1.  Introduction

Within the University, acetylene is used for oxyacetylene cutting and welding and to a much lesser extent in chemical synthesis and in analytical instrumentation.  Besides being an extremely flammable gas, acetylene is also highly unstable.  Under the right conditions it will decompose explosively, even in the absence of oxygen.  Decomposition is usually triggered by heat, for example, if the acetylene cylinder is involved in a fire or if a flashback from an acetylene flame travels back to the cylinder.  The danger from acetylene cylinders is unique, because it persists for some time (24 hours or more) after the fire has been extinguished.  Acetylene cylinder explosions are violent and extremely destructive, and they pose a special risk to the emergency services.

Because of its dangerous properties, the use of acetylene is covered by explosives legislation and is the subject of two codes of practice published by the British Compressed Gases Association (BCGA).  Both the legislation and the codes of practice are complex and can only be briefly summarised in this policy, though the full texts are in the Safety Office library.

Departments are asked to notify the Safety Office of the location of all acetylene cylinders.  Departmental contingency plans must show the location of acetylene cylinders for the benefit of the emergency services.

This policy summarises the precautions needed for safe use of acetylene in its common applications.  Many references are made to British Standards, simply so that users can seek assurances from suppliers that they are using suitable equipment (there is normally no need to consult the text of the Standards).

2.  Care of acetylene cylinders

In its free state acetylene will begin to dissociate into its constituent elements at pressures above 0.62 bar; violent decomposition may occur.  Therefore, acetylene cylinders differ from other cylinder gases, in that they do not contain compressed or liquefied acetylene.  Instead, they contain acetylene dissolved in acetone absorbed onto a porous mass within the cylinder.  The porous mass is designed to slow or inhibit any decomposition, usually providing time for emergency action in the event of mishap.

Care must be taken to avoid mishandling acetylene cylinders, by dropping, jarring or rolling them.  They must always be stored and used upright.  If these precautions are not taken, then voids may develop in the porous substance, allowing detonation to initiate, e.g. by mechanical shock.  Acetylene cylinder storage should comply with the conditions set out in 4 (b) below.

Where acetylene cylinders are exposed to external threat from fire, there is always the danger of explosive detonation, putting those tackling the fire at risk.  Because of this danger, the location of all acetylene cylinders must be clearly marked on the department’s contingency plan, for the information of the emergency services.

3.  Oxyacetylene cutting and welding equipment

The safe use of oxyacetylene (and other oxy-fuel) equipment is fully covered in a code of practice (CP7) produced by the BCGA.  This code includes advice on maintenance as well as on equipment; on the safety precautions to be taken in the working area; on cylinder handling and storage; and on emergency procedures.

CP7 contains a number of minimum safety standards and requirements, as follows.

(a)  Pressure regulators must conform to BS EN 585 or BS 7650.  Pressure adjusting screws should be set to the zero pressure position when the regulator is not in use.  Outlet pressures should be set no higher than needed for the work.

(b)  Hoses must conform to BS EN 559 and should be no longer than is necessary for the work.  Temporary extensions may be used (see below), but they must be removed when no longer needed.  Hoses must not be coiled while in use, e.g. around the cylinder or trolley (a fire in a coiled hose is difficult to extinguish).

(c)  Hose connections must conform to BS EN 560.  Any temporary extensions must be made using hose couplers to BS EN 560.  Reusable worm drive hose clips must not be used to make connections with acetylene, nor must copper pipe be used to couple hoses.

(d)  In the case of blowpipes, supplier’s operating instructions for use and maintenance must be followed.

(e)  Suitable protective clothing must be used.  Eye protection must be to BS 1542 Class 2 with lenses to BS 679.  Other protective clothing (eg coveralls, gloves) may be needed, as determined by a risk assessment.

(f)  Additional safety devices (non-return valves and flame arrestors) conforming to BS EN 730 are required as follows.  Note that only flame arrestors approved by the HSE may be used with acetylene (an up to date list is available from the Safety Office).

(i)  For hoses shorter than 3m and with a bore less than 6.3mm

(a)  a non-return valve must be fitted to each blowpipe connection

(b)  a flame arrestor and cut-off valve of adequate recommended inlet pressure must be fitted to each regulator outlet connection.

(ii)  For hoses longer than 3m and/or with a bore greater than 6.3mm

(a)  a non-return valve must be fitted to each blowpipe connection

(b)  a flame arrestor and a cut-off valve (temperature- or pressure-activated) must be fitted to both the acetylene and the oxygen cylinders’ regulator outlet connections.

(c)  Alternatively, both a non-return valve and a flame arrestor may be fitted to each blowpipe connection.  In this case, a suitable flame arrestor and a cut-off valve must also be fitted to each regulator outlet connection.

