WHAT IS THE TERM PURGING IN WELDING?
Removal of oxygen, water vapour and any other gases or vapours that might be harmful to the welding.Purging a pipe prior to a weld is the act of displacing the atmosphere of a pipe or vessel and replacing it with inert gas.
Purging is critical to the success of welding stainless steel, titanium and their alloys for the nuclear, aerospace,petrochemical, pharmaceutical, semi-conductor, Oil & Gas, and food & beverages industries requires the proper tools and a clean environment to create a perfect weld environment.
WHY WE NEED PURGING&WHY PREVENTING OXIDATION IS IMPORTANT
To prevent the oxidation during welding. (For the best weld appearance and life)
When welding stainless steel, titanium and other corrosion resistant materials, an inert environment for welding is high priority to retain the metals corrosion resistant properties.
During the welding of steels, oxidation occurs in the weld seam if oxygen reaches this area unhindered. This situation is even more acute with Stainless Steel, Titanium and other reactive metals and alloys. The resulting oxidized surfaces are no longer corrosion-resistant, and further treatment is necessary.
Different ways to Remove oxidation or prevent oxidation in future.
Grinding, Pickling acids, Brushing & new welding cleaning machine (electro chemical process).
However, in some cases such as pipe welding, removing oxidation by above methods after welding is difficult or not economical to do.Difficult to remove oxides located in an area almost impossible to access.
Prevent it from occurring in the first place:
Instead of removing oxidation, another solution is to prevent it from occurring in the first place, as the old saying goes “Prevention is better than Cure” in essence.
Purging is a proactive rather than reactive approach which will provide a better outcome.
HOW PURGING WORKS
With purging, an inert gas is used to protect the weld area until it has cooled to the point that oxidation no longer will occur. It removes the contaminating air from the piping and vessels to reduce chances of corrosion.
Argon is the most commonly used purging gas due to its low cost compared to the other gases that are available. Argon displaces the gases you do not always want near your weld pool such as nitrogen, carbon dioxide, and oxygen.
Argon is heavier than air so it should have introduced at the bottom of the chamber and the air to exit towards the top of the chamber.
Also when purging, maintaining a positive pressure is preferred, a positive pressure will displace other gases and maintain the integrity of the purge gas.
We should see to it that no further oxygen penetration is possible, monitor the same using oxygen monitor and start welding.Purging should continue until the weld seam gets cooled to get the desired oxide free weld.
Different Purging technique & solution provided by Mehta Sanghvi& Company
- Water Soluble film B. Inflatable pipe purging system C. The Triple Seal System
- Gas cylinders E. Purge Plugs / Nylon Plugs G. Trailing Shields / Gas Drag
F. Gas Chambers / Flexible welding enclosuresH. Welding Backing tape