A small storage tank (standpipe) in Blacksburg, Virginia. Photo by Francis Martin.
Importance of Storage Facilities
Reservoirs, Standpipes and Elevated Tanks
Importance of Storage Facilities
One of the most integral parts of the distribution system is the storage facility. Storage tanks are used as a means of providing head for a water distribution system. During periods of low consumption, water is pumped to elevated tanks. During periods of high consumption, the stored water is drawn upon to augment pumped water, allowing fairly uniform rates of pumping and mitigating diurnal variations. Storage tanks also provide excess capacity for fire protection. The National Board of Fire Underwriters grades cities upon their fire defense abilities. Quantity of storage is one of the major factors in providing adequate fire protection.
Reservoirs, Standpipes and Elevated Tanks
Reservoirs, standpipes and elevated tanks are the three basic types of storage tanks. A reservoir is a flat-bottomed cylindrical tank with a shell height equal to or smaller than its diameter. A standpipe is a flat bottomed cylindrical tank with a shell height greater than its diameter. An elevated tank has a supporting structure which elevates the lower operating level to provide additional head.
Elevated storage tanks are usually made of steel. Within the last 100 years, over 100,000 steel storage tanks have been constructed, by far exceeding storage vessels of any other type.
The capacity of the elevated tank depends upon the load characteristics of the system, which are carefully studied before a decision is made. Tanks can be cylindrical, either on the ground (as seen above) or elevated, or spherical, or a host of other shapes. Although storage tanks are often considered unsightly to many nearby residential dwellers, storage tanks can be an artform. Some of the world's most interesting storage tanks can be seen in Bernd and Hilla Becher's Water Towers (MIT Press, 1989).
Design Considerations
Water systems have been developed based on safety, hydraulic principles and technology. Water systems must be reliable in maintaining water pressure, equalization of demand on supply sources. The range of head means the vertical distance from the lower capacity to the overflow, between which levels the required capacity is provided. The capacity of these water tanks are used to regulate the size of existing mains, and makes provisions for normal demand. The water supplied by these tanks must never be interrupted by power failure. It should be design to provide uninterrupted water service during power outages at all times.
The AWWA Standard D100 guides the construction of the majority of steel storage tanks. Extra tank height can be specified to provide additional earthquake resistance. Tension members are also added to resist wind load and earthquake load. The two basic type of roofs are the supported and self-supported. The choice of roof for a reservoir or standpipe influences the cost. The cone roof is generally the least expensive and the ellipsoidal is the most costly.
A soils investigation should precede selection of the tank design. Borings investigation takes place based on standard minimum guidelines. Unusual site conditions may dictate the need for additional borings. Figure 1 shows the minimum soil investigation guidelines for elevated storage tanks (from AWWA D100).
The allowable soil bearing pressure specified in the soils investigation determines the maximum height of the reservoir. In general, reservoirs whose shell height ranges from 40-56 ft are the most economical.
Five types of foundations are used for flat bottom tanks. The most common is the granular berm. This foundation consists of coarse stone or gravel which extends 3 ft. beyond the tank shell. The foundation must be protected from weathering and runoff from the tank.
Addtional information on Storage Tank design is provided in Welded Steel Water Storage Tank Manual and Steel Tanks for Liquid Storage from the Steel Plate Fabricators Association.
Elevated tanks are distinguished by the number of support members and the geometric form of the tank container. The shape of the container is often a function of the storage capacity and forms the basis for industry nomenclature. Smaller tanks are often double ellipsoidal, with an ellipsoidal top and bottom and cylindrical sides. The medium capacity tanks have an ellipsoidal roof, cylindrical shell and cone-supported torus bottom. Large capacity tanks utilize a cone of fluted riser-supported torus bottom whose continuously curved surface extends and blends into the ellipsoidal roof to form a geometrically pleasing shape. When aesthetics are a concern, single pedestal tanks can be specified. These are most commonly spherical in shape.
See the Various Types of Tanks!
One of the considerations for the construction of a water tank is the location. To maintain adequate pressure, an elevated storage tank provides this need in an area of flat terrain. If, however, the terrain is such that it can be placed on top of a hill of sufficient height and convenience, a ground storage tank can be considered. Reservoirs and standpipes placed on strategic hills often save money. According to AWWA D100, as the availability of land decreases, the cost generally increases. This requires the purchaser of the tank to provide a site which has sufficient room to build the structure by customary methods. The tank should be located away from building, heavy traveled public areas, parking lots, and livestock to limit liability from damages or injustices to a third party resulting from construction activities. The tank site must be free from all overhead obstructions and power lines. Power lines located near the tank must be a minimum of feet (40) feet from the tank. The final property boundary of the tank should be located at a sufficient distance from the foundation to facilitate maintenance of the tank and site. If a permanent site of adequate size is not available, then the purchaser should provide a temporary construction easement. The tank site should also have very good drainage which initially reduces construction cost and in the long term prevents possible foundation failure due to the intrusion of surface water. Other important factors for the location of a tank are: the accessibility of the site - there must be an adequate amount of space to allow for the stockpiling of material during construction and future growth - rapidly growing residential areas require a great domestic allowance as well as fire flow protection.
In general, the sizing of tanks is done based on economy, while the selection is done based on industrial standards. For example, elevated tanks are normally sized in increments of 50,000 gallons through a 300,000 gallon capacity. Between 300,000 and 500,000, 100,000 gallon increments are used, while above 500,000 gallons 250,000 gallon increments. Ground storage tanks are normally sized so that the shell height (Figure 2) is a multiple of 8ft and the diameter a multiple of 10 ft. so that the fabricator can utilize plates of a standard dimension (8 ft.x 10ft.).
Figure 2. Shell Height
The capacity of a storage tank must always be sized to meet the daily consumption of the area that is being served. To do that, the system must be perfectly balanced, allowing constant pumping during the day and at the same time provide a constant pressure head. The typical demand curve for a municipal system, with diurnal variations, influences the filling and draining of the tank. Figure 3 illustrates typical tank use. The lowest demand is in the early hours of the morning and the greatest demand is late afternoon and early evening.
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