By Dr Michael Fletcher of Delta Consultants
Discussing experience with weld purging on stainless steel pipes.
Few would disagree with the fact that making high quality welds in stainless steel 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 repeatable sound 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. Unfortunately, the additional costs of post weld treatment such as mechanical removal of defects go unrecorded.
They also 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. Some suppliers actually specialise in inert gas purging and offer a wide range of solutions. They have taken advantage of leading edge technology in the design and manufacture of fully integrated, rugged and reliable inflatable purge systems.
Quick?Purge® equipment has been used extensively and internationally during the welding of stainless steel for the LNG industries and for gas transmission pipework.
Advanced polymers are used to provide flexibility and thus ensure good sealing inside the pipe. The flexibility has the added bonus of affording ease of insertion and post weld removal. Additionally these materials also offer abrasion resistance and are thus suitable for demanding onsite work and repeated use.
These solutions have been widely acknowledged by the world leaders in liquid natural gas plant construction. Bechtel (Sabine Pass), Whessoe (Milford Haven), BP (Valhall) and Agip (Kuryk) have all acknowledged the expertise and products now available. They have also recognised and appreciated the potentially large cost savings accruing from low inert gas usage and faster purge times.
The Quick?Purge® systems were used extensively by Whessoe to help ensure the production of high quality welds during the fabrication of stainless steel pipe?work for a massive LNG terminal at Milford Haven in west Wales. A recent major order has been issued to supply purging equipment for use during the US$ 1 billion Agip?led offshore programme in Kazakhstan. The BP group has benefited from using advanced purging techniques within the US$ 84 million Norwegian Valhall gas field.
A range of weld purge systems designed and manufactured in the UK have been supplied to accommodate pipe sizes from 25 ? 1800 mm: these represent the most advanced ancillary equipment available anywhere at the present time.
In addition to the application to austenitic stainless steel, the Argweld® pipe weld purging system is being used during the fabrication of titanium alloys, duplex stainless steel and 'pipe?in?pipe' joints.
Producing welded joints of the highest integrity for these applications requires that inert gas protection of the weld root be provided to prevent oxidation.
Argon is fed into inflatable seals on both sides of the joint to provide a reliable and rapid inert gas coverage.
For measurement and control of residual oxygen content instruments to monitor oxygen levels in the purge gas have been developed. The PurgEye® equipment can measure levels as low as 10 ppm (0.001%) and offers data recording software for quality control purposes.
Typical equipment for inert gas weld purging
Proven commercial systems are available globally and covering all pipe diameters between 25 and 1800 mm. Specialised versions have been developed that offer ease of use combined with speed of purging for the gas pipeline sector. For example the fully integrated QuickPurge® equipment meets the demand for robustness coupled with minimum gas use. PurgElite®, released this year, has been developed to allow small diameter pipes be welded.
Even with the most careful design and application of commercial purge systems there are occasions when leaks occur between the pipe wall and the inflated seal. Ensuring that the residual purge gas has an acceptably low oxygen content, typically 0.01% for stainless steel, instruments have been developed specifically to measure low oxygen levels. Whereas previous monitors have evolved from general purpose devices that measure high oxygen content in mixed gases, these recent instruments meet the singular requirements of the welding industry and can also be used under the demanding conditions prevailing during site welding.
Case histories have been used to make comparisons between commercial pipe purging systems and 'do it yourself' alternatives. It is quite clear from this analysis that a very strong case can be made for more widespread use of commercial systems. The capital cost of engineered purge systems is not included in the analysis but this cost can be quickly amortised since the systems can be used repeatedly. Engineered systems are much less likely to leak and therefore lead to defective welds. There is a high cost attached to remedial work when welds need to be dressed to remove debris.
On balance, the potential savings are high but the biggest attraction is the major increase in weld quality that accrues from the elimination of oxidation and weld defects. Exploiting advanced technology in the design and manufacture of purge systems has allowed purging technology to develop very rapidly over the last ten years. These systems have become progressively easier to use and innovative designs offer real opportunities to produce joints faster and at lower cost.
Note: Argweld® Quick Purge®, PurgEye® and PurgElite® are registered trademarks of Huntingdon Fusion Techniques (HFT).
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