Oxygen-fuel gas welding requirements

The maximum pressure for generating, piping (except approved cylinder manifolds), or using acetylene is 15 psig (103 kPa gauge) or 30 psia (206 kPa absolute). See 1910.253(a)(2).

• The 30 psia absolute limit is intended to prevent unsafe uses of acetylene in pressurized chambers (for example, caissons or tunnels).
• This pressure restriction does not apply to acetylene stored dissolved in a solvent in DOT-approved cylinders or to acetylene intended for chemical use.
• Employers must ensure any on-site acetylene generation, piping, or use complies with these limits to avoid explosive hazards.

No — the use of liquid acetylene is prohibited. See 1910.253(a).

• Employers and workers must not use acetylene in liquid form for welding or cutting operations.
• Use only cylinders and equipment approved for acetylene gas handled at the permitted pressures.

You must use only approved apparatus such as torches, regulators or pressure‑reducing valves, acetylene generators, and manifolds. See 1910.253(a)(3).

• "Approved" means equipment designed and maintained for the intended welding/cutting service.
• Use regulators, hoses, and torches that are rated and maintained for their gases and pressures; damaged or nonapproved devices increase the risk of leaks, backflow, and explosions.

No — devices or attachments that facilitate mixing air or oxygen with flammable gases before the burner are not allowed unless they are specifically approved for that purpose. See 1910.253(a)(1).

• Mixtures of fuel gases and air or oxygen may be explosive; this rule eliminates pre‑burner mixing except in standard torches or approved devices.
• Do not install unauthorized tees, fittings, or improvised mixers upstream of the torch.

Employers must instruct and judge competent the workers placed in charge of oxygen or fuel‑gas supply equipment and distribution piping before leaving them in charge. See 1910.253(a)(4).

• Employers must provide rules and written instructions covering operation and maintenance and make them readily available.
• "Competent" means the person has been trained and evaluated to safely operate and maintain the specific supply equipment and piping systems.

Compressed gas cylinders must be legibly marked with either the chemical or trade name of the gas, using stenciling, stamping, or labeling that is not readily removable. See 1910.253(b)(1)(ii).

• Markings should be durable and, whenever practical, located on the cylinder shoulder per 1910.253(b)(1)(iii).
• Proper marking prevents accidental mixing or misuse of gases during welding and cutting operations.

Cylinder connections must comply with the American National Standard "Compressed Gas Cylinder Valve Outlet and Inlet Connections, ANSI B57.1—1965," which is incorporated by reference in [1910.6]. See 1910.253(b)(1)(iii) and 1910.6.

• Use the correct, compatible connections to prevent cross‑connection of gases (for example, oxygen vs. acetylene).
• Follow manufacturer and supplier guidance on proper fittings and adapters; using incompatible connections can cause leaks or hazardous gas mixtures.

All cylinders with a water weight capacity over 30 pounds (13.6 kg) must have means to attach a valve protection cap or must have a collar or recess to protect the valve. See 1910.253(b)(1)(iv).

• Valve protection prevents damage to the valve during handling and transport and reduces the risk of sudden release of gas.
• Keep valve caps in place (hand‑tight) except when cylinders are in use or connected for use per 1910.253(b)(2)(iv).

Inside buildings, cylinders must be stored in a well‑protected, well‑ventilated, dry location at least 20 feet (6.1 m) from highly combustible materials and should be kept in assigned places away from elevators, stairs, or gangways. See 1910.253(b)(2)(ii).

• Storage areas should prevent cylinders from being knocked over or damaged and should restrict access to unauthorized persons.
• Keep cylinders away from heat sources such as radiators per 1910.253(b)(2)(i).

Empty cylinders must have their valves closed, and valve‑protection caps (where designed to accept a cap) must always be in place, hand‑tight, except when cylinders are in use or connected for use. See 1910.253(b)(2)(iii) and 1910.253(b)(2)(iv).

• Closing valves on empty cylinders prevents leakage and accidental release.
• Keep protection caps on during storage and movement to protect valve integrity.

