The oxy-acetylene system of blowpipe welding is at present employed in two forms, which may be described as the high and low pressure systems.

The first to be introduced was the high pressure system, by which both gases are delivered to the blowpipe under pressure. Oxygen is supplied from an ordinary trade cylinder, and acetylene from a dissolved acetylene cylinder. Although the system is called " high pressure " a low pressure (injector) type blowpipe may be used, by simply adjusting the acetylene pressure regulator (which will be necessary with either a low, or high pressure, blowpipe when using dissolved acetylene), to deliver the gas at its minimum pressure. This system supplies engineers with the means of using the oxy-acetylene blowpipe in its most portable form. It has many useful applications, and for repair work generally, especially on board ship, it is well adapted.

Here, however, we are more particularly concerned with the low pressure system of welding, which is largely used in this country. In this latter method only oxygen is required under pressure, acetylene being taken from any ordinary generator of approved design.

All blowpipes employed with the low pressure system of oxy-acetylene welding are constructed on the injector principle, with which users of oxy-coal-gas blowpipes have for many years been familiar. In all injector blowpipes the oxygen, under a pressure varying from 7 to 401b. per square inch, is made to draw the necessary quantity of the combustible gas into the blowpipe, and then deliver both gases well mixed and under sufficient pressure through the burner nozzle.

Composition Of The Flame

In order to properly appreciate the value of the blowpipe, the welder should have some knowledge of the physical properties, and calorific value, of the flame obtained with it.

Acetylene (C2H2) is nearly as heavy as air, its density being 13 where air is taken at 15. It is an endothermic gas which is decomposed at the moment of combustion into its elements carbon and hydrogen, and by this decomposition alone about 300 B.T.U.'s are generated per cubic foot of the gas. The remaining heat in the flame is furnished mainly by the combustion in oxygen of the carbon into carbon dioxide, and in a lesser degree by the combustion of the hydrogen into water vapour, the total heat generated per cubic foot of acetylene being 1,500 B.T.U.

The flame thus produced has in its centre a small white cone, at the apex of which the temperature is about 6,000° Fahr. This flame consists almost entirely of carbon monoxide, which is being converted at its extremity into carbon dioxide. Round the flame is a relatively cool jacket of hydrogen, which, not being able to combine with oxygen at the very high temperature in the immediate neighbourhood of the flame, remains temporarily in the free state, and thus protects the inner zone from loss of heat whilst excluding the possibility of oxidation, a difficulty other methods of welding have to contend with.

Theoretically, 21/2 volumes of oxygen are required for the complete combustion of 1 volume of acetylene. In practice, however, with the oxy-acetylene blowpipe it is found that the best welding results are obtained with about equal volumes of oxygen and acetylene.

The heat, in addition to being greater than that obtainable from any other blowpipe system, is also more concentrated, whilst the flame is luminous, and in the form of a short steady jet. Illustrations indicating the correct adjustment of the flame are given in Fig. 221.

The power of a blowpipe is recognised by the number of cubic feet (or litres) of acetylene gas it will burn in one hour.

Fig. 222 shows a low pressure welding equipment (without the acetylene generator) coupled up ready for use.

The complete low pressure oxy-acetylene welding outfit consists of five units :-

The blowpipe.

The oxygen pressure regulator. The hydraulic back pressure valve.

The acetylene supply generator.

The oxygen cylinder.

An oxy-coal-gas brazing or lead burning outfit is similar, but the fuel gas supply may be drawn from a convenient gas main, thus dispensing with the acetylene generator and hydraulic back pressure valve ; the complete outfit will then consist of three units only :-

Oxy-coal-gas brazing blowpipe or an Oxy-coal-gas lead burning blowpipe.

The oxygen pressure regulator.

The oxygen cylinder.

Fig. 223 shows a complete high pressure oxy-acetylene welding outfit, consisting of five units :-

The blowpipe.

The oxygen pressure regulator.

The acetylene pressure regulator.

The acetylene supply, D.A. cylinder.

The oxygen cylinder.

We will now proceed to illustrate and describe the various units starting with :-

The Blowpipe

The Universal oxy-acetylene blowpipe is illustrated by Fig. 224. It is of the interchangeable type and is supplied in two sizes of shank or handle :-

(a) " P" type shank taking welding heads sizes 1-10.

(ft) " R" type shank taking welding heads sizes 8-15.