TPI July 2022

Purge dams Much larger pipe sizes can be purged effectively using inflatable dams. These can be placed either side of the joint or used as single units for welding flanges/fittings and inflated with inert gas or air. Once inflated, inert gas is passed into the space between the dams to flush out air.

Figure 7 : Water vapour can be detected by using a dew point monitor

The free hydrogen liberated is known to give rise to porosity in the weld. Detection and removal of water vapour in the purge gas is therefore crucial if hydrogen porosity is to be avoided.

Conclusions Detection of oxygen and water vapour within the purge zone is essential if oxidation and porosity are to be avoided. Effective seals placed either side of the joint coupled with the use of instruments to continuously monitor oxygen and water vapour content in the purge gas are necessary. References 1. Porosity in Welding – Defects/Imperfections in Welds. The Welding Institute. Job Knowledge 1999 2. The solubility of gases in liquid metals and alloys. Austin, Chang, Fitzner, Min-Xian, Zhang. Progress in Materials Science. Volume 32, Issues 2–3, 1988 3. Sieverts Law. The absorption of gases by metals. Zeitschrift für Metallkunde. 21:37–46. Sieverts A (1929) 5. John C Cox, Digital Refining, Processing, Operations and Maintenance. March 2008. https://worldwidescience.org/topicpages/a/ argon+leak+detector.html 6. Ofem U, Effect of Back Purging Oxygen Content on Weld Discolouration and Corrosion Resistance …. Industrial Member Report. The Welding Institute Core Research Programme. Member report 1092/2017 4. Henry’s Law. https://chem.libretexts.org/@go/page/1599

Figure 5 : Large pipe dams are available up to 3m diameter to accommodate large diameter tubes and pipes or large pressure vessels

Weld purge gas monitors Oxygen

In order to ensure compliance with the required oxygen level within the weld purge zone it is desirable to measure the oxygen content continuously during the welding operation. Sensitive instruments have been developed specifically for weld purging applications. These purge monitors, of which a range to meet differing requirements is available, are robust devices capable of measuring oxygen levels as low as 10ppm.

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www.huntingdonfusion.com

Figure 6 : Purge monitor in use during pipe welding

Dr M J Fletcher is a qualified Metallurgist with extensive experience in welding and nondestructive testing. He works as an independent consultant, providing support to a wide range of manufacturing industry on a global basis.

Water and water vapour Dew point monitors have been designed to satisfy the requirement of the weld purge industry. Temperatures in the arc zone during welding are high enough to cause breakdown of water into its constituents.

Huntingdon Fusion Techniques www.huntingdonfusion.com

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TUBE PRODUCTS INTERNATIONAL July 2022

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