Equipment for Oxyacetylene Welding
Essential equipment components
Torch
The basic oxyacetylene torch comprises:
- torch body (or handle)
- two separate gas tubes (through the handle connected to the hoses)
- separate control valves
- mixer chamber
- flame tube
- welding tip
NB The cutting torch requires two oxygen
supplies to the nozzle, one mixed with fuel gas for preheating and a separate
oxygen flow for cutting.
Hoses
Hoses are colour-coded red for acetylene and blue (UK) or green
(US) for oxygen. Oxygen fittings on the hose have a right-hand thread while
acetylene is left-handed.
Gas regulators
The primary function of a gas regulator is to control gas
pressure. It reduces the high pressure of the bottle-stored gas to the working
pressure of the torch, and this will be maintained during welding.
The regulator has two separate gauges: a high pressure gauge for gas in the
cylinder and a low pressure gauge for pressure of gas fed to the torch. The
amount of gas remaining in the cylinder can be judged from the high pressure
gauge. The regulator, which has a pressure adjusting screw, is used to control
gas flow rate to the torch by setting the outlet gas pressure. Note
Acetylene is supplied in cylinders under a pressure of about 15 bars psi
but welding is carried out with torch gas pressures typically up to 2 bars.
Flame traps
Flame traps (also called flashback arresters) must be fitted
into both oxygen and acetylene gas lines to prevent a flashback flame from
reaching the regulators. Non-return spring-loaded valves can be fitted in the
hoses to detect/stop reverse gas flow. Thus, the valves can be used to prevent
conditions leading to flashback, but should always be used in conjunction with
flashback arresters.
A flashback is where the flame burns in the torch body, accompanied by a
whistling sound. It will occur when flame speed exceeds gas flow rate and the
flame can pass back through the mixing chamber into the hoses. Most likely
causes are: incorrect gas pressures giving too low a gas velocity, hose leaks,
loose connections, or welder techniques which disturb gas flow.
Identification of gas cylinders
An oxygen cylinder is colour-coded black
and the acetylene cylinder is maroon. Oxygen and acetylene are stored in
cylinders at high pressure. Oxygen pressure can be as high as 230 bars.
Acetylene, which is dissolved in acetone contained in a porous material, is
stored at a much lower pressure, approximately 15 bars.
The appropriate regulator must be fitted to the cylinders to accommodate
cylinder pressures. To avoid confusion, oxygen cylinders and regulators have
right-hand threads and acetylene cylinders and regulators have left-hand ones.
Typical gas pressures and flow rates for C-Mn steel:
| Steel thickness (mm) |
Nozzle size |
Acetylene |
Oxygen |
| Pressure (bar) |
Consumption (l/min) |
Pressure (bar) |
Consumption (l/min) |
| 0.90 |
1 |
0.14 |
0.50 |
0.14 |
0.50 |
| 1.20 |
2 |
0.14 |
0.90 |
0.14 |
0.90 |
| 2.00 |
3 |
0.14 |
1.40 |
0.14 |
1.40 |
| 2.60 |
5 |
0.14 |
2.40 |
0.14 |
2.40 |
| 3.20 |
7 |
0.14 |
3.30 |
0.14 |
3.30 |
| 4.00 |
10 |
0.21 |
4.70 |
0.21 |
4.70 |
| 5.00 |
13 |
0.28 |
6.00 |
0.28 |
6.00 |
| 6.50 |
18 |
0.28 |
8.50 |
0.28 |
8.50 |
| 8.20 |
25 |
0.42 |
12.00 |
0.42 |
12.00 |
| 10.00 |
35 |
0.63 |
17.00 |
0.63 |
17.00 |
| 13.00 |
45 |
0.35 |
22.00 |
0.35 |
22.00 |
| 25.00 |
90 |
0.63 |
42.00 |
0.63 |
42.00 |
Selection of correct nozzles
Welding torches are generally rated
according to thickness of material to be welded. They range from light duty (for
sheet steel up to 2mm in thickness) to heavy duty (for steel plate greater than
25mm in thickness). Each torch can be fitted with a range of nozzles with a bore
diameter selected according to material thickness. Gas pressures are set to give
correct flow rate for nozzle bore diameter. Proportions of oxygen and acetylene
in the mixture can be adjusted to give a neutral, oxidising or carburising
flame. (See the description of oxyacetylene processes) Welding is normally
carried out using a neutral flame with equal quantities of oxygen and acetylene.
Equipment safety checks
Before commencing welding it is wise to inspect
the condition and operation of all equipment. As well as normal equipment and
workplace safety checks, there are specific procedures for oxyacetylene.
Operators should verify that:
- flashback arresters are present in each gas line
- hoses are the correct colour, with no sign of wear, as short as possible
and not taped together
- regulators are the correct type for the gas
- a bottle key is in each bottle (unless the bottle has an adjusting screw)
It is recommended that oxyacetylene equipment is checked at least
annually - regulators should be taken out of service after five years. Flashback
arresters should be checked regularly according to manufacturer's instructions
and, with specific designs, it may be necessary to replace if flashback has
occurred.
For more detailed information the following legislation and codes of practice
should be consulted:
- UK Health and Safety at Work Act 1974
- Pressure Systems and Transportable Gas Containers Regulations
- British Compressed Gases Association, Codes of Practice
- BOC Handbook
Copyright by TWI, 1999
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