Welding Torch and Blow Pipe

Welding Torch and Blow Pipe

Welding Torch and Blow Pipe - Oxygen and the fuel gas having been reduced in pressure by the gas regulators are fed through suitable hoses to a welding torch which mixes and controls the flow of gases to the welding nozzle or tip where the gas mixture is burnt to produce a flame for carrying out gas welding operation. There are two types of welding torches, namely: (i) High pressure (or equal pressure) type. (ii) Low pressure (or injector) type. High pressure blowpipes or torches are used with (dissolved) acetylene stored in cylinders at a pressure of 8 bar. Low pressure blowpipes are used with acetylene obtained from an acetylene generator at a pressure of 200 mm head of water (approximately 0.02 bar).
(a) Working of a low pressure blowpipe: It is termed as a low pressure blowpipe because it can be operated at low acetylene pressures; it is frequently used with acetylene generators. As acetylene is of low pressure, it is necessary to use oxygen at a high pressure (2.5 bar). The oxygen enters the mixing chamber through a passage located in the centre of the torch. The oxygen passage is surrounded by the one carrying the acetylene. The high pressure oxygen passes through a small opening in the injector nozzle, enters the mixing chamber and pulls (or draws) the acetylene in after it. An advantage of low pressure torch is that small fluctuations in the oxygen supplied to it will produce a corresponding change in the amount of acetylene drawn, thereby making the proportions of the two gases constant while the torch is in operation.
	(b) Working of a high pressure blowpipe: In this type of blowpipe both the oxygen and acetylene are fed to the blow pipe at equal pressures and the gases are mixed in a mixing chamber prior to being fed to the nozzle tip. The equal pressure or high pressure type of blowpipe is the one most generally used because (i) It is lighter and simpler. (ii) It does not need an injector. (iii) In operation, it is less troblesome since it does not suffer from backfires to the same extent. To change the power of the welding torch, it is only necessary to change the nozzle tip (size) and increase or decrease the gas pressures appropriately.
Welding Nozzles or Tips Depending upon the design of the welding torch (or the blow pipe) the interchangeable nozzles may consist of : (i) Either, a set of tips which screw onto the head of the blowpipe, or (ii) As a set of gooseneck extensions fitting directly onto the mixer portion of the blowpipe. The welding nozzle or tip is that portion of the torch which is located at the end of the torch and contains the opening through which the oxygen and acetylene gas mixture passes prior to ignition and combustion. A welding nozzle enables the welder to guide the flame and direct it with the maximum ease and efficiency. The following factors are important in the selection of appropriate welding nozzle:
(i) The position of the weld. (ii) The type of joint. (iii) Job thickness and the size of welding flame required for the job. (iv) The metal/alloy to be welded. To provide for different amounts of heat, to weld metals of different thicknesses, welding tips are made in various sizes. The size of a welding tip is determined by the diameter of the opening or orifice in the tip. As the orifice size increases, greater amounts of the welding gases pass through and are burnt to supply a greater amount of heat. The choice of the proper tip size is very important to good welding. A chart giving sizes of tips for welding various thicknesses of metal alongwith oxygen and acetylene pressures used is generally provided by the manufacturers.

Care of Welding tips
(i) All welding tips are made of copper and may be damaged by careless handling.
(ii) Nozzles should never be dropped or used for moving or holding the work.
(iii) Nozzle seat and threads should be absolutely free from foreign matter in order to prevent any scoring when tightening on assembly.
(iv) Nozzle orifice should only be cleaned with tip cleaners specially designed for this purpose.

Gas Lighter A gas (spark) lighter provides a convenient, safe and inexpensive means of lighting the torch. Match sticks should never be used for this purpose because the puff of the flame produced by the ignition of the acetylene flowing from the tip is likely to burn the welder's hand. Spark lighters are constructed from flint and steel.

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