Purging for Tubes and Pipes for Improving your Technique
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.
Weld joints that require a root gap or which exhibit bad end matching, both of which characteristics provide an unwanted leak path for the purge gas, can be sealed by taping.
Oxygen and moisture levels in the purge gas should be checked using appropriate equipment. Whilst 0.1% residual oxygen is a suitable working level for materials such as some stainless steels, the level needs to be lower than 0.01% when welding the more sensitive alloys based on titanium and other reactive metals. Recent research indicates that levels for welds in the pharmaceutical, dairy and semiconductor industries need to be below 0.001%.
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.
On joints which are not fully sealed to restrict leakage a higher flow rate will be necessary to avoid contamination. Towards the end of the weld run however, as the joint becomes permanently sealed, the gas flow rate will need to be reduced to avoid over-pressurisation.
Weld Purge Techniques
There are six principle methods available for pipe purging.
Screwed up paper
Open Pipe Operation
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, plastic foam 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.
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.
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 up to 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 considerable.
These are suitable for low volume weld production where time is not of the essence and access to joints is readily available. Paper and engineering plastic versions are available but paper versions may not be suitable for site work because of their fragility. Time, dexterity and patience are required during application and the film can only be inserted as far away from the pipe access as an engineer can reach.
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 where the system needs to be used either side of a sharp pipe bend.
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.
The HFT 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 anywhere. Systems are available to accommodate diameters between 25 and 600 mm.
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.
Responding to industry feedback high temperature versions of these inflatable systems are now available to meet the requirements for welds requiring pre- and post- weld heating. They can be used at temperatures up to 300ºC for several hours without deterioration.
Recent innovative work by HFT designers has led to the introduction of several further advances in equipment production. A patented control valve called PurgeGate® has been added. This prevents over-inflation even when the operator attempts to increase purge gas pressure.
RootGlo® illuminating tape is now fixed at the centreline of the purge system. This glows brightly inside the pipe to help welders position the equipment accurately. IntaCal® is a pre-calibrated gas release valve and replaces alternative expensive vales that need pre-setting prior to each weld procedure.
Weld Purge Monitors® are now an essential part of the purging process and provide accurate information on purge gas oxygen content. The latest instruments from HFT are compatible with data recording systems and can provide audible alerts when the required oxygen content has been reached and even stop the welding process if this level exceeds that set by the welding procedure.
Providing precise data on cost comparisons between the different purging techniques is difficult not least because the pipe diameter and wall thickness have a profound influence on the cost. Furthermore it is impractical to use some types of removable inserts in sealed pipes or in pipes and tubes with diameters much below 100 mm.
An illustration of costs can be provided by examining the differences between soluble film (claimed to be cheap) and engineered inflatable systems can be seen.
|TYPE OF PURGE SYSTEM||NUMBER OF WELDS||COST FOR 30 MM PIPE DIAMETER US$||COST FOR 750 MM PIPE DIAMETER US$|
|Pre-cut Soluble Paper||1||17||170|
|Commercial Weld Purge System||1||270||700|
Comparison of costs between soluble paper and commercial weld purge system
It is clear from the table that where several welds have to be made on similar pipe diameters there can be genuine cost savings when using inflatable pipe purging systems as the sealing medium. Add this to the technical advantages of reliable sealing and ease of use and the inflatable purge system concept can be seen to offer significant attractions.
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