Nitrogen Purging SWMS

Nitrogen purging is a routine HVAC and process piping activity used to displace oxygen, moisture and combustible vapours from refrigeration lines, chillers, brazed copper pipework, pressure vessels and process systems prior to commissioning, brazing or hot work. Because nitrogen is colourless, odourless and non-irritating, an oxygen-deficient atmosphere can form silently inside plant rooms, risers, pits, ceiling voids and adjacent confined spaces, causing rapid collapse and asphyxiation with no warning to the worker. Under WHS Regulation 2025, nitrogen purging is high risk construction work where it occurs in or adjacent to a confined space or where atmospheric oxygen may fall below 19.5 percent. A documented Safe Work Method Statement is mandatory before work commences, must be developed in consultation with workers, kept available at the workplace, and reviewed after any incident, control failure or change in plant configuration.

Hazards identified

7 hazards covered, sorted by priority.

Control measures

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

1. 1Elimination — Where commissioning permits, eliminate nitrogen purging by using pre-charged factory-sealed line sets or vacuum dehydration alternatives that do not introduce inert gas into occupied areas.
2. 2Elimination — Remove the need for confined space entry by designing purge vent points to atmosphere outside the building envelope through hard-piped vent stacks above roof level.
3. 3Substitution — Substitute high-volume nitrogen flow rates with regulated low-flow trickle purging using calibrated flow meters and pressure regulators set well below pipework MAWP.
4. 4Engineering — Install fixed and personal oxygen monitors with audible/visual alarms set at 19.5% and 18% in all plant rooms, risers and ceiling voids where purging vents discharge.
5. 5Engineering — Fit pressure relief valves, blow-off discs and inline regulators on all nitrogen supply lines, with restraints and whip checks on every hose coupling.
6. 6Engineering — Provide mechanical ventilation at minimum 6 air changes per hour in the purge zone and direct vent discharge via flexible ducting to external atmosphere.
7. 7Administrative — Issue a confined space entry permit and hot work permit where applicable, conduct pre-start atmospheric testing, and implement standby person with rescue plan and comms.
8. 8Administrative — Train operators to AS 2865 confined spaces and AS 4839 first aid in remote situations, brief the SWMS at pre-start, and prohibit lone working during active purging.
9. 9PPE — Wear safety glasses to AS/NZS 1337, cryogenic gloves for liquid nitrogen handling, hearing protection during purge venting, and steel-capped boots when handling cylinders.
10. 10PPE — Where O2 may fall below 19.5% and engineering controls cannot guarantee atmosphere, use supplied-air respirators to AS/NZS 1715/1716 — never filtering respirators in oxygen-deficient atmospheres.

Applicable Codes of Practice

High-Risk Construction Work triggered

Worked example

On a six-storey commercial office fitout, a mechanical services subcontractor is commissioning the chilled water and VRF refrigerant pipework on Level 4. At the 6:30am pre-start, the supervisor opens the Nitrogen Purging SWMS on a tablet in the site shed and walks the two-person crew through each hazard line by line. The crew identify that today's purge will vent into the Level 4 plant room — a partially enclosed space with limited natural ventilation — which immediately triggers the confined space category. They select the engineering control of routing flexible duct from the purge vent through the louvred external wall, set up a fixed oxygen monitor at head height near the work zone, and confirm the personal O2 monitors on each operator are bump-tested and reading 20.9%. The standby person is nominated, radio channel confirmed, and the rescue plan reviewed. Both workers sign onto the SWMS electronically. Two hours into the purge, the personal monitor on the lead technician alarms at 19.8% — the SWMS-mandated response is followed: nitrogen supply isolated at the regulator, crew withdraw to the corridor, mechanical ventilation increased, and atmosphere re-tested before re-entry. The supervisor records the deviation against the SWMS control register and adds an additional purge break interval for the afternoon session, signed off by the crew before resuming work.

Related legislation

• WHS Act 2011 (model)
• WHS Regulation 2025
• AS 2865 — Confined spaces

Frequently asked questions