This case study explores the critical principles of weld purging in stainless steel tube and pipe welding, highlighting the importance of eliminating atmospheric gases to achieve high-quality welds. Despite the availability of advanced purging systems, many fabricators continue to use outdated and ineffective methods, leading to weld defects, increased costs, and compromised product quality.
The study compares various purging techniques, focusing on the limitations of flexible disc systems versus the superior performance of inflatable seals. Inflatable systems, such as the Argweld® range, provide controlled inert gas flow, easy deployment, thermal resistance, and effective oxygen monitoring, ensuring welds are free from oxidation and other defects.
The Argweld® PurgElite® and QuickPurge® II systems are highlighted as industry-leading solutions for tube and pipe diameters ranging from 25 mm to 1800 mm. These systems offer significant advantages, including reduced purge times, improved weld quality, and overall cost savings, making them the preferred choice for critical industrial applications.
The study concludes that investing in reliable inflatable purging systems and precise oxygen monitoring is essential for achieving consistent, high-quality welds, particularly in demanding industries such as LNG, pharmaceuticals, and petrochemicals.
The Importance of Eliminating Atmospheric Gases
In the world of stainless-steel fabrication, maintaining weld quality is paramount, especially when dealing with tube and pipe welding. Despite the availability of professionally designed weld purging systems, many fabricators still rely on unreliable, makeshift devices to prevent oxidation of the weld root. This is a surprising fact, especially given the demanding nature of LNG storage and distribution industries. Using plastic, foam, cardboard, and even newspaper for purging is outdated and can be far more costly in the long run than using engineered systems.
High-quality welds between stainless steel tubes, pipes, and vessels require eliminating atmospheric gases from the weld zone. Oxygen, and to a lesser extent nitrogen, around the molten weld can lead to several severe defects:
Post-weld imperfections, such as these, are not just cosmetic; they can lead to product contamination, particularly in sectors where cleanliness is critical, such as food processing, pharmaceuticals, and petrochemicals. Removing these imperfections is time-consuming and expensive, involving methods like pickling, electrochemical cleaning, or mechanical techniques that may leave behind unwanted residues.
Basic Principles of Weld Purging
The key to ensuring a quality weld root in tubular joints lies in removing air from the fusion zone and providing a blanket of inert gas around it—this is the essence of weld purging. The selection of the optimal purging gas is crucial, with argon being the most used due to its inertness and relative cost-effectiveness. However, attempts to cut costs further by using cheaper gases like nitrogen can be detrimental, as nitrogen is not genuinely inert and can lead to weld defects.
Examination of Purging Equipment
Various purging techniques have been developed, but two primary methods are worth comparing: flexible disc systems and inflatable seals.
Flexible Disc Systems
Flexible disc systems are based on natural, silicone, and synthetic rubber. They are marketed as cost-effective and easy to use, but they have significant limitations:
Inflatable Seals
The only reliable and versatile purging systems are those based on inflatable seals. Over the past decade, significant design improvements have made these systems highly efficient. Modern inflatable systems address vital issues such as:
Advanced Inflatable Purging Systems
The Argweld® range of inflatable purging systems sets the industry standard. These systems are used globally on demanding projects where weld quality is critical. They cover tube and pipe diameters from 25 mm to 1800 mm, with specific systems like the PurgElite® for smaller diameters and the QuickPurge® II for larger diameters.
Both systems are constructed from materials that withstand high temperatures and have low outgassing rates, making them suitable for critical applications. Inflatable systems like these can significantly reduce purge times, improve weld quality, and ultimately lower costs.
Cost Comparison: Inflatable Systems vs. Makeshift Solutions
Using cheap, makeshift solutions like plastic foam dams is tempting, especially under commercial pressures. However, these solutions often lead to higher overall costs due to their inefficiency. A comparison between using an inflatable system like the QuickPurge® II and foam dams reveals substantial time and cost savings, particularly for larger pipe diameters. While the upfront cost of an inflatable system might be higher, the savings in time and gas and reduced rework make them far more cost-effective in the long run.
The Role of Oxygen Monitoring
Even small amounts of oxygen in the purge gas can cause significant issues, so using a reliable purging system with effective sealing is essential. Monitoring the oxygen content is also crucial. Instruments like the PurgEye® Weld Purge Monitors® are designed to measure oxygen levels as low as ten ppm (0.001%), ensuring that the purge environment is maintained at the necessary low oxygen levels for high-quality welds.
Summary
In summary, inflatable purging systems, such as those offered by Argweld®, provide a reliable, efficient, and cost-effective solution for tube and pipe welding. These systems ensure the welding environment is free from atmospheric contamination, leading to better weld quality, reduced rework, and overall cost savings. When combined with advanced oxygen monitoring, these systems represent the best practice in weld purging technology.
