Part 2 Pipe and Tube Weld Purging
In Part 1 Dr Fletcher set out to show many inconsistencies in the approach to weld purging, especially in the choice of gases used.
His conclusions were as follows:
Much more definitive work is now essential if fabricators are able to proceed with confidence in consistently producing welded joints capable of meeting the demanding standards imposed by service conditions. In the meantime the precautionary note from the American Welding Society offers the best advice with a conclusion that the shielding gas needs to be matched to the metal composition.
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 fallacious. 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 15 - 20 l/min
Whilst 0.05% (50 ppm) residual oxygen is suitable for ensuring zero colour welds in materials such as stainless steels, the level needs to be lower than 0.02% when welding the more sensitive alloys based on titanium and other reactive metals.
The Weld Purge Process
Once the quality of the gas in the dammed volume has reached the required level, gas flow can be reduced to about 5 l/min for the welding operation. On a more practical level it should just be possible to feel the gas flow from the exit point. Excessive flow can cause the internal pressure in the pipe to rise and create concavity in the weld root geometry and in more extreme cases can cause complete ejection of the molten weld pool.
Weld Purge Techniques
There are six principle methods available for pipe purging.
- Screwed up paper
- Open Pipe Operation
- Soluble Discs
- Expandable Plugs
- Flexible Discs
- Inflatable Systems
Generally the simple and low-cost solutions may well serve to offer limited protection but they are far from being totally reliable.
It’s hard to believe that the use of screwed-up newspaper or cardboard discs to block the pipe on each side of the joint and rely on this as an effective seal is still considered by some to be adequate. Even if they don’t burst into flames during the welding cycle the problem of removal after completion of the joint is rarely considered.
Furthermore most paper products contain moisture and this can emit water vapour as the purged volume temperature rises and contaminate the weld.
For small diameter tubes, say up to 10 mm, the use of continuous inert gas flow without seals is not uncommon. Air is replaced by incoming inert gas. However it overlooks the possibility of turbulence and thus entrapment of oxygen. Continuous gas flow can also be costly.
The disc and gasket dam uses rubber or foam sandwiched between a pair of wooden or metal discs and some adjustment to diameter can be effected by applying axial pressure. This gasket technique is not collapsible and after welding the discs must be pulled out past the weld root, an operation that may cause difficulties. Cheap, yes, Reliable, no!
Again foam is highly porous and continually allows atmospheric gas and vapour through, as dammed volume removes the atmospheric gases producing a low partial pressure which the gas on the outside of the foam is trying hard to constantly equalise.
|Home-made Foam Dam Systems|
Expandable pipe plugs can be very effective and cheap. These are widely used for pipe pressure testing and volume production means that cost is low. The sealing area is large and the time involved in preparation is relatively small. Plugs are available covering diameters upto 1000 mm and whilst the smaller plugs are available with nylon bodies, aluminium and steel are used above 150mm. The larger and therefore heavier versions may thus be difficult to insert and remove if the joint line is more than 500 mm from the access point.
Soluble barriers cut to pipe size and glued to the internal diameter provide some degree of protection and the discs can be removed by flushing with water after use. The bond to the pipe is prone to leakage however and the time and skill involved in preparation can be expensive.
Flexible ‘floppy’ discs connected by a flexible tube are available. They can be deployed quickly and removal after welding is easy since the assembly can be withdrawn past the weld. Reliability is suspect however because disc to pipe sealing depends upon a very small contact area. Some have a semi-rigid connecting tube and this may be unsuitable.
Improvements on the compressed gasket have been introduced as cost-effective solutions to weld purging.
None the less, they still only fit a very limited range of internal tube or pipe dimensions and sealing depends entirely on the disc edge being true - even minor variations in diameter and roughness cannot be accommodated.
The only totally reliable purging systems are those based on inflatable seals. Contemporary equipment has been designed to provide fully integrated control of purge pressure. Inert gas use is minimal and they are compatible with site use where multiple welds of the same diameter need to be made quickly and efficiently.
Both QuickPurge® and PurgElite® products exploit the latest engineering developments in abrasion resistant fabrics and low profile gas control valves. They are compatible with standards laid down by the nuclear industry.
The family of QuickPurge® equipment was introduced in 2004 to meet a growing global demand for fully integrated inflatable weld purge systems. The family covers the range between 150 and 2400 mm diameter and is designed for repeat use under site conditions. QuickPurge reduces inert gas use by minimising the purge volume.
Following the success of QuickPurge®, HFT developed a complementary product range, PurgElite®, that includes the smallest integrated purge system currently available. Systems are available to accommodate diameters between 25 and 200 mm.
Thermally resistant versions of the QuickPurge® Systems are available for use when welding pre-heating and post-heating are required. They can accommodate temperatures up to 300ºC for 24 hours. This allows these devices to remain in place in the hot pipe work while controlled cooling is taking place to prevent undesirable metallurgical characteristics being developed.
If rapid deployment and retrieval, multiple use and reliability coupled with rugged design and effective all-round sealing are required then inflatable systems offer the best options. They are available to cover all pipe sizes between 25 and 2400 mm diameter.
- Analysis of the results of recent study of reliability, effectiveness and cost of using weld purging techniques.
Stainless Steel World 2013
- Highly innovative developments in purging meet the most demanding applications.
World Pipelines 2013 Oct
- Gas Purging for Weld Root Control during Pipeline Fabrication
Kuwait International Pipeline Conference 2010
- Analytical review of innovative products designed to provide inert gas protection
American Welding Society Spring Conference 2010
- Major Advances in Weld Purge Technology
World Pipelines 2014
Latest developments in weld purging
- Weld Purging for Tubes and Pipes
Stainless Steel World 2014
A helpful and informative series of technical publications has been produced by Huntingdon Fusion Techniques as White Papers. These are based on practical applications of weld purging and are available free on request from www.huntingdonfusion.com .
By Dr. Michael J. Fletcher M.Sc. Metallurgy
This White Paper is Successfully Published in Worldwide Magazine Stainless Steel World:
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