6. The system is now ready for leak testing as shown in Fig. 1.39 .
1.17.3 Fire hazards
To reduce the risk of fi re and explosions:
■ Do not position gas cylinders and hoses where sparks can fall on them.
■ Avoid the use of wooden structures and other combustible materials in the vicin- ity of welding operations.
Table 1.1 Colour coding for gas cylinders (based on BS 349)
If a cylinder has no neck band its contents are neither poisonous nor fl ammable.
If a cylinder has a red neck band or it is wholly coloured red or maroon its contents are fl ammable.
If a cylinder has a yellow neck band its contents are poisonous.
If a cylinder has both a red and a yellow neck band its contents are both fl ammable and poisonous.
Full details can be found in BS 349 Identifi cation of the Contents of Industrial Gas Containers .
Whilst on site, the cylinders should be kept clean so that they can be easily identifi ed. On no account must the colour of any cylinder be changed by any person either on site or in the workshop. Acetylene cylinders must never be stacked horizontally. Here are some examples of colour coding.
Gas Ground colour of cylinder Colour of neck bands
Acetylene Maroon None
Air French grey None
Ammonia Black Signal red and golden yellow
Argon Peacock blue None
Carbon monoxide Signal red * Golden yellow
Helium Middle brown None
Hydrogen Signal red * None
Methane Signal red * None
Nitrogen French grey Black
Oxygen Black None
* The name of the contents is also painted clearly on the cylinder in a contrasting colour.
■ Keep fi re-fi ghting equipment ready to hand. A responsible person should keep the site under observation for at least half an hour after completion of the work in order to watch for, and deal with, any out beak of fi re. There is always the risk of combustible material smouldering for hours before a fi re breaks out.
1.17.4 Explosion risks
The heat required for gas welding and cutting operations is produced by the com- bustion of a suitable fuel gas with oxygen. Usually this is oxygen mixed with acety- lene as this give the hottest fl ame (see Table 1.2 ). However, other fuel gases may be used for reasons of convenience and economy where a lower temperature will suf- fi ce as when brazing and fl ame cutting.
Explosions can occur when acetylene gas is present in air in any proportions between 2% and 82%. You do not need an oxygen-rich atmosphere to cause an explosion. Acetylene will also explode when under unduly high pressure even when air or oxygen is not present. The working pressure for acetylene should not exceed 0.62 bars. This is why acetylene is dissolved in acetone and absorbed into porous concrete in acetylene cylinders.
Another potential cause of explosions in any oxy-fuel gas welding equipment is a fl ash-back. This generally occurs because of faulty equipment or incorrect usage, mainly when lighting up the torch. Always light up with fuel gas only and gradu- ally increase the fl ow of oxygen until the correct fl ame conditions are attained.
Search for leaks at connections
SOAP
&
WATER
Soap and water solution applied by brush is a good indicator
Escaping gases can be detected by sound or feel, or smell in the cases of ACETYLENE
Places where leaks may occur
Figure 1.39 Testing for leaks
Other causes of fl ash-backs are due to dipping the nozzle tip into the weld pool, or putting the nozzle tip against the work so as to prevent the free fl ow of gas mixture.
Manufacturers ’ literature should be consulted before using gas welding equipment.
Flash-backs can be prevented by the use of ‘ hose protectors ’ as shown in Fig. 1.40 . Note that all items of oxygen equipment have right-hand threads and that all items of acetylene equipment have left-hand threads to prevent the possibility of incorrect connection.
Copper tube or fi ttings made from copper must never be used with acetylene.
All valves and fi ttings must contain no more than 70% copper. This is because pure copper reacts with acetylene to form a highly explosive compound called copper acetylide which can be detonated by heat or abrasion. The only exception to this rule is the welding or cutting nozzle itself. This is to prevent overheating.
Explosions can also occur as a result of dust, grit, oil or grease getting into the socket of the cylinder. The outlet sockets of cylinder valves should be examined for cleanliness before fi tting the regulators. Not only should the sockets be carefully cleaned, but the cylinder sockets should be blown through by momentarily opening
Table 1.2 Welding gas mixtures
Fuel gas Maximum fl ame temperature with air (°C) with oxygen (°C)
Acetylene 1755 3200
Butane 1750 2730
Coal gas 1600 2000
Hydrogen * 1700 2300
Propane 1750 2500
* The oxy-hydrogen fl ame has an important application in under- water cutting processes.
