Characteristics of Measuring Weld Purge Results from a Distance.
Circumstances occasionally arise, particularly during on-site welding of pipelines, when it becomes necessary to measure purge gas oxygen content using instrumentation located some distance from the joint.
The accepted method of monitoring is to extract gas down a tube and take measurements at the exit of this exhaust tube.
Whilst this technique yields good results when the tube length is short, if below one metre, readings become inconsistent and unreliable once the sampling distance exceeds this.
In order to establish the reasons for this variation in readings Huntingdon Fusion Techniques, HFT® has initiated laboratory research but based on commercially available equipment.
The Test Set-Up is shown below.
The preliminary results confirm the feedback from field observations concerning erratic readings but also reveal some alarming statistics. The delay in readings when the measurement distance exceeds one metre can be up to 30 minutes to reach an oxygen level in the region of 100 ppm even when the actual level is only 25 ppm measured at 1 metre. The user can thus be misled by a very large margin and the welding process thus extended considerably.
Data in the above chart has been obtained by sampling oxygen content at 1 (series 3), 10 (series 2) and 20 metres (series 1) and shows the change in oxygen content in parts per million with time over a 30 minute period.
The efficiency of the HFT® system is indicated by the fact that an oxygen level of 100 ppm measured at 1 metre is reached in approximately 3 minutes. However at 10 metres this oxygen level is not reached for 20 minutes.
The reasons for the very significant delays can be attributed to a number of factors;
1. Gas Flow Velocity
The HFT® monitor used in the research incorporates 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.
2. Sampling Tube Gas Friction
Unlikely to be significant with the polyurethane material used but nevertheless contributory.
3. Sampling Tube Diameter
Possibly better laminar flow with increased diameter but even slower gas flow rate.
4. 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.
This preliminary research will be supported by further, more comprehensive, study but the results clearly show that measurements taken beyond one metre distance are unreliable and misleading.
Recognising this problem, and the consequent delay and cost of welding involved;
Huntingdon Fusion Techniques, HFT® has developed an electronic weld purge monitor that allows a sensing head to be placed at the nearest point to the purge exhaust, i.e. just behind an inflatable pipe purge system (See diagram) while the monitor displaying the results can be up to 1,000 metres from the joint.
This unique instrument, the Argweld® PurgEye Remote Weld Purge Monitor®, provides instant and accurate weld purge readings, eliminating delays in oxygen level measurement even at 1000 metres distance from the weld region.
Sampling tube: 6 mm od/5 mm id polyurethane
Oxygen measurement: HFT® PurgEye 500® . Pump extraction manufacturers pumping speed 4l/m unthrottled
Inert gas: grade 5 argon (99.999%A - O2 <2ppm)
1 Technical specifications available via HFT® website www.huntingdonfusion.com
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