(iii)  For hoses much longer than 3m, two flame arrestors should be fitted; one at the blowpipe connection and one at the regulator outlet.  Because this may mean that the system’s flow capacity is reduced, affecting blowpipe and cut-off valve operation, the supplier’s advice should be taken to ensure safe operation.

(g)  Other hazards may be present in the use of oxyacetylene equipment and measures should be in place to control the associated risks.  Examples include exposure to toxic fumes; work in confined spaces; oxygen depletion or enrichment; work on vessels or tanks which have contained flammable, explosive or toxic materials; or fires caused by sparks.

(h)  Hoses, regulators, flashback arrestors, check valves and nozzles should be inspected and maintained regularly and replaced if damaged (see Appendix for details).

4.  Flashback

Flashback can occur if there is a flammable mixture of oxygen and acetylene already in the hose when the torch is lit.  If unchecked, the mixture will ignite and the flame will travel back from the torch, through the hoses and regulators and into the cylinder.  Decomposition of acetylene may be triggered in the hose, the regulator or the cylinder.

(a)  Prevention of flashback

(i)  Training - ensure that only properly trained and experienced persons use the equipment.

(ii)  Use the correct lighting-up procedure – purge the hoses of gas (one at a time) for a few seconds; close the blowpipe valve after purging; use the correct gas pressures and nozzle sizes for the work; use a spark igniter to light the gas.

(iii)  Flashback arrestors – fit approved arrestors to the regulators on both the acetylene and the oxygen cylinders (for hoses longer than 3m fit them to the blowpipe as well).

(iv)  Non-return valves (check valves) – fit them to the torch to prevent gas backfeeding into the hoses.

(v)  Nozzles – poorly maintained nozzles cause turbulent gas flow, increasing the risk of flashback; overheated nozzles (caused by holding them too close to the workpiece) can cause flashback.

(b)  Dealing with a flashback

(i)  Close both acetylene and oxygen cylinder valves immediately, if it is safe to do so.

(ii)  Once the acetylene is shut off, the flame should extinguish itself.  If the flame cannot be put out at once, then the building or area should be evacuated and the fire alarm activated.

(iii)  Check any acetylene cylinder which has been involved in a flashback or affected by a fire.  If it becomes warm, or starts to vibrate, then evacuate the building or area and activate the fire alarm.

The fire service must be informed immediately of the involvement of an acetylene cylinder in the fire.

5.  Other applications of acetylene

The use of acetylene at pressures greater than 1.5 bar is forbidden without the specific approval of the HSE.  Their approval is not required for installations operating between 0.62 and 1.5 bar, provided that the conditions of Certificate of Exemption No 2 1989(2), made under the Explosives Act (1875) (Exemptions) Regulations 1979 are complied with.  This document is available in the Safety Office.

The distribution of acetylene between 0 and 1.5 bar is covered in code of practice CP6 issued by the BCGA.  Its requirements are briefly summarised here.

(a)  Permanently piped systems

Wherever reasonably practicable, acetylene used to supply equipment etc in laboratories should be distributed by a permanently piped system, with the cylinders stored in the open air or in a safe, properly ventilated location. All such storage locations must be clearly visible and accessible.  The construction of the system, including the materials of construction, must be in accordance with CP6.  The Safety Office will give advice on storage, especially where acetylene for piped distribution cannot be stored in the open air.  Note that cylinders connected to permanent systems should not be kept within occupied rooms.

(b)  Non-permanent systems

Cylinders of acetylene for non-permanent use should be kept in the appropriate cylinder storage area when not in use.  Where this is not reasonably practicable, then they may be kept in the workroom provided

(i)  the smallest practicable number and size of cylinders is kept

(ii)  there is a high standard of permanent, natural high and low level ventilation

(iii)  the cylinders are kept away from any fire hazard (e.g. stores of flammable or oxidising materials) or heat (e.g. radiators or heaters)

(iv)  there is a well defined area set aside, clearly visible and accessible, and marked for acetylene cylinder storage

(v)  wherever reasonably practicable, cylinder valves are closed when the gas is not in use

(vi)  suitable warning notices (available from the Safety Office) are displayed outside the room.

6.  Fires involving acetylene

Fire extinguishers should always be readily available.  With oxyacetylene equipment, leakage of acetylene from faulty hoses or hose connections is the most common cause of fires.  There is serious danger from fires whose heat affects the acetylene cylinder or from flashbacks where a flame travels back into the cylinder via the hose.

Where ignition occurs because of leakage, the cylinder valve should be closed, then the fire extinguished as soon as possible.  If this sequence is not possible, then a CO2 or dry powder extinguisher should be aimed, not at the flame, but in the same direction and behind it.  Once the flame is out, the valve should be closed to avoid re-ignition.

If the fire cannot be extinguished like this, further attempts should not be made.  The fire alarm must be raised and the building evacuated.  The fire service must be informed immediately of the involvement of an acetylene cylinder in the fire.


May 2001