Section 9 — Storage Tanks

Containment Dikes for Storage Tanks and Spheres

IPE Engineering Practice IPE-EP-9-5-1

Document number: IPE-EP-9-5-1 · Section: 9 — Storage Tanks

SCOPE

• This Practice governs the general requirements for the design and construction of containment dikes, and for the spacing of storage tanks.
• Any deviation to this Practice must be approved by the procedure described in EP 1-1-3.
• An asterisk (*) indicates that a decision by the Owner or Owner's Engineer is required, or that additional information is furnished by the Purchaser.
• A revision bar indicates all changes made to this Revision.

2.0 REFERENCES

The latest edition of the following standards and publications are referred to herein,

STANDARDS AND PUBLICATIONS

DEFINITIONS

• Atmospheric Tank - A storage tank that has been designed to operate at pressures from atmospheric through 0.5 psig measured at the top of the tank.
• Combustible Liquid - Per NFPA 30, a liquid having flash point at or above 100° F. Combustible liquids are subdivided as follows:
• Class II includes those liquids having flash points at or above 100°F and below 140°F.
• Class lIlA includes those liquids having flash points at or above 140°F and below 200°F.
• Class IlIB includes those liquids having flash points at or above 200°F.

• Contractor - Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
• Dike - A soil or concrete wall used to retain and/or divert accidental discharge of product from aboveground tanks.
• Diversion Wall - A soil or concrete wall that directs spills to a safe disposal area.
• Flammable Liquids - Per NFPA 30, a liquid having a flash point below 100°F and having a vapor pressure not exceeding 40 psia at 100°F is known as a Class I liquid. Class I liquids are subdivided as follows:
• Class IA includes those having flash points below ?3°F and having a boiling point below 100°F.
• Class IB includes those having flash points below ?3°F and having a boiling point at or above 100°F.
• Class IC includes those having flash points at or above ?3°F and below 100°F.
• Flash Point - The minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture with the air near the surface of the liquid or within the tank. The flash point shall be determined using the appropriate test in accordance with ASTM testing procedures as specified in NFPA 30.
• Impervious Soil - A soil, usually clay, that when compacted has a coefficient of permeability equal to or less than 10-? cm/sec.
• Inspector - A Inflection Point Engineering, LLCappointed engineer or inspector.
• Liquid - Any material which has a fluidity greater than that of 300 penetration asphalt when tested in accordance with ASTM D5, Test for Penetration for Bituminous Materials.
• Owner - IPERefining Company.
• Owner's Engineer - A Inflection Point Engineering, LLCappointed engineer.
• Permeable Soil - A soil, usually sand or gravel, that allows passage of liquid and which has a coefficient of permeability of 10-4 cm/sec or greater.
• Toe Wall - A low soil, concrete, or masonry wall required for the retention of small leaks or spills.
• Vapor pressure - The pressure, measured in pounds per square inch absolute exerted by a liquid as determined by ASTM D323.

GENERAL

• The design and construction of diked areas for above ground petroleum storage tanks is not uniform throughout the industry and has been guided mainly by fire codes which provide some degree of uniformity in impoundment area, volumetric capacity, and tank spacing. However, current trends in environmental legislation suggest that soon the design and construction of dikes will be driven by environmental requirements to protect the ground water in case of spills; therefore, this Practice will have to be revised periodically to reflect new environmental regulations. Users of this Practice shall assure themselves that the most current regulations are considered.

• A diked system consists of the main dikes, dike basin, and intermediate dikes.
• The main dikes are used to retain accidental spills of products from aboveground tanks to prevent the liquid from endangering adjacent facilities, property, or waterways; to control storm water; and when necessary, to protect tanks from high water. The basin, the area within the dikes, provides a barrier to minimize infiltration of substances that may otherwise pollute the groundwater. Intermediate dikes are constructed whenever two or more tanks storing Class I liquids are located in a common dike area and any one is over 150 feet in diameter. Their main purpose is to contain minor spills to minimize contamination and fire exposure within the whole basin.

ENGINEERING DESIGN

Impounding Dikes

Dikes must be designed to be impervious to contain a hydrocarbon spill for a period of time adequate to allow recovery and cleanup. The dikes shall be capable of withstanding a full hydrostatic head of the stored product. Dikes shall be constructed mainly of earth; however, if space limitations preclude construction of earth dikes, concrete or steel can be used. Masonry is not recommended since it is prone to settlement cracks. Earth dikes are recommended as the standard design; therefore, construction details of those dikes are given below. Construction of other types of dikes will require site specific evaluation from a geotechnical point of view.

Earth Dikes

The construction of earth dikes should meet the following requirements:

• Soils that can be easily eroded or are too pervious should not be used to construct dikes. These soils include rock flour, silt, sand, and gravel. A material will probably be satisfactory if, when wet, it can be rolled between the palms of the hands into a 1/8-inch diameter thread without crumbling; generally, most clay type soils will meet this requirement, and will exhibit low permeability when properly compacted. The dike material will be properly compacted if it is compacted to 90 percent of the maximum density obtained in ASTM D155?. In areas where only sands and gravels are available, an 8 to 12-inch thick clay blanket should be placed on the slope of the dike, or a 1 to 2 feet wide clay core should be built within the dike. However, the clay blanket on the slope of the dike is typically easier and cheaper to construct than a core.
• Before dike construction is started, vegetation and topsoil shall be removed and the surface scarified 6 to 8 inches deep and 1 foot wider than the proposed dike base. The area shall then be recompacted. However, if the in situ soil is sand, the sand should be removed to a depth of 18 inches before placement of dike materials.
• The slope of the earth dike should be consistent both with the shear strength of the material of which the dike is constructed and with the shear strength of the foundation soil. The side slope should not be steeper than 1.5 horizontal to 1 vertical. However, in areas where the foundation soils have low shear strength, a slope stability analysis shall be conducted.
• The top of the dike should be at least 2 feet wide.

