This case study examines the critical role of weld purging in the LNG (liquefied natural gas) industry, where the quality and integrity of welds are paramount. Despite the availability of advanced weld purging systems designed to prevent oxidation and contamination, many fabricators still rely on outdated and ineffective methods, leading to subpar weld quality and increased costs.
The study compares two primary types of weld purging equipment: flexible discs and inflatable seals. While flexible discs are inexpensive and easy to deploy, they often prove unreliable due to poor sealing capabilities, leading to significant gas leakage and potential weld defects. On the other hand, inflatable seals provide superior sealing, flexibility, and thermal resistance, making them ideal for demanding welding environments.
The Argweld® range of inflatable weld purging systems by Huntingdon Fusion Techniques Ltd, including PurgElite®, QuickPurge®, and HotPurge® systems, is highlighted as an industry-leading solution. These systems offer reliable sealing, reduced purge times, and enhanced weld quality, making them particularly well-suited for the stringent requirements of LNG projects.
The study concludes that while inflatable purging systems may have a higher upfront cost, they offer significant long-term savings by reducing gas wastage, improving weld quality, and minimising the need for rework. Thus, they are the superior choice for weld purging in critical LNG applications.
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The Importance of Weld Purging
The quality and integrity of welds are paramount in LNG (liquefied natural gas) storage and distribution. Despite the availability of advanced weld purging systems designed to prevent oxidation of the weld root, many fabricators still rely on unreliable homemade devices or unsuitable products like plastic, foam, cardboard, or even screwed-up newsprint. While seemingly cost-effective, these archaic practices often lead to subpar results and increased costs due to the need for post-weld repairs and cleaning.
High-quality joints between stainless steel cylindrical sections such as tubes, pipes, and vessels can only be achieved by eliminating atmospheric gases, particularly oxygen, from the weld area. The presence of oxygen, and to a lesser extent nitrogen, during welding can lead to several defects:
Removing these post-welding imperfections can be time-consuming and expensive. Processes like pickling, electrochemical cleaning, or mechanical techniques are required, all of which have limitations and risks.
Basic Principles of Weld Purging
To achieve high-quality welds, the fusion zone must be shielded from air by a blanket of inert gas, known as weld purging. The selection of the optimal purge gas, typically argon, due to its inertness and cost-effectiveness and the control of gas flow rate and pressure are critical factors that should be defined in the welding procedure.
Comparison of Weld Purging Equipment: Flexible Discs vs. Inflatable Seals
Flexible Discs
Flexible disc systems made from natural, silicone, and other synthetic rubbers have been developed specifically for weld purging. These systems are marketed as relatively inexpensive, quick to deploy, and easy to remove post-weld. However, in practice, they often prove unreliable because they rely on a tiny contact area between the disc and the pipe, which can lead to significant leakage of inert gas.
Rubber Gasket Dam: This system involves a rubber, foam, or similar disc sandwiched between two wooden or metal discs. The diameter can be adjusted by applying axial pressure, but these systems are not fully collapsible. After welding, pulling the discs out past the weld root can be difficult and potentially damaging.
Multiple Floppy Discs: While these systems are an improvement over the rubber gasket dams, they still fit only a limited range of pipe diameters and depend on the precision of the disc edge for sealing. They also contain metal parts that can scratch polished surfaces, leading to a loss of corrosion resistance. Additionally, purchasing a disc system for each pipe diameter can become uneconomical compared to more versatile systems.
Inflatable Seals
Inflatable seals represent the most reliable and versatile solution for weld purging. These systems have been carefully engineered to address the issues of controlled inert gas pressure and flow, easy deployment and removal, thermal resistance, and leak-tight access for oxygen monitoring. They provide a large contact area with the pipe, ensuring excellent sealing, and are designed to withstand the rigours of demanding welding environments.
Advantages of Inflatable Seals:
The Argweld® Inflatable Systems by Huntingdon Fusion Techniques Ltd
Huntingdon Fusion Techniques Ltd has set a global standard with their Argweld® range of inflatable weld purging systems. These systems cover pipe diameters from 25 to 2400 mm and are used by central international fabricators on demanding projects.
PurgElite® Systems: These systems cover 25 to 600 mm pipe diameters and are the most miniature inflatable systems globally. They can be equipped with PurgeGate® to prevent over-inflation.
QuickPurge® Systems: Designed for larger pipes (200 to 2400 mm), these systems use unique materials to withstand higher temperatures when placed closer to the weld zone. They also feature thermal-resistant versions for applications requiring preheating and post-heating.
HotPurge® Systems: These are designed to remain in place during welding and controlled cooling processes, accommodating temperatures up to 300°C for 24 hours.
The Economic Argument for Inflatable Systems
While it might seem cost-effective to use homemade or low-cost solutions for weld purging, these methods often lead to higher overall costs due to gas wastage, poor weld quality, and the need for rework. In contrast, inflatable purging systems are more economical in the long run, providing better control over gas flow and pressure, reducing purge times, and ensuring higher-quality welds with fewer defects.
Conclusion
Inflatable purging systems, such as those in the Argweld® range, ensure a high level of sealing reliability, which is crucial in maintaining low oxygen levels during welding. These systems are particularly well-suited for the demanding conditions of LNG projects, where weld quality is paramount. By investing in proven, engineered solutions, fabricators can achieve significant cost savings, improved weld quality, and reduced rework, making these systems superior for weld purging in critical applications.
