What is a weld purge system and why do I need one?
A weld purge system displaces oxygen from the back side (root side) of a weld joint before and during welding. Without back-purging, oxygen and nitrogen react with the hot weld pool to cause oxidation ('sugaring') in stainless steel and discoloration or porosity in titanium and other reactive metals. The purge system consists of inflatable dams or bladders that isolate a section of pipe or a joint cavity, plus a gas inlet to fill that cavity with argon or nitrogen before you strike the arc.
What purge gas should I use — argon or nitrogen?
Argon is the preferred purge gas for most stainless steel, duplex, and nickel alloy pipe welding, and it is required for titanium. Nitrogen can be used satisfactorily for purging the back side of welds in austenitic stainless steel, copper, and copper alloys where some minor surface discoloration is acceptable. For semiconductor tubing and ultra-high-purity applications, the backup purge gas purity may be specified at 99.999% or higher.
How much gas flow is needed for back-purging a pipe joint?
Backup purge gas flow rates range from 0.5 to 42 L/min (1 to 90 cfh) depending on the volume of air to be displaced. A general rule is to flush the purge cavity with four times its volume in inert gas to achieve a relatively inert atmosphere; high-quality applications require more volume changes. Once purging is complete, reduce the flow to maintain only a slight positive pressure inside the cavity during welding — excessive pressure can cause weld pool blowout or root concavity.
What does 'heat resistant' mean in the context of these purge systems?
Heat-resistant purge dams and bladders are made from materials (silicone, high-temp foam, or ceramic fiber) rated to withstand the radiant heat of the weld without melting, off-gassing, or losing their seal. Standard foam or rubber dams degrade rapidly near the weld zone. Heat-resistant systems maintain their integrity through multi-pass welds and can be repositioned for subsequent joints. The 'heat resistant' rating typically covers continuous temperatures from 300–700°F (150–370°C) depending on material.
How do I set up a purge system for a circumferential pipe joint?
Place one inflatable dam inside the pipe on each side of the joint — typically 6–12 in. beyond the weld on each side. Connect the purge gas inlet to one dam and run an exhaust vent to the other side so gas flows through and out. Tape all other open joints in the pipe section to prevent gas loss. Introduce argon from a low point so it displaces the lighter air upward toward the vent. After purging (at least 4× the cavity volume), reduce flow and begin welding with the purge running. After root and first fill passes are complete, the back purge may be discontinued depending on quality requirements.
