National
Drinking Water Clearinghouse
West Virginia University
P.O. Box 6064
Morgantown, WV
26506-6064
Tech
Brief
Reservoirs, Towers, and Tanks
Drinking Water Storage Facilities
by
Vipin Bhardwaj
Technical Assistance Specialist
Summary
After water leaves the treatment plant but before it reaches the customer, it
must be adequately and safely stored. This Tech Brief explores the various aspects
of water storage. A water distribution system transports water from the treatment
facility to the user. The distribution system should supply water, without impairing
its quality, in adequate quantities and at sufficient pressures to meet system
requirements.
The facilities that make up the distribution system include finished water storage;
pumping, transmission and distribution piping supply mains; and valves.
Storage facilities—such as reservoirs, towers, and tanks—provide
storage for treated water before it is distributed. The water distribution system
should have storage so that it is capable for basic domestic purposes, commercial
and industrial uses, and to accommodate the flows necessary for emergencies
such as fire fighting.
Storage Reservoir Functions
Service reservoirs provide the following functions:
• provide a reserve of treated water that will minimize interruptions of
supply due to failures of mains, pumps, or other plant equipment;
• help maintain uniform pressure;
• provide a reserve of water for fire fighting and other emergencies;
• act as a relief valve on a system of mains supplied by pumping;
• permit a reduction in the size of distribution mains below that which
would be required in the absence of a reservoir; and
• allow pumping at the average rather than peak flow rate.
Classification of Storage Requirements
Storage volume requirements are classified by function: operating, equalizing,
fire and/or emergency, and dead-storage volumes. Engineers must consider these
individual volume components in combination to determine the total volume of
storage capacity that is required for any system. The total storage required
is typically the sum of all these functions.
The
Illustration to the left: Figure 1 - Storage Volume Classification
Rather than requiring both fire and emergency storage, some local fire and state
agencies allow systems to use the larger of either fire or emergency storage
volumes. These local agency requirements need to be determined on a case-by-case
basis before designing a storage reservoir. Figure 1 illustrates the
different components of the total storage volume.
Operating Storage—This is the difference in volume between the “pump
on” and “pump off” levels when the tank is normally being used
and the sources of supply pumps to the storage tank are off.
Equalizing Storage—This storage component is used when the source
pump capacity is less than the peak system demands. The storage is needed so
that water production facilities can operate at a relatively constant rate.
Daily peak rates determine the volume, compared to the average daily demand
and source capacity. |
Fire Storage—The volume of water stored within the water system
for fighting fires is known as “fire storage.” The storage volume
required varies with the size of city and with the size, type, and classification
of construction within the service area. Storage volume requirements for fire
fighting should be determined based upon state and local fire districts and
municipalities. Fire authorities often refer to the latest edition of the Guide
for Determination of Required Fire Flow, published by the Insurance Services
Office to determine local fire flow requirements.
The typical minimum municipal fire flow requirement is 500 to 1,000 gallons
per minute (gpm) for two hours for single family residential areas. Commercial
and industrial areas fire flows can be as high as 8,000 gpm or more for many
hours. Typical fire storage requirements are shown in Table 1. Engineers
should base the actual capacity needs upon local fire flow requirements.
The Type of Development | Storage Volume, gallons | Storage Volume, ML
Low-density residential, 2 hr at 500 gpm | 60,000 | 0.23
Built-up residential, 2 hr at 1,000 gpm | 120,000 | 0.45
Light commercial, 4 hr at 2,000 gpm | 480,000 | 1.8
Commercial, 4 hr at 4,000 gpm | 960,000 | 3.6
Emergency Storage—This storage is used to provide water to the
system during other unusual or emergency conditions. Emergency storage volume
depends upon the likelihood of supply interruption and the time required to
make repairs or arrange for an alternative water supply.
Dead Storage—Storage in tanks or reservoirs that cannot be drawn
out or used beneficially because of piping elevations or low pressures is
known as dead storage. Dead storage is typically most significant in tall
standpipe-type tanks where water in the bottom of the tank cannot be used
because of low system pressure.
Storage Volume Dedicated to Contact Time—Finished water stored
in clearwells at water treatment plants is sometimes used to meet the disinfection
contact time. In these cases, the amount of fixed volume used to meet the
disinfection requirement should also be considered in the total volume and
operational limits of the reservoir.
Daily Storage Volume Use Varies
Water use is greater during daylight hours—typically peaking in
the mid-morning and early-evening hours. Stored water is withdrawn during
these peak demand hours of the day and is replenished during minimum-demand
times in the late-night and early-morning hours.
