•When flammable materials cannot be moved, use sheet metal shields or guards to keep the sparks away from burnables.
•Prevent sparks from falling into holes or cracks in wooden buildings.
•Do not use tarps or fabric covers to protect other materials from sparks as they will catch fire.
•If cutting on a wooden floor, first sweep it clean, then wet it down before starting cutting.
•Avoid using excessive oxygen pressure while cutting as this will propel sparks farther and make more of them.
•Plan ahead where hot metal will fall when cut; be especially careful to avoid your legs, feet, gas hoses, cylinders, and regulators.
•Have fire extinguishers, buckets of sand, or water on hand should a fire start.
•Make sure jacketed or hollow parts are vented before beginning cutting operations.
What is best way to cut into a sealed tank or container?
Never cut into a sealed container regardless of its size. Even if the container is clean and empty, penetration of the shell could release hot gases or send the cutting flame back toward the welder. If the container is empty and contains no residual vapors, vent it to atmosphere by opening a valve, hatch, bung or drilling a hole, then proceed to cut or weld. See Figure 2–30.
Figure 2–30How to cut into a clean container
How can cutting or welding be done on a tank or container which has contained flammable materials?
An even more dangerous situation results when the vessel contains residual flammable vapors, whether it is vented to atmosphere or not. This will almost certainly result in an explosion. Flood the vessel with water to just below the cutting or welding point. Get the container cleaned usually by boiling with a caustic if the container is small or purged with a non-flammable gas like nitrogen, carbon dioxide or steam. Have the vessel checked for lack of explosive vapors by a qualified person. Then begin cutting or welding. See Figure 2–31.
Figure 2–31Using nitrogen or carbon dioxide to purge oxygen from a container which has held flammable materials
Plasma Arc Cutting
How does plasma arc cutting work and what are its applications?
Plasma arc cutting, AWS designation PAC, is an arc cutting process that uses a constricted arc and removes molten metal with a high-velocity jet of ionized gas issuing from a constricting orifice. There are two variations:
•The first variations are in low-current plasma systems which use the nitrogen in compressed air for the plasma and are usually manual.
•The second variations are in high-current plasma systems which use pure nitrogen for the plasma and are usually automatic.
The PAC torch works very much like the plasma arc welding torch performing keyhole welding, except that the keyhole is not allowed to close. Plasma heat input is very high and melts the work metal. Then the plasma jet blows away the molten material completing the cut. Some PAC systems inject water into the plasma to reduce fumes and smoke; others perform the cutting under water to reduce noise and airborne metal vapor.
What are the capabilities of PAC?
High-current PAC systems cut
Figure 2–32PAC schematic.
What are PAC’s advantages?
•PAC cuts all metals.
•Cutting action is so rapid that despite the high heat input, there is a smaller heat affected zone than in most other processes.
•PAC can pierce metals cleanly without the starting hole needed by OAC.
•PAC is ideal for cutting parts under computer-driven control.
•With its 30,000°F (16,600°C) plasma, it cuts materials with melting points too high for OAC.
•All positions can be used.
•Surface smoothness of the cut edges is equal to or better than OAC.
What are the drawbacks to PAC?
•Equipment may be expensive. Small units today are more cost effective than in the past.
•Metal vapor produced from the cutting must be captured.
•Thick cuts are normally done under water so the metal vapor can be captured; the water container must then be periodically cleaned usually requiring a HAZ MAT crew.
What safety rules should be followed?
All of the safety rules suggested in OAC cutting should be applied to PAC including:
•This process used electricity with voltage ranges from 150 to 400 volts of direct current; this equipment must be properly grounded to avoid electric shock.
•Keep electrical circuits dry.
•Keep all mechanical electrical connection tight; this includes the work lead. Poor electrical connection can cause over heating and fires.
•Proper ventilation is required to prevent inhalation of hazardous metal vapors and gases.
•When securing the equipment always be sure the power supply has been properly shut down and the torch placed in back on it’s proper insulated storage position.
Laser Beam Cutting
How does laser beam cutting work and what are its applications?
Laser beam cutting, AWS designation LBC, is a thermal process using laser beam energy to cut materials by melting or vaporizing. A gas is sometimes used to assist in the removal of melted or vaporized material. Cutting and drilling power densities in the range of 6.5 × 106 to 6.5 × 108 W/in2 (104 to 106 W/mm2) are achieved. Lasers can also drill holes using higher power densities and shorter dwell times than when cutting. Hole dimensions range from 0.0001 to 0.060 inches (0.0025 to 1.5 mm). Although a high-power CO2 laser cuts carbon steel up to 1 inch thick (25 mm), good quality cuts are made on material
