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Subchapter 1. Unfired Pressure Vessel Safety Orders
Article 6. Anhydrous Ammonia

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§501. Installation of Aboveground Storage Tanks for Other Than Refrigeration Service.

(a) Stationary tanks shall be installed aboveground on firm masonry or concrete foundations, or on full length skids providing a bearing area limiting foundation loadings to not more than 2,000 pounds per square foot. Unless supported by continuous skids welded to pads on the tank shell, aboveground horizontal storage tanks shall have no more than 2 points of support longitudinally. Where necessary, tanks shall be securely anchored or weighed to prevent floating.

A tank may be installed on saddles, directly on concrete foundations, or it may be supported by properly designed metallic structural supports. When the tank is installed directly on concrete foundations, a corrosion pad shall be provided at thee points of contact with the foundation. These corrosion pads shall be at least 3/16 inch thick, and shall have a width at least equal to the width of the foundation at the point of contact with the tank, but in no case shall they be less than 8 times the shell thickness in width.

When corrosion pads or pad-type steel supports are used, the pad shall have rounded or semielliptically shaped ends, and shall be attached to the shell with a continuous seal weld. Corrosion pads shall be not less than 1/4 the circumference of the shell in length.

Unless installed on concrete pads, skid tanks shall be placed on engineered soil or road base topped with well-drained gravel or oiled sand, capable of supporting 2,000 pounds per square foot.

Sufficient space shall be provided between stationary storage tanks to permit access for fire fighting. This space shall not be less than 5 feet for tanks over 1,200-gallon capacity. Tanks exceeding 1,200-gallon capacity shall be so installed that the bottom outside surface of the tank is at least 12 inches above ground level.

(b) All tanks over 1,200-gallon capacity shall be installed on foundations in such a manner as to permit expansion and contraction. Every tank shall be so supported as to prevent the concentration of excessive loads on the tank at the points of support. All foundations and fastenings shall be designed to provide reasonable safety under imposed loadings, including wind, earthquake, vibrations, etc.

(c) Containers shall bee located outside of buildings, or in buildings or sections thereof especially equipped for this service. Consideration shall be given to the adverse health effects of ammonia, as well as the adjacent fire hazards, when selecting the location for a storage container.

Storage areas shall be kept free of readily ignitible materials such as waste, weeds, and long dry grass.

Stationary storage tanks shall be located with relation to property lines, residential buildings, highways, etc., in accordance with thee following table:

Minimum distance (feet) from tank to
Capacity of tank in U.S. gallons Line of property adjoining which may be built upon Highway or main track of railroad Residential building
1,200 and less 50 25 50
Over 1,200 to and including 30,000 50 50 50
Over 30,000 to and including 70,000 65 50 65
Over 70,000 to and including 500,000 100 50 100
Over 500,000 150 50 150

Minimum distance (feet) from tank to
Capacity of tank in U.S. gallons Any Building or area used by the public for deliberation, worship, education, entertainment, amusement, or awaiting transportation Hospital, jail or or other similar institutions Open well, reservoir, or other source of potable water
1,200 and less 250 750 50
Over 1,200 to and including 30,000 400 1,000 100
Over 30,000 to and including 70,000 520 1,300 100
Over 70,000 to and including 500,000 800 2,000 200
Over 500,000 1,000 2,500 200

Electrical equipment and wiring for use in ammonia installations shall be general purpose or weather resistant as appropriate.

Electrical systems shall be installed and maintained in accordance with the Electrical Safety Orders.

(d) Where vehicle impact is possible or likely stationary tanks shall be protected against vehicle damage by a rugged fence, suitable crash posts, curbs, or other acceptable protection. All storage tanks located where unauthorized tampering is possible shall be surrounded with a rugged steel fence or equivalent, or all liquid and vapor outlets shall be kept effectively locked when not in use.

(e) All stationary storage tanks exceeding 1,200 gallons capacity shall have the loading and unloading connections secured to a concrete bulkhead or equivalent designed to withstand a horizontal pull of not less than 2,000 pounds in any direction, unless other suitable protection is provided. This bulkhead shall not be located underneath the tank. The loading and unloading connections shall be firmly secured to this bulkhead and the piping between the bulkhead and tank shall be installed in a manner to provide for expansion, contraction, jarring, vibrations, settling, etc. For skid tanks exceeding 1,200 gallons capacity, such loading and unloading connections shall be securely fastened to the skid or to the tank supports.

NOTE: 2,000 pounds may not be adequate for all sizes of loading and unloading hoses and connections.

(f) Where excess-flow valves [See Section 507(e)] are used, liquid and vapor lines shall be at least full size from the excess-flow valve in the tank to the point of discharge or an additional excess-flow or equivalent shall be located as close to the point of pipe size reduction or other restriction as is practical, unless the excess-flow valve in the tank is designed to operate at the reduced flow condition, in which case the excess-flow valve in the tank may suffice.

Where excess-flow valves are used and 2 or more tanks are installed in battery, with common loading and/or unloading lines, the common liquid and vapor lines shall be fitted with excess-flow valves or equivalent (unless the common line has a cross-sectional area equal to or in excess of the combined areas of the individual tank lines; or unless the excess-flow valves are designed to operate at the reduced flow condition, in which case the excess-flow valve in the tank may suffice). Where additional excess-flow valves are used in common loading and/or unloading lines, the additional excess-flow valve or equivalent shall bee located as close to the point of restriction as practical.

NOTE: A quick-closing manually operated valve may be considered equivalent to an excess-flow valve at the point of pipe size reduction or other restriction providing:

(1) It is equipped with a means of closing the valve manually from a point remote from the delivery connection.

(2) The loading and/or unloading line in which is located is secured to a bulkhead complying with Section 501 (e).

(3) The quick-closing valve is in the pipeline on the tank side of the bulkhead.

(g) Tanks installed in battery shall be so installed that the top surfaces of the tanks are substantially in the same horizontal plane.

(h) The following minimum equipment shall be installed, properly maintained, and readily available for use at all stationary storage tanks in readily accessible locations.

(1) At least two full face respiratory devices in compliance with Section 5144; preferably one self contained breathing apparatus, and one NH(3) gas mask with spare canister.

(2) One pair NH(3) resistant gloves.

(3) One pair NH(3) resistant boots.

(4) NH(3) resistant pants and jacket and/or slicker.

(5) One quick-acting deluge shower and bubble fountain or other method of simultaneously washing both eyes with clean water.

The minimum water supply shall be a 50-gallon container of clean water.

(6) One first-aid kit in compliance with Section 3400 or 3439.

(7) One fire extinguisher conforming to the provisions of California Administrative Code, Title 19, Chapter 1, Subchapter 3, and having a rating of not less 40B-C.

The employer shall have a program of periodic inspection of the above emergency equipment to maintain it in serviceable condition.


1. Amendment filed 12-8-72 as procedural and organizational; effective upon filing (Register 72, No. 50).

2. Amendment of subsections (a), (c), (d), and (h) filed 3-28-75; effective thirtieth day thereafter (Register 75, No. 13).

3. Amendment of subsection (a) filed 8-6-76; effective thirtieth day thereafter (Register 76, No. 32).

4. Amendment of subsection (f) filed 4-1-77; effective thirtieth day thereafter (Register 77, No. 14).

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