is stored, it is used up by 11 a.m. In the afternoon, the demand exceeds the supply by 0.42 MG (1,590 m )
by 6 p.m. and must be drawn from storage that is replenished by midnight. Hence, the required storage is the sum of the
3
morning excess and afternoon deficiency, or 0.82 MG (3,104 m ). This storage volume as a percentage of total draft equals
3
3
3
[(3,104 m )∕(5 × 3,785 m )] × 100 = 16.4%,where 5 × 3,785 m = 5MG.
b. For steady supply during the 12-h period from 6 a.m. to 6 p.m., the draft plotted in Fig. 8.6b exceeds the supply by 0.84 MG
3
(3,180 m ) between midnight and 6 a.m. and must be drawn from storage. In the afternoon, the supply exceeds the demand
3
by 0.86 MG (3,255 m ) by 6 p.m., but this excess is required to furnish water from storage between 6 p.m. and midnight.
3
Therefore, total storage equals 3,180 + 3,255 = 6,435 m , or [(6,435)∕(5 × 3,785)] × 100 = 34% of the day’s consumption.
8.3.2 Fire Reserve emergency reserve is sometimes made no more than 25% of
the total storage capacity, that is, the reservoir is assumed
Based on IFC recommendations on observed durations of
to be drawn down by one-fourth its average depth. If supply
serious conflagrations, it is recommended that distributing
lines or equipment are expected to be out of operation for
reservoirs be made large enough to supply water for fighting
longer times, higher allowances must be made.
a serious conflagration for (a) 4 h for fire flows of more than
4,000 gpm (252 L/s), (b) 3 h for fire flows of 3,000–3,750
gpm (189–237 L/s), and (c) 2 h for fire flows of 2,750 gpm
8.3.4 Total Storage
(174 L/s) and less (see Table 4.13). The resulting fire reserve
may not always be economically attainable, and design val- The desirable total amount of storage is equal to the sum
ues may have to be adjusted downward to meet local finan- of the component requirements. In each instance, economic
cial abilities. Changing community patterns, moreover, may considerations dictate the final choice. In pumped supplies,
make for changing requirements in the future. cost of storage must be balanced against cost of pumping,
and attention must be paid to economies affected by operating
pumps more uniformly and restricting pumping to a portion
8.3.3 Emergency Reserve
of the day only. In all supplies, cost of storage must be bal-
The magnitude of this storage component depends on (a) anced against cost of supply lines, increased fire protection,
the danger of interruption of reservoir inflow by failure of and more uniform pressures in the distribution system.
supply works and (b) the time needed to make repairs. If Storage facilities should have sufficient capacity, as
shutdown of the supply is confined to the time necessary for determined from engineering studies, to meet domestic
routine inspections during the hours of minimum draft, the demands and, where fire protection is provided, fire flow