• The slopes of the dike should be protected against erosion caused by wind or/and rain. This can be accomplished by any standard procedure such as by inhibiting plant growth, or by covering the slope with soil cement, or by spreading the slopes with cutback asphalt, or by covering the slope with crushed stone or shell. It is important to remember that in dry areas vegetation may be fire hazard, therefore, its use is not recommended in the refineries.
• The height of the dike should be generally restricted to 6 feet above the interior grade. However, if required, dikes may be higher than 6 feet above interior grade where provisions are made for normal access and necessary emergency access to tanks, valves and other equipment, and safe egress from the diked enclosure.
• The Flammable and Combustible Liquids Code, NFPA 30, requires that provisions be made for normal operation of valves and for access to tank roof(s) without entering below the top of the dike, when the average height of the dike enclosing tanks containing Class I liquids is over 12 feet, measured from interior grade. These provisions may be met through the use of remote operated valves, elevated walkways, or similar arrangements.

Tank Basins

• For environmental reasons, consideration should be given to making the tank basin, the area within the dikes, impervious when one or more of the following conditions exist:
• Permeable soils such as sands and gravels.
• Potential for contamination of an aquifer that is used for drinking water.
• Potential for contamination of a wetland, waterway or other surface waters.
• Presence in the immediate area of buildings or houses that can be affected by product seeping from the tank basin.
• Presence of toxic chemicals in the product stored in the tank.
• (*)Clay type soils, if available, should be the first choice as the material to be used as the impervious barrier to make the tank basin impermeable. If clay is not available, a synthetic membrane should be used as the impermeable barrier. Guidelines for the construction of a clay barrier are given in Paragraph 5.4. The use of a synthetic membrane is site specific and therefore its use and construction requirements should be discussed with the Owner's Engineer, as the need arises.
• The basin should be graded to provide a slope of not less than 1% away from the tank shell to the dike base and/or any drainage basins.

Construction Guidelines for a Clay Blanket

• Remove the topsoil, organic material, debris, and other unsuitable materials from the basin area. Topsoil is the top layer of soil that can support vegetation.
• Prior to placing the clay blanket, the cleared area should be scarified 6 to 8 inches deep, wetted if necessary, and then rolled with 6 to 8 passes of a suitable heavy roller.

• The clay blanket in the containment area should be at least 12 inches thick and should be compacted to a minimum dry density of at least 95 per cent of the maximum standard Proctor (ASTM D698) density, and should be compacted at a moisture content of 1% to 2% above optimum moisture content. The liquid limit and plasticity limit of the clay shall comply with the requirements of Paragraph 4.3 of EP 4-2-2.
• The clay blanket should be covered with about 6 inches of sand to inhibit moisture loss from the clay to minimize cracking of the clay due to the formation of desiccation cracks.
• Where the upper 5 feet of the natural subsoil has a coefficient of permeability to water of less than 10-5 cm/sec, a clay blanket is not required.

CAPACITY OF DIKED AREA

Atmospheric Storage Tanks

• For Class l, ll, or lIlA liquids and crude oils, the volumetric capacity of the diked area shall not be less than the greatest amount of liquid that can be released from the largest tank within the diked area, assuming a full tank. To allow for volume occupied by tanks, the capacity of the diked area enclosing more than one tank shall be calculated after deducting the volume of the tanks, other than the largest tank, below the height of the dike.
• For Class IlIB liquids there are no minimum retention capacity requirements for fire protection purposes; however, a peripheral toe wall shall enclose tanks in such services.
• Where the terrain slope from tankage containing combustible or flammable liquids or crude oil is towards critical areas, means shall be provided to prevent dike overflow from reaching these areas. Preferentially, this shall be accomplished by installing curbs, diversion ditches, dikes or walls, or by regrading the terrain. Increasing the dike height to contain the capacity of the largest tank is also acceptable.

Refrigerated Storage Vessels

• Single vessel in diked enclosures: the capacity of the dike shall not be less than 100% of the capacity of the enclosed vessel.
• Paired vessels: two vessels of similar basic design (i.e., both spheres or both tanks), regardless of capacity, may be paired and enclosed by a single peripheral dike, provided the following requirements are fulfilled:
• Capacity of the peripheral dike shall be 100% of the capacity of the larger vessel, allowing for the displacement of the second vessel.
• An intermediate dike shall be provided between paired vessels. Intermediate dikes must be at least 18 inches high.
• The pairing principle may be extended to include three vessels, but only in the case of an odd number of vessels. For example, nine vessels may be arranged in three groups of two vessels each and one group of three, but not three groups of three vessels. Where the pairing principle is extended to include three vessels, the capacity of the peripheral dike shall be 100% of the capacity of the largest vessel, allowing for the displacement of the remaining two vessels, and the vessels shall be separated by intermediate dike

Pressure Storage Vessels (Spheres)

• Spheres shall be enclosed in individually diked areas.
• Capacities of dikes for spheres shall be as follows:
• For vessels storing flammable liquids with a Reid Vapor Pressure (RVP) of 100 psia or less retention capacity of the diked area surrounding each vessel shall be a minimum of 50% of the capacity of the enclosed vessel.
• For vessels storing flammable liquids with RVP greater than 100 psia retention capacity of the diked area surrounding each vessel shall be a minimum of 25% of the capacity of the enclosed vessel.

7.0 LAYOUT AND SPACING REQUIREMENTS

Layout and spacing requirements for storage tanks and spheres are covered in EP 3-

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