The Art of Welding. William Galvery
maintains the constant pressure needed by the torch, even though the cylinder supply pressure drops greatly. For example, an oxygen cylinder may contain oxygen at 2,250 psi (155 bar) and the torch requires about 6 psi (0.4 bar) to operate. Similarly, a full acetylene tank may contain gas at 225 psi (15.5 bar) and the torch needs fuel gas at 6 psi (0.4 bar).
There are single-stage and two-stage regulators available. The two-stage regulator’s advantage is that a higher volume of gas may be withdrawn from the cylinder with less pressure fluctuation than produced by a single-stage regulator. The combination of two regulators working together in series maintains a very constant torch pressure over wide cylinder pressure changes. Its disadvantage is cost. They are only needed when large gas volumes are needed as with multiple stations or rosebud tips.
Figure 3-15 This is an oxygen tank regulator
Torches, Tips, and Hoses
Shown below is the most common oxyacetylene torch design. Other designs are available. Some have very small flames for jewelry and instrument work, while others take no accessories and are much lighter in weight than standard torch designs to reduce operator fatigue. Some torch handles can accommodate cutting heads.
Matching the size of the flame, which is controlled by the torch tip, and the resulting volume of gas to the thickness of the metal in the weld is important. Too much flame and the base metal around the weld may be damaged, too little and there is inadequate heat to melt metal for full penetration.
Figure 3-16 Tank hoses are sold in 25 and 50-foot lengths
Figure 3-17 Oxyacetylene torch handle
Figure 3-18 Cutting head attachment
Figure 3-19 Oxyacetylene torch and tip
Figure 3-20 Welding tip
Photos courtesy of Hobart Welders.
Torch Tip Sizes
There is no industry standard; each torch manufacturer has its own numbering system. Cross-reference tables compare each manufacturer’s tip sizes with numbered drill sizes.
The American Welding Society (AWS) has been urging tip manufacturers to stamp tips with the material thickness size to eliminate the confusion of tip size numbers. The AWS C4.5M Uniform Designation System for Oxy-Fuel Nozzles calls for tips to be stamped with the name of the manufacturer, a symbol to identify the fuel gas, the maximum material thickness, and a code or part number to reference the manufacturer’s operating data; many manufacturers are not in compliance. Most companies making welding tips do provide information booklets available to cross reference their tip sizes to tip drill sizes. See Table 3-2.
Table 3-2 Matching welding tip size to weld material thickness
Finding the Drill Size of a Tip
Using a tip cleaner find the round file which fits into the tip easily but snuggly then check the drill size of that file listed on the body of the tip cleaner cover.
TOOL TIP
Cleaning Tips
When sparks from the weld puddle deposit carbon inside the nozzle and on the tip face, they act as spark plugs and cause premature ignition of the gas mixture. Torch tips should be cleaned at the start of each day’s welding and whenever flashback occurs, the flame splits, or when the sharp inner cone no longer exists. To clean, select the largest torch tip cleaning wire file that fits easily into the nozzle and use the serrated portion to remove any foreign material. Be careful not to bend the tip cleaner file into the tip which can cause the cleaning file to break inside the tip; if the breaks inside the tip it is nearly impossible to remove. Also be sure not to enlarge the existing hole. Then touch up the face of the tip with a file or emery cloth to remove any adhering dirt. Use compressed air or oxygen to blow out the tip. Never use a twist drill to clean the tip; it will cause bell-mouthing.
Figure 3-22 Here is a collection of torch cleaning files.
Photo courtesy of Hobart Welders.
Flashback and Backfire
Flashback occurs when a mixture of fuel and oxygen burns inside the mixing chamber in the torch handle and reaches the hoses to the regulators or cylinders. Such burning in the hoses is extremely dangerous and will lead to serious injury. If either through operator horseplay (like turning on both the acetylene and the oxygen with the torch tip blocked) or through regulator failure, an explosive mixture of acetylene and oxygen is forced back toward the cylinders. When the torch is lit, this explosive mixture will go off. See Figure 3-23.
Figure 3-23 How flashback can occur
Flashback is easily prevented by installation of flashback arrestors consisting of both a check valve and a flame arrestor. These devices are about the diameter of the gas hoses and about 1 3/4 inches long. Some newer torch designs incorporate check valves and flashback arrestors into the torch handle itself. Some arrestors fit between the regulator and the hose. See Figure 3-24. The best arrestors include a thermally-activated, spring-loaded shut-off valve which closes on sensing a fire.
A backfire is a small explosion of the flame at the torch tip. The biggest hazard is that the detonation from the tip may blow molten weld metal five to ten feet from the weld and injure someone. Also, a series of repeated, sustained backfires, which can sound like a machine gun, may overheat the tip or torch, permanently damaging them.
Figure 3-23 Reverse-flow check valve flashback arrestor cross section
Figure 3-25 The flashback arrestor on the right is for an oxygen line; the one on the left is for acetylene.
Photo courtesy of Hobart Welders.
The most frequent cause of backfire is pre-ignition of the mixed acetylene and oxygen. Here are the most common causes of pre-ignition and their solutions:
•The mixed welding gases are flowing out through the tip more slowly than the flame front burns and the flame front ignites the gas in the tip and/or mixing chamber causing a pop. Solution: Slightly increase both the oxygen and acetylene pressures and if this results in too large a flame for the job, reduce the torch tip size.
•The tip may be overheated from being held too close to the weld or from working in a confined area like a corner. Solution: Let the tip cool off and try again holding the tip farther from the weld pool.
•Carbon deposits or metal particles inside the tip