Inerting & Purging (Nitrogen / Inert Atmosphere) SWMS

Inerting and purging is the deliberate replacement of a flammable or reactive atmosphere inside a vessel, tank, pipe or system with an inert gas — usually nitrogen — to make it safe for hot work or maintenance, and the later re-purging back to a breathable atmosphere. It is high risk construction work under the NSW WHS Regulation 2025 because it is carried out on and in confined spaces and creates a contaminated, oxygen-deficient atmosphere (s291), so a SWMS is mandatory under s299 and a confined space entry permit is required under Part 4.3. An inert atmosphere is the most deceptive hazard in confined-space work. Nitrogen is colourless, odourless and gives the body no warning: it simply displaces oxygen, and a worker who enters an inerted space, or is engulfed by an inert-gas release, loses consciousness within a couple of breaths and dies within minutes. Worse, the instinct to rescue kills — an unprotected rescuer who rushes in collapses the same way, and inert-gas incidents characteristically claim multiple victims. The SWMS therefore treats every inerted space as immediately lethal, controls the gas at its source, prohibits entry into an inert atmosphere except under air-supplied breathing apparatus, manages venting so the displaced and residual gas cannot pool in surrounding low-lying or enclosed areas, and builds a non-entry rescue capability. Entry and atmospheric control are to AS 2865, and SafeWork NSW is the regulator.

Hazards identified

7 hazards covered, sorted by priority.

Control measures

Hierarchy-of-controls order: elimination → substitution → isolation → engineering → administrative → PPE.

1. 1Elimination — Complete the task without entry — internal inspection and cleaning by remote means — so no one enters the inert space at all
2. 2Substitution — Substitute mechanical or water displacement for nitrogen inerting where the process allows a non-asphyxiant method
3. 3Engineering — Continuous oxygen monitoring with audible alarm inside the space and at the vent, and forced re-ventilation before any breathable entry
4. 4Engineering — Controlled venting routed to atmosphere away from pits, trenches, intakes and enclosed areas so inert gas cannot pool
5. 5Engineering — Pressure relief and flow control on the inerting supply to prevent over-pressure or implosion
6. 6Administrative — Confined space entry permit that prohibits entry into an inert atmosphere except under air-supplied breathing apparatus
7. 7Administrative — Positive re-purge verification — oxygen proven to normal at multiple points — before any breathable entry is authorised
8. 8Administrative — Non-entry rescue system and a standby attendant; rescuers never enter an inert space on air without their own supply
9. 9PPE — Air-supplied breathing apparatus (not air-purifying respirators, which do not protect against oxygen deficiency) for any inert-atmosphere entry
10. 10PPE — Full-body harness and retrieval line, plus cryogenic gloves and face protection where liquid nitrogen is handled

Applicable Codes of Practice

High-Risk Construction Work triggered

Worked example

At 6:00 am during a refinery shutdown, a crew from Meridian Process Services is set to enter a vessel that was nitrogen-inerted overnight for hot work. The supervisor's first rule is stated at the toolbox: an inerted vessel is treated as instantly lethal, and no one enters on the strength of a clock or a hunch. The vessel is re-ventilated with forced air, then oxygen is measured at the manway, mid-height and the base — all three must read normal before entry, and they don't yet at the base, so ventilation continues. The vent line is checked to confirm it discharges to open air well clear of a nearby pit, because pooled nitrogen there would kill someone outside the vessel. Only when oxygen holds at normal throughout, with the monitor live inside, does the permit authorise entry — and even then the entrant wears air-supplied breathing apparatus and a retrieval harness, with a non-entry rescue rigged and a standby attendant who is briefed never to follow a collapsed worker in. The job done, the space is re-monitored before the permit is closed.

Related legislation

• WHS Act 2011 (model)
• WHS Regulation 2025
• Confined Spaces Code of Practice
• AS 2865 Confined spaces

Frequently asked questions