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Subchapter 1. Unfired Pressure Vessel Safety Orders
Article 7. Compressed and Liquefied Natural Gas System

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§533. Location and Installation of Underground Tanks.

(a) General. Tanks and regulating equipment shall not be buried below ground unless written permission to do so has been obtained from the Division and the following special conditions are complied with:

(1) The proposed location complies with Order 531(a)(1), (2) and (3).

(2) The tank shall be set with the top at least 24 inches below the surface of the ground, shall rest on firm foundations and shall not be installed under roadways.

(3) Before any back-filling is done, the installation shall be inspected by a qualified inspector and shall comply with all applicable sections of these Orders.

(4) The back-filling shall be done in such a manner that there will be at least 6 inches of water-washed sand free from soil, rocks or gravel immediately adjacent to the tank. The sand shall be well tamped into place during the back-filling operation.

(5) All buried tanks shall be constructed for underground services and the manufacturer shall provide means for lowering the tank into place without damage to the tank or corrosion-resistant coating.

(6) If the valves and fittings are located in a curb box, adequate provision shall be provided for proper drainage and ventilation of the curb box.

(7) No curb box shall be larger than necessary for the operation and maintenance of the valves and fittings located therein nor more than 3 feet in depth.

(8) All buried tanks shall be completely uncovered for an inspection of all external surfaces at intervals of at least once every 3 years.

The intervals may be changed at the discretion of the Division if a test plate is buried adjacent to the tank and shows no appreciable corrosion on inspection after 3 years of service; provided, however, that every tank shall be uncovered at least every 15 years. The test plates shall be of the same material as the shell of the tank and shall be at least 1/4 of the tank diameter in length and 6 inches wide and shall be buried in water-washed sand at least as deep as the lowest surface of the tank, and shall not be coated. Test plates shall be provided in accordance with the following table:

(A) Tanks 10 feet or less in length, one test plate.

(B) Tanks more than 10 feet but less than 20 feet in length, 2 test plates, one on each side near opposite ends.

(C) Tanks 20 feet or more in length, one test plate for each 10 feet of length, placed on alternate sides at 10-foot intervals.

(9) No tank used underground shall be reinstalled aboveground until it has been inspected and found to be in compliance with requirements for an aboveground installation.

(10) Where necessary, tanks shall be securely anchored or weighted to prevent floating.

(b) Compressed Natural Gas. The exterior surfaces of CNG tanks shall be covered with an acceptable corrosion-resistant material.

Tanks will be considered acceptable when prepared and coated as follows:

(1) Clean and prime.

(2) Coat with asphalt enamel.

(3) Wrap with 40-pound felt.

(4) Coat with asphalt enamel.

(5) Wrap with 40-pound felt.

(6) Coat with asphalt enamel.

(7) Wrap with Kraft paper.

Equivalent corrosion protection may be accepted at the discretion of the Division.

(c) Liquefied Natural Gas.

(1) LNG vessels utilizing a vacuum jacket shall be so installed that a vacuum leak can be detected, and so piped that means can be provided to restore the vacuum or minimize the leak.

(2) Consideration shall be given to the design and installation of the supporting members and surrounding materials in the possible freezing zone to prevent damage to the tank and its supports by low temperature effects of LNG spillage or by frost-heaving.

(3) LNG vessels having an outer jacket made of a material which may be subject to corrosion shall be protected in accordance with (b) above.

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