Inside a building, fuel‑gas cylinders not in actual use or not attached ready for use are limited to a total gas capacity of 2,000 cubic feet (56 m³) or 300 pounds (135.9 kg) of liquefied petroleum gas; if storage exceeds those amounts, you must provide a separate room or compartment meeting the requirements in 1910.253(f)(6)(i)(H) and 1910.253(f)(6)(i)(I), or keep cylinders outside or in a special building. See 1910.253(b)(3).

• Special rooms must have no open flames for heating or lighting and be well ventilated.
• Plan cylinder inventories so on‑site storage stays within limits or provide compliant storage spaces if larger amounts are necessary.

Acetylene cylinders must be stored valve end up. See 1910.253(b)(3)(ii).

• Storing acetylene valve up helps keep the acetone solvent and the gas in the proper configuration inside the cylinder, preserving safety device function.
• Secure cylinders to prevent tipping or falling.

Oxygen cylinders in storage must be separated from fuel‑gas cylinders or combustible materials by a minimum of 20 feet (6.1 m) or by a noncombustible barrier at least 5 feet (1.5 m) high with a fire‑resistance rating of at least one‑half hour. See 1910.253(b)(4)(iii).

• Maintain the 20‑foot separation or install the specified barrier to reduce fire spread risk.
• Keep oxygen cylinders away from oil, grease, and other materials that could accelerate fire per 1910.253(b)(4)(i).

Oxygen cylinders must not be stored near highly combustible materials (especially oil and grease), near reserve stocks of carbide and acetylene or other fuel‑gas cylinders, or in an acetylene generator compartment. See 1910.253(b)(4)(i).

• Oxygen supports combustion; even small contaminations with oils/greases can cause rapid ignition.
• Keep oxygen cylinders clean and in designated, separated storage areas.

If a liquid oxygen system will supply gaseous oxygen for welding or cutting and has a storage capacity over 13,000 cubic feet of oxygen connected in service or ready for service, or more than 25,000 cubic feet including unconnected reserves, it must comply with the provisions of NFPA No. 566—1965 as incorporated by [1910.253(b)(4)(iv)]. See 1910.253(b)(4)(iv).

• These volume thresholds are measured at 14.7 psia and 70 °F.
• Large liquid oxygen systems require special design, siting, and safety measures per NFPA to reduce fire and explosion risks.

No — cylinders, cylinder valves, couplings, regulators, hose, and apparatus must be kept free from oily or greasy substances, and oxygen cylinders or apparatus must not be handled with oily hands or gloves. See 1910.253(b)(5)(i).

• Oils and greases can ignite violently in oxygen‑enriched atmospheres.
• Establish clean handling procedures and inspect gloves and tools to prevent contamination.

When transporting cylinders by crane or derrick you must use a cradle, boat, or suitable platform; slings or electric magnets may not be used, and valve‑protection caps (where designed to accept a cap) must always be in place. See 1910.253(b)(5)(ii)(A).

• The valve cap protects against valve damage during lifting.
• Use proper lifting gear and secure the cylinders to prevent tipping or impact.

Unless cylinders are secured on a special truck, regulators must be removed and valve‑protection caps (when provided) must be put in place before cylinders are moved. See 1910.253(b)(5)(ii)(D).

• Removing regulators prevents damage to both regulators and valves during transport.
• Always secure cylinders during movement to avoid falls or strikes.

Cylinders must be kept far enough away from the actual welding or cutting operation so that sparks, hot slag, or flame will not reach them; if that is not possible, fire‑resistant shields must be provided. See 1910.253(b)(5)(ii)(I).

• Position cylinders upwind and away from hot work as a best practice.
• Use noncombustible shields or relocate cylinders to prevent heat or sparks from contacting them.

Before attaching an oxygen regulator, you must attach a regulator to the cylinder valve; however, before connecting the regulator the valve shall be opened slightly for an instant and then closed, and you should always stand to one side of the outlet when opening the cylinder valve. A hammer or wrench must not be used to open cylinder valves; if valves cannot be opened by hand, notify the supplier. See 1910.253(b)(5)(ii)(P) and 1910.253(b)(5)(ii)(Q).

