General gas storage procedures
1 Any person in charge of storage of compressed
gas cylinders should know the regulations
covering highly flammable liquids and
compressed gas cylinders as well as the characteristics
and hazards associated with individual
gases.
2 It is best to store full or empty compressed gas
cylinders in the open, in a securely fenced compound,
but with some weather protection.
3 Within the storage area oxygen should be stored
at least 3 m from fuel gas supply.
4 Full cylinders should be stored separately from
the empties, and cylinders of different gases,
whether full or empty, should be segregated
from each other.
5 Other products must not be stored in a gas
store, particularly oils or corrosive liquids.
6 It is best to store all cylinders upright, taking
steps, particularly with round bottomed cylinders,
to see that they are secured to prevent them
from falling. Acetylene and propane must never
be stacked horizontally in storage or in use.
7 Storage arrangements should ensure adequate
rotation of stock.
Acetylene cylinders
1 The gas is stored together with a solvent (acetone)
in maroon painted cylinders, at a pressure
of 17.7 bar maximum at 15 °C. The cylinder
valve outlet is screwed left-handed.
2 The hourly rate of withdrawal from the cylinder
must not exceed 20 per cent of its content.
3 Pressure gauges should be calibrated up to 40.0
bar.
4 As the gas is highly flammable, all joints must
be checked for leaks using soapy water.
5 Acetylene cylinders must be stored and used in
an upright position and protected from excessive
heat and coldness.
6 Acetylene can form explosive compounds in
contact with certain metals and alloys, especially
those of copper and silver. Joint fittings
made of copper should not be used under any
circumstances.
7 The colour of cylinders, valve threads, or markings
must not be altered or tampered with in
any way.
Oxygen cylinders
1 This gas is stored in black painted cylinders at
a pressure of 200/230 bar maximum at 15 °C.
2 Never under any circumstances allow oil or
greases to come into contact with oxygen fittings
because spontaneous ignition may take place.
3 Oxygen must not be used in place of compressed
air.
4 Oxygen escaping from a leaking hose will form
an explosive mixture with oil or grease.
Gas welding, gas cutting and plasma arc cutting 269
5 Do not allow cylinders to come into contact
with electricity.
6 Do not use cylinders as rollers or supports.
7 Cylinders must not be handled roughly, knocked
or allowed to fall to the ground.
Safe usage and handling of gas
Cylinders
Leaks
As a matter of routine, check for leaks. Test with a
solution of 1 per cent Teepol in water; apply with a
brush. Never use a naked flame to trace leaks. In
hoses, leakages, cuts or local surface damage may
be repaired by cutting out faulty sections and
inserting an approved coupling. Worn ends should
be cut back and refitted with the appropriate hose
and connections and clips. Discard hoses that show
signs of general deterioration.
If an acetylene valve shows a minor leak and it
cannot be stopped by closing the valve or tightening
the gland nut, eliminate all sources of ignition,
move the cylinder outside to a safe area and contact
your supplier.
If a cylinder is leaking and on fire, and you suspect
the valve is damaged, do not attempt to extinguish
the fire yourself. Call the fire brigade and
evacuate the area. Use a fire extinguisher to put out
any fire caused.
Fire: action to be taken
Gas cylinders involved in a fire may explode.
Therefore the key actions to be taken are as
follows:
1 Evacuate the surrounding area to a minimum
distance of 100 metres.
2 Call the fire brigade immediately.
3 Advise persons between 100 and 300 metres
from the cylinder to take cover.
4 Any attempt to fight the fire should be done
from a protected position using copious quantities
of water.
5 When the fire brigade arrives, inform them of
the location, the number of gas cylinders directly
involved in the fire, and the names of the gases
they contain.
6 Cylinders which are not directly involved in the
fire and which have not become heated should
be moved as quickly as possible to a safe place,
provided this can be done without undue risk to
personnel. Make sure the valves of these cylinders
are fully closed.
7 As soon as possible, inform your gas supplier
of the incident.
8 Be aware of the fact that, even after the fire has
been extinguished, some cylinders which have
been heated can explode, particularly acetylene
cylinders. Therefore do not approach any cylinder
until the key actions given below have been taken.
9 When the cylinder content are unknown, treat
as acetylene cylinders.
Acetylene cylinders in a fire: key actions
Refer to Figure 9.26.
