Purging and inerting both work by removing one leg of the fire triangle rather than by controlling ignition sources, which makes them fundamentally different from every other explosive atmosphere control — and also means an incomplete purge can leave a system in a more dangerous state than not purging at all.

Purging versus inerting

Purging generally means displacing a flammable atmosphere from a vessel or system, typically with an inert gas such as nitrogen, to bring it below the lower explosive limit before work begins. Inerting means maintaining an inert atmosphere in the system during operation so an explosive mixture cannot form in the first place. The distinction matters because the two have different success criteria: a purge is verified complete and then work proceeds, while inerting must be continuously maintained and monitored for as long as the protection is required.

Passing through the explosive range

Introducing air into a vessel containing flammable vapour, or introducing flammable material into a vessel containing air, takes the mixture through the explosive range on the way to either extreme, and this transitional period is the most hazardous part of the operation. Purging with an inert gas first, so the transition happens between flammable-rich and inert rather than between flammable-rich and air, avoids passing through the explosive range entirely, which is the core reason inert purging exists rather than simply ventilating with air.

The asphyxiation hazard inerting creates

An inerted vessel or space is oxygen-deficient by design, and this creates a life-threatening asphyxiation hazard that is invisible, odourless and can cause loss of consciousness within a very small number of breaths without any warning sensation. Nitrogen asphyxiation incidents frequently involve a second casualty attempting rescue, since the hazard gives no sensory warning to the person entering. Any inerted space must be treated as a confined space entry with full atmospheric testing and rescue arrangements, never entered on the basis that the flammability hazard has been controlled.

Verifying purge completion by measurement

Purge completion needs verifying by direct measurement at multiple points, including low points and dead legs where heavier vapour can persist, rather than by elapsed time or volume calculation alone. A vessel with internal structure, baffles or branch connections can retain pockets of flammable atmosphere well after the calculated purge volume has passed through, and single-point sampling at a convenient location can read clear while such a pocket remains.

Re-establishing the normal atmosphere afterward

Returning a system to service requires the same care in reverse, and reintroducing flammable material into a vessel still containing air, or admitting air into an inerted vessel before it has been purged of flammable residue, both recreate the explosive range transition the original purge was designed to avoid.

For confined space entry requirements that apply to inerted spaces, see Rope Access and Confined Space.

Related standards

The standards below set the test methods and performance levels behind the equipment referenced in this note.

• EN 136 — Full face masks
• EN 140 — Half masks and quarter masks
• EN 1149 — Electrostatic properties of protective clothing

Frequently asked questions