Backfire is the momentary retrogression of the flame into the torch that is usually signaled by a popping sound. The flame may either extinguish or re-ignite at the end of the tip. Sustained backfire (flashback) is the retrogression of flame into the torch with continued burning inside the torch. This event can be identified by an initial popping sound followed by a squealing or hissing sound caused by continued burning inside the torch. When it occurs, ambulance the torch oxygen valve should be turned off immediately and then the fuel gas valve.
If backfire continues to occur, try the torch or the tips, or both, should be removed from service for cleaning or possible repair.
Flashback is the return of the flame through the torch and into the hose and/or regulator and can potentially cause an explosion at any point in the system. It may also reach the cylinder. This event is caused by oxygen and fuel mixing in one side of the oxy-fuel system and subsequently being ignited at the tip and by reverse flow of one gas into the other side of the system. When it occurs, the torch oxygen valve should be turned off immediately and then the fuel gas valve.
The mostly widely accepted manner to light the torch is to open the fuel gas valve slightly and light the gas with a spark lighter. Adjust the gas until a stable flame is maintained at the end of the tip. Open the oxygen preheat valve slowly and increase the flow until the desired flame is achieved.
The immediate work area must be clean from all contaminants. This includes removal of dirt, grease, oil, rust, paint, plastic coverings, etc., from the surface of the parts being welded. The method of cleaning depends on the material to be removed and the location of the work piece. For most construction and production equipment, steam cleaning is recommended. When this is not possible, solvent cleaning can be used. Blast cleaning with abrasives is also used. For small parts, pickling or solvent dip cleaning can be used and, finally, power tool cleaning with brushes, grinding wheels, disc grinding, etc., can be employed. The time spent cleaning a weld repair area will pay off in the long run. Be sure that all solvents have been removed before applying heat.
Before lighting tip, purge all hoses and torch passages by briefly opening and closing each torch valve. Vent gases safely. NOTE: mixed gases in the torch or the hose, etc. can cause a backfire or flashback. Purging must be done before each torch lighting and especially after a cylinder change.
Flame heating involves igniting fuel gases mixed with air or oxygen. Examples include flame straightening, flame hardening, hot forming and heating in conjunction with welding. The flame is used for melting purposes in flame brazing. Acetylene, propane, propylene, and natural gas are used as fuel gases for flame heating. The flame temperature and intensity depend on the fuel gases used and oxygen mixing ratio.
Always use the proper size tip for the job. Never use an over-sized tip and then cut the fuel and oxygen back to work on a small job. This may cause tip damage, overheating, backfire and flashback.
The preheat flame ports and the cutting oxygen orifice are sized for the thickness range of metal that the tip is designed to cut.
Cutting tips are precision-machined copper alloy parts of various designs and sizes. They are held in the cutting torch by a tip nut.
Cutting tips have metal-to-metal seating, so be sure to tighten the tip nut at 9lbs. of torque. Do not over tighten.
Acetylene cutting tips are usually one piece with drilled and swaged flame parts.
The torch cutting tip contains a number of preheat flame ports and a center passage for the cutting oxygen. The preheat flames are used to heat the metal to a temperature where the metal will react with the cutting oxygen.
When a piece is cut by an oxygen cutting process, a narrow width of metal is progressively removed. The width of the cut is called a Kerf. Kerf width is a result of the type of tip used, the tip size, the flow rate of oxygen and preheating gases and the speed of cutting.
For a plate thickness of ½” or more, the cutting tip should be held perpendicular to the plate. When cutting a thinner plate, the tip can be tilted in the direction of the cut. Tilting increases the cutting speed and helps prevent slag from freezing across the kerf.
A damaged seating surface on either the tip or the head can create a dangerous condition, resulting in a backfire or flashback. This may damage the cutting torch. If the seating surface of a tip becomes damaged, do not use it. Discard the damaged tip.
One piece tips are used with acetylene and are madder from copper alloy to withstand the heat of the cutting process. The copper alloy is machined, drilled and swaged over special wires to produce exact holes for preheats and cutting oxygen bores. Tolerances must be controlled carefully to produce stable flames.
Two-piece tips consist of an outer shell and a splined insert. The splined insert is used to eliminate the need to drill numerous preheat holes required for cooler and slower burning gases. Different spline configurations allow different fuel gases to burn more efficiently.
One-piece tips have oxygen bores that come in two configurations: a straight bore using 40 to 60 PSI for hand cutting and a divergent bore that has a tapered or flared outlet hole for use in machine cutting. The divergent-bore tips use higher outlet pressure of 70 to 100 PSI and allow for 25% increase in cutting speed over straight-bore tips.
The six outer jets of a cutting tip are for oxygen and acetylene (Preheat Flames) and the central jet carries only oxygen. The preheat flames are not intended to melt the metal, but to bring it to its ignition temperature.
