Steel Water Storage Tanks: A Selection Guide

This data has been prepared to assist the buyer and designer in the selection of a steel water storage tank appropriate to local needs.

The sizes and design data are examples of typical vessels.

Why Buy a Welded Steel Water Storage Tank?

Welded steel water storage tanks satisfy the four basic criteria of low cost, reliability, versatility and availability. Steel structures can be constructed in a variety of shapes, limited only by the imagination of the designer. The three primary categories of storage tank design are reservoirs, standpipes or elevated tanks. In addition, special tank designs are increasingly requested by end users.

When selecting a water storage tank to fill the needs of a city or industry, a comprehensive evaluation should be made of all the alternate designs available. Members of the Steel Tank Institute/Steel Plate Fabricators Association (STI/SPFA) give important factors, such as potential service and economic considerations, the careful attention they deserve.

It is the purchaser’s responsibility to select the size and type of tank to best meet a set of operating conditions. The rules of the American Water Works Association (AWWA) are then applied to the task of designing the appropriate tank. The purchaser will want to consider the following factors in detail.

Cost

What is the total cost of ownership over the tanks expected service life? A value engineering analysis shows that welded steel water storage tanks are the least costly alternative, even if you liberally assume that concrete tanks incur no maintenance expenses.

Reliability

How reliable is the tank over the period of time it is expected to remain in service? A welded steel water storage tank, when constructed and tested according to specifications and properly maintained, is the most reliable type of tank. The steel need only be maintained on its surface. There will be no internal deterioration or progressive degradation of the tank material. Further, the ductility of steel provides a significant measure of safety against unexpected forces resulting from earthquakes, tornadoes and subsidence.

Versatility

How can a tank be fabricated to create a unique visual impact that furthers the image of a community or corporation? How can a tank be built to address unusual needs for storage operation? Steel offers unmatched versatility in both form and function.

Availablility

How available are welded steel water storage tanks? Because of the relatively large number of qualified vendors, welded steel water storage tanks can be constructed to meet your schedule requirements at competitive prices. Find a member of the STI/SPFA.

Design and Construction

Virtually all water storage vessels in the United States are designed and constructed in accordance with Standard D100 of the American Water Works Association. This is a public standard to be used, without reservation, by anyone involved in water storage.

A water storage tank usually is located in the midst of a heavily populated area, often on the highest elevation available. Public safety is the underlying concern of AWWA Standard D100, a safeguard that has existed since 1935. Anyone dealing regularly wth water storage tanks should obtain a copy of the complete standard.

AWWA D100 offers two different design bases, the standard (or basic) design and an alternate design as outlined in Section 14. The alternate basis permits higher design stresses in return for a more refined engineering design, more rigorous inspection, and the use of shell plate steels with improved toughness. Section 14 is applicable only for reservoirs and standpipes.

Engineering and Materials

How Steel is Supplied

The production of steel plate is a closely controlled procedure, from furnishing the raw material to shipment of the completed product. Steel is inspected at every point of manufacturing to assure uniformity of strength and thickness. This uniformity is very important to the design and engineering of a steel water storage tank. There is no guesswork as to tensile strength, ductility or flexiblility.

Ductility also gives steel the ability to withstand abnormal shocks such as earthquakes, tornadoes, and extreme temperature changes. In this respect, steel surpasses all other materials because it has maximum strength with maximum ductility. The minimum tensile strength of steel normally used for water storage vessels is 55,000 pounds per square inch.

Reliability is a necessity for any water storage vessel. Engineers can depend on steel to do the job for which it was designed, and its reliability includes the ability to withstand many unexpected or emergency conditions that would spell disaster for concrete tanks.

The provision for bottle-tight joints is another area in which steel excels. Joints in steel shell plates are completely watertight and will stay that way for the life of the vessel.

Building a steel water storage tank is a controlled process. All materials used in its construction are subjected to rigid examination. Its erection is carried out to an established standard by a proven technique. Its performance characteristics will be predictableon the day it is completed and after 10 years of service. This is the owner’s assurance that a steel tank will carry the workload as soon as the last plate is welded. And it will continue to do so, even under weathering and conditions of service and stress that would be cause for grave concern if the tank were constructed of any other material.

All steel plate is manufactured to American Society for Testing and Materials (ASTM) Standards, a national set of specifications. These standards are revised periodically to report new specifications and changes to existing ones. This ensures uniformity of product purchased anywhere in the country. Mill test reports are available for every piece of steel that is manufactured in the United States.

