Seismic Considerations for Flammable Gas Piping

In our July 2006 article, we described methods for lowering the risk of a fire after an earthquake. One portion of the article discussed ways to protect piping used to transfer flammable liquids and gases, though in a general manner. In this month’s article, we discuss specific protection criteria for flammable gas piping.

Site Arrangement:

Transmission piping at the site should not cross at right angles to known earthquake faults. Crossing at an oblique angle allows for lateral movement and reduces the possibility of pipe shearing.

Gas Valves:

Install a seismic gas shutoff valve at the supply side to each individual building utilizing fuel gas with pressures at or below 60 psi. Fuel gases include methane (natural gas), propane and butane. Locate this valve outside buildings, downstream of the pressure regulator, and in a clearly marked and accessible location, in accordance with the manufacturer’s instructions.

For fuel gas in excess of 60 psi, install safety shutoff valves actuated by seismic switches or seismic actuated pneumatic valves. This also applied to for flammable gases arranged in cylinder packs, manifold or dispensing rack systems and gas cabinets regardless of the system pressure. Arrange earthquake actuated safety devices for automatic operation and to safely shutdown the flow of gas in the event of strong ground motion.

Pipe Materials, Connections and Support:

Cast iron should not be used for valves, fittings, or supports due to its brittleness relative to other materials.

Threaded pipe should not be used. Welded piping systems can withstand the cyclic, horizontal, vertical and rotational forces of an earthquake much better than threaded piping, which may fail after several cycles.

Hangers with shock absorbing devices should be provided for piping subjected to seismic forces imposed by other equipment. Tags should be attached to piping having springs so that the hangers can be reset to their proper tension.

Flexible couplings have a history of being susceptible to leakage during an earthquake and should be avoided.

Piping Arrangement:

Clearance should be provided where piping passes through foundations, walls, and floors. If one part of the piping is secured rigidly and the other is free to move, sway bracing should be provided.

Seismic protection for Schedule 40 or greater steel pipe inside buildings:

Provide separate metered gas supply mains for building sections separated by a seismic joint. If impractical, use corrugated stainless steel tubing for fuel gas applications, or provide seismic separation assemblies in piping crossing seismic joints.

Use welded steel piping designed in accordance with ASME B31.3 and local building code requirements for flammable gas service. For fuel gas service, use of corrugated stainless steel tubing is acceptable where allowed by local authorities.

Pipe support should be used in accordance with the following guidelines:

• Where U-hooks are used for support, provide vertical restraint to the last hanger on horizontal piping.
• Keep piping at least 1 ft. away from unbraced pipe of equal or larger diameter. If unavoidable, provide bracing to the unbraced lines. Keep piping away from unrestrained fixtures and equipment by the same distance.
• Pipes that are supported by simply resting on structural elements or on trapeze hangers should be restrained to that element/hanger.
• Provide retaining straps for C-clamps used to attach hangers to the structural members.
• Do not use powder-driven fasteners to attach hangers to the building structure.

Provide sway bracing for flammable gas piping systems of nominal pipe size 1 in. or larger.

• Risers and vertical pipe runs should have four-ay bracing near their tops and bottoms, as well as intermediate points depending upon the pipe diameter. If passing through a multistory building using pipe sleeves at each floor (less than 30 ft. spacing) the intermediate bracing are not needed.
• Horizontal pipe runs need lateral sway braces: within 6 ft. of the start and end of pipe runs; within 6 ft. of changes in horizontal direction; within 2 ft. of vertical drops; within 2 ft. of concentrated mass piping elements (valves, meters, pressure regulators, etc.); and, at regular intervals (30 ft. max. for 2 in. or less diameter, 40 ft. max. for greater than 2 in. diameter). Exceptions for lateral braces include: where the hanger rods are less than 6 in. and the pipe diameter is less than 5 in.; and, where the pipe is supported directly against the structure using U-bolts.
• Provide at least one longitudinal sway brace per horizontal pipe run, plus additional braces at a maximum 80 ft. spacing.
• A lateral brace for one horizontal pipe may act as longitudinal bracing for a connected pipe (and vice-versa) provided: the brace is attached to the larger pipe; it is installed within 2 ft. of the change in direction; and, it is designed to handle the forces created by both pipes.
• Provide bracing for pipes on trapeze hangers using the same requirements as single pipes on hangers. Locate braces to coincide with trapeze locations and properly restrain pipes to the trapeze. Determine the seismic load for trapeze hangers based on the weight of all the pipes on the trapeze, and place braces symmetrically to prevent twisting of the trapeze.
• The design of the braces (loads, components, connection method) should be similar to the design of bracing for automatic sprinkler systems (NFPA 13 or FM Global Property Loss Prevention Data Sheet 2-8), noting the weight of the flammable gas is negligible.

Connections to equipment, tanks, etc. should have flexibility such as flexible pipe compatible with the application and rated for the required operating pressure (allow for 6 in. of open slack), or, engineered systems such as expansion loops with welded joints. Support downstream rigid piping so it does not defeat the point of flexibility. Vertical drops to the equipment should be braced about ¾ the way down the drop; avoid drops greater than 20 ft. unless specifically designed to provide restraint without creating excessive stresses at the top of the drop. Do not restrain the drop to the equipment or mezzanine as this may result in differential movement on the drop.

Where piping passes through walls or floors, provide clearances as noted in the table below. Seal openings with mastic or a weak, frangible mortar if needed. If the pipe passes through a firewall, fill the space with acceptable material such as mineral wool held in place with a pipe collar. When the wall material is frangible, such as gypsum board, and the wall is not required to have a fire rating, clearance is not needed. Where providing the recommended clearances is not possible, provide flexibility on both sides of the wall by installing flexible piping or ball joints.