Standards are minimal requirements designed to protect employees, the public and the environment. Depending on a tank’s intended use, the product being stored and the method of construction, one or more of the following standards may be referenced in the tank specifications: American Water Works Association (AWWA), National Fire Protection Association (NFPA), Factory Mutual (FM), American Petroleum Institute (API), Occupational Safety and Health Administration (OSHA), and U.S. Environmental Protection Agency (EPA).
Storage tanks can be designed to hold any product, but the most common products stored are water, petroleum and other chemical liquids. Welded steel tanks constructed to store potable drinking water should be designed and maintained in accordance with American Water Works Association AWWA D100 standards and National Fire Protection Association (NFPA 22) if a portion of the water will also serve as fire protection.
Tanks that supply water for private fire protection only may be designed in accordance with NFPA 22 or Factory Mutual (FM) standards and inspected according to NFPA-25 standards.
Tanks that hold petroleum-based products or other liquid chemicals may refer to American Petroleum Institute-650 and API-620 standards, and most often require an API-653 inspection, set forth by the EPA and performed by a certified API inspector. If the tank holds flammable or combustible liquids, then it also must be inspected in accordance with pertinent NFPA standards.
Product Being Stored
After a tank’s intended use is determined, the product being stored should be carefully analyzed. Material Safety Data Sheet (MSDS) or Safety Data Sheets (SDS) can provide information about the product being stored. Information listed on these sheets include
• product and company identification
• composition of ingredients
• identification of hazards
• first aid, fire fighting, and accidental release measures
• handling and storage
• exposure controls and personal protection
• physical and chemical properties
• stability and reactivity
• toxicological and ecological information
• other regulatory information
Information obtained from these sheets can help determine specific storage requirements.
Specific Storage Requirements
To maintain the ideal temperature for the product being stored, the tank may need to be insulated, heated, cooled or built with a double wall. Double walls can be constructed to hold insulation between two walls like with cryogenic tanks, and they are most often constructed because of limited space, proximity to bodies of water, or a heightened risk with the product stored. Leak detection couplings and other visual alarm systems can also be added for visual leak detection, but they should be inspected regularly.
The capacity needs and pressure requirements should also be one of the first items to address, because it can affect where the tank may be located, if the tank can be shop-built or field-erected, and the overall design of the tank dimensions. Tank dimensions can affect the size and number of manways required, ventilation and roof options available.
Roof selections such as open top, floating, fixed and selfsupported can also affect the amount of freeboard required and impact the maximum and minimum fill levels. For example, an internal floating roof has travel constraints at the top and bottom of the tank to consider. A cone roof has rafters that interfere with the high water level, and center columns take from the useable capacity. However, an aluminum self-supporting dome roof does not have rafters or a center column to interfere with the useable capacity or water levels and therefore could be a better option for maximum performance.
Tanks are subject to internal corrosion from the product being stored and external corrosion from exposure to environmental elements. Tanks that will be exposed to harsh environments such as a continuous spray of sea salt may need extra corrosion protection. Construction material such as carbon steel, stainless steel and duplex stainless steel options should be explored with coatings and cathodic protection to determine the most cost effective solution for meeting specific corrosion allowance requirements.
Many factors should be considered when determining the best tank location, but most are related to availability, soil composition, seismic site class and seismic used group of the tank. Extra freeboard or anchorage may be needed depending on the specific location, seismic zone and wind speed.
Ensure Maximum Performance
After the ideal location for the tank has been selected and construction is complete, much information about the tank will have been collected. This information should be organized and maintained with copies of every inspection, records of repairs and modifications. Maintaining the complete history of the tank will give the owner a clearer view of past repairs, and areas of concern can be monitored more closely for the future.
Regular inspections should be conducted by an experienced and reputable tank company, and regular maintenance and repairs may be needed to maintain the tank. Tank companies that design, manufacture, construct, modify, repair and inspect welded steel ground storage tanks in accordance with all standards should be contacted to answer questions and provide services relating to storage tanks. Full-service tank companies tend to have the experience and flexibility needed to customize and maintain any tank for maximum performance.
Please contact Pittsburg Tank & Tower at (270) 826-9000 or visit our website at www.watertank.com for more information about storage tanks.