The X's and O's of oxyfuel
Tips for safety and success
If properly used and well-maintained, this equipment can cut, heat, and weld metals safely. But damaged, worn, or improperly handled equipment can lead to safety hazards such as fire and explosions that can cause serious injury or death. To know how to handle and care for the equipment and devices found in oxyfuel cutting and welding, you have to first be familiar with them and the kind of care they need.
Oxyfuel cutting and welding equipment mixes flammable fuel gases and oxygen under pressure to support a flame. All fuel gases are flammable. While oxygen is not flammable, it vigorously accelerates the combustion of fuel gases and combustible materials. If properly used and well-maintained, oxyfuel equipment can cut, heat, and weld metals safely. But damaged, worn, or improperly handled equipment can lead to safety hazards such as fire and explosions that can cause serious injury or death. To know how to handle and care for the equipment and devices found in oxyfuel cutting and welding, you first have to be familiar with them and the kind of care they need.
Fuel Gas Safety
Because each fuel gas has different characteristics for operating, storing, and transporting, it is important that each be identified by its correct name, for example acetylene, liquid propane (LP), or propylene.
Propane (LP) and propylene gas cylinders must be transported, stored, and used in an upright position to maintain a gaseous state for safety devices and to prevent liquid from reaching pressure reducing regulator. Acetylene cylinders must be transported, stored, and used in an upright position to avoid discharge of acetone during use.
If a fuel gas cylinder valve is leaking, close the valve on the cylinder immediately if it is open. If the valve still leaks or the fuse plug (or other relief device) or the cylinder itself leaks:
- Immediately take the cylinder outdoors, away from possible sources of ignition.
- Identify the cylinder with a tag that indicates the location of the leak, and remove the cylinder from service.
- Post "No Smoking" warning signs in the area.
- Notify your cylinder supplier to pick up the damaged cylinder.
Handling Acetylene Safely. If an acetylene cylinder is not equipped with a multiturn shutoff valve, leave an acetylene cylinder wrench on the open valve and remove it after you close the valve. Use only approved wrenches, generally available from your cylinder supplier. You also should be aware of the recommended acetylene withdrawal rate.
Safe operating procedures call for a withdrawal rate not to exceed 1/10 the capacity of the acetylene cylinder per hour during intermittent use. For full withdrawal of the contents of the cylinder on a continuous basis, the flow rate should be no more than 1/15 the capacity of the cylinder per hour. Always consult the manufacturer's specification sheet for the flow requirements.
Remember, never discharge fuel gas near any flame, spark, or other source of ignition.
Although oxygen is not a flammable gas, precautions must be taken to protect both the equipment and you from potential harm. Materials that do not burn in air usually can burn in oxygen. Those that burn slowly in air can ignite easily and burn violently in an oxygen-enriched atmosphere. Oxygen is not compressed air and should never be used as a substitute for compressed air.
Handling Oxygen Safely. Never use oxygen to ventilate confined spaces. Use air to replace atmospheric oxygen consumed by welding or cutting. Cylinder and equipment leaks should be prevented, particularly in confined spaces.
Oxygen-enriched (more than 23 percent) or oxygen-depleted (less than 19 percent) atmospheres are potentially dangerous.
Do not work in or create such a hazardous condition.
Avoid grease, oil, oil-bearing materials, greasy gloves and rags, and other combustibles around oxygen equipment that can readily ignite. Oxygen never should be substituted for air in air-operated tools, in oil preheating burners, to start an internal combustion engine, to purge pipelines, or to build pressure in a confined container.
Identify oxygen equipment for oxygen use only, and never allow it to become contaminated by use in any other gas service. If equipment is used for other gas service, never use it for oxygen.
It is dangerous to clean clothing with oxygen stream or hang clothing on oxygen cylinders. Clothing saturated with oxygen burns intensely when ignited. Should clothing become saturated, do not weld, cut, or start any kind of spark or flame. Remove the clothing from the work area for cleaning.
When not in use, all oxygen cylinders must be stored in an upright position at least 20 ft. from fuel gas cylinders. Any readily combustible material, particularly grease or oil, cannot be within the oxygen storage area. If the 20-ft. distance is not available to separate the oxygen cylinders from the fuel gas supply, a fire barrier having a fire resistance rating of at least hour and height no less than 5 ft. must be installed between the oxygen and fuel gas cylinders.
