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

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§541. Safety Relief Devices.

(a) General.

(1) Unless covered elsewhere in these Orders, every natural gas tank shall be provided with one or more spring-loaded safety relief valves set to open as follows with relation to the maximum allowable working pressure:
Type of Vessel Minimum Safety Relief Valve Setting Maximum Safety Relief Valve Setting
ASME Code Constructed Vessels with a factor of safety of 5 or more 100% 125%
ASME Code Constructed Vessels built with a factor of safety of less than 5 100% 110%
DOT Cylinders --
Safety Relief Devices

As approved by Bureau of Explosives

All safety relief valves required by this Order shall be ASME labeled except for safety relief devices on DOT cylinders and safety relief valves as required in 537(c)(3), which may be ASME or Underwriters' Laboratories rated and stamped.

(2) Each safety relief valve shall be plainly marked by the manufacturer of the valve with the pressure in pounds per square inch at which the valve is set to start to discharge and the discharge capacity in cubic feet per minute (cfm) air.

(3) No shutoff valves shall be installed between the safety relief valves and the tank without written permission from the Division except that a shutoff valve may be used on multiple valve installations where the arrangement of the valves will provide full required flow through the safety relief valves at all times. The opening or connection between the tank and safety relief valve or valves shall have at least the combined areas of all connected safety relief valve inlets.

(4) Safety relief valves shall be so arranged that they will have an unobstructed full size discharge to a to a safe place, and shall be so arranged that escaping gas will not impinge on the vessel, valves or fittings.

(5) Except for safety valves that are integral with service valves, safety relief valves on tanks and cylinders shall be installed in a vertical position and shall be fitted with suitable raincaps.

(6) Safety relief valves for natural gas service shall not be fitted with lifting devices. The adjustment if external shall be provided with means for sealing the adjustment to prevent tampering by unauthorized persons. If at any time it is necessary to break such seal, the valve shall be removed from service until it has been reset and sealed. Any adjustments necessary to natural gas safety relief valves shall be made by the manufacturer or other companies having competent personnel and adequate facilities for the repair, adjustment and testing of such valves. The organization making such adjustment shall attach a permanent tag with the setting, capacity and date. All safety relief devices shall be tested at least annually and maintained in proper operating condition.

(7) If pressure regulators are used, a safety relief device shall be provided on the low-pressure side of each final stage regulator and on the low-pressure side of all other regulators unless the piping and/or fittings from the low-pressure side of such regulators to the high-pressure side of the next stage regulation are suitable for full tank pressure.

(8) Safety relief valves for surge tanks containing vapor only shall have a relieving capacity of not less than the maximum input from the system, vaporizer or reducing valves.

(9) Fusible plugs and/or rupture discs are prohibited for primary relief devices except as permitted in (b)(3) below.

(10) On underground installations, the discharge from safety relief valves shall be vertically upward and shall be piped to a point at least 10 feet aboveground, and shall meet the requirements for the location of tanks as specified in Order 531(a)(1), (2) and (3).

(b) Compressed Natural Gas.

(1) The minimum required rate of discharge of the safety relief valves for aboveground CNG tanks shall be at least equal to any input from the system, whether stored or being compressed.

(2) Safety relief valves located on stationary tanks of over 166,000 cubic feet capacity, shall be fitted with ferrous standpipes which will discharge at least 2 feet above the top of the tank, but not less than 10 feet above ground level. These standpipes shall not be threaded at the open end, and shall be fitted with suitable drain openings. Return bends and pipe fittings at the upper ends of safety relief valve discharge pipes are prohibited. Standpipes shall be adequately braced or otherwise supported if over 4 feet in length.

(3) Safety relief devices for DOT cylinders shall be provided in accordance with DOT regulations.

(4) Safety relief valves for underground CNG tanks shall have a relieving capacity of not less than that required for aboveground tanks.

(c) Liquefied Natural Gas.

(1) The minimum relieving capacity of the safety relief valves for LNG tanks shall be at least equal to the capacity marking on the vessel nameplate required in 523(c)(4).

(2) Safety relief capacity shall be determined for exposure to external fire and shall be sufficient to prevent pressure in the tank from exceeding 120 percent of the allowable working pressure for tanks built with a factor of safety of less than 5 and 150 percent for tanks built with a factor of safety of 5 or more. For calculating, the applicable formula in Appendix A shall be used to determine minimum relieving capacity.

(3) Safety relief valves for underground LNG tanks shall have a relieving capacity at least equal to the relief valve capacity markings on the vessel nameplate. When the capacity is not based on exposure of the vessel to fire, such underground tanks shall not be uncovered until they are free of liquid.

(4) Unless otherwise provided for in these Orders, safety relief valves shall have direct communication with the vapor space of the vessel.

(5) Safety relief valves located on stationary tanks over 2,000 gallons capacity shall be fitted with ferrous or suitable copper alloy standpipes which will discharge at least 2 feet above the top of the tank but not less than 10 feet above ground level. These standpipes shall not be threaded at the open end, and shall be fitted with suitable raincaps and shall be provided with a drain opening at their lower end unless the safety relief valves discharging into the standpipes are fitted with suitable drain openings. Return bends and pipe fittings at the upper ends of safety relief valve discharge pipes shall be prohibited. Standpipes shall be adequately braced or otherwise supported if over 4 feet in length.

(6) The outer tank of LNG vessels shall be provided with one or more relief devices set to open at not more than the maximum design working pressure of the outer tank. The discharge area shall be not less than .00024 square inch per pound of water capacity of the inner tank.

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