Oxyacetylene welding is a fusion-welding process. The coalescence of metals is produced by an oxygen-acetylene flame; Extreme heat is concentrated on the edges or on the edge and surface of the piece of metal being joined until the molten metal flows together. The type of joint design determines whether a filler metal should be used to complete the weldment or not. Filler metal is added by inserting it into the molten puddle of the base metals. The puddle then solidifies, after cooling making the weld bead.
The extremely high heat depends on the mixture of two types of gaseous substances, oxygen and acetylene. The oxygen supports higher combustion, the acetylene is the fuel for the combustion.
Oxy-Acetylene Welding Plant (High Pressure System)
The term oxy acetylene welding plant refers to the basic equipment needed to weld. An oxyacetylene station includes an oxyacetylene outfit, a welding table, ventilation, lighting, and other necessary room equipment. The equipment necessary for an oxyacetylene outfit may vary depending upon the welding operations performed. In general, a welding outfit will consist of the following equipment.
(a) Gas Cylinder. It is required for storing the gases used for welding. Two types of cylinders are required for storage of gases used for welding.
(i) Oxygen cylinder.
(ii) Acetylene cylinder.
(b) Oxygen and Fuel Gases. Oxygen and acetylene are the two gases used for welding. In high pressure system, we use compressed acetylene at high pressure. That is why it is called high pressure system. In low pressure system we use acetylene, produced by acetylene generator at very low pressure.
(c) Regulators. It should be equipped with high pressure indicator or gauges. Cylinder and hose fittings are also required. One regulator with gauges and fittings is required for the oxygen cylinder and one regulator with gauges and fittings for the acetylene cylinder.
(d) Hoses. It should be equipped with appropriate fittings like hose connector, jubilee clip, hose protector, etc.
(e) Welding Torch (Blow Pipe). It should be fully equipped with all accessories, like gas control knobs and Nozzles.
(f) Trolley. It is required to transport the Oxy-Acetylene Welding Plant from one place to another.
(g) Spark Lighter. It is required to ignite the flame in the torch.
(h) Protective Clothing. During welding, it is common to experience spatter and sparks flying quite a distance. Protective Clothing protects the limbs of the welder against burning by molten metal, molten flux, spatter and sparks.
The equipment for a complete outfit is shown connected and ready for use in image below.
Gas cylinder is a steel container, used to store different gases at high pressure with safety. Gas cylinders are called by name of the gas stored.
Oxygen Gas Cylinder. It is a seamless steel container used to store oxygen gas safely and in a large quantity under a maximum pressure of 150 kg/cm2 (2100 PSI). It is used for gas welding and cutting. (1 Kg/cm2 =14 PSI).
Constructional Features of Oxygen Cylinder. It is made from seamless drawn steel and tested with a water pressure of 225 kg/cm2(3150 PSI). The cylinder top is fitted with a high-pressure valve made from high quality forged bronze. The cylinder valve incorporates a pressure safety device, which consists of a pressure disc, which will burst before the inside cylinder pressure becomes high enough to rupture the cylinder body. The cylinder valve outlet socket fitting has right hand thread, to which pressure regulator is fitted. The cylinder valve is also fitted with a steel spindle to operate the valve for opening and closing. A steel cap is screwed over the valve to protect it from damage during transportation.
The cylinder body is painted black and the valve outlet is screwed right hand. The capacity of the cylinder may be 3.5m3-8.5m3. Oxygen cylinders of 7m3 capacity are commonly used. The oxygen cylinders are filled with oxygen gas under a pressure of 120-150kg/cm2 (1680-2100PSI). The cylinders are tested regularly and periodically. The wall thickness of the cylinder is 6.5mm.
Acetylene Gas Cylinder. It is a steel container used to store high pressure acetylene gas safely in dissolved state for gas welding or cutting purpose.
Constructional Features of Acetylene Cylinder. The acetylene gas cylinder is made from seamless drawn steel tube or welded steel container and tested with water pressure of 100 kg/cm2 (1400 PSI). The cylinder top is fitted with a high-pressure valve made from high quality forged bronze. The cylinder valve outlet socket fitting has standard left-hand threads, to which acetylene pressure regulator is fitted. The cylinder valve is also fitted with a steel spindle to operate the valve for opening and closing. A steel cap is screwed over the valve to protect it from damage during transportation. The body of the cylinder is painted maroon and the valves are screwed left handed; to make this easily recognizable thea are chamfered or grooved. The capacity of the dissolve acetylene cylinder may be 3.5 m3– 8.5 m3.
The base of the cylinder is fitted with a safety valve which will melt at a temperature of approx.100º C. In case the cylinder is subjected to high temperature, the safety valve will melt and allow the gas to escape, before the pressure increase enough to harm or rapture the cylinder fuse plugs are also fitted on the top of the cylinder.
Method of Charging Dissolved Acetylene Gas Cylinder. The storage of acetylene gas in its gaseous form under pressure above 1kg /cm2 is not safe. A special method is used to store acetylene safely in cylinders. The cylinders are filled with porous substances such as Kopak wool, asbestos dissolved in acetone.
Acetylene gas is then charged in the cylinder, under a pressure of approximately 15kg/cm2. The liquid acetone dissolves the acetylene gas in large quantity as safe storage medium; hence, it is called dissolved acetylene. One volume of liquid acetone can dissolve 25 volumes of acetylene under normal atmospheric pressure and temperature. During the gas charging operation one volume of liquid acetone dissolves 25X15=375 volumes of acetylene gas at less than 15 kg/cm2 pressures at normal temperature.
Difference between Oxygen and Acetylene Cylinders
|(a) Painted black|
(b) Convex bottom
(c) Right hand threads
(d) Gas stored at 120 atmospheric pressure
(e) Gas stored is commercial oxygen
(f) Gas carrying pipe is black in colour
|(a) Painted maroon|
(b) Has a concave bottom
(c) Left hand threads
(d) Gas stored at 15 atmospheric pressure
(e) Gas stored is dissolved acetylene.
(f) Gas carrying pipe is red in colour
Safety of Gas Cylinders
Oxy-acetylene welding is safe if it is properly handled, but it may become a great destructive power if handled carelessly. It is important that the operator be familiar with all the safety rules before handling gas cylinders.
(a) Keep the cylinders free of oil, grease or any type of lubrication.
(b) Check leakage before use.
(c) Open cylinder valves slowly.
(d) Never fall or trip over gas cylinders.
(e) Keep the cylinders away from exposure to high temperature.
(f) Store full and empty cylinders separately in a well-ventilated place.
(g) Mark the empty cylinders (EMPTY) with chalk.
(h) Do not try to repair a leak. Put on the protection cap when the cylinder is not in use.
(i) Cylinder should always be kept in upright position and properly chained when in use.
(j) Always use a proper cylinder key to open or close the valve.
(k) Never apply undue force to open a cylinder valve. Avoid the use of hammer or wrench.
(l) Smoking or naked lights should be strictly prohibited near gas cylinder.
(m) Stand aside when opening the cylinder.
(n) Do not remove the cylinder keys from the cylinder during welding. It will help to close the cylinder quickly in case of any emergency.
(o) Always check and clean the cylinder valve socket before fitting the regulator.
This is a device used to couple a number of cylinders in order to draw gas evenly and prevent the drawing out of acetone which will occur if the acetylene is drawn out too rapidly. The acetylene is not to be drawn out at a rate, which will empty the cylinder in less than 5 hours. If it is necessary to consume acetylene at the rate greater than this, a number of cylinders are to be coupled together using the manifold.