(b) The
usefulness of elevated storage is
is hydraulically connected to the distribution
shown in figure 1-1. The system illustrated in
system, and the volume of water in the tank tends
figure 1-1 (A) (without elevated storage) requires
to maintain system pressures at a uniform level.
storage at the plant sufficient to provide for
When water use is high and pumping facilities
system demand rates in excess of the plant produc-
cannot maintain adequate pressures, water is dis-
tion rate, assuming the plant is operated at a uni-
charged from elevated tanks. Conversely, when
form rate. The pump station forces water into the
water use is low, the pumps, which operate within
service main, through which it is carried to three
a reasonably uniform head-capacity range, supply
load areas: A, B, and C. Since all loads on the
excess water to the system and the elevated stor-
system are met without the use of elevated stor-
age is refilled.
age, the pump station must be capable of supply-
e. Provision of emergency water supplies.
ing the peak rates of water use to Areas A, B, and
(1) Firefighting flows. This demand can occur
C, simultaneously, while maintaining the water
at any time, but may well coincide with other
pressure to Area C at a sufficient level. The mini-
large water demands on the system. Necessary
mum recommended pressure in the distribution
flows for firefighting purposes are as given in TM
system under peak nonemergency flow conditions
5-813-6/AFM 88-10, Vol. 6, and TM 5-813-7/AFM
2
is 40 lb/in . Figure l-l(B) assumes the construc-
88-10, Vol. 7. Storage and distribution facilities
tion of an elevated storage tank on the service
main between Areas B and C, with peak loads in
will include capacity for required firefighting flows
Area C and part of the peak load in Area B being
at adequate pressures at any point of the installa-
satisfied from this tank. The elevation of the tank
tion.
ensures adequate pressures within the system. The
(2) Other emergencies. Water storage must pro-
storage in the tank is replenished when water de-
vide an emergency supply of water in the event
mands are low and the pump station can fill the
the water treatment plant, distribution pumps, or
..
tank while still meeting all flow and pressure re-
a principal transmission main is out of service.
quirements in the system. The figure 1-1 (B) ar-
The amount of emergency storage required de-
rangement reduces required capacity of the distri-
pends on the reliability of the system and the
bution pumps.
extent of other safeguards incorporated into the
system, i.e., finished water interconnections with a
(c) Most elevated storage tanks "float" on
the distribution system. That is, the elevated tank
municipality (for either normal or emergency use).
1-2
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