Published on March 5, 2014
Oxyacetylene Welding (OAW) • by WAQAS AHMED
Oxyacetylene Welding (OAW) The oxyacetylene welding process uses a combination of oxygen and acetylene gas to provide a high temperature flame.
Oxyacetylene Welding (OAW) • OAW is a manual process in which the welder must personally control the the torch movement and filler rod application • The term oxyfuel gas welding outfit refers to all the equipment needed to weld. • Cylinders contain oxygen and acetylene gas at extremely high pressure.
Typical Oxyacetylene Welding (OAW) Station
Oxygen Cylinders • Oxygen is stored within cylinders of various sizes and pressures ranging from 20002640 PSI. (Pounds Per square inch) • Oxygen cylinders are forged from solid armor plate steel. No part of the cylinder may be less than 1/4” thick. • Cylinders are then tested to over 3,300 PSI using a (NDE) hydrostatic pressure test.
Oxygen Cylinders • Cylinders are regularly re-tested using hydrostatic (NDE) while in service • Cylinders are regularly chemically cleaned and annealed to relieve “jobsite” stresses created by handling .
Oxygen Cylinders • Oxygen cylinders incorporate a thin metal “pressure safety disk” made from stainless steel and are designed to rupture prior to the cylinder becoming damaged by pressure. • The cylinder valve should always be handled carefully
Pressure Regulators for Cylinders • Reduce high storage cylinder pressure to lower working pressure. • Most regulators have a gauge for cylinder pressure and working pressure.
Pressure Regulators for Cylinders • Regulators are shut off when the adjusting screw is turn out completely. • Regulators maintain a constant torch pressure although cylinder pressure may vary • Regulator diaphragms are made of stainless steel
Pressure Regulators Gauges Using a “Bourdon” movement • Gas entering the gauge fills a Bourdon tube • As pressure in the semicircular end increases it causes the free end of the tube to move outward. • This movement is transmitted through to a curved rack which engages a pinion gear on the pointer shaft ultimately showing pressure.
Regulator Hoses • Hoses are are fabricated from rubber • Oxygen hoses are green in color and have right hand thread. • Acetylene hoses are red in color with left hand thread. • Left hand threads can be identified by a grove in the body of the nut and it may have “ACET” stamped on it
Check Valves & Flashback Arrestors • Check valves allow gas flow in one direction only • Flashback arrestors are designed to eliminate the possibility of an explosion at the cylinder. • Combination Check/ Flashback Valves can be placed at the torch or regulator.
Acetylene Gas • Virtually all the acetylene distributed for welding and cutting use is created by allowing calcium carbide (a man made product) to react with water. • The nice thing about the calcium carbide method of producing acetylene is that it can be done on almost any scale desired. Placed in tightly-sealed cans, calcium carbide keeps indefinitely. For years, miners’ lamps produced acetylene by adding water, a drop at a time, to lumps of carbide. • Before acetylene in cylinders became available in almost every community of appreciable size produced their own gas from calcium carbide.
Acetylene Cylinders • Acetylene is stored in cylinders specially designed for this purpose only. • Acetylene is extremely unstable in its pure form at pressure above 15 PSI (Pounds per Square Inch) • Acetone is also present within the cylinder to stabilize the acetylene. • Acetylene cylinders should always be stored in the upright position to prevent the acetone form escaping thus causing the acetylene to become unstable.
Acetylene Cylinders • Cylinders are filled with a very porous substance “monolithic filler” to help prevent large pockets of pure acetylene form forming • Cylinders have safety (Fuse) plugs in the top and bottom designed to melt at 212° F (100 °C)
Acetylene Valves • Acetylene cylinder shut off valves should only be opened 1/4 to 1/2 turn • This will allow the cylinder to be closed quickly in case of fire. • Cylinder valve wrenches should be left in place on cylinders that do not have a hand wheel.
Oxygen and Acetylene Regulator Pressure Settings • Regulator pressure may vary with different torch styles and tip sizes. • PSI (pounds per square inch) is sometimes shown as PSIG (pounds per square inch -gauge) • Common gauge settings for cutting – 1/4” material Oxy 30-35psi Acet 3-9 psi – 1/2” material Oxy 55-85psi Acet 6-12 psi – 1” material Oxy 110-160psi Acet 7-15 psi • Check the torch manufactures data for optimum pressure settings
Types of Flame • There are three distinct types of oxy-acetylene flames, usually termed: – Neutral – Carburizing (or “excess acetylene”) – Oxidizing (or “excess oxygen” ) • The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.
FLAME Settings • Oxygen is turned on, flame immediately changes into a long white inner area (Feather) surrounded by a transparent blue envelope is called Carburizing flame (30000c) • Addition of little more oxygen give a bright whitish cone surrounded by the transparent blue envelope is called Neutral flame (It has a balance of fuel gas and oxygen) (32000c) • Used for welding steels, aluminium, copper and cast iron • If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter and more fierce is called Oxidizing flame • Has the highest temperature about 34000c • Used for welding brass and brazing operation
Pure Acetylene and Carburizing Flame profiles
Neutral and Oxidizing Flame Profiles
Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing flame.
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