1 Never approach or attempt to move cylinders.
2 From a safe position, drench the entire surface
of all cylinders with water for at least one hour
after the fire has been extinguished. Do not use
a jet of water of such strength that it would
knock over a free-standing cylinder.
3 Check visually from a safe position. If steam is
seen to be coming from the surface of the cylinder
when water spray is interrupted, continue
spraying with water. Then check at hourly intervals
until it is seen that steaming has ceased.
4 Once steaming has stopped, observe from a safe
distance whether the surface of the cylinder
remains wetted. If patches dry quickly, continue
to cool with water and observe again after half an
hour. Repeat this operation until all surfaces
remain wetted after water spray is stopped. Pay
particular attention to the centre cylinders of manifold
cylinder pallets (MCPs) and to any cylinder
where some difficulty has been experienced in
maintaining a good supply of cooling water.
5 Once all cylinder surfaces remain wetted after
the water spray is discontinued, check using the
bare hand that the cylinder remains cold for
30 minutes. Wait a further 30 minutes and
check again: if any part of the cylinder feels
warm to the touch, reapply the cooling water
for 30 minutes and repeat the procedure until
cylinders remain cold for one hour.
6 When you are satisfied that the entire surface
has remained cold for one hour, submerge the
cylinder in water carefully, avoiding shocks and
bumps. Normally after 12 hours immersion the
cylinder will be safe to be disposed of by the
gas supplier.
270Repair of Vehicle Bodies
Figure 9.26Acetylene cylinders in fires (BOC Ltd )
Ventilation and oxygen-enriched areas
Whenever cylinder gases are used, constant and
thorough ventilation should be maintained. This
is particularly important when they are used in
confined spaces.
The normal content of air is 21 per cent. If this
becomes enriched to 25 per cent there is an
increase in the speed at which materials will burn.
At 30 per cent the typical characteristics of an
oxygen-fed fire are apparent. The fire is in two
phases: an initial flash fire, followed by local
burning at a number of points. Fires in oxygenenriched
atmospheres are very difficult to extinguish,
and can spread rapidly across combustible
Gas welding, gas cutting and plasma arc cutting 271
materials from a single point source such as a
spark from a cigarette.
Nitrogen, argon and carbon dioxide, if allowed to
replace the oxygen in the atmosphere, can cause
asphyxiation. The dangers occur typically when gas
is released in a confined space. It is especially important
to beware of argon and carbon dioxide; both are
heavier than air, and will displace air in confined
spaces or spaces with no ventilation at floor level.
General equipment safety
All equipment should be subjected to regular periodic
examination and overhaul. Failure to do so
may allow equipment to be used in a faulty state,
and may be dangerous.
Rubber hose Use only hose in good condition,
fitted with the special hose connections attached by
permanent ferrules. Do not expose the hose to heat,
traffic, slag, sparks from welding operations, or oil
or grease. Renew the hose as soon as it shows any
sign of damage.
Pressure regulators Always treat a regulator
carefully. Do not expose it to knocks, jars or sudden
pressure caused by rapid opening of the cylinder
valve. When shutting down, release the
pressure on the control spring after the pressure in
the hoses has been released. Never use a regulator
on any gas except that for which it was designed,
or for higher working pressures. Do not use regulators
with broken gauges.
Welding torch When lighting up and extinguishing
the welding torch, the manufacturer’s instructions
should always be followed. To clean the nozzle use
special nozzle cleaners, never a steel wire.
Fluxes Always use welding fluxes in a well ventilated
area.
Goggles These should be worn at all times during
welding, cutting or merely observing.
Protection Leather or fire-resistant clothing should
be worn for all heavy welding or cutting. The feet
should be protected from sparks, slag or cut material
falling on them.
Backfiring and flashbacks
Of welding torch
A welding torch is said to backfire when it goes
out with a loud pop, and then relights itself immediately,
providing the heat of the job is sufficient to
ignite the acetylene. Backfires result from defective
equipment, incorrect pressures or incorrect
lighting up; or careless use of the welding torch,
permitting the nozzle to touch the work, overheating
the nozzle tip, or working with a loose nozzle.
Usually the backfire is arrested at the mixer or
injector of the welding torch. If prompt action is
taken, turning off first the oxygen then the acetylene
valve, no damage occurs and the welding
torch may be relit provided that the cause of the
trouble has been eliminated.