Loads to Be Considered

Other than the loading from the tank full of water, there are three loads that must be considered in the design of a tank-wind, snow and earthquake. Wind load generally is based on an assumed wind velocity of 100 MPH, although a higher velocity is used in some instances. Snow load is assumed to be 25 PSF of horizontal projected area. This loading generally is used even in areas where snow does not occur.

Because of their flexibility, steel tanks have an excellent safety record in earthquakes. Steel has the ability to absorb large amounts of energy without fracture. The purchaser should specify if provisions for earthquake resistance should be included in the design. The design percentage used, unless otherwise specified by a local code stipulated by the purchaser, shall be determined by the location of the tank with respect to the zones by the seismic risk map published in AWWA Standard D100.

Control of Fabrication and Welding

Just as the production of steel plate is under strict ASTM controls, shop fabrication is performed in accordance with the latest AWWA requirements. Likewise, field erection of a steel water storage tank is performed according to strict specifications regarding assembly, welding and inspection.

The erection contractor must have qualified welding procedures, and all welders and welder operators must pass qualification tests in accordance with Section IX of the ASME Code. Welded joints generally are inspected by the radiographic method, although the sectioning method also is used. These inspection procedures are detailed in Section II of AWWA Standard D100.

Certification of Protective Coatings

Coating systems applied to internal surfaces in steel water tanks have been tested and certified in accordance with National Sanitation Foundation Standard No. 61. This certification provides assurance that the coatings pose no potentially adverse health effects to drinking water. Similar testing and ce11ification for concrete tanks is not available.

Guarantee

The terms and conditions of each manufacturer’s guarantees will vary. Most manufacturers repair, replace or correct any defective material or workmanship for varying periods of time. Many steel tanks built over 60 years ago are still in use today. TI1is testifies to the effectiveness, reliability and economy of steel structures. Proper maintenance, improved paints, and cathodic protection systems have greatly increased the life expectancy of steel tanks.

Types of Vessels

When additional storage capacity is needed in the water system, engineers must select the storage structure that best suits the circumstances. The quantity of storage required, the local topography, and system pressure must all be analyzed in making this selection.

New design and engineering concepts and materials make possible water storage facilities that combine utility with aesthetic considerations. Often they pennit a dual use of community land-for recreational and other purposes, as well as for waler storage.

The three basic types of steel water storage structures; reservoirs. standpipes and elevated tanks, are available in a variety of eye-appealing designs that can blend into your community’s landscape.

Reservoirs

A reservoir, a tank with a diameter greater than its height, is a common type of water storage structure. It usually is built on high ground so the water will flow by gravity. Of the three types of steel water tanks, a reservoir generally is the most economical to fabricate and erect because of its low height and relative simplicity.

The roof style can be conical, toriconical, dome or ellipsoidal. Standard accessories may include shell and roof manholes, screened roof vent, inside and/or outside ladders, painter’s trolley rail, and connections for pipes as required.

Steel reservoirs weigh only about 1/8th as much as concrete ones of the same capacity. The lower weight of steel structures is especially advantageous when the construction site is difficult to reach, as the cost of moving materials to the site generally is proportional to the weight.