• "Cracking" the valve (open slightly then close) clears dust and debris from the outlet before attaching the regulator.
• Standing to one side protects you if a sudden high‑pressure discharge occurs.
• Never force a valve open with tools—contact the supplier for service.

Before connecting a regulator to a fuel‑gas cylinder valve, the valve must be opened slightly and closed immediately while standing to one side of the outlet; before removing a regulator the cylinder valve must be closed and the gas released from the regulator. See 1910.253(b)(5)(iii)(C) and 1910.253(b)(5)(iii)(D).

• Cracking the valve keeps dust/debris from entering the regulator and reduces the chance of a flashback or leak.
• Always bleed the regulator before removal to avoid sudden releases and to allow safe disconnection.

No — safety devices must not be tampered with. See 1910.253(b)(5)(iii)(H).

• Employers must ensure safety devices remain in place and in working order; disabling, modifying, or bypassing them is prohibited under the standard.

No — fuel-gas must have its pressure reduced through a suitable regulator attached to the cylinder valve or manifold before being used through torches or other devices. See 1910.253(b)(5)(iii)(I).

• This prevents excessive pressure at the torch and reduces the risk of leaks, flashbacks, or equipment failure.

The cylinder valve shall always be opened slowly. See 1910.253(b)(5)(iii)(J).

• Opening slowly reduces the chance of a pressure surge that could damage regulators, cause hose failure, or create unsafe flow conditions.

An acetylene cylinder valve shall not be opened more than one and one-half turns of the spindle, and preferably no more than three-fourths of a turn. See 1910.253(b)(5)(iii)(K).

• This limits the speed of shutoff and helps ensure the valve can be closed quickly in an emergency.

If a special wrench is required it must be left in position on the stem of the valve while the cylinder is in use so the fuel-gas flow can be quickly turned off in an emergency, and at least one such wrench must be immediately available for manifolded or coupled cylinders. See 1910.253(b)(5)(iii)(L).

• For manifolded or coupled cylinders, ensure at least one wrench is present and accessible at the manifold.

Such cylinders should be plainly tagged, and the supplier should be promptly notified and his instructions followed as to their return. See 1910.253(b).

• Tagging identifies the cylinder as defective or out of service and prevents accidental use.
• Follow supplier instructions for return or repair to ensure safe handling.

Fuel-gas cylinders connected to one manifold inside a building are limited to a total capacity not exceeding 300 pounds of liquefied petroleum gas or 3,000 cubic feet of other fuel-gas. See 1910.253(c)(1)(ii).

• Keep records or calculations of aggregate capacity to demonstrate compliance.

If the aggregate capacity connected to one manifold exceeds the indoor limit, the cylinders must be located outdoors or in a separate building or room constructed in accordance with 1910.253(f)(6)(i)(H) and 1910.253(f)(6)(i)(I). See 1910.253(c)(1)(iii).

• Design the room/building per the referenced construction provisions if indoor storage above the limit is necessary.
• Prefer locating large manifolds outdoors to simplify compliance.

Yes — more than one manifold may be located in the same room provided the manifolds are at least 50 feet apart or separated by a noncombustible barrier at least 5 feet high with a fire-resistance rating of at least one-half hour. See 1910.253(c)(1)(iii).

• Use measured spacing or an approved fire-resistant barrier to meet the requirement.

High-pressure fuel-gas manifolds must be provided with approved pressure regulating devices. See 1910.253(c)(1)(v).

• Ensure regulators are rated and approved for the intended pressure and gas service.

No — oxygen manifolds shall not be located in an acetylene generator room. See 1910.253(c)(2)(ii).

• Keep oxygen systems physically separated from acetylene generator areas to reduce combined hazard risks.

Oxygen manifolds must be separated from fuel-gas cylinders or combustible materials by a minimum distance of 20 feet or by a noncombustible barrier at least 5 feet high with a fire-resistance rating of at least one-half hour. See 1910.253(c)(2)(iii).

• This separation reduces the chance of an oxygen-fueled fire involving nearby combustibles.

An oxygen manifold connected to cylinders having an aggregate capacity of more than 6,000 cubic feet of oxygen should be located outdoors or in a separate noncombustible building. See 1910.253(c)(2)(iv).

