In the second part of Dr. Fletcher's series, the focus shifts to proper pipe and tube welding purging techniques. Effective pre-purging and weld purging are critical to ensuring weld quality, particularly in high-stakes applications involving sensitive alloys like stainless steel and titanium.
Pre-Purge Process:
The pre-purge process displaces air within the pipe or dam before welding, with a recommended gas flow rate of 15-20 l/min. The goal is to achieve low residual oxygen levels—typically 0.05% (50 ppm) for stainless steel and below 0.02% for titanium. Misconceptions about flow rates leading to faster purging are addressed, emphasising that higher rates can cause turbulence and extend purging time.
Weld Purge Techniques:
Six methods of pipe purging are discussed:
Among these, inflatable systems are highlighted as the most reliable, providing robust sealing, minimising gas usage, and allowing for rapid deployment, especially in demanding industries like nuclear.
Products Mentioned:
Pipe and Tube Weld Purging
In Part 1, Dr. Fletcher highlighted several inconsistencies in weld purging practices, especially regarding the selection of shielding gases. He concluded that fabricators need more definitive guidance to produce consistently high-quality welded joints that meet the rigorous standards required by various service conditions. Until such guidance is available, the American Welding Society's recommendation remains the best advice: the shielding gas must match the metal being welded.
The Pre-Purge Process
Before welding, the pre-purge process is essential for displacing air within the pipework or dam volume. Several factors, including pipe diameter, purge volume, and the maximum permissible oxygen level, determine the duration of this process. A common misconception is that increasing the purge flow rate will reduce the purge time. Higher flow rates can increase turbulence, leading to unwanted mixing of purge gas with air, thus potentially extending the purge time. Generally, the pre-purge flow rate and time should allow for about five-volume changes in the pipe system, with a typical gas flow rate of 15-20 l/min.
For materials such as stainless steel, a residual oxygen level of 0.05% (50 ppm) is typically sufficient to prevent discolouration during welding. However, more sensitive alloys, such as those based on titanium, require oxygen levels below 0.02% to avoid contamination.
The Weld Purge Process
Once the desired gas quality within the dammed volume is achieved, the gas flow can be reduced to approximately five l/min for the welding operation. A practical guideline is that the gas flow should be barely perceptible at the exit point. Excessive flow can raise internal pipe pressure, potentially leading to concave weld root geometry or, in extreme cases, complete ejection of the molten weld pool.
Weld Purge Techniques
There are six principal methods for pipe purging:
While simple, low-cost solutions may offer some protection, they are generally unreliable for high-quality welds.
For instance, some still consider the use of screwed-up newspaper or cardboard discs to block the pipe on each side of the joint adequate, but this method is far from reliable. These materials risk catching fire during welding, and their removal after completing the joint is often problematic. Moreover, paper products usually contain moisture, emitting water vapour as temperatures rise, contaminating the weld.
Continuous inert gas flow without seals is sometimes used for small-diameter tubes, but this approach can overlook the potential for turbulence and oxygen entrapment. Additionally, constant gas flow can be costly.
The disc and gasket dam, which uses rubber or foam sandwiched between wooden or metal discs, offers some adjustability in diameter through axial pressure. However, this method is not collapsible, making post-weld removal difficult. Foam is also highly porous, allowing atmospheric gases and vapours to seep, compromising the weld quality.
Expandable pipe plugs can be effective and are commonly used for pipe pressure testing. These plugs cover diameters up to 1000 mm, and while smaller plugs often use nylon bodies, larger ones are made of aluminium or steel. However, the larger versions can be cumbersome to insert and remove, mainly if the joint line is far from the access point.
Soluble barriers provide some protection and can be removed by flushing with water after use. However, their bond to the pipe can be prone to leakage, and the time and skill required for preparation can be costly.
Flexible ‘floppy’ discs connected by a flexible tube are available and can be quickly deployed, with easy removal after welding. However, their reliability is questionable, as their effectiveness depends on a minimal contact area for sealing.
The most reliable purging systems are based on inflatable seals. Modern systems are designed to provide fully integrated control of purge pressure, minimising inert gas usage. They are suitable for site use where multiple welds of the same diameter must be completed quickly and efficiently.
QuickPurge® and PurgElite® products utilise the latest abrasion-resistant fabrics and low-profile gas control valve advancements. These systems meet the stringent standards of the nuclear industry.
Conclusion
Inflatable systems offer the best solutions for applications requiring rapid deployment, multiple use, reliability, rugged design, and effective all-around sealing. They are available for pipe sizes ranging from 25 to 2400 mm in diameter.
Additional Reading