The
Illustration to the right: Figure
2 - Hourly Water Use Variation
Figure 2 illustrates the hourly variation in daily water use (diurnal
variation) that might occur in a typical residential community on the day
of maximum water use for the year. The shape of the diurnal curve of water
demand will vary significantly between different cities because of differences
in climates and local economies. Local design data should be obtained for
each water system to determine storage needs. However, with an adequate source
of supply capacity, equalizing storage of approximately 22 percent of the
maximum daily demand is typical for small residential areas.
Storage Tank Shape and Volume
Water towers can be made of concrete or steel and can take various forms.
The most suitable form for concrete towers is a cylinder with a curved shaped
bottom or with a flat bottom. Steel tanks may have a spherical or dome shaped
bottom. The shape chosen is usually a compromise between function, construction
and maintenance costs, and aesthetics.
The lowest water level in the tank is determined according to the pressure
requirements in the pipeline. The pressure in the pipelines may vary depending
on the type of community and pressure needs of different areas in a city.
Typically, minimum acceptable water system pressures are 35 to 40 pounds per
square inch (psi) and maximum pressures are 100 to 120 psi.
To keep pumping costs low, water depth in the tank is generally kept small.
Due to structural considerations, the depth is kept equal to the diameter.
The Illustration below: Figure
3 - Service Reservoir
Locating
Storage Reservoirs
A service reservoir stores the water and supplies it at the
required pressure to the farthest point in the area. In view of the cost of
pipelines and uniform pressure distribution, the reservoir should be located
near the center of the service area. In flat areas, it is relatively easy to
build the water tower at the center. In hilly areas, however, it may be more
advantageous to select the highest point for the construction of an elevated
tank, which may lie at one end of the area instead of the center.
Apart from the center, the tank or tower can be situated between the area and
the source of supply (pumping or gravity flow). When the service reservoir lies
between the area and the source, all the water must pass through the elevated
tank before flowing through the area. (See figure 3A.)
The pressure in the water supply system depends upon the water level in the
service reservoir. A water supply system needs to guarantee a minimum pressure
even at the most remote point in the area. Therefore, it is essential that the
hydraulic gradient line always be above the required pressure.
When water is supplied from an impounded high-level reservoir, the service
reservoir may function as a pressure-reducing device. (See figure 3B.)
This reduces the possibility of damage to the pipes due to high hydrostatic
pressure. When the area lies between the source and the service reservoir,
then most of the requirements are met by direct pumping and the excess water
flows to the service reservoir. (See figures 3C and 3D.) In this system
there may be larger fluctuations in the supply pressure.
Construction Materials
Most states permit steel and concrete construction materials. All piping, joints,
and fittings should conform to American Water Works Association (AWWA) specifications.
Welded steel water tanks should confirm to American National Standards Institute
(ANSI)/AWWA Standard D100. Factory-coated bolted steel tanks should confirm
to ANSI/AWWA D103. Wire and strand wound, circular, pre-stressed concrete tanks
should confirm to ANSI/AWWA Standard D110.
The storage tanks should be painted or have cathodic protection. The AWWA standards
for painting exclude the use of paints that might add toxic materials to the
stored water. The paint, both external and internal, should comply with the
standards prescribed by AWWA D101 and D102.
Other Considerations
All water reservoirs should be covered to protect the stored water against contamination.
Overflow pipes should be brought down near the ground surface and discharged
to minimize erosion. The storage structure should be designed so that there
is water circulates. There should be a convenient access to the interior for
cleaning, maintenance, and sampling. Rigid storage reservoirs should be vented.
References:
Pizzi, Nicholas G, 1995. Hoover’s Water Supply and Treatment, 12th
Edition. National Lime Association: Arlington, VA.
HDR Engineering, Inc. 2001. Handbook of Public Water Systems. 2nd Edition.
John Wiley and Sons, Inc.: New York, New York.
Hobbs, Aubrey Thomas (ed.) 1969. Manual of British Water Engineering Practice.
Vol 2. W. Heffer and Sons, Ltd: Cambridge, England.
Middle Brooks, E Joe, et al. 1977. Water Supply Engineering Design. Ann
Arbor Science Publishers: Ann Arbor, MI.
For more information, comments about this fact sheet, or to suggest
topics for future Tech Briefs, contact Bhardwaj at (800) 624-8301 or via e-mail
at vbhardw2@wvu.edu.
Vipin Bhardwaj, technical assistance specialist, received his bachelor’s
degree in chemical engineering and has recently finished his master’s
degree in agriculture at West Virginia University.