Acetylene cutting tips are usually manufactures with four or six preheat holes and produced to allow light, medium and heavy preheats for use with clean, dirty, or rusted plate. They are solid coppery, one piece. Rivet washing tips use a low-velocity cutting oxygen stream to blow rivets through a plate without damaging it. Tips also are available for gouging out welds, cutting sheet metal, and other specialized uses. One-piece tips can also be used with methylacetylene-propadiene (MAPP®) and propylene. They have eight preheat holes to provide the extra heat required for these gases. However, two-piece tips generally provide the best results with MAPP® gas and propylene.
To start a cut on an edge, place the preheat flames halfway over the edge, holding the end of the flame cones 1/8” above the surface of the material to be cut. When the top corner reaches a reddish yellow, the cutting oxygen valve is opened and the cutting process begins.
It is often necessary to begin a cut at some point other than on the edge of a piece of metal. This technique is known as piercing. Piercing requires a larger preheat flame than the one used for an edge start. The tip should be angled and lifted up as the cutting oxygen valve is opened. The torch is held stationary until the cutting jet pierces through the plate.
Rivet washing tips use a low-velocity cutting oxygen stream to blow rivets through a plate without damaging it. Burn the rivet heads off by placing the tip perpendicular to the rivet and not by trying to cut the rivet head off from the side as you will likely score and damage the base steel material. After removal of the rivet head, the rivet shank needs to be removed by mechanical means only.
The oxygen fuel gas-cutting torch can be used for gouging. Special gouging tips are available and they should be selected based on the particular geometry of joint preparation. It is possible, by closely watching the cut surface, to find and follow cracks during the flame gouging operation. The edges of the cracks will show since they become slightly hotter.
Rivets can be easily removed by "washing" one head off with an acetylene torch. Next, a hole is burned along the rivet longitudinal axis. Then, while still hot, the rivet can be driven out with a pin punch and a two-pound hammer. This process completely removes the rivet and leaves a clean hole in original condition.
Oxy-fuel gas cutting machines are either portable or stationary. Portable cutting machines are primarily used for straight line cutting. They can carry one or more heavy-duty machine torches.
When oxy-fuel cutting, the metal is heated until it glows orange~ (1800°F= 982°C), and then a lever on the torch is pressed to pass a stream of oxygen through the work-piece to burn the steel away where the cut is desired. The iron-oxide product of this combustion process falls to the floor as dust. Once the process is started properly, there should be no globs of melted steel under the work-piece. No melting should occur.
Cutting is initiated by heating the edge of the steel to near melting point using the pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen from the central jet. The steel is instantly oxidized into molten iron oxide, producing the cut. Robotic oxy-fuel cutters sometimes use a high-speed divergent nozzle. This uses an oxygen jet that opens slightly along its passage. This allows the compressed oxygen to expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need very accurate positioning above the work. Their ability to produce almost any shape from large steel plates gives them a secure future in shipbuilding and in many other industries.
The ideal cut is a narrow gap with a sharp edge on either side of the work piece. Overheating the work piece and thus melting through it, causes a rounded edge.
Robotic oxy-fuel cutters sometimes use a high speed divergent nozzle. This uses an oxygen jet that opens slightly along its passage. This allows the compressed oxygen to expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need very accurate positioning above the work.
Beveling allows a full-penetration weld (if required). It gives the torch tip access to deep-lying base metal, prepares a well-defined, uniform container for molten metal and ensures quality welds. Bevel angles and root openings are essential for adequate weld penetration in thick plates or pipes. The joint design must specify all relevant dimensions and angles. the joint design and root opening specification are both part of the weld procedure.
FUEL / GAS
For proper combustion during cutting, propane (C3H3) requires 4 to 4-1/2 times its volume of preheat oxygen.
Oxygen used for cutting should have a purity of 99.5% or higher. Low purity reduces the efficiency of the cutting operation.
Natural gas produces a lower flame temperature and a lower heating efficiency. Specialty tips designed to provide a heavy preheat flame should be considered.
One volume of propylene requires 3.6 volumes of torch supplied oxygen for maximum flame temperature.
The advantages of oxy-fuel gas cutting include: Equipment is very portable and can be used in the field. Cutting direction can be changed rapidly on a small radius during operation. Large plates can be cut rapidly in place by moving the torch rather than the plate. Equipment costs are lower than machine tools.
Propane is used regularly for oxygen cutting in a number of plants because of its availability and its much higher total heat value (MJ/m³) than natural gas. For proper combustion during cutting, propane requires 4 to 4-1/2 times its volume of preheat oxygen. This requirement is offset somewhat by its higher heat value. It is stored in liquid form and easily transported to the work site.
In oxyacetylene gouging, equal quantities of oxygen and acetylene are used to set a near-neutral preheating flame. When the preheating flame and oxygen jet are correctly set, the gouge has a uniform profile and its surfaces are smooth.