Product category:
Machinery and Production Equipment
News Release from: Huntingdon Fusion Techniques | Subject: Argweld
Edited by the Engineeringtalk Editorial
Team on 09 June 2004
Water-soluble film forms barrier in weld
purging
Creating a controlled environment around a weld inside a pipe or tube can be a problematic process: Darren Sewell, Managing Director of Huntingdon Fusion Techniques, discusses the options.
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 contributes to better efficiency and overall cost effectiveness 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
This article was originally published on Engineeringtalk on 24 Jun 2004 at 8.00am (UK)
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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.
Further reading
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Argweld weld purge film has now been formally approved for use by the US Nuclear Management Company.
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 the 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.
Although 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% 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 10 litre/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 that 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 place 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, in the food and drinks industries for example, is that paper leaves a residue of relatively large fibres and sludge which can remain after washing and cause filters to become blocked - quite apart from the risk of harbouring bacteria and other contaminants.
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 35um film is truly water-soluble and biodegradable.
Analysis by IR spectrometry has shown the level of residual organic product after dissolution to be negligible.
Although 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 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.
Darren Sewell is Managing Director of Huntingdon Fusion Techniques Ltd and has a long history of development and company management in the field of weld purging. Request a free brochure from Huntingdon Fusion Techniques ...
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