• Plan site layout or use separate noncombustible enclosures when capacity exceeds this threshold.

Yes — oxygen bulk supply systems with storage capacity over 13,000 cubic feet (connected in service or ready for service) or more than 25,000 cubic feet including reserves must comply with NFPA No. 566-1965, and high-pressure oxygen manifolds must have approved pressure-regulating devices. See 1910.253(c)(2)(v) and 1910.253(c)(2)(vi).

• For very large supplies, follow NFPA 566 requirements and ensure regulators are suitable for high-pressure oxygen service.

Low-pressure oxygen manifolds must be suitable for use at 250 psig, have a minimum bursting pressure of 1,000 psig, be protected by a safety relief device that relieves at a maximum of 500 psig, and must display the sign "LOW-PRESSURE MANIFOLD DO NOT CONNECT HIGH-PRESSURE CYLINDERS MAXIMUM PRESSURE-250 PSIG (1.7 MPA)." See 1910.253(c)(3)(i) and 1910.253(c)(3)(v).

• Post the required sign conspicuously at each low-pressure manifold to prevent improper connections.

The fluid used for testing oxygen manifolds shall be oil-free and not combustible. See 1910.253(c)(3)(iv).

• Using oil-free, noncombustible test fluids avoids introducing substances that could ignite in oxygen service.

No — portable outlet headers shall not be used indoors except for temporary service where conditions preclude a direct supply from outlets located on the service piping system. See 1910.253(c)(4)(i).

• Use fixed service piping for normal, long‑term supply; reserve portable headers for temporary needs only.

Each outlet on the service piping that feeds a portable outlet header must have a readily accessible shutoff valve; master shutoff valves for both oxygen and fuel-gas are required at the entry end of the header; and fuel-gas portable headers must have an approved hydraulic back-pressure valve at the inlet unless an approved pressure‑reducing regulator, an approved back‑flow check valve, or an approved hydraulic back‑pressure valve is installed at each outlet. See 1910.253(c)(4)(ii), 1910.253(c)(4)(iv), and 1910.253(c)(4)(v).

• These shutoffs and backflow devices prevent unintended gas flow and cross‑contamination between systems.

Each service outlet on portable outlet headers must have a valve assembly that includes a detachable outlet seal cap that is chained or otherwise attached to the body of the valve. See 1910.253(c)(4)(vi).

• Keep caps attached so outlets are protected from contamination and caps are not lost.

When acetylene cylinders are coupled, approved flash arresters must be installed between each cylinder and the coupler block; for outdoor use of portable manifolds with three or fewer coupled cylinders, one flash arrester installed between the coupler block and regulator is acceptable. See 1910.253(c)(5)(iii) and 1910.253(c)(5)(iv).

• Use approved flash arresters at the required locations to reduce the risk of flame propagation into cylinders.

Acetylene and liquefied fuel-gas cylinders must be manifolded in a vertical position, and the pressure in cylinders connected to and discharged simultaneously through a common manifold shall be approximately equal. See 1910.253(c)(5)(v) and 1910.253(c)(5)(vi).

• Vertical positioning and matched pressures help ensure safe and even gas withdrawal from manifolded cylinders.

Pipe shall be at least Schedule 40 and fittings at least standard weight in sizes up to and including 6‑inch nominal; copper tubing shall be Types K or L in accordance with ASTM B88‑66a. See 1910.253(d)(1)(i)(A)(1) and 1910.253(d)(1)(i)(A)(2).

• Use appropriate schedules and tube types to meet the standard's minimum strength and quality requirements.

No — unalloyed copper shall not be used for acetylene or acetylenic compounds except in listed equipment. See 1910.253(d)(1)(iii)(B).

• Use approved materials (e.g., steel or wrought iron) for acetylene piping unless the equipment is specifically listed for copper use.

Hose connections and hose complying with paragraph (e)(5) may be used to connect a manifold pressure regulator to piping when the working pressure of the piping is 250 psi or less and the length of the hose does not exceed 5 feet; hose shall have a minimum bursting pressure of 1,000 psig. See 1910.253(d)(1)(ii)(C) and the hose bursting pressure requirement in 1910.253(d)(1)(ii)(C).

