The intense heat generated by the oxy-acetylene flame has resulted in various constructions of blowpipes for this welding system.

It is, however, not practicable to compress acetylene in cylinders, as may be done with hydrogen, because of its explosive qualities when compressed, Claude and Hess having devised the acetone solution method.

At first an ordinary form of blowpipe was used for burning acetylene with oxygen. The two gases were kept separate up to the nozzle of the blowpipe, that is to say, to the actual point of combustion.

With acetylene, however, a serious inconvenience appeared. A heavy deposit of carbon took place, the flame was suppressed, and, indeed, a kind of carbon mushroom resulted. It was therefore necessary to resort to some sort of blowpipe in which the mixture of gases might be made in the interior of the apparatus. But with acetylene a serious accident was to be feared, as the flame might flash back into the blowpipe and cause an explosion.

It is known that to prevent a back draught of the flame the issuing gas must ordinarily have a velocity higher than that of the explosion wave, which, for a mixture of oxygen and acetylene, is extremely high. Practical experience proves that it is not necessary to attain this point because of the very small section of the blowpipe nozzle. A velocity of flow equal to about 150 metres a second is sufficient to prevent the flame from travelling back into the apparatus.

In further prevention of this return of the flame the interior was filled with a porous material, but this made it necessary to increase the pressure to the equivalent of 3 to 4 metres of water.

A problem still to be solved was that of utilising acetylene not under pressure, that is, generated immediately in the carbide apparatus. Fouche found a very interesting solution by introducing the oxygen into the apparatus through an injector, which draws a flow of acetylene with it. The acetylene enters through extremely small tubes, which do not permit the passage of the flame.

There are now in general use two types of oxy-acetylene blowpipes:

High-pressure type.

Low-presure type.

The High-pressure blowpipe is one in which both the gases are used under pressure. The oxygen is supplied from an ordinary trade oxygen cylinder, and the acetylene from a cylinder in which it is dissolved in acetone absorbed by a porous material.

The Acetylene Illuminating Company, Limited, give the following information as to their high-pressure interchangeable blowpipes: -

Interchangeable Blowpipes (High-pressure).

No. 3 is fitted with water circulation round the tip.

The standard sizes of the patent high-pressure blowpipes are as follows: -

Note. - 28 litres = 1 cubic foot approximately.

The Low-pressure blowpipe is one in which acetylene is used direct from a gas-holder, and the oxygen from a trade cylinder. This system is suitable for use in workshops situated in districts where dissolved acetylene cannot be obtained.

Low-pressure Blowpipes.

By the high-pressure type the adjustment of the flame is far easier with both gases under pressure; once the adjustment is made right it remains so; a more intimate mixing of the two gases is obtained than in the low-pressure type, and this secures higher efficiency. This is a point of great importance, as it is found that with the high-pressure blowpipe considerably less acetylene is required to do a fixed quantity of work than is necessary with a low-pressure blowpipe.

For some time experiments have been going on to perfect a blowpipe in which the consumption of gas could be regulated so as to do away with the necessity of having a large number of different sized blowpipes. Up to the present this has been found to be quite impossible in the low-pressure type of blowpipe, but blowpipes on the high-pressure system admit the gas consumption to be altered over a wide range by merely changing the nozzle. In this type of blowpipe both gases are used at the same pressure, which has the great advantage that if, while the work is going on, the orifice of the nozzle becomes partly choked by scale from the metal, the composition of the flame remains the same, but in the case of a low-pressure blowpipe, where the oxygen is introduced at a high pressure and in a constant quantity, and the acetylene at a low pressure, any choking of the orifice will result in an alteration of the composition of the flame and consequent oxidation of the metal.

The Acetylene Illuminating Company give (on pages 153 to 157) tables of practical instruction for using their blowpipes.

When a blowpipe is working properly the length of the small white cone of the flame for the different blowpipes should be as in the following table: -

The approximate internal diameter in inches of pipe required between the acetylene apparatus and the hydraulic back-pressure valve:

Speed at which Work can be Welded and Proper Size of Blowpipe to Use. - The table on p. 157 shows the approximate size of blowpipe to be used on different thicknesses of metal and the approximate rate at which work can be welded. Allowance must be made for the skill of the workman, the character of the job, and the type of blowpipe employed. The sizes of blowpipes given in the table are of the high-pressure type; it will be found that a larger blowpipe will be required when low-pressure plants are in use.

The Fouche blowpipe is a French invention based upon the injector principle. It was introduced a few years since, and has recently undergone some further improvements.

Experience has already confirmed, to a certain extent, what science has taught, that a proper weld is vitally dependent upon various matters, the foremost amongst which are the absolute purity of the gases employed, the composition and continuity of their mixture and its velocity at the nozzle of the blowpipe, the pressure and temperature of the flame and its application upon the metals to be welded. Furthermore, consideration must also be taken as to the thickness as well as the chemical and physical properties of the welding metal.