Figure 1.40 Hose protector
the cylinder valves before fi tting the regulators. This is known as cracking the valves orcracking the cylinders as shown in Fig. 1.41 .
1.17.5 Protective equipment (goggles)
Goggles must be worn to protect the welder’s eyes from heat, glare and particles of hot metal. These should be fi tted with an approved fi lter lens complying with BSEN 169 and the grade of fi lter selected should be suitable for the size of the welding fl ame and the thickness of the metal being welded (see Table 1.3 ). Figure 1.42 shows typi- cal eye protection suitable for gas welding. Note that the lenses of welding goggles are always of composite construction. The front lens is made of clear shatter-proof glass for safety reasons and ease of replacement if damaged. Behind the shatter-proof glass is the tinted lens which can be changed in accordance with the recommenda- tions found in Table 1.3 . This tinted lens is softer, more easily scratched and more expensive than the clear glass that protects it.
1.17.6 Protective equipment (overalls)
Normal workshop overalls are usually made from cotton/polyester mixtures that are not fl ame resistant materials. However, overalls worn when welding should be made from a fl ame resistant cloth and should be kept buttoned or zipped up to the neck so that particles of hot metal and slag cannot lodge inside them. Sleeves and trousers should not have cuffs which can trap globules of hot metal and hot slag. In addition, fl ame resistant head gear and a chrome leather apron should be worn together with Figure 1.41 Cracking the cylinder
Always leave cylinder keys in the valves when working so that they may be quickly turned off in any emergency
Use correct key to avoid unnecessary strain on the valve Blow out the cylinder valve socket before
connecting the regulator, ‘cracking open’
the cyliner valve momentarily
OFF
ON
leather gloves. Metal tongs should be used for holding or moving hot metal. Chrome leather gauntlet gloves and leather spats and a safety helmet should also be worn when welding overhead or on vertical surfaces. This full protective clothing should always be worn when fl ame cutting.
Table 1.3 Filters for gas welding and cutting
Grade of fi lter required Recommended for use when welding Welding
without fl ux
Welding with fl ux
3/GW – Thin sheet steel
– 3/GWF Aluminium, magnesium and aluminium alloys. Lead burning, oxyacetylene cutting
4/GW – Zinc-base die castings. Silver soldering. Braze welding light gauge copper pipes and light gauge steel sheet – 4/GWF Oxygen machine cutting – medium sections. Hand cutting,
fl ame gouging and fl ame descaling 5/GW – Small steel fabrications. Hard surfacing
– 5/GWF Copper and copper alloys. Nickel and nickel alloys. Heavier sections of aluminium and aluminium alloys. Braze welding of un-preheated heavy gauge steel and cast iron
5/GW – Heavy steel sections. Preheated cast iron and cast steel.
Building up and reclaiming large areas
– 6/GWF Braze welding of preheated cast iron and cast steel
Figure 1.42 The essential features of good quality welding goggles
1.18 Arc-welding equipment (mains operated)
This equipment is designed to change the high-voltage alternating current mains supply into safe, low-voltage, heavy current supply suitable for arc welding. Figure 1.43 shows some examples of some typical arc welding sets.
It can be seen that the output of arc-welding sets can have an alternating current (a.c.) wave-form or be a direct current (d.c.) that fl ows continuously in one direc- tion. The output potential should be between 50 volts and 100 volts for safety but the current may be as high as 500 amperes. Figure 1.44 shows a typical alternating current arc-welding circuit. It can be seen that, in addition to the transformer which reduces the voltage, there is a tapped choke to control the current fl ow to suit the gauge of electrode used.
1.18.1 Hazards associated with mains-operated arc-welding equipment
The hazards associated with mains-operated arc-welding equipment are listed in Table 1.4 which should be read in conjunction with the schematic circuit diagram Figure 1.43 Mains-operated arc welding equipment. (a) Single operator mobile a.c. welding set; (b) motor generator d.c. welding set; (c) transformer-rectifi er, d.c. welding set
shown in Fig. 1.45 . The circuit divides into two parts:
1. The primary (mains-voltage) circuit which should be installed and maintained by