The Weld Purge Process
Once the quality of the gas in the dammed volume has reached the required level, gas flow can be reduced to about 5 l/min for the welding operation. On a more practical level it should just be possible to feel the gas flow from the exit point. Excessive flow can cause the internal pressure in the pipe to rise and create concavity in the weld root geometry and in more extreme cases can cause complete ejection of the molten weld pool.

On joints that are not fully sealed to restrict leakage a higher flow rate will be necessary to avoid contamination. Towards the end of the weld run however, as the joint becomes permanently sealed, the gas flow rate will need to be reduced to avoid over-pressurization.

Process costs — Providing precise data on cost comparisons between the different purging techniques is difficult not least because the pipe diameter and wall thickness have a profound influence on the cost. Furthermore it is impractical to use removable inserts in sealed pipes or in pipes and tubes with diameters much below 100 mm.

It is impractical therefore to be specific across the entire spectrum of diameters and welding procedures, other than to say that savings can be significant when commercial purging systems are employed. As an example, users report that a 900 mm diameter pipe can be fully purged to less than 0.1% oxygen in under 10 minutes when using the ‘Argweld^™ commercial systems illustrated. There are reported savings in excess of 80% on purge time compared with alterative purging systems so that inert gas usage can be reduced dramatically as a result. (‘Argweld’ is a registered trade name of Huntingdon Fusion Techniques Ltd.)

Typical analysis has been made on pipe diameters between 100 and 300 mm and these are presented in Table 1.

Table 1. Indicative times to reduce oxygen to 0.1% in pipes with and without the use of gas dams.

It is clear from this basic analysis that where several welds have to be made on similar pipe diameters there can be genuine cost savings when using inflatable weld purge systems as the sealing medium. Add this to the technical advantages of reliable sealing and ease of use and the inflatable purge concept can be seen to offer significant attractions.

Table 2. Comparison of costs between soluble paper and commercial weld purge system. In making these comparisons, average costs for inert gas and labor in the USA have been taken. Soluble film can be used only once but the inflatable purge system is re-usable, so the number of welds planned can influence unit weld cost significantly. The use of soluble film requires a greater labor involvement.

It is clear from this basic analysis that where several welds have to be made on similar pipe diameters there can be genuine cost savings when using commercial inflatable pipe purging systems (see Figs. 3, 4, 5 and 6.) Add this to the technical advantages of reliable sealing and ease of use and the inflatable purge system concept can be seen to offer significant attractions.

References
Techniques for GTAW Welding Success.
(Miller Electric Manufacturing Co., Appleton, Wis.)

Purging for Pipe/Tube Welds
(James A McMaster, AWS Welding Journal.)