WP-27 Weld Purging on LNG Projects

Flexible Discs versus Inflatable Seals

41W-QuickPurgePipeWeldPurgeSystemDespite the fact that professionally designed weld purging systems have been available for some time it's surprising that fabricators of stainless steel tube and pipe work still employ unreliable home-made devices or unsuitable products, to prevent oxidation of the weld root.

Even on prestigious and demanding work such as LNG storage and distribution the use of plastic, paper foam, cardboard and even screwed up newsprint proliferates.

It's a fallacy that such relatively archaic practices are more cost-effective than the use of widely available, proven, properly engineered systems.

Defining the problem

Joints of high quality between stainless steel cylindrical sections such as tubes, pipes and vessels can only be made by ensuring that atmospheric gases, and in particular oxygen, are eliminated. 

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WP-25 Help with Best Practice for Purging

Advanced Training Courses - Safety Critical Welding

nuclearWelding skills and the introduction of new technologies are focus areas behind a series of recommendations set out in the "2012 Nuclear Construction Lessons Learned" study report.

The report, published by the Royal Academy of Engineering, notes the need for advanced training in some welding practices*.

'Welding is recognised as an area at high risk of skills shortage, with an ageing and retiring demographic'

A guidance document includes a recommended approach to Quality Assurance, referencing ASME and RCC/ETC codes/standards with detailed discussion of the relevance of the ISO 3834 framework welding-specific Quality Assurance. 

Whilst this significant study is nuclear-specific it highlights the urgent need for specialist training in all sectors where safety critical welds are produced.  Such industries include the petrochemical, pharmaceutical, food and beverage sectors and indeed the power generation industry generally.

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WP-21 Help with Pristine Weld Purging of Tubes

Demand for Help with Inert Gas

Manufacturers involved in the biofuel, semiconductor, aerospace, food production and pharmaceutical industries are all concerned with handling under aseptic conditions. The presence of any debris and contamination in finished assemblies is unacceptable and quality control procedures must be rigidly adhered to.

Nowhere is this more significant than in fabrication of tubular sections of processing plant. Attention to detail here is crucial since even small installations may involve kilometers of pipework. Mistakes during fabrication can therefore be costly.

Demand for help with inert gas purging to meet the stringent quality control requirements set by fabricators has led to the development of a wide range of ancillary products. Global leader in design and manufacture of weld purging equipment is Huntingdon Fusion Techniques Ltd, a company that has specialised in meeting the requirements of end users for 35 years.

The HFT product range includes inert gas pipe weld purging equipment covering diameters from 12 to 1800 mm. It also manufactures advanced monitors designed specifically to measure and control oxygen content of purge gases used in welding applications. 

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WP-19 Why Should You use a Weld Purge Monitor®

figure 1. PurgEye100 IP65 WeldPurgeMonitorWhat is Weld Purging?

Preventing oxidation and contamination of joints during fusion welding demands that special attention be given to local protection using an inert gas.

Whilst the welding torch provides a protective cover to the top of the fusion zone, inert gas coverage is also required in the region of the weld root.

If this is not done, the weld root and local metal will at best discolour but invariably will become heavily oxidised.

The solution to the problem is to provide a weld purging technique. This is most easily done by sealing the weld region using inflatable purge bladders.

Other techniques are available - expanding plugs, tapes and membranes are often used. The sealed region can then be filled with inert gas such as argon or helium and this provides a very effective insurance against oxidation during welding.

Typical expanding plug manufactured in corrosion resistant nylon and rubber.

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WP-17 Why should you use Inert Gas Purging when welding Stainless Steel, Titanium and Nickel Alloys?

16W-PurgEliteWeldPurgeSystemsWhy should you use Inert Gas Purging when welding Stainless Steel, Titanium and Nickel Alloys?

Preventing oxidation and thus avoiding discolouration and weld underbead discontinuities in tubes and pipes can be achieved readily by purging the inside with inert gas. This simple procedure saves both time and cost, both of which can be considerable. Consider the thoughts of one specialist.

'It's difficult persuading our supervisors to allow us to take the time to purge lines during the welding process! Instead, engineers weld the lines, then use a die grinder with a flapper wheel to clean the "grapes" off the inside of the pipe! What an incredible waste of time.

Purging really makes more sense in the long run, especially since products going through these lines get snagged or contaminated.

The troubleshooting process is confused because the pipe fitter may not see the correlation, nor does the machine operator unless there is good exchange of information!'

Greg Godek - Integrity Piping Solutions Inc

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WP-15 Post Weld Purging -Why is it necessary

38W-QuickPurgePipeWeldPurgeSystemWhilst most welders recognise the need for inert gas purging when making quality joints, it is not obvious why purging should be continued after the weld has been completed. 

The answers are clear:

  1. Hot metal oxidises in the presence of air so the weld and any metal surrounding it will discolour until it has cooled below the oxidation temperature for the metal being welded. This applies equally to the top and bottom weld bead.

  2. Although nominal oxidation, i.e. a light straw shade, is quite acceptable in some cases, and in any event can perhaps be cleaned at a later stage, some applications require total absence from contamination of this type. This is particularly so in the pharmaceutical and semiconductor industries where installations are normally completed in the 'as welded' condition.

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WP-11 Tube and Pipe Welding

High quality welds in pipes and tubes can only be assured if the weld underbead is protected from oxidation. The various techniques available through which protection can be given is referred to as purging.

The Pre-Purge Process

A pre-purge is used to displace air present in the pipework system or dam volume. Numerous factors control the pre-purge time such as pipe diameter, purge volume and maximum permitted oxygen level.

A common misconception is that increasing the purge flow rate will reduce the purge time. This is falacious. Increase in flow rate increases turbulence and results in unwanted mixing of purge gas and air and can actually extend the purge time. As a general rule, the pre-purge flow rate and time should allow for about five volume changes in the pipe system or dam volume, but a typical gas flow rate will be in the region of 20 l/min.

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