TYPES OF STORAGE
zones filling the tanks and being passed to higher
2-1. General
pressure zones through adjacent pump stations.
Required storage capacity at military installations
is met by use of elevated or ground storage. Ele-
2-3. Elevated Storage
vated storage, feeds the water distribution system
a. General. Elevated storage is provided within
by gravity flow. Storage which must be pumped
distribution system to supply peak demand rates
into the system is generally in ground storage
and equalize system pressures. In general, elevated
tanks. Clearwell storage, which is usually part of a
storage is more effective and economical than
water treatment plant, is not included in comput-
ground storage because of the reduced pumping re-
ing storage unless sufficient firm pumping capac-
quirements, and the storage can also serve as a
ity is provided to assure that the storage can be
source of emergency supply since system pressure
utilized under emergency conditions, and then
requirements can still be met temporarily when
only to the extent of storage in excess of the 24-
pumps are out of service.
hour requirements of the treatment plant. Clear-
well storage is used to supply peak water demand
b. Type. The most common types of elevated
rates in excess of the production rate, and to pro-
storage are elevated steel tanks, and standpipes.
vide a reservoir for plant use, filter backwash
An example of a conventional elevated steel tank
supply, and water supply to the system for short
is given in figure 21. In recent years, elevated
periods when plant production is stopped because
tanks supported by single pedestals, such as shown
of failure or replacement of some component or
in figure 22, have been constructed where esthet-
unit of treatment.
ic considerations are an important part of the
design process. (See American Water Works Asso-
2-2. Ground Storage
ciation D100, Standard for Welded Steel Tanks for
Water Storage (app A).)
a. General. Ground storage is usually located
--
c. Standpipe. A standpipe is a tall cylindrical
remote from the treatment plant but within the
tank normally constructed of steel or reinforced
distribution system. Ground storage is used to
concrete. Only the portion of the storage volume of
reduce treatment plant peak production rates and
a standpipe that meets the requirements of d
also as a source of supply for repumping to a
below is considered useful storage for pressure
higher pressure level. Such storage for repumping
equalization purposes. The lower portion of the
is common in distribution systems covering a large
storage acts to support the useful storage and to
area, because the outlying service areas are
provide a source of emergency water supply.
beyond the range of the primary pumping facili-
d. Elevated storage. Elevated storage tanks
ties.
should be located in the areas having the lowest
b. Type. Ground storage tanks or reservoirs,
system pressures during intervals of high water
below ground, partially below ground, or con-
use to be effective in maintaining adequate system
structed above ground level in the distribution
pressures and flows during periods of peak water
system, may be accompanied by pump stations if
demand. These are those of greatest water demand
not built at elevations providing the required
or those farthest from pump stations. Elevated
system pressure by gravity. However, if the ter-
tanks are generally located at some distance from
rain permits, this design location of ground tanks
the pump station(s) serving a distribution pressure
at elevation sufficient for gravity flow is preferred.
level, but not outside the boundaries of the service
Concrete reservoirs are generally built no deeper
area, unless the facility can be placed on a nearby
than 20-25 feet below ground surface. If rock is
hill. Additional considerations for siting of elevat-
present, it is usually economical to construct the
ed storage are conditions of terrain, suitability of
storage facility above the rock level. In a single
subsurface soil and/or rock for foundation pur-
pressure level systems, ground storage tanks
poses, and hazards to low-flying aircraft. Elevated
should be located in the areas having the lowest
tanks are built on the highest available ground, up
system pressures during periods of high water use.
2
to static pressures of 75 lb/in in the system, so as
In multiple pressure level systems, ground storage
to minimize the required construction cost and
tanks are usually located at the interface between
heights.
pressure zones with water from the lower pressure
2-1
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