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  Tank Farm Tank Bottoms

     It is important to have foundation knowledge of tank bottoms, just as it is to have foundation knowledge of hazardous materials procedures before approaching an incident. While the actual construction will have little merit at your stage of the response, it is interesting to understand what complexities you are dealing with. Most tank base problems only come into play for the responder when they fail and either launch the tank, or spill the entire contents into their respective secondary dike retainment areas. In each scenario, emergency response operations will quickly change if either of these two events occurs. Understanding the construction of common tank bottoms will greatly enhance your ability to assess conditions at an emergency incident.  Because of the vast differences in climate and soil conditions it is not practical to establish design data suitable for all situations. Tank bottom design should follow established guidelines such as ANSI/ACI 318 {American National Standards Institute/American Concrete Institute} for applicable stress values and material specifications. If the area uses a non-concrete base, the applicable standards should be referenced.

     For original construction, the subsurface constituents must be known to determine the total amount of settlement and the expected results. The sub-grade must be able to support the tank and contents. Total settlement must not place stress on piping or produce gauging inaccuracies above engineered amounts. The settlement should afford the tank bottom to settle below the grade for the immediate area. If the soil is inadequate to carry this load many engineering techniques can be used to either strengthen or replace the questionable subsurface. If this is not an option, concrete ring-walls can be constructed to support the tank walls and its content. The drawings show an example of ring-wall type construction. If natural materials are sufficient, both structures should follow the general formation of tank bottom construction by offering a 1 foot base above ground level. This provides for settlement, weather drainage, and aids in keeping the bottom dry and free of corrosion. After the specifics of foundations are followed from API 650 {American Petroleum Institute} it is suggested that the finished tank bottom grade be “crowned” from the outer perimeter to its center with a minimum slope of 1 inch for every 10 feet of travel. This crown compensates for slight settlement and facilitates easier clean out drainage and removal of waterous sludge that accumulates in storage tank bottoms with age.

     The additional drawings illustrate tank bottom improvements to both new construction and retrofitting

of existing tank bottoms. The top drawing shows the addition of a secondary barrier to prevent primary corrosion and subsequent leaks. This polymer membrane can be injected through existing tank shell support walls underneath a sand barrier. Another advantage is the possibility of the now intermediate sand, becoming helpful in cathodic protection for the original tank bottom.

     The lower drawing shows a secondary steel bottom for new construction tanks. The medium in this case assists in reduction of corrosion to the inner tank bottom. If this area is unsealed, accelerated, localized corrosion is a high probability. The secondary bottom excels in the area of leak detection. Not only is it much easier to detect leaks, but the resultant clean-up costs are substantially smaller for the tank farm owner.

     Tank inspections are generally performed at 10 to 20 year increments by inspectors following the API 650 standards and amendments. For many contractors local, state, and federal differences has caused confusion when tank construction is undertaken, but by using API 650 as the default guide you will be erring on the side of proven safety and experience. The foundation is the core of strength. When dealing with new neighbors contemplating tank farm construction or refurbishment, stress this point to them. As a FLBSS your responses will be safer and overall damage to the environment less in the advent of a major spill or fire.

                       Haz Mat Mike    


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