If liquid oxygen (portable cryogenic cylinders) is used, cylinders must be transported, stored, and used in an upright position to maintain its gaseous state for safety devices and to prevent liquid from reaching pressure reducing regulator.
The Cutting Torch
Before each use, inspect the seating surfaces of the torch and connections for signs of wear and damage. Don't attempt to repair the torch yourself. If the equipment is damaged, send it to the manufacturer or authorized repair center. You should also purge lines daily before lighting to remove air and other contaminants from hoses. Open each torch valve long enough for the pure gas to expel any gas mixtures. Shut one valve before opening the other. Never purge in a confined space, in the presence of flame or any other source of ignition, or toward people. Point the tip away from yourself and others when lighting and using the torch. Always use a spark-lighter, pilot light, or other approved device to light the torch.
When you are not using the torch, be sure to leave it in a safe position to prevent accidental dislodging that may open valves or cause damage. Close the cylinder valves on both the fuel gas and oxygen. Do not store compressed-gas cylinders in unventilated cabinets or vehicles. Improperly closed valves can cause a combustible mixture of gases to accumulate in an unventilated area. This mixture will explode when ignited or can reduce the oxygen content in the breathing air, risking possible asphyxiation.
Pressure Regulators for Oxyfuel Applications. Always use a compressed-gas regulator with the gas for which it was designed. If the regulator is not working properly—for example, if gas leaks externally; if the regulator delivery pressure continues to rise with the downstream valve closed; or if the gauge pointer does not move off the stop pin when pressurized or return to the stop pin after pressure is released—remove the faulty regulator from service immediately.
Flashbacks and Backfires
Sparks, flying slag, fumes, hot metal, and heat are under control in most cases, but when the fuel gas and oxygen mixture becomes unstable, flashbacks and backfires can occur, causing damage to the equipment and possible harm to you or others. A flashback is a burning of the flame back into the tip nozzle or into or through the torch handle. It is also called a sustained burning in the tip or torch. If it doesn't cause fire or hose rupture, it may produce a hissing or squealing sound, usually at the mixer.
A flashback can be caused by faulty equipment or operating practices, which can include:
- Failure to purge the oxyfuel equipment before and after use.
- Incorrect pressures for both the oxygen and the fuel gas.
- Damaged equipment, especially mixer assemblies, tip nozzles, or adapter seats.
- Hoses in poor condition or with inad-equate flow capacity.
- Restricted flow in tip nozzles, mixers, or torch orifices.
- Overheated tip or torch.
A backfire is a loud noise produced by the explosion of gases at the cutting or welding tip, usually following a minor flashback of the flame, extinguishment, and reignition at the tip. Repeated backfires can cause the tip to overheat and eventually cause a sustained flashback into the equipment.
Common causes of backfires are:Working too close to the workpiece or touching it with the equipment. Particles that enter the tip and restrict or impede gas flow. Working conditions, such as working in corners, that reflect heat onto the equipment. Incorrect gas flow.
Oxyfuel Safety Devices
You can protect yourself and your equipment from flashbacks and backfires by incorporating the appropriate safety devices, such as flashback arresters, which are designed to enhance operating procedures and help protect you and your equipment in dangerous conditions.
Approved protective equipment should be installed in the oxyfuel gas system to prevent the backflow of oxygen into the fuel gas supply system; the passage of a flashback into the gas supply system; and the development of back pressure higher than the pressure rating of the system components.
Reverse-flow check valves connected to equipment prevent backflow of unwanted gases into the gas supply system. While check valves will prevent backflow, they should not be relied upon as a substitute for flame arresters because they are not intended to stop backfires or flashbacks.
For maximum safety, use flashback arresters to help prevent and protect against flashbacks and backfires. If not stopped, a flashback can melt the equipment. In the very worst case, the flashback could travel back to the cylinder and cause an explosion. Properly designed flashback arresters have a sensitive nonreturn valve that stops the gas flow and a fine sintered filter that quenches the flame, plus an internal thermal cutoff valve. This valve stops the flow of gas before ignition occurs upstream.
Remember, no safety device can replace practicing safe operating procedures and making sure your oxyfuel torch and equipment are well-maintained.
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