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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 act like spark plugs prematurely igniting the mixed gases. Solution: Let the tip cool, then clean it thoroughly with your tip cleaning kit.
How do you select filler metal (welding rod)?
Usually the filler metal is a close match to the base metal. Sometimes the filler metal will have deoxidizers added which will improve the weldmore than just a base metal match. Rod diameters vary from 1/16 to 3/8 inch diameter. The prefix R in the description of the oxy-acetylene welding wire means rod which is followed by two or three numbers designating the ultimate tensile strength of the as welded filler material in thousands of pounds per square inch (ksi). See Table 3-3.
Table 3-3 Oxyacetylene steel welding rods
Procedures and Material Selections
Before welding, remove all surface dirt, scale, oxide, grease, and paint. Refer to Table 3-4 for suggestions on welding common metals. See Figure 3-25.
Figure 3-25 Before welding, clean metal thoroughly to remove grime and mill scale.
Table 3-4 Information for welding various metals
SR = Slightly Reducing SO = Slightly Oxidizing N = Neutral
Figure 3-26 Be sure you have the right type flame for the job at hand. For additional information on torch flames see pages 55 and 56.
Here are the steps for beginning the OAW process.
•Put on your welding safety equipment: tinted safety goggles (or tinted face shield), cotton or wool shirt and pants, high-top shoes, and welding gloves at a minimum.
•Make sure the valves on previously used or empty cylinders are fully closed and their valve protection covers are securely screwed in place. Then remove the empty cylinders from the work area and secure them against tipping during the wait for a refill shipment. Secure the newly replaced or full cylinders to a welding cart, building column, or other solid anchor to prevent the cylinders from tipping over during storage or use.
•Momentarily open each cylinder valve to the atmosphere and reclose the valve quickly purging the valve; this is known as cracking a valve. Cracking serves to blow out dust and grit from the valve port and to prevent debris from entering the regulators and torch.
•With a clean, oil-free cloth, wipe off the cylinder valve-to-regulator fittings on both cylinders to remove dirt and grit from the fittings’ connection faces and from the fittings’ threads. Do the same to both regulators’ threads and faces. Remember, never use oil on high-pressure gas fittings. Oxygen at high pressures can accelerate combustion of oil into an explosion.
•Make sure reverse-flow check valves are installed on the torch or the regulators.
•Check to see that both the oxygen and acetylene regulator pressure adjustment screws are unscrewed, followed by threading each regulator to its respective cylinder. Snug up the connections with a wrench. Caution: Oxygen cylinder-to-regulator threads are right-handed; so are oxygen hose to-torch screw fittings. Acetylene cylinder-to-regulator fittings and acetylene hose-to-torch fittings threads are left-handed.
•Stand so the cylinders are between you and the regulators, S-L-O-W-L-Y open the oxygen cylinder valves. Open the oxygen cylinder valve until it hits the upper valve stop and will turn no further. Also standing so the cylinders are between you and the regulator, open the acetylene cylinder valve gradually and not more than 1 1/2 turns. If there is an old-style removable wrench on the acetylene cylinder, keep it on the valve in case you must close it in an emergency. Standing so that the cylinders are between you and the regulators offers some protection should the regulator fail and the housing and gauges explode.
Figure 3-27 With the tank valves closed, back out the pressure adjusting screws on the regulators. When opening the tank valves, stand to the side or behind the tank in case the regulator malfunctions and explodes.
•Look at the high-pressure—cylinder side—pressure gauges to indicate about 225 psi (15.5 bar) in the acetylene cylinder and 2,250 psi (155 bar) in the oxygen cylinder. Note: 1 bar = 1 atmosphere = 14.5 psi = 0.1 MPa. Cylinder pressures vary with ambient temperature. The pressures given above are for full cylinders at 70°F (21 °C).
•Purge each torch hose of air separately: Open the oxygen valve on the torch about three-quarters of a turn, then screw in the pressure control screw on the oxygen regulator to your initial pressure setting—about 6 psi (0.4 bar). After several seconds, close the torch valve. Do the same for the acetylene hose. Comment: We do this for two reasons, (1) to make sure we are lighting the torch on just oxygen and acetylene, not air, and (2) to set the regulators for the correct pressure while the gas is flowing through them.
Caution: never adjust the acetylene regulator pressure above 15 psi (1 bar) as an explosive disassociation of the acetylene could occur.
•Recheck the low-pressure gauge pressures to make sure the working pressures are not rising. If the working pressure rises, it means the regulator is leaking. Immediately shut down the cylinders at the cylinder valves as continued leaking could lead to a regulator diaphragm rupture and a