• Limit hose length and use hoses rated for the service pressure to reduce rupture risk.

Joints in steel or wrought iron piping shall be welded, threaded, or flanged; joints in brass or copper pipe shall be welded, brazed, threaded, or flanged, and if socket type they must be brazed with silver‑brazing alloy or similar high melting point filler metal (not less than 800 °F). See 1910.253(d)(2)(i) and 1910.253(d)(2)(ii).

• Avoid prohibited fittings (e.g., gray or white cast iron) and follow brazing temperature requirements for socket joints.

After assembly, piping shall be thoroughly blown out with air, nitrogen, or carbon dioxide to remove foreign materials; for oxygen piping, only oil‑free air, oil‑free nitrogen, or oil‑free carbon dioxide shall be used, and oxygen piping and fittings shall be washed with a suitable solution that removes grease and dirt without reacting with oxygen. See 1910.253(d)(3)(ii) and 1910.253(d)(3)(vii) and 1910.253(d)(3)(viii).

• Use hot water solutions of caustic soda or trisodium phosphate for cleaning when appropriate.
• Ensuring oil‑free condition prevents ignition hazards in oxygen service.

Under 1910.253(d)(3)(x) you must not weld or cut on an acetylene or oxygen pipeline until the line has been purged. Only oil-free air, oil-free nitrogen, or oil-free carbon dioxide may be used to purge oxygen lines, and when purging flammable gas lines or other parts of equipment, open lights or other ignition sources must be kept away from uncapped openings. See 1910.253(d)(3)(x) and 1910.253(d)(3)(ix).

• Use only the specified oil-free gases to purge oxygen lines to avoid creating a combustible mixture.
• Keep all ignition sources well away from uncapped openings while purging to prevent fires or explosions.

Reference: 1910.253(d)(3)(x) and 1910.253(d)(3)(ix).

Under 1910.253(d)(5)(i) piping systems must be tested and proved gastight at 1½ times the maximum operating pressure and thoroughly purged of air before being placed in service, and materials used to test oxygen lines must be oil-free and noncombustible. Flames must not be used to detect leaks, per 1910.253(d)(5)(ii).

• Test pressure: 1.5 × maximum operating pressure.
• Purge: Remove air thoroughly before placing lines in service.
• Test medium for oxygen lines: must be oil-free and noncombustible (to avoid ignition hazards).
• Leak detection: do not use open flames; use safe methods such as soapy water or approved leak-detection fluids.

Reference: 1910.253(d)(5)(i) and 1910.253(d)(5)(ii).

Under 1910.253(e)(3)(ii) employers must install approved protective equipment in fuel-gas piping to prevent (A) backflow of oxygen into the fuel-gas supply, (B) passage of a flash back into the fuel-gas supply, and (C) excessive back pressure of oxygen into the fuel-gas system. Station outlets must have a check valve, pressure regulator, hydraulic seal, or combination to prevent backflow, per 1910.253(e)(4)(i).

Key points:

• Functions required: backflow prevention, flashback (flame) arresting, and back-pressure relief; these may be combined in one device or provided by separate devices (1910.253(e)(3)(ii)).
• Location options: protective equipment shall be located in the main supply line, at the head of each branch line, or at each location where fuel gas is withdrawn, as shown in Figures Q-1 to Q-3 (1910.253(e)(3)(ii)(C)(1)).
• Back-pressure relief: a pressure-relief device must be set no higher than the rating of the backflow or flashback device and be located downstream of those devices; its vent must be at least as large as the inlet and installed without low points that trap moisture (1910.253(e)(3)(ii)(C)(4)).
• Station outlets: each outlet (including portable headers) must have a shutoff valve upstream of other outlet equipment and a detachable outlet cap; if pipeline protective equipment is installed at the station outlet per Figure Q-3, no additional check valve or regulator is required (1910.253(e)(4)(i),(ii),(iii),(vii)).

References: 1910.253(e)(3)(ii), 1910.253(e)(3)(ii)(C)(1), and 1910.253(e)(4)(i).