Purge Assemblies

A purge assembly is a device that seals a section of pipe or tube and fills it with inert gas (typically argon or nitrogen) during TIG (GTAW) welding of the root pass. This prevents the back side of the weld from being oxidized by atmospheric oxygen and moisture, which causes sugaring (rough, porous oxidation) on stainless steel and contamination defects on titanium and other reactive metals. Purging is required any time the root pass of a pipe weld cannot be oxidized — especially on food-grade, pharmaceutical, and high-purity piping.

Argon is the most reliable purge gas for stainless steel — it is inert to all metals and heavier than air, so it displaces air downward when introduced from the bottom of the purge zone. Nitrogen is an acceptable lower-cost alternative for austenitic stainless (304, 316 grades) as it does not react significantly with these alloys, but it should not be used for duplex stainless, titanium, or nickel alloys. Argon purity should be 99.995% or better for critical applications such as semiconductor or pharmaceutical piping.

Use a purge monitor (oxygen analyzer) to measure residual oxygen in the purge zone. For standard stainless steel, oxygen levels should be below 100 ppm (0.01%) before welding; for high-purity or discoloration-sensitive work, below 50 ppm is preferred. As a practical rule without a monitor, flush with at least four times the volume of the purge zone with inert gas before beginning, then maintain a slight positive pressure during welding. Never weld on stainless without confirming purge adequacy — the resulting oxidation cannot be removed by post-weld cleaning alone.

AWS Welding Handbook (9th ed.) recommends backup purge flow rates of 0.5 to 42 L/min (1 to 90 cfh) depending on the volume being purged. A typical small-diameter pipe purge (2–4 in. / 50–100 mm NPS) uses 5–15 cfh during the purge phase; once oxygen is displaced, reduce to 2–5 cfh just to maintain positive pressure. Excessive flow pressure during the final inch of root closure can blow out the weld pool — reduce flow as you close the joint.

Most inflatable purge assemblies and water-soluble dam plugs are designed for single-use or limited reuse — check the manufacturer's instructions. Inflatable bladder-style assemblies can typically be reused many times if not damaged. Select a dam by pipe ID (inside diameter), not nominal size; for example, a 2-inch nominal pipe has an ID that varies by schedule (Sch. 40 is 2.067 in. / 52.5 mm, Sch. 80 is 1.939 in. / 49.3 mm). Measure your actual pipe ID for a proper seal.

For pipe root passes, you purge the ID (inside diameter) to protect the root bead from the back side. The OD (outside) is protected by the TIG shielding gas flowing from the torch nozzle. However, for highly reactive metals like titanium or for very thin-wall tubing, trailing shields — extended gas coverage behind the torch — are used on the OD to protect the hot weld metal as it cools. The AWS GTAW handbook recommends trailing shields for titanium even on outside-diameter passes.