acetylene guidance by jizhen1947


									                           UNIVERSITY OF BRIGHTON


1. Introduction

Within the University, acetylene is used for oxyacetylene cutting and welding.
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
guidance note.

Departments and Schools are asked to notify the Health & Safety Office of
the location of all acetylene cylinders. Contingency plans must show the
location of acetylene cylinders for the benefit of the emergency services.

This guidance 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)

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/school contingency plan and in the
Premises Information Box, 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

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

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

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
clipsmust 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 (e.g. 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.
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

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.

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
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.

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 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.
When it is suspected that acetylene may be involved in a fire the Fire
Service will initiate a 200 metre exclusion zone around the incident for 24
hours and will only fight the fire defensively. The consequences of such
action are clearly very serious and any activities involving the use of
acetylene should be risk assessed, and where possible the use of
alternative systems for welding and hotwork should be considered.

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