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Subchapter 7. General Industry Safety Orders
Group 20. Flammable Liquids, Gases and Vapors
Article 145. Tank Storage

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§5595. Drainage, Dikes and Walls for Aboveground Tanks.

(a) The area surrounding a tank or group of tanks storing Class I, Class II or Class IIIA liquids shall be provided with drainage as in (b), or shall be diked as provided in (c), to prevent accidental discharge of liquid from endangering employees or facilities. Tanks storing Class IIIB liquids do not require drainage or dikes.

(b) Where protection is by means of a natural or man-made drainage system, such systems shall comply with the following:

(1) A slope of not less than 1 percent away from the tank toward the drainage system shall be provided.

(2) The drainage system shall terminate in vacant land or other area or in an impounding basin having a capacity not smaller than that of the largest tank served. This termination area and the route of the drainage system shall be so located that, if the flammable or combustible liquids in the drainage system are ignited, the fire will not seriously expose tanks or adjoining property.

(3) The drainage system, including automatic drainage pumps, shall not discharge to adjoining property, natural water courses, public sewers, or public drains unless the discharge of flammable or combustible liquids would not constitute a hazard, or the system is so designed that it will not permit flammable or combustible liquids to be released.

(c) Where protection is accomplished by retaining the liquid around the tank by means of a dike, the volume of the diked area shall comply with the following requirements:

(1) 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.

(2) Walls of the diked area shall be of earth, steel, concrete or solid masonry designed to be liquid-tight and to withstand a full hydrostatic head. Earthen walls 3 feet or more in height shall have a flat section at the top not less than 2 feet wide. The slope of an earthen wall shall be consistent with the angle or repose of the material of which the wall is constructed. Diked areas for tanks containing Class I liquids located in extremely porous soils may require special treatment to prevent seepage of hazardous quantities of liquids to low lying areas or waterways in case of spills.

(3) Except as provided in (4) below, the walls of earthen dikes shall be restricted to an average interior height of six feet above interior grade.

(4) Dikes may be higher than an average of six feet above interior grade where provisions are made for normal and necessary emergency access to tanks, valves and other equipment, and safe egress from the diked enclosure.

(A) Where the average height of the dike containing Class I liquids is over 12 feet high, measured from interior grade, or where the distance between any tank and the top inside edge of the dike wall is less than the height of the dike wall, provisions shall be made for normal operation of valves and for access to tank roof(s) without entering below the top of the dike. These provisions may be met through the use of remote operated valves, elevated walkways or similar arrangements.

(B) Piping passing through dike walls shall be designed to prevent excessive stresses as a result of settlement or fire exposure.

(C) The minimum distance between tanks and toe of the interior dike walls shall be five feet.

(5) Where provision is made for draining water from diked areas, drainage shall be provided at a uniform slope of not less than one percent away from tanks toward a sump, drainbox or other safe means of disposal located at the greatest practical distance from the tank. Such drains shall normally be controlled in a manner so as to prevent flammable or combustible liquids from entering natural water courses, public sewers, or public drains, if their presence would constitute a hazard. Control of drainage shall be accessible under fire conditions and outside the dike.

(6) No loose combustible material, empty or full drum or barrel, shall be permitted within the diked area.

(7) Each diked area containing two or more tanks shall be subdivided preferably by drainage channels or at least by intermediate curbs in order to prevent spills from endangering adjacent tanks within the diked area as follows:

(A) When storing normally stable liquids in vertical cone roof tanks constructed with weak roof-to-shell seam or approved floating roof tanks or when storing crude petroleum in producing areas in any type of tank, one subdivision for each tank in excess of 10,000 bbls. and one subdivision for each group of tanks (no tank exceeding 10,000 bbls. capacity) having an aggregate capacity not exceeding 15,000 bbls.

(B) When storing normally stable flammable or combustible liquids in tanks not covered in subparagraph (1), one subdivision for each tank in excess of 100,000 gallons (2,500 bbls.) and one subdivision for each group of tanks (no tank exceeding 100,000 gallons capacity) having an aggregate capacity not exceeding 150,000 gallons (3,570 bbls.).

(C) When storing unstable liquids in any type of tank, one subdivision for each tank except that tanks installed in accordance with the drainage requirements of the Standard for Water Spray Fixed Systems for Fire Protection, NFPA No. 15-1973, shall require no additional subdivision. Since unstable liquids will react more rapidly when heated than when at ambient temperatures, subdivision by drainage channels is the preferred method.

(D) The drainage channels or intermediate curbs shall be located between tanks so as to take full advantage of the available space with due regard for the individual tank capacities. Intermediate curbs, where used, shall be not less than 18 inches in height.

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