Few would disagree with the fact that making high quality welds demands skill. Most metallurgical obstacles have been overcome through progressive development of filler materials. Contemporary equipment for use in GTAW and GMAW is probably as good as it is likely to get. Of the few difficulties remaining, the production of sound and oxide-free weld underbeads is arguably the most underestimated.
One of the principle requirements of the pipeline industry is a weld underbead that is physically positive, i.e., no undercut, smooth and free of oxidation and debris such as ‘grapes’ or ‘sugaring’. These defects can usually be removed by post weld grinding and polishing but this practice can be very expensive. Far better to address the problem through the use of dedicated inert gas weld purging techniques. They provide a stable, oxygen-free environment throughout the welding cycle and thus eliminate oxidation.
The purging solution has been recognised for many years but only recently have the welding accessory manufacturers developed dedicated equipment.
The last few months have seen the launch of several innovative improvements to the range of internationally approved weld purge systems manufactured by Huntingdon Fusion Techniques Ltd. Exploiting current engineering developments in materials, electronics and specialist products such as gas valves, HFT’s designers, encouraged by customer feedback, have been able to make significant improvements to its range of purge equipment and instrumentation.
Three principle products form the thrust of HFT’s Argweld® purge range; QuickPurge®, PurgElite® and PurgEye®.
The Argweld® Product Range was developed to help speed up the welding process for engineers involved in the fabrication of pipes and tubes. This was realised by using a design, which allows for easy and positive insertion into position and by limiting the purge volume. The product range has been used extensively and internationally during the welding of stainless steel gas and oil transmission pipework.
The QuickPurge® body employs synthetic fabrics throughout and incorporates an integral protective lining below the weld zone to resist thermal damage. Hoses used to transmit inert gas to and from the purge cavity and to provide inflation pressure are made from engineering grade nylon. Brass gas fittings are located well outside the weld zone so that post weld radiography and ultrasonic inspection can take place with the purge system left in place.
It is impractical to be specific across the entire spectrum of diameters and welding procedures other than to say that savings are significant. 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. There are reported savings in excess of 80% on purge time compared with alternative purging systems so that gas usage can be reduced dramatically as a result.
PurgElite® systems have been developed to help speed up the welding process for engineers involved in the fabrication of pipes and tube lines and assemblies up to 300 mm diameter.
The revolutionary product range replaces an earlier one that has been manufactured and marketed successfully by HFT world-wide for many years and is widely recognised as a robust, easy to use, welding ancillary that offers considerable savings in time and inert gas.
The ‘Elite’ development incorporates many advances in engineering technology and extends the size range to include purge systems as small as 25 mm diameter.
The inflatable components employ low vapour pressure synthetic fabrics with low outgassing rates throughout and incorporate a connecting hose with a protection sleeve to resist thermal damage. Hoses used to transmit inert gas to and from the purge cavity and to provide inflation pressure are made from engineering grade nylon.
PurgEye® monitors were developed by HFT® specifically for the weld purge industry to provide sensitive oxygen measuring instrumentation. Most fabricators, but stainless steel fabricators in particular, need to ensure that the weld root is protected against oxidation. If the root becomes oxidised, the affected metal may need to be cleaned and this can be an expensive operation. Protection is best effected by providing inert gas purging. Of course the purge gas itself needs to be free of oxygen and a measure of the oxygen content of the purged volume is thus a crucial need for the welder.
For routine welding of the majority of stainless steels there is a requirement for the purge gas to contain less than 0.1% oxygen or 1000 parts per million (ppm). For these applications the HFT® PurgEye® monitors are more than adequate. More sensitive materials such as titanium alloys and some special stainless steels may require oxygen levels to be below 0.01% or 100 ppm.
A major US Nuclear Site employs QuickPurge® and Argweld® Weld Purge Monitors® to fabricate stainless steel duct work ranging in size from 150 to 600 mm.
Before using QuickPurge® the practice was to purge a complete pipe system.
Other types of stand-alone dams were unsuitable because of the risk of dams being accidentally left in place leading to costly obstruction issues. Water soluble dams were not allowed because post-weld flushing with water was unacceptable.
A number of observations have been made;
- Argon usage has been reduced by up to 90%.
- Productivity has increased dramatically and overheads reduced due to the elimination of wasted down-time with welders spending hours waiting for purges.
- A significant benefit has been the elimination of a major jobsite safety issue. Before using QuickPurge®, they would end up with hundreds of feet of ducting that were filled with argon. When the argon was to be expelled from the lines it produced a serious safety issue requiring the evacuation of areas due to dangerously lowered oxygen levels caused by the exhausted argon.
Pennecon Energy in Newfoundland selected Argweld® purge systems for a large piping project.
Pennecon was awarded a contract that required hundreds of welds on titanium and stainless steel piping with sizes up to 1500 mm diameter and they were in the process of gearing up a brand new welding facility with multiple welding stations to handle the job.
The company evaluated several alternatives for effective and efficient methods of weld purging in order to achieve a purge below 50 parts per million of oxygen. Following testing and evaluation and after studying the return on investment that the Argweld® products offered with reduced operating costs, a decision was made to move forward with the procurement of QuickPurge®II systems and PurgEye® Weld Purge Monitors®.
They are now using the equipment extensively on 250, 450 and 500 mm pipe joints and plan to continue to use other Argweld® equipment as they move into other pipe sizes.
PurgeGate® has been added to all QuickPurge® systems. This patented device prevents inflatable systems from being over-pressurised even when operators try to increase pressure to increase the flow of weld purge gas.
The RootGlo® illuminating tape is now sewn on the centreline of the QuickPurge® system. Inside the dark pipe, the centering band glows brightly to give welders the opportunity to see all aspects of the purge device alignment and the quality of the weld root as it is being welded.
IntaCal® is a calibrated gas release system to purge the interspace between the dams. IntaCal® removes the need for expensive and complicated valve systems that need presetting before each weld. This time-consuming process is now eliminated, as is the previous problem of incorrectly setting valves which frequently led to bursting dams.
Weld Purge Monitors® ,the exhaust gases are fed directly to the weld purge monitor and can be set to alert the user automatically when the required oxygen level has been reached.
Fig 1. Effective gas seals need to be provided on either side of the joint and these need to be far enough away to be unaffected by the temperature rise during welding. This schematic section illustrates how QuickPurge® equipment works
Fig 2. Engineers prepare to fabricate large diameter pipework on the Isle of Grain Natural Gas installation using QuickPurge®
Fig 3. PurgEye® monitors have been specifically designed for indicating oxygen levels in inert gas during weld purging. These rugged instruments have a measuring range down to 1 part per million on an alpha-numeric display.
Fig 4. PurgElite® inflatable purge system designed to accommodate tube and pipe sizes between 25 and 300 mm diameter. The 25 mm version is the worlds’ smallest fully integrated purge system.
Advances in weld purging systems for pipeline fabrication have led to the development of equipment that meets the requirement for reliability and robustness under site conditions.
Without appropriate purging, oxidation of the weld underbead will usually require removal and the cost of this is significant.
A recent document published in the UK by the Royal Academy of Engineering (Reference: www.twi.co.uk/news-events/news/2012-07. Guidance on Best Practice for Welding ) directs welding engineers to recognise the significance of appropriate training. In the case of purging they must address the need for education on the necessity for purging and the choice and application of appropriate purging techniques.
Author: Michael Fletcher
Dr M J Fletcher is a qualified metallurgist with extensive experience in welding and non-destructive testing.
He works as an independent consultant, providing support to a wide range of manufacturing industry on a global basis.
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