Oxyacetylene welding, cutting and heating

Guidelines are given on the principal health and safety considerations for oxyacetylene welding to ensure safe welding practices.

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The oxyacetylene process

The oxyacetylene process produces a high temperature flame, over 3000 degrees C, by the combustion of pure oxygen and acetylene. It is the only gas mixture hot enough to melt steel; other gases (propane, LPG or hydrogen) can be used for lower melting point non-ferrous metals, for brazing and silver soldering and as a preheating/piercing gas for cutting.

Safe storage

Gases are normally supplied under high pressure in steel cylinders; in the UK, the colour coding for the cylinders is in the process of being harmonised across Europe. For acetylene the shoulder of the cylinder is maroon and for oxygen the shoulder is white, although black oxygen cylinders will remain in circulation for some time. The cylinder should also have a label marked with the type of gas. To prevent the interchange of fittings between cylinders containing combustible and non-combustible gases, oxygen cylinders have a right-hand and acetylene have a left-hand thread. All cylinders are opened by turning the key or knob anticlockwise and closed by turning them clockwise.

Oxygen will cause a fire to burn more fiercely and a mixture of oxygen and a fuel gas can cause an explosion. It is, therefore, essential that the oxygen cylinders are separated from the fuel gas cylinders and stored in an area free from combustible material.

Safe practice and accident avoidance

• Store the cylinders in a well-ventilated area, preferably in the open air
• The storage area should be well away from sources of heat, sparks and fire risk
• Cylinders should be stored upright and well secured
• Oxygen cylinders should be stored at least 3m from fuel gas cylinders or separated by a 30 minute fire resisting barrier
• The store area should be designated 'No Smoking'.

Handling compressed gases

Cylinders are fitted with regulators to reduce the gas pressure in the cylinder to the working pressure of the torch. The regulator has two gauges, a high pressure gauge for the gas in the cylinder and a low pressure gauge for the gas being fed to the torch. The gas flow rate is controlled by a pressure adjusting screw which sets the outlet gas pressure. The BCGA Code of Practice CP7 recommends the gauges are checked annually and replaced every 5 years.

Factors to be considered are that the gas system is suitable for the pressure rating and the hoses are connected without any leaks. Valve threads should be cleaned before screwing in the regulator. The valve of an acetylene cylinders can be opened slightly to blow out the threads but the threads in oxygen cylinders are best cleaned using clean compressed air (the threads on hydrogen cylinders must always be blown out using compressed air).

As oxygen can react violently with oils and grease, lubricating oils or sealant for the threads must not be used.

Safe practice and accident avoidance

• Cylinders are very heavy and must be securely fastened at all times
• Cylinder valves or valve guards should never be loosened
• Check the regulator is rated for the pressure in the cylinder
• When attaching the regulator to the cylinder the joints must be clean and sealant must not be used
• Before attaching a regulator, the pressure adjustment screw must be screwed out to prevent unregulated flow of gas into the system when the cylinder valve is opened

Using compressed gases

Gases are mixed in the hand-held torch or blowpipe in the correct proportions. Hoses between regulator and torch should be colour coded; in the UK, red for acetylene and blue for oxygen. Hoses should be kept as short as possible and users should check periodically that they are not near hot or sharp objects which could damage the hose wall. Acetylene cylinders must always be used upright.

When connecting the system, and at least at the start of each shift, hoses and torch must be purged to remove any inflammable gas mixtures. It is essential the oxygen stream does not come into contact with oil which can ignite spontaneously. Purging should also not be carried out in confined spaces.

The torch should be lit with a friction lighter or stationary pilot flame to avoid burning the hands; matches should not be used and the flame should not be reignited from hot metal, especially when working in a confined space.

The cylinders should not become heated, for example by allowing the torch flame to heat locally the cylinder wall. Similarly, arc welding too close to the cylinder could result in an arc forming between the cylinder and workpiece/electrode.

