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Use of water-soluble film as a barrier material in weld purging
Use of water-soluble film as a barrier material in weld purging
Keywords: Welding, Materials
Creating a controlled environment around a weld inside a pipe or tube can be a problematic process, but using water-soluble plastic film as a purge barrier reportedly contributes to better efficiency and overall cost effectiveness. Darren Sewell, Managing Director of Huntingdon Fusion Techniques, discusses the options.
It is ever more desirable for installations and facilities managers to employ techniques and materials that contribute to better safety and are more environmentally friendly. It goes almost without saying that operating costs have to be cut at the same time – and quality improved!
In the specialised but now widespread field of stainless steel or titanium welding, particularly in pipes and tubes, repeatable and clean, strong welds are critical. Several disciplines are involved in achieving this, including weld purging, or the control of environmental oxygen levels around the weld in order that the finished join is free from coking, cracking and porosity or uneven penetration of the weld material prevented. In order to retain the purge gas in place, a variety of means can be deployed and, increasingly, operators are using barriers that are readily soluble and can be flushed from the system once the welds are complete.
These barriers are made from water-soluble film or paper. The option of soluble film provides key benefits for the operator, the material dissolves fully, leaving no residue of pulp and negligible organic matter; also the material is more environmentally friendly since it is not made from wood.
Weld purging is pushing out from inside pipes and tubes of air with inert gas, generally argon. The inert gas is kept flowing into the inside of the pipe, tube or around other weld surfaces throughout the welding process.
Whilst quality and reproducibility are becoming ever more important, cost reduction is, as always, high on the agenda in every boardroom. With weld purging, it is now vital to both control the quality of the weld and the quantity of inert gas being used during the process. Simply pouring the gas into a large diameter pipeline could cost hundreds of dollars per weld. Creating a closed environment by damming either side of the weld can be achieved easily and cheaply but not always efficiently; bad purging practice is still common, using such materials as rag, foam bungs, or wooden discs with the obvious loss in gas and ingress of oxygen-laden air.
Such materials can introduce an added problem. Any porous material will contain some amount of moisture and the vapour released into a warm and unsaturated gas contaminates the space and allows unwanted reactions at the point of weld.
It is necessary for a good purge to have positive pressure inside the dammed space. This pressure is decreased as the weld reaches its climax, preventing blow-out of the final weld material being deposited but in the meantime, a constant pressure is needed for consistency. The only way to achieve this is with a virtual seal at the dams and a release-aperture of known area to allow for desired leakage. This is also important in achieving the correct oxygen level – typically 0.1 per cent volume – as, even with oxygen meters being employed, the likelihood of unpredictable gas levels is high.
In order to achieve the right conditions, virtual seals are often achieved with inflatable bags where the purge gas, heavier than air, is “poured” in at a nominal rate of 10l/min or less, so as not to mix the gases and prevent the air from being removed, so the bigger the space, the more gas and time are expended. However, accessibility for removal is not always possible and the means to properly dispose of dams after welding makes the use of disposable material necessary.
This problem, as with many others, is solved by using membrane-type barriers which can be flushed away with water. Many pipework installations are washed or flushed with water as a matter of routine so this is not an additional process. A dam is placed either side of the weld, as with other methods but, because the surface is flat across the pipe or tube, a narrow purge space can be accomplished with little purge gas required and little time expended in filling it. Pressure inside the void is more easily controlled, as is the actual level of oxygen by means of an oxygen meter.
Where it is appropriate to use such a barrier method, the choice is between the old industry standard of paper or the technically superior approach, water-soluble plastic film. Water- soluble paper has been seen as adequate in many instances, although it produces significant disadvantages in many applications.
The main problem is that paper leaves a residue of relatively large fibres and sludge which can remain after washing and cause filters to become blocked. Paper is also becoming more expensive and, with the ecological implications of its use, increasingly operators are looking for a better alternative that carries lower operating costs.
PVOH, a British-made plastic manufactured in 35 μm film is truly water-soluble and bio- degradable. Analysis by IR spectrometry has shown the level of residual organic product after dissolution to be negligible. Whilst water temperature has a bearing on the rate of dissolution, PVOH film has been shown to go into solution faster than paper in most instances.
Supplied as a roll and with a kit including water- soluble adhesive, PVOH is FDA approved for wrapping of foodstuffs and therefore, suitable for applications where contaminants would present a problem with costly implications. The material is easily cut to size and holes for the insertion of gas are easily made with little risk of tearing or splitting. Dams made with PVOH can be washed away immediately the weld has cooled or left in place until a complete pipework system is completed ready for testing.
Specialist Huntingdon Fusion Techniques manufactures PVOH weld purging barrier kits under the trade name Argweld. Full instructions for use are included and the company also produces a free booklet, “Guide to Weld Purging”, available from the company or its distributors.
Details available from: Huntingdon Fusion Techniques. Tel: +44 (0)1554 836836; E-mail: firstname.lastname@example.org