Gas welding and cutting process is a common in many engineering factories. The accidents happening during these processes are very typical and involve number of injuries. Amongst the various types, the backfires and flashback are the more common. Backfire occurs in the torch itself and never reaches the fuel gas source. The backfire reaches only into the welding or cutting torch tip and it never travels beyond the torch tip.
But the flashback is the more dangerous type and in this case the flame clearly reaches the fuel gas supply source through cutting torch and hoses. The flame burns back into or even beyond the mixing chamber and may travel through hoses.
Backfire is the momentary retrogression or burning back of the torch flame, into the welding or cutting torch tip accompanied by a loud popping sound and also accompanied by the immediate reappearance or the complete extinguishments of the flame.
Flashback: – Flashback is the retrogression or burning back of the flame into or beyond the mixing chamber accompanied by a hissing or squealing sound and a characteristic smoky, sharp pointed flame. A flashback can moves as fast as 330 ft/second.
If backfire, as defined above occurs, and if the torch is not shut off immediately, and if the conditions causing the backfire are not corrected, then flashback occurs. Flashback is burning of gases within the supply system.
In case of flashback the flame action is having an explosive violence. The flame clearly flashes back into the torch and burns insides with a shrill hissing or squealing noise.
A flashback can occur in oxygen hose as well as in acetylene hose also. In case of an acetylene pipeline, the flash travels all the way back to water seal.
Precautions to be taken when backfire occurs:
• Close both the oxygen and acetylene station or cylinder valves immediately. The very important thing is that in case of flashback do not change setting of torch valves or regulator adjusting screws.
• Keep the equipment intact. Disconnect the outfit from valves and movie aside. Do not disassemble the outfit or alter setting or regulator adjusting screw.
• Do not crimp or squeeze hoses to stop flash back. The reason is that in case of flashback, the fire may be in the hose and it can explode in you hands.
Causes of Flashback-
• A flashback happens when the torch tip close or fuel line pressure drops. When this happens the reverse flow mixes oxygen and the fuel gas (acetylene) in the same line.
• Flashback occurs when oxygen and fuel gas mix in a supply line and are ignited by relighting a torch after the torch tip has clogged, without first cleaning the lines or by cylinders without first closing the regulators and cleaning the lines.
• Sometimes the return valve (check valve) is fouled by tiny particles of foreign material and in that case the return valve does not seal tight to stop reverse flow of gas which can carry flame along with it back from the torch.
• Flashback and subsequent explosion occurs when damaged tips are used.
• Flashback can occur if the cylinder valves are turned off before turning off the torch valves after the work is over.
• Flashback occurs when the torch valve is not turned off when the cylinder is empty.
• Sometimes flashback is due to an incorrect acetylene or oxygen pressure.
Causes of Backfire
• Backfire is caused due to holding the torch tip too close to the work or allowing hot slag to build up on tip.
• Backfire occurs when the torch is operated at lower pressures then required for the tip used for cutting purpose.
• Backfire occurs when the tip is over heated.
• Backfire occurs by an obstruction in the tip.
General causes of backfire and flashback
• Draining cylinders dawn to the working pressure of the tip in use.
• Improper lighting procedure.
• Excessive oxygen pressure.
• Insufficient acetylene supply.
• Sharp kinks in hoses.
• Pinched, cut or burn hoses.
• Dirt in hoses and torch.
• Loose connection.
• Failure to purge hoses and torch before trying to light the torch.
• Tips that have been cut back more than 3/8 inch or that have had the enlarged.
• Taped hoses or connections.
• Porous or leaky hoses.
• Defective regulators.
• Leaky torch valves or valves that cannot be closed tightly.
• Wrapped torch heads or nicked seats in head.
• Nicked or bruised tip seats.
• Broker or cracked hose-glands generally caused by the use of impact tools such as hammer and chisels.
• Loose or dirty tips.
Remedies against backfire/Flashback
• Never hold the burning tip too close to the work.
• Never operate the torch at pressures (of oxygen an acetylene) which are lower than the required pressure for the particular torch tip used. Never allow the tip too over heat.
• Turn of the torch valve when the cylinder is empty.
• Always try to stop the flashback it reaches the fuel source (acetylene cylinder).
• Never use damaged tips.
• Never allow hot slag (which is formed while cutting the metal) to build up an the torch tip.
• Always change the cylinders by first closing the regulators and clearing the lines.
• Always clean the tip of the torch before using the same for cutting purpose.
• Never turn off cylinders valves while torch valves are open.
Equipments which are used to guard against Flashback & Backfires.
Flashback arrester-as welding regulator-
This device prevents the explosions from flashback through regulators, cylinders and supplies. With this arrester installed downstream from the regulator and cylinder, pressure from the flashback activities a by-pass valve, shutting the flame away from the tank, through spiral tubing and extinguishing the flame.
This device stops the reverse flow of oxygen or fuel gas and prevents flashback from entering into the regulator or pipeline and at the same it stops the fuel gas or oxygen supply.
This device is installed between the torch and regulator.