Although very little UV is emitted, the welder must wear tinted goggles. The grade of filter is determined by the intensity of the flame which depends on the thickness of metal being welded; recommendations for filters according to the acetylene flow rate are given in the table (BSEN 169:1992).

Grade of filter recommended according to the acetylene flow rate:

Safe practice and accident avoidance

• When cleaning the cylinder threads, connecting the regulator and purging the hoses, protect face and eyes by wearing the appropriate head shield
• Use a suitable welding shield equipped with the appropriate ocular protection filter
• Wear non-combustible clothing
• Ensure the cylinder is not heated by the flame or by stray arcs from adjacent electrical equipment

Leak detection

Joints and hoses should be checked for leaks before any welding is attempted. Whilst acetylene may be detected by its distinctive smell (usually at levels of less than 2%) oxygen is odourless.

Leak detection is best carried out applying a weak (typically 0.5%) solution of a detergent in water or a leak detecting solution from one of the gas supply companies. It is applied to the joints using a brush and the escaping gas will form bubbles. On curing the leak, the area should be cleaned to remove the residue from the leak detecting solution. Leaks in hoses may be repaired but approved replacement hose and couplings must be used in accordance with BSEN 560:1995 and BSEN 1256:1996.

Backfire and flashback

A backfire (a single cracking or 'popping' sound) is when the flame has ignited the gases inside the nozzle and extinguished itself. This may happen when the torch is held too near the workpiece.

A flashback (a shrill hissing sound) when the flame is burning inside the torch, is more severe. The flame may pass back through the torch mixing chamber to the hose. The most likely cause is incorrect gas pressures giving too low a gas velocity. Alternatively, a situation may be created by a higher pressure gas (acetylene) feeding up a lower pressure gas (oxygen) stream. This could occur if the oxygen cylinder is almost empty but other potential causes would be hose leaks, loose connections, or failure to adequately purge the hoses.

Non-return valves fitted to the hoses will detect and stop reverse gas flow preventing an inflammable oxygen and acetylene mixture from forming in the hose. The flashback arrestor is an automatic flame trap device designed not only to quench the flame but also to prevent the flame from reaching the regulator.

Backfire or flashback procedure

After an unsustained backfire in which the flame is extinguished:

• close the blowpipe control valves (fuel gas first)
• check the nozzle is tight
• check the pressures on regulators
• re-light the torch using the recommended procedure

If the flame continues to burn:

• close the oxygen valve at the torch (to prevent internal burning)
• close the acetylene valve at the torch
• close cylinder valves or gas supply point isolation valves for both oxygen and acetylene
• close outlets of adjustable pressure regulators by winding out the pressure-adjusting screws
• open both torch valves to vent the pressure in the equipment
• close torch valves
• check nozzle tightness and pressures on regulators
• re-light the torch using the recommended procedure

If a flashback occurs in the hose and equipment, or fire in the hose, regulator connections or gas supply outlet points:

• isolate oxygen and fuel gas supplies at the cylinder valves or gas supply outlet points (only if this can be done safely)
• if no risk of personal injury, control fire using first aid fire-fighting equipment
• if the fire cannot be put out at once, call emergency fire services
• after the equipment has cooled, examine the equipment and replace defective components

When a backfire has been investigated and the fault rectified, the torch may be re-lit. After a flashback, because the flame has extended to the regulator it is essential not only to examine the torch, but the hoses and components must be checked and, if necessary, replaced. The flashback arrestor should also be checked according to manufacturer's instructions and, with some designs, it may be necessary to replace it. BCGA Code of Practice CP7 recommends that non-return valves and flashback arrestors are replaced every 5 years.

The information was prepared in collaboration with Roger Sykes, Health & Safety Executive, Newcastle upon Tyne.

This Job Knowledge article was originally published in Connect, May 1997. It has been updated so the web page no longer reflects exactly the printed version.