Inerting & Purging

Before welding, cutting or entering a tank, vessel or pipe that held fuel or chemicals, the flammable vapors must be removed or the oxygen must be reduced so a fire or explosion can't happen. Purging pushes out the flammable atmosphere with another substance. Inerting reduces oxygen with an inert gas. Both save lives, and both create new hazards, especially asphyxiation from the inert gas.

• Pipefitters & Welders
• Tank Cleaning & Repair Crews
• Refinery & Chemical Workers
• Fire Watch & Safety Staff

Terms and methods

• Purging: replacing the contents of a tank or piping, for example, flammable vapor with air (to make it safe to enter) or with an inert gas (to make it safe to weld on). Methods include displacement with inert gas, steaming, water-filling and ventilation.
• Inerting: reducing the oxygen level to below the point where combustion can occur, usually using nitrogen, carbon dioxide or argon. The target oxygen level depends on the fuel and the safety margin set by the procedure (for many flammable gases, keeping oxygen well below 8 to 10 percent is typical, but the procedure and standards decide).
• Water filling: filling a tank with water leaves little or no vapor space, so hot work can proceed on the upper area. Use caution with fuel on the surface and trapped vapors.
• Steam purging and cleaning, where appropriate for the material, removes vapors and residues but can create static, pressure and heat hazards.
• Ventilation with fresh air removes vapors in a tank to be entered. See Ventilation.
• Common standards include NFPA 69 (explosion prevention systems, including inerting), NFPA 326 (safeguarding tanks and containers for entry, cleaning or repair) and NFPA 51B (fire prevention during hot work). See Hot Work Permits and Hot Work on Drums & Tanks.

Main hazards

• Asphyxiation from inert gas: nitrogen, argon and carbon dioxide are odorless and displace oxygen, causing sudden unconsciousness and death in seconds without warning. This kills workers in tanks and in rooms where inert gas leaks. See Inert Gas Asphyxiation and Oxygen-Deficient & Enriched Atmospheres.
• Fire and explosion if purging is incomplete, if pockets of vapor remain (in dead legs, scale, sludge and insulation), or if air leaks into an inerted system.
• Static electricity from steam, nitrogen and fast gas flows near flammable liquids. See Static, Bonding & Grounding.
• Pressure and vacuum damage: overpressure from purge gas or steam, or vacuum collapse when steam condenses.
• Toxic vapors released to the atmosphere during venting, such as hydrogen sulfide and benzene. See Benzene.
• Heat and cold: hot steam or cold liquid nitrogen burns. See Cryogenic Liquids.

Safe procedure

1. Plan and write the procedure. Identify the contents, hazards, vessel volume, vents, purge gas source, flow rate, duration and target gas concentrations, and who does what. Use a permit. See Permit to Work.
2. Isolate the tank or pipe from other sources of fuel, with lockout, blinds or spools. See Group Lockout.
3. Choose the purge medium and method that suits the contents, and prepare the system with safe vent locations that release gas away from workers and ignition sources.
4. Purge for enough volume or time (often several volume changes) as the procedure specifies, controlling flow rates to avoid static and pressure problems. Make sure inlets and outlets cover the whole space and dead legs.
5. Test the atmosphere with calibrated gas detectors, at multiple points and levels, for oxygen, flammable gas and toxics. Don't trust a single reading, and continue testing during work. See Flammable Gas Detection and Gas Detectors.
6. Keep inerting going during hot work if the procedure requires, with continuous oxygen monitoring, and stop at once if conditions change.
7. For entry, after inert purging you must re-ventilate with fresh air and test until oxygen is safe before anyone goes in. A tank that is inert is not safe to enter without supplied air. See Tank Cleaning & Vessel Entry and Confined Spaces.

Protecting workers around inert gas

• Post warning signs and barricade areas where inert gas is vented or used, including outdoors near vents and low spots.
• Keep people out of the area except essential workers, and monitor oxygen in the work area with personal oxygen monitors with alarms.
• Use supplied air or SCBA for any entry into an inert or unknown atmosphere, with an attendant and rescue plan. Never enter to rescue someone without the right equipment. See Respiratory Protection.
• Train workers on how inert gas kills and on emergency response.
• Keep fire protection and a fire watch in place during hot work after purging. See Hot Work in Occupied Buildings.

Inerting and purging checklist

• A written procedure and permit cover the contents, method, flow rate and vents
• The system is isolated and locked out
• The atmosphere is tested at several points for oxygen, flammables and toxics
• Inert gas areas are barricaded and posted, with oxygen monitors in use
• Nobody enters an inerted space without supplied air; re-ventilation and testing precede entry

Last updated: October 5, 2026