Tech Talk - Training
Introduction to TIG Welding
Inert gas welding is a process of arc welding in which the molten weld pool is shielded from atmospheric contamination and oxidation by shielding it with an inert gas such as Argon. Two common inert gas welding processes include GTAW (Gas Tungsten Arc Welding) and GMAW (Gas Metal Arc Welding (GMAW); commonly know as TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) welding; respectively.
Purpose of Pipe Purging
The objective of pipe purging is to shield the back of the joint from atmospheric contamination. It is a two-stage operation, involving pre-purge and purging during welding. The pre-purge displaces air inside a pipe with an inert gas; the purge during welding maintains the oxygen free backing gas achieved by the pre-purge and prevents air ingress into the system.
Effectiveness of purging depends upon:
- An oxygen content inside the pipe that can be tolerated by the material during welding, without detrimentally affecting joint performance.
- The purge gas containment system.
- Pipe diameter, pre-purge gas flow rate and the pre-purge time.
There are several purge dam alternative methods available including:
- EZ Purge® pre-formed water soluble purge dams
- Aquasol® water soluble paper dam
- Fiback™ fiberglass weld backing tape
- Water-soluble film
- Removable rubber gasket
- Removable hinged discs
- Removable expansion bags
- Thermally disposable cardboard discs
The most suitable approach depends on access, pipe diameter, available equipment, joint configuration and economic factors.
Purge Gas Containment
Two approaches can be applied to purging pipes: purging the entire system, or localized purging.
Total purging of the pipe system is normally only applied to small diameter pipes, where the volume inside the pipes is not considered a significant gas cost factor.
Localized purging is used more often due to excessive cost of noble gas. This normally involves use of purge dams, which are placed in the pipe during joint assembly. There are several different types of dams that may be used, including:
- The all new, patented EZ Purge®. EZ Purge® is a pre-formed, sel-adhesive water soluble purge dam that dissolves instantaneously during hydro-test. Available in sizes from 2" to 36", this is the simplest solution.
- Aquasol® water-soluble paper can be formed into dams of any size that are taped inside the pipe. After, welding, these are dissolved by water flowing through the pipe.
- Fiback™ is a fiberglass weld backing tape that eliminates or reduces the need for purging.
- Removable dams, such as hinged collapsible, expansion bags (inflatable bladder dams) rubber gasket discs. These are retrieved following welding by using attached pull cords or rods.
- Thermally disposable dams, such as cardboard discs. These are normally reduced to ash during post weld heat treatment, and subsequently washed away.
Conventional purge dams such as inflatable rubber bladders, rubber gaskets or cardboard discs traditionally must be placed far from the weld zone, anywhere from 10-20 feet. Remote placement of the purge dams may require large amounts of purge gas to be introduced to fully displace the oxygen in the weld zone, making the use of the conventional purge dams cost prohibitive.
To overcome this problem, the assembly of water-soluble purge dams was introduced. The greatest advantage of water-soluble purge dams is that they can be placed in close proximity (6 to 8 inches) from a weld zone, then removed following welding during hydrotesting or flushing by dissolving them with water or steam introduced through the pipe. The purging process is simplified and expedited and must less inert gas is used.
This process has been even further enhanced with the introduction of EZ Purge®. Since EZ Purge® is pre-formed, there is no need to measure, cut or construct a purge dam. Save time and money with EZ Purge®.
Water Soluble Paper
An alternative to traditional cardboard purging systems, Aquasol® water-soluble paper and tape are extremely easy to use, inexpensive, practical and provide a safe work environment for welders. Aquasol® is made of environmentally friendly and non-toxic sodium carboxy methyl cellulose and wooden paper that dissolves rapidly and completely in most liquids including water. Aquasol® paper is available in sheets, rolls and pressure sensitive tapes.