Steel Water Storage Reservoir

Typical Steel Water Storage Reservoirs

Capacity Range of Sizes Available Range of Sizes Available
U.S. Gallons Cubic Meters Feet - Inches Meters
Diameter Height to HWL Diameter Height to HWL
50,000 189 19-3 24-0 5.9 7.3
60,000 227 21-0 24-0 6.4 7.3
75,000 284 23-6 24-0 7.2 7.3
100,000 379 23-6 32-0 7.2 9.8
27-0 24-0 8.2 7.3
125,000 473 26-0 32-0 7.9 9.8
30-3 24-0 9.2 7.3
150,000 568 28-6 32·0 8.7 9.8
33-0 24-0 10.0 7.3
200,000 757 33-0 32-0 10.0 9.8
38-3 24-0 11.7 7.3
250,000 946 37-0 32-0 11.3 9.8
42-9 24-0 13.0 7.3
300,000 1,135 40-6 32-0 12.3 9.8
46-9 24-0 14.3 7.3
400,000 1,515 46-6 32-0 14.2 9.8
54-0 24-0 16.5 7.3
500,000 1,890 46-6 40-0 14.2 12.2
52-0 32-0 15.9 9.8
60-6 24-0 18.4 7.3
600,000 2,270 51-0 40-0 15.6 12.2
57-0 32-0 17.4 9.8
750,000 2,840 57-0 40-0 17.4 12.2
64-0 32-0 19.5 9.8
1,000,000 3,785 66-0 40-0 20.1 12.2
74-0 32-0 22.6 9.8
1,500,000 5,680 80-6 40-0 24.5 12.2
90-6 32-0 27.6 9.8
2,000,000 7,570 93-0 40-0 28.4 12.2
104-6 32-0 31.9 9.8
3,000,000 11,360 114-0 40-0 34.7 12.2
127-6 32-0 38.9 9.8
4,000,000 15,140 131-6 40-0 40.1 12.2
147-6 32-0 44.9 9.8
5,000,000 18,930 147-0 40-0 44.8 12.2
165-0 32-0 50.3 9.8
7,500,000 28,390 180-0 40-0 54.9 12.2
201-6 32-0 61.4 9.8
10,000,000 37,850 233-0 32-0 71.0 9.8
208-0 40-0 63.5 12.2

Standpipes

Steel Water Storage Standpipe

Standpipes are ground storage tanks that are taller than their diameter. They usually are built where there is little natural elevation and the extra height is needed to create pressure for water distribution.

It is important that standpipes be designed as a pleasing addition to the community skyline. Steel, because of its strength and formability, can be fabricated into many interesting and attractive shapes.

The shape of the roof may be conical, toriconical, dome or ellipsoidal, with or without ornamental trim. Standard accessories may include shell and roof manholes, a roof vent, painter’s trolley rail, fixed outside ladder, and connections or pipes as required. Inside ladders are not recommended in locations where sustained freezing weather can be expected. Safety devices for long ladders are required by law in all locations.

The standpipe’s flat bottom, like the reservoir’s is not designed to carry stress. It acts as an impervious, flexible membrane that assures reliable, "bottle-tight" construction. It usually is 1/4 inch steel plate lap welded on the top side only. These lap seams are tested for tightness using a vacuum box and a soap solution. The bottom frequently is crowned upward toward the center to accommodate a modest amount of subsoil consolidation without producing excessive tensile stressses.

Typical Steel Water Storage Standpipes

Capacity Range of Sizes Available Range of Sizes Available
U.S. Gallons Cubic Meters Feet - Inches Meters
Diameter Height to HWL Diameter Height to HWL
50,000 189 14-9 40-0 4.5 12.2
60,000 227 16-2 40-0 4.9 12.2
75,000 284 18-0 40-0 5.5 12.2
100,000 379 19-0 48-0 5.8 14.6
125,000 473 21-3 48-0 6.5 14.6
150,000 568 23-3 48-0 7.1 14.6
200,000 757 24-10 56-0 7.6 17.1
250,000 946 27-9 56-0 8.5 17.1
300,000 1,135 28-5 64-0 8.7 19.5
400,000 1,515 32-10 64-0 10.0 19.5
500,000 1,890 34-7 72-0 10.5 21.9
600,000 2,270 37-10 72-0 11.5 21.9
750,000 2,840 42-6 72-0 12.9 21.9
1,000,000 3,785 46-4 80-0 14.1 24.4
1,500,000 5,680 56-9 80-0 17.3 24.4
2,000,000 7,570 65-6 80-0 20.0 24.4
2.500,000 9,465 69-10 88-0 21.3 26.8
3,000,000 11,360 76-6 88-0 23.3 26.8
4,000,000 15,140 84-6 96-0 25.8 29.3
5,000,000 18,930 94-6 96-0 28.8 29.3

Elevated Tanks

Double Ellipsoidal Steel Elevated Tank

Elevated steel water tanks have two primary components-the tank itself and its supporting structure. They ordinarily are used where natural ground elevation is not sufficient to assure distribution of water by gravity at suitable pressure.

Because of their height and supporting structure, elevated tanks offer virtually unlimited possibilities for attractive design. Steel’s strength and versatility make it the natural material for construction of safe, economical and eye-catching structures.

