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Safety critical when using oxyacetylene process |
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 Commonly used for welding, cutting, brazing and heating, the oxyacetylene process is still one of the best techniques available for many applications.
However, acetylene is an unstable hydrocarbon that must be handled with
care. Consequently, it is very important to respect all safety measures
and applicable regulations when using, transporting or storing this
gas.
Respecting the correct rules-of-use and the safety measures that exist
in each country and province, and using the equipment safely, depends
on the worker’s knowledge, ability and professional training. Since the
apparatus is now easier-to-use and safer than ever, because of major
innovations introduced by gas equipment manufacturers, the oxyacetylene
flame (oxygen and acetylene combined) remains one of the most
convenient methods for welding and cutting steel. Advantages include
low cost and portability compared to other techniques, particularly
electrical processes.
The use of oxygen and acetylene gases presents a number of potential
hazards: high pressure, flammability, asphyxiation and oxygen
enrichment. Amongst possible occurrences, it sometimes happens that the
flame momentarily burns back into the torch tip and makes a loud
popping sound. This is called a backfire. Occasionally the flame
relights by itself and the worker can continue working, but at other
times the flame does not relight and torch valves should be shut off
immediately. A backfire is normally caused by using the wrong tip
pressures, by overheating the torch or by touching the tip to the
workpiece.
A persistent high-pitched squealing sound, with sparks issuing from the
tip, indicates a more serious backfire that can create an explosive
situation. The flame has entered the tip and continues to burn inside
the torch at the gas mixing point. It is extremely important to shut
the oxygen valve off first, as quickly as possible, then to shut the
fuel gas valve, to prevent damage to the torch. Before attempting to
relight, make sure the torch has cooled and verify the complete set-up.
An improper working pressure or defective equipment can be the cause of
sustained backfire.
A flashback can occur when lighting the torch if a mixture of oxygen
and fuel gas is present in the system. In this case, the flame burns so
fast that the user has no time to shut the torch valves before it
passes through the torch and into the hose. The most feared
consequences of a flashback are either the bursting of hoses or that
the burning progresses back to the regulators and cylinders. To avoid
flashbacks it is mandatory to always purge the hose lines, one at a
time, to make sure there is no mixed gas in the hoses before lighting
the flame.
Check valves and flashback arrestors should be installed on the
appropriate equipment. A check valve will help prevent the formation of
an explosive gas mixture and a flashback arrestor will stop the flame
if a flashback does occur. Different provinces in Canada have different
regulations specifying where these safety devices shall be located.
Some require installation at the regulator outlet, while others require
them at the torch inlet. Be sure to check the local requirement.
In order to meet the safety challenges posed by the oxyacetylene
process, Air Liquide has developed Altop and Minitop oxygen and
acetylene cylinders.
Traditionally, regulators have been separate devices that are installed
onto the cylinder valve outlet. They are often found in bad condition
that can lead to accidents. Air Liquide has developed a new type of
regulator that is integrated with the cylinder valve. Designated Altop
for large size cylinders and Minitop for small cylinders, these
regulator/valve combinations come with the cylinder, forming a complete
package. Each time an Altop or Minitop cylinder is returned for refill
it is verified at the filling plant. No longer do users need to provide
and maintain their own regulators. Of course, all the important safety
rules and instructions still apply when it comes to operating torches,
tips, and accessory devices with Altop and Minitop cylinders.
Altop and Minitop cylinders make transportation between jobs much safer
and more convenient. Their designs incorporate built-in protective caps
that remain permanently in place. Valve protective caps are mandatory
when transporting cylinders to prevent any damage to the valve. A major
leak from a damaged valve could turn a compressed gas cylinder into a
rocket. Just try to imagine an oxygen cylinder with an inside pressure
of over 2200 lb/in2. As a result, with standard cylinders the user must
remove the regulators from the cylinders to install the safety caps
before moving them. This takes time and increases the risk of
contaminating or damaging the regulator while it is not installed on
the cylinder.
Other unique features of Altop and Minitop make them safer and easier
to use. For example, both have a built-in gauge to tell the user how
much gas remains in the cylinder. Altop also has a built-in delivery
pressure gauge, while Minitop has a calibrated delivery
pressure-adjusting knob. To control the flow of gas there is an ON-OFF
lever that makes it easy to shut down your torch for a few minutes
without losing your pressure settings.
This article was written by Tom Dixon, program development manager, flame and cutting processes, Air Liquide Canada Inc.
airliquide.com
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