Water-soluble plastic film barrier boosts weld-purging efficiency.

Darren Sewell, managing director, Huntingdon Fusion Techniques Ltd., edited by Ron Lucas, managing editor

An Argweld purge-film kit available from Huntingdon Fusion includes 70 ft. 2 of film, two bottles of adhesive, a cutting knife, and an instruction manual.

A water-soluble membrane barrier is used as a dam inside a pipe to control purge-gas use, pressure, and oxygen levels.

Weld purging materials and practices typically cause companies to lose money, waste time, and hurt the environment. Knowing this, installations and facilities managers are always looking for techniques and materials that are safe, and environmentally friendly, while cutting operating costs and improving quality. In addition, they need repeatable, clean, strong welds, especially with stainless steel and titanium welding pipe and tube applications.

Fortunately, advances in weld-purging techniques let shops have the best of both these worlds.

Weld purging is the control of environmental oxygen levels around the weld. It is key in helping prevent coking, cracking, porosity, and uneven weld penetration in finished joints. In the process, inert gas, typically argon, is kept flowing inside of the pipe, tube, or around other weld surfaces, throughout the welding process. There are a variety of methods used to keep the purge gas in place.

Increasingly, companies employ membrane barriers such as water-soluble film or paper that they can flush from the system once welds are complete.

Bad purging practices
Weld purging is advantageous as it requires controlling the quantity of inert gas used during the process — simply pouring gas into a large-diameter pipeline could cost a company hundreds of dollars per weld. Operators can already easily and cheaply create a closed environment by damming either side of the weld. But they can't always do this efficiently. Thus, bad purging practices are common. For example, welders sometimes use rags, foam bungs, or wooden discs, resulting in lost gas and the ingress of oxygen-laden air.

The problem is these porous materials typically contain moisture. Once heat is applied, it's released as vapour into the warm and unsaturated gas, contaminating the gas and allowing unwanted reactions at the point of weld.

Creating positive pressure
A good weld purge provides constant positive pressure inside the dammed space for consistency during welding. The purge must also permit the pressure to decrease as the weld process reaches its climax in order to prevent a blow-out of the final weld material deposited. The only way to achieve this is with a virtual seal at the dams and a release-aperture of known area to allow for the desired leakage — also important in achieving the correct oxygen level (typically 0.1% volume), since even with oxygen meters there is a good risk of that levels may vary.

For the right welding conditions, inflatable bags are often used as virtual seals. In this application, the heavier-than-air purge gas is "poured" in at a nominal rate of 10 liters/min., or less, so as not to mix gases and to prevent air from being removed. Thus, the bigger the space, the more gas and time are expended. However, operators can't always reach the seals to take them out, and they often lack the means to properly dispose of the dam material after completing the job.

Switching to membrane-type barriers, which can be flushed away with water, solves this problem. Since many pipework installations are routinely washed or flushed with water, flushing does not represent an additional process. As with other methods, this procedure requires a dam on either side of the weld. However, because the surface is flat across the pipe or tube, the resulting narrow purge space requires less purge gas and time expended to fill it. Void pressure is controlled more easily, as are the oxygen levels when using an oxygen meter.

Paper or plastic?
Using water-soluble paper is adequate in many instances, although it does have significant disadvantages. In the food and drink industries, for example, paper leaves a residue of relatively large fibers and sludge that often remains after washing, causing blocked filters and harboring bacteria and other contaminants. Paper has also become more expensive and, adding the ecological implications of its use, shops are increasingly looking for lower-cost alternatives. On the other hand, soluble film dissolves fully, leaving no pulp residue and little organic matter. In addition, the material is environmentally friendly.

For example, poly (vinyl alcohol) (PVOH), a British-made in 35-µ-thick plastic film is water-soluble and biodegradable. Infrared spectrometry analysis has shown the level of residual organic product after dissolution is negligible. While water temperature has a bearing on the dissolution rate, in most instances PVOH film goes into solution faster than paper.

Welders can easily cut this material to size or make holes for gas insertion with little risk of tearing or splitting. In addition, dams made with PVOH can be washed away immediately once the weld has cooled, or left in place until the complete pipework system is ready for testing.

Supplied as a roll or kit including water-soluble adhesive, PVOH is FDA-approved for the wrapping of foodstuffs. This makes it well-suited for applications where contaminants pose costly implications.

About the author
Mr. Sewell is managing director of Huntingdon Fusion Techniques Ltd. with extensive development and company-management experience in weld purging. For more information write: Huntingdon Fusion Techniques, Stukeley Meadow, Gwscwm Road, Burry Port, Carmarthenshire SA16 OBU U.K.; phone: 01554 836836; e-mail: sales@huntingdonfusion.com