Elevated tanks can be categorized into several different types. The various diameters and head ranges for those shown in the accompanying diagrams and tables are only representative, and may vary with individual fabricators. Specific diameter/head range combinations should be decided by the tank fabricator within the limits indicated in the tables. Height should be specified by the purchaser as the dimension between the top of the foundation and the overflow level of the tank. Further dimensions, which are a function of the fabricator’s standard, should not be specified.

The double ellipsoidal tank has a cylindrical side wall, an ellipsoidal bottom and roof, and a top capacity level in the roof several feet above the top of the cylindrical shell.

Typical Double Ellipsoidal Steel Elevated Tanks

Capacity Range of Sizes Available
U.S. Gallons Cubic Meters Feet Meters
Diameter Head Range Diameter Head Range
25,000 95 18-20 12.5-15.5 5.5-6.1 3.8-4.7
30,000 114 18-20 15.0-16.5 5.5-6.1 4.6-5.0
40,000 150 22-23 15.0-17.0 6.7-7.0 4.6-5.2
50,000 189 22-24 18.0-20.0 6.7-7.3 5.5-6.1
60,000 227 22-25 19.0-23.0 6.7-7.6 5.8-7.0
75,000 284 26-30 16.0-24.0 7.9-9.1 4.9-7.3
100,000 379 28-30 20.0-25.0 8.5-9.1 6.1-7.6
125,000 473 30-32 23.0-28.0 9.1-9.7 7.0-8.5
150,000 568 32-34 24.5-29.5 9.7-10.4 7.5-9.0
200,000 757 36-38 28.0-29.5 11.0-11.6 8.5-9.0

Column and Rafter Supported Cone Roofs

Column and Rafter Supported Cone Roofs The column and rafter supported roof generally is the most economical one for reservoirs. The roof has minimum slope for adequate drainage, is easy to walk on and provides easy access to the roof hatch for interior inspection. Column loads are spread to a safe limit by column bases, and concrete footings under the columns usually are not required.

Column and Rafter Supported Roof with Knuckle

Column and Rafter Supported Roof with Knuckle This tank is similar to one with a supported roof in every respect, except that the transition from the shell plate to the roof plate is a smooth curve rather than a sharp break. The knuckle plate is a dished section.

Self-Supporting Dome Roof (or Umbrella Roof)

Self-Supporting Dome Roof (or Umbrella Roof) Plates are butt welded or lap welded and are supported directly on the top angle and shell plate. This roof is used where an uncluttered interior and smooth exterior appearance is desired. Structural stiffeners may be used internally on large diameter roofs to avoid excessive plate thickness.

Self-Supporting Ellipsoidal Roof

Self-Supporting Ellipsoidal Roof This roof is not a true ellipse. It is formed with two radii with major and minor axis proportions of approximately 2:1. The transition from shell to roof is a smooth unbroken curve. This roof is suitable for large and small diameter reservoirs and standpipes. It is pleasing to the eye and free of internal structural members. Larger diameter tanks usually have radial stiffening members.

Special Applications

Such applications generally fit into the following categories:

Aesthetics

The appearance of a steel water tank may be enhanced to create special effects by blending its design with local architecture or creatively using paint for a dramatic visual image. Some tanks include non-structural elements such as pilasters or facades. Where a truly unique impact is desired, special shapes may be incorporated into the tank design.

Unique Operations

Steel water tanks may be designed to accommodate special operational requirements. One example of this is the dual purpose tank that provides more than one storage compartment. A typical configuration includes a lower storage compartment, very similar to a pipe, which also supports an elevated tank. Each functions as a separate water storage vessel. Where applicable, the dual purpose tank offers potential savings in both property and maintenance costs by combining two tanks into one structure.

The flexibility of steel tanks also allows for alternate applications that address community concerns or local building standards. One such example would be a buried, or partially buried, steel tank. If a low profile tank is needed and the demand for water storage exceeds the available site conditions, steel tanks can be - and have been - successfully buried.

Today’s tank coatings and the integrity of steel will provide countless years of low cost water storage. In contrast to other materials, steel tanks built to the AWWA D100 standard will also provide assurance that your tank will be leak-free.

For more information about special application tanks, contact a STI/SPFA member tank company.

More Information

For more information, contact the STI/SPFA member companies in STI/SPFA’s Field Erected Tank Section. These are industry-leading organizations with years of experience in design, engineering, fabrication, erection, painting and maintenance of steel water tanks.

“STI technologies have better industry recognition than any of our individual brands.”

Greg Scalet
Steel Tank & Fabricating Corp.

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