The importance of gas purging when welding stainless steels has long been recognised, particularly in the more sensitive sectors such as petrochemical, pharmaceutical, cryogenics, food and drink and semiconductor. The presence of oxygen causes, at best, an unsightly appearance but, more significantly, oxide deposits on the joints can become detached and lead to serious product contamination 1,2 3,4.
Recent research undertaken by technologists at Huntingdon Fusion Techniques, HFT® has revealed some startling evidence. Whilst existing Weld Purge Monitors® can be applied confidently to take measurements close to the welding source i.e. within one metre, they have been shown to have shortcomings beyond this distance.
Recognising the need for more precise remote monitoring, HFT® has developed a unique purge gas measuring instrument with which accurate and rapid observations can be made up to one kilometre from the weld. For quality control purposes the instruments can be integrated into a recording and data processing system.
Measuring oxygen content
Although electrochemical or amperometric principles have been in use for many years, those ‘wet cell’ concepts are quite unsuitable for continuous operation in industrial environments. Later work witnessed the introduction of instruments based on zirconium oxide sensors and these form the basis for contemporary oxygen Weld Purge Monitors®.
Weld Purge Oxygen Monitoring
Recent investigations have shown that oxygen levels considerably lower than those previously considered adequate for good welding results, are necessary. Whilst a recommended maximum of oxygen in purge gas of 100 ppm has hitherto been considered acceptable, this new work shows that levels of 20 ppm and less, are essential if loss of corrosion resistance is to be avoided.
These observations have led to the development of Weld Purge Monitors® capable of accurately measuring oxygen levels down to 10 ppm. Furthermore they are robust, accurate and offer greatly extended sensor life.
The simplest instruments rely on gas flow through a tube connecting the purge volume and the analyser, (Fig. 1) but improved models also incorporate manual or pumped extraction to improve speed and provide a consistent gas flow.
Basic oxygen monitoring instrument. These are robust but can only be used reliably when positioned close to the weld joint i.e. less than one metre.
Most stainless steel welding operations, especially tube and pipe fabrications in the petrochemical, food, cryogenics and pharmaceutical sectors, require measurements to take place some distance from the joint. For example, when there are long runs in a pipework fabrication a tube from the purge gas exhaust will be dragged through and remain connected to the monitor at the starting point. It is not unusual for this to take place over several metres. Clearly, the monitor will be required to measure oxygen content from the most extreme joint.
This situation is aggravated by an increasing trend for quality engineers operating remotely from the welding location to monitor and control purge gas composition prior to releasing permission to weld: this leaves the welder to concentrate on purge system positioning and welding. On a complex and remote situation the quality engineer could be several hundred metres from the joint being welded.
The HFT® work, Appendix 1, clearly shows that measurements taken beyond five metres distance are unreliable and misleading. Recognising this problem, and the consequent delay and cost of welding involved, Huntingdon Fusion Techniques, HFT® engineers have developed an advanced electronic Weld Purge Monitor®.
Appendix 1: Research Methodology and Results
Appendix 1: Experimental set-up for research programme.
Methodology and Experimental data
- Sampling tube: 6mm od/5mm id polyurethane.
- Weld Purge Monitor®: HFT® PurgEye® 200 IP65 used for 150 mm measurements.
- Weld Purge Monitor®: HFT® PurgEye® 300 Nano used for all other measurements.
- Inert gas: grade 5 argon (99.999%A - O2 <2ppm).
This unique instrument, the Argweld® PurgEye® 1000 Remote Weld Purge Monitor®, (Fig.2.) eliminates delays in oxygen level measurement even at 1000 metres distance from the weld region.
Oxygen content is measured close to every weld and signals are transmitted electrically via a data cable to a display facility. For quality control purposes this is integrated into a recording and data processing system.
The latest digital monitor overcomes the delay involved when used remotely from the joint. It incorporates data processing and storage facilities for integration with quality control programmes.
Results confirm the feedback from field observations concerning erratic readings but also reveal some alarming statistics. The delay in readings when the measurement distance is five metres can be up to 15 minutes to reach an oxygen level in the region of 0.1% even when the actual level is only 0.0025%. The user can thus be misled by a very large margin and the welding process thus extended considerably.
The reasons for the very significant delays were attributed to a number of factors;
Gas flow velocity. The HFT® Weld Purge Monitors® used in the research incorporate an extraction pump but the manufacturer’s pumping speed is based on unthrottled measurement at the pump inlet. Even at one metre this speed is likely to be much reduced. Increasing the pumping speed however will lead to turbulence and aggravate the readings even more.
Sampling tube gas friction. Unlikely to be significant with the polyurethane material used but nevertheless contributory.
Sampling tube diameter. Possibly better laminar flow with increased diameter but even slower gas flow rate
Sampling tube material. Polyurethane is probably the material offering least resistance to gas flow and unlikely to contribute to oxygen pick-up from the tube wall.
The efficiency of HFT® Weld Purge Systems is indicated by the fact that an oxygen level below 0.01% (100 ppm) measured against the purge zone is reached in less than 15 minutes. However at 30 metres this oxygen level has still not been recorded after an hour. Even at 10 metres it takes over 30 minutes to achieve 0.01% oxygen. Note: 0.1% = 1000 ppm.
PurgEye® 1000 Remote Weld Sensing Head mounted inside QuickPurge® Pipe Purging System.
- Microbiologically influenced corrosion of stainless steel.
Titz. 2nd symposium on orbital welding in high purity industries, La Baule, France.
- Effects of purge gas purity and chelant passivation on the corrosion resistance of orbitally welded 316L stainless steel tubing.
Pharmaceutical Engineering. Vol 17 Nos 1 & 2 1997.
- Considerations for Orbital Welding of Corrosion Resistant Materials to the ASME Bioprocessing Equipment Standard Henon, Arc Machines Inc. Stainless Steel America conference 2008.
- Heat Tint Poses Corrosion Hazard in Stainless Steel.
Fletcher, Welding Journal December 2014.
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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