Compressed gas cylinders and oxy-fuel equipment

Oxy-fuel welding and cutting, and many other hot work processes, depend on compressed gases stored in cylinders: oxygen, acetylene, propane, propylene, natural gas, hydrogen, argon, carbon dioxide, nitrogen and gas mixtures. Each cylinder holds a great deal of energy. A full oxygen cylinder is pressurized to about 15 to 20 MPa (2,200 to 3,000 psi); if its valve is snapped off, it can become a projectile that punches through walls. Fuel gases can feed large fires and explosions. Oxygen makes everything burn faster. Acetylene is unstable under pressure. This lesson covers how to store, move, connect, use and shut down cylinders and oxy-fuel equipment safely.

Know the gases

Different gases bring different hazards. The cylinder label and safety data sheet identify the contents; color alone is not reliable, because cylinder colors are not fully standardized between suppliers.

Storing cylinders

Cylinders should be stored upright, secured against falling with chains, straps or racks, with valve protection caps in place when not connected. Storage areas should be well ventilated, dry, away from heat sources, exits, elevators and stairways, and protected from vehicle impact and tampering. Full and empty cylinders should be separated and marked.

Oxygen and fuel gas cylinders in storage must be separated so that a fire involving one does not quickly involve the other.

OSHA 1910.253(b)(4)(iii) requires oxygen cylinders in storage to be separated from fuel gas cylinders or combustible materials, especially oil or grease, by at least 20 ft, or by a noncombustible barrier at least 5 ft high with a fire-resistance rating of at least one-half hour. Fire codes and NFPA 55 also limit quantities of flammable gas stored inside buildings.

Storage rules do not apply to cylinders that are connected and in use on a cart or manifold, but those cylinders still need to be secured and kept away from the hot work, sparks and heat.

Moving and handling cylinders

Cylinders are heavy, awkward and easily damaged. They should be moved with a cylinder cart or hand truck designed for them, with the valve protection cap in place and the regulator removed unless the cart is designed to carry cylinders with regulators attached. Never roll a cylinder on its side, drag it, slide it, drop it, lift it by the cap, or use a magnet or sling to lift it unless a cradle designed for the purpose is used. When cylinders are carried in vehicles on public roads, transportation of dangerous goods rules apply: cylinders must be secured, the load ventilated, and documentation and placards provided where required.

Under the DOT Hazardous Materials Regulations, 49 CFR, some small quantities carried as materials of trade qualify for reduced requirements, but cylinders still need to be secured and transported with ventilation. Many fires and explosions have started in vans when a leaking fuel gas cylinder filled the closed cargo area and a light switch or cigarette ignited it.

Setting up oxy-fuel equipment

An oxy-fuel outfit consists of an oxygen cylinder and a fuel gas cylinder, a regulator on each, hoses, check valves, flashback arrestors and a torch with welding tips or a cutting attachment. Setting it up correctly prevents most failures.

• Inspect the cylinders, valves and threads for damage, oil and grease. Never use oil or grease on oxygen equipment.
• Briefly open and close each cylinder valve (crack it) to clear dirt, standing to the side and pointing the outlet away from people and ignition sources. Do not crack fuel gas valves near flames or sparks, and never crack hydrogen valves.
• Attach regulators that match the gas and the cylinder connection; oxygen and fuel connections have different threads to prevent mix-ups.
• Fit flashback arrestors and check valves, usually at the regulator outlet and often at the torch as well. Several provinces and states require flashback arrestors by law.
• Use hoses in good condition, color-coded green for oxygen and red for fuel gas, with the correct fittings. Do not use tape to repair hoses.
• Open cylinder valves slowly, with the regulator adjusting screw backed off, standing to the side of the regulator, not in front of it. Open acetylene valves no more than about one and a half turns, and leave any valve wrench on the valve so it can be closed quickly.
• Test all connections for leaks with a leak-detection solution approved for oxygen service, never with a flame.

Flashback and backfire

A backfire is a momentary burning back of the flame into the torch tip, with a sharp pop, after which the flame may go out or relight. A flashback is far more dangerous: the flame burns back through the torch and into the hose or even the regulator, often with a squealing or hissing sound. Flashbacks happen when gases mix in the wrong place, for example when oxygen flows back into the fuel line because of incorrect pressures, a blocked tip, or failing to purge lines before lighting.

Check valves stop reverse gas flow. Flashback arrestors stop a flame front from traveling back through the equipment, and many also include a thermal shutoff and check valve. Both must be installed in the right direction and inspected or tested at the intervals the manufacturer specifies. If a flashback occurs, shut off the oxygen valve on the torch, then the fuel valve, then the cylinder valves, and do not relight until the cause is found and the equipment inspected.

Acetylene: special rules

Acetylene becomes unstable and can decompose explosively when compressed above about 15 psi gauge (103 kPa), so it is stored dissolved in acetone within a porous mass inside the cylinder. OSHA 1910.253 prohibits generating, piping or using acetylene at pressures above 15 psig. Store and use acetylene cylinders valve end up so acetone does not enter the regulator and hose. If a cylinder has been lying on its side, stand it upright for the time the supplier specifies before use. Limit the withdrawal rate, commonly to no more than about one-seventh of the cylinder's contents per hour, or manifold several cylinders together for high flow. Copper and alloys with more than about 65 % copper must not be used in acetylene service because they can form explosive acetylides.

Using and shutting down the equipment

Light the torch with a friction lighter, never a match or cigarette lighter, following the manufacturer's sequence, usually fuel first and then oxygen. Keep hoses out of the path of sparks, slag and traffic, and away from sharp edges. Never leave a lit torch unattended or lay it down while lit. Keep cylinders at least several meters from the hot work, or shielded, so sparks and slag cannot reach them.

When work stops for a break or at the end of the job, close the torch valves, close the cylinder valves, bleed the lines and back off the regulator adjusting screws. Never leave gas in hoses inside closed spaces, such as tanks or confined spaces, during breaks; remove the torch and hoses or disconnect them at the cylinder, because a small leak can fill the space with fuel gas or oxygen while no one is there.

Inspecting hoses, regulators and torches

Oxy-fuel equipment wears out. Hoses crack, become brittle or are cut by slag and sharp edges. Regulators develop creeping pressure, cracked gauges and leaking seats. Torch valves leak and tips become clogged. A daily visual check before use should look for cuts, burns and worn spots on hoses, damaged fittings, cracked gauges and dirty or damaged tips. If a regulator's delivery pressure keeps rising with the torch closed, it is creeping and must be taken out of service. Equipment should be serviced by qualified technicians at the manufacturer's intervals, and flashback arrestors tested or replaced as the manufacturer requires.

Arc welding equipment needs similar care: inspect cables for damaged insulation and loose connections, keep the work return clamp clean and tight, and make sure welding machines are grounded and their covers in place. Damaged cables can cause shocks and arcs that start fires far from the intended work.