Cleaner welds

Louise Davis

Weld Purging (back purging) describes the removal of oxygen from the vicinity of a weld before, during and after welding. For pipe and tank welding of stainless steels, titanium and chrome steels, it is good practice to restrict the area required to be purged, usually with an inflatable tandem weld purging system.

For the purging of short pipe pieces and dished end or dome end welds however, the tandem devices are unsuitable. Welders over the years have created single dams to block the ends of pipes and cylindrical tanks using easily obtainable, locally available material.

Ron Sewell, Chairman at Huntingdon Fusion Techniques said: “Even today, many companies are still allowing their technicians to spend many hours fabricating dams made of foam, cardboard, adhesive tape, wood, paper and so on. These materials contain a high percentage of water, which is very undesirable to have in the presence of a weld.”

“As these old fashioned, poor quality dams and materials are warmed by the welding operation, they start to outgas their water vapour, which starts to circulate around the weld joint and combine with the weld pool to cause porosity and oxidation, leading to metallurgical defects.”

“In addition, the purging process takes many times longer to reach a low enough level for welding to begin at which point the inert gas flow needs to be raised to a much higher level to combat the outgassing.”

Using the technology and skills that they have obtained over 43 years of design and manufacture, Huntingdon Fusion Techniques has created a series of low cost, but professionally manufactured Weld Purge Dams, using low vapour pressure materials that seal tightly around the circumference of the volume to be purged.

Available for pipe diameters from 6 to 88 inches (150 mm up to 2,235 mm), the HFT Single Ended Inflatable Weld Purge Dams are made of low vapour pressure materials specially selected for optimum weld purging results to provide the lowest oxygen reading in the fastest possible time with the minimum use of expensive argon gas.

The Weld Purge Dams are fitted with a dual inflation and purge hose, an additional purge gas hose and an exhaust which can be connected to one of the HFT Weld Purge Monitors.

The additional purge gas hose is suitable for introducing extra purge gas into the weld zone at any time to cool welds to meet interpass temperature specifications, or to provide more inert gas in the event of titanium, zirconium or special stainless applications needing a guaranteed zero colour weld.

Once the Dam is inflated using the purge gas and seals all around the internal circumference of the pipe, the excess inert purge gas spills out and purges the space around the weld joint. The air (oxygen) is then released out and into the open through a series of venting exhausts on the Dam.

Four pull tags are located around the circumference of the Weld Purge Dam, so rope or pull wires can be attached for retrieval of the deflated system, as required. By producing bright shiny welds, there will be no porosity and there will be no loss of corrosion resistance caused by oxidation.

Apart from the benefit of having a metallurgically sound weld, the difficulties of cleaning an oxidised weld are eliminated, saving vast amounts of money in labour and material costs as well as the disposal costs where acids are concerned.

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