Sometimes a backfire may pass beyond the injector
and travel back into either the oxygen or fuel
gas hoses. This is termed flashback, and its effect is
more serious as immediate damage to hoses and
regulators may result; there is also a risk of injury
to the operator. A flashback should be suspected if
there is a squealing or hissing noise, sparks coming
from the nozzle, heavy black smoke, or if the welding
torch gets hot. If the flame burns back far
enough it may burst through the hose.
With oxy-fuel gas equipment, flashbacks can
and do occur because the recommended pressures
and procedures have not been observed, and
because of nozzle blockage, faulty equipment or
leaking equipment. One of the main causes of
flashback is due to backfeeding, which occurs
when higher-pressure gas feeds back up a lowerpressure
stream. The presence of hose check valves
will prevent the oxygen and fuel gas mixing in the
hose and subsequently causing fire, injury and
damage.
A flashback arrester is a device designed to
quench the flashback. When it incorporates a cutoff
valve, this will automatically shut the gas flow.
These multifunction devices afford an additional
safeguard, particularly in locations where a fire following
flashback could not be tolerated, such as
garages, body shops and other workplaces with
flammable or hazardous materials.
9.14 Plasma arc cutting
Plasma cutting process
And equipment
The plasma cutting process relies on the fact that if a
gas or mixture of gases, such as air, is subjected to a
very high temperature it becomes ionized: negative
electrons are separated from the atom, which is then
272Repair of Vehicle Bodies
positively charged. This ionized state of the gas is
called ‘plasma’, and in this state the gas is electrically
conductive.
The high temperature necessary to create the
plasma is achieved, in the case of plasma cutting,
by a standing electric arc. This is constricted
by forcing the plasma through a small nozzle
which increases the temperature of the arc to over
24 000 °C and concentrates it on to a very small
area. When this plasma is directed at a conductive
material the arc is transferred through the plasma
(transferred arc operation) to the material. The
high energy of the arc melts the material which, as
long as it is within the cutting range of the equipment,
will be displaced by the gas flow.
In order to initiate the standing arc it is necessary
to produce an ionized path in the gases. This
is achieved either by applying a very high voltage
at a high frequency between the electrode and the
tip and work, causing a high-frequency spark, or
by momentarily touching the electrode and nozzle
together and then quickly breaking the contact,
causing an arc between the electrode and the nozzle.
As soon as the gas between the tip and the
nozzle is ionized, the main arc will ignite.
Plasma cutting and welding torches are designed
with a small orifice which constricts the arc. Gas
flows under pressure through the arc, and is subsequently
heated to an exceptionally high temperature.
The heated gas cannot expand owing to the
constriction of the nozzle, and it is forced out to
form a supersonic jet hotter than any known flame
or a conventional electric arc. The jet melts down
and patially vaporizes the base material, and the
force of the jet blasts away the molten metal. The
plasma jet is controlled by adjusting current, gas
velocity and type of gas.
The distance between electrode and workpiece is
made electrically conductive (ionization) by an
auxiliary arc (pilot arc) between electrode and
plasma nozzle. As the intensity of the high-voltage
impulses of the pulse generator is not sufficient,
part of the cutting current (limited by a resistor) is
admitted for the sufficient ionization of the pilot
arc distance. When the ionized gas jet contacts the
workpiece the main circuit is closed and the cutting
process is introduced (see Figure 9.27).
Clean, dry workshop air can be used when severing
thin sheet steel, but it is recommended to use
an argon/hydrogen mix or nitrogen when cutting
gauges in excess of 5 mm, and for stainless steels,
as these gases produce a higher thermal conductivity
and will transfer more heat to the material to be
cut. Reference to the manufacturers’ recommendations
are essential, as the correct cutting speed
must be maintained to ensure the quality of the cut.
The equipment can be grouped as follows:
High-output units (approximately 50 amperes)
operating on input voltages of 380/415 V three
phase, fitted with water-cooled torches and operating
with workshop air or special cutting gases
(see Figure 9.28).
Figure 9.27Simplified diagram of plasma arc cutter
(Motor Insurance Repair Research Centre)
Figure 9.28Plasma cutting unit: three phase,
40 amperes (Olympus Welding Supplies Ltd )
Gas welding, gas cutting and plasma arc cutting 273
There are two types of electrode available for the
plasma cutting torches. The first is a tungsten electrode
intended for cutting with nitrogen or an
argon/hydrogen mix. The second is an electrode
intended for cutting with air (see Figure 9.31).