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276 Chapter 8 Pumping, Storage, and Dual Water Systems

5.0 5.0
Storage 0.86 MG Draft or
Storage
Supply and demand, MG 3.0 Draft or demand Supply and demand, MG 2.0 Uniform inflow or
4.0
4.0
0.42 MG
demand
3.0
Uniform inflow
or supply
for 24 h
2.0
supply for 12 h
1.0
Storage
0.40 MG 1.0 Storage 0.84 MG
0 0
12 4 8 12 4 8 12 12 4 8 12 4 8 12
Mid- A.M. A.M. Noon P.M. P.M. Mid- Mid- A.M. A.M. Noon P.M. P.M. Mid-
night night night night
(a) (b)
Figure 8.6 Determination of Equalizing, or Operating, Storage by Mass Diagram See Example 8.5.
Uniform inflow, or supply, (a) extending over 24 h; (b) confined to 12 h. Total storage 0.40
0.42 0.82 MG; (b) total storage 0.84 0.86 1.70 MG
Conversion factor: 1 MG 1 million gallons 3.785 ML 3,785 m 3

Achieving a steady supply at the rate of maximum daily use will ordinarily require an
equalizing storage between 15% and 20% of the day’s consumption. Limitation of supply
to 12 h may raise the operating storage to an amount between 30% and 50% of the day’s
consumption.

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 the 12 h from 6 a.m. to 6 p.m.

Table 8.2 Observed Drafts for Example 8.5
(a) Time 4 A.M. 8 A.M. noon 4 P.M. 8 P.M. midnight
(b) Draft, MG 0.484 0.874 1.216 1.102 0.818 0.506

(c) Cumulative draft, MG 0.484 1.358 2.574 3.676 4.494 5.000
Conversion factors: 1 MG 1 million gallons 3.785 ML 3785 m 3

Solution 1 (U.S. Customary System):
(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 ex-
ceeds 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