When welding stainless steel, zirconium or titanium tubes and pipes, the oxygen must be purged down to a level where the alloy will not oxidise since this can lead to a loss of corrosion resistance properties. This is usually under 100 ppm for stainless steel and under 50 ppm for titanium.
The Pre-Purge Process
Displacing the air in the pipework is essential. Numerous factors control the pre-purge time such as pipe diameter, purge volume and maximum permitted oxygen level.
The Weld Purge Process
Once the quality of the gas in the dammed volume has reached the required level, gas flow can be reduced. 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.
Inflatable Tube and Pipe Purging Systems
The only totally reliable purging systems are those based on inflatable seals. Restricting the volume required to be purged is also extremely cost effective.
QuickPurge® Systems are manufactured in sizes 150 up to 2,235 mm diameter and are designed for repeated use under site conditions.
Following the success of QuickPurge, PurgElite® was introduced, which includes the smallest integrated purge system currently available anywhere. Systems are available to accommodate diameters between 25 and 600 mm.
Responding to industry feedback, high temperature versions of these inflatable systems are now available to meet the requirements for welds needing pre- and post- weld heating. HotPurge® can be used at temperatures up to 300ºC for several hours without deterioration.
A patented control valve called PurgeGate® has been added to most systems and available as an accessory on others. This unique one-way valve prevents over-inflation even when the operator attempts to increase purge gas pressure.
RootGlo® illuminating tape is now fixed at the centreline of all purge systems. This glows brightly inside the pipe to help welders position the equipment accurately. IntaCal® is a pre-calibrated gas release valve and replaces alternative expensive vales that need pre-setting prior to each weld procedure.
Weld Purge Monitors® are now an essential part of the purging process and provide accurate information on purge gas oxygen content. The latest instruments from HFT are compatible with data recording systems and can provide audible alerts when the required oxygen content has been reached and even stop the welding process if this level exceeds that set by the welding procedure. With a monitor for all applications and budgets, the PurgEye® range now accurately reads down to 10 ppm.
More product information is available on our website.
