Can nitrogen or instrument air be used for Ex p pressurisation?

Yes. IEC 60079-2 defines the protective gas as air or an inert gas, and names nitrogen, carbon dioxide and argon as examples of inert gas. Clean air of instrument quality is the usual choice; nitrogen is permitted, and is required in some cases with an internal source of release, but it turns the enclosure into an asphyxiation hazard that must be marked and managed. The gas actually permitted is the one stated in the manufacturer's instructions.

Short answer

• The certificate and instructions state which protective gas may be used. IEC 60079-2 treats air of normal instrument quality and nitrogen as the usual choices; any other specified gas must not reduce the protection or harm the enclosed equipment.
• The gas must be non-flammable, non-toxic, and essentially free of moisture, oil, dust and other contaminants. It must not contain more oxygen than air.
• Air must be drawn from a non-hazardous area, usually where contamination by process releases is unlikely.
• Nitrogen or another inert gas is required where IEC 60079-2 demands it: for example a liquid release from an analyser's containment system, leakage compensation with an internal gas release, and static pressurisation.
• An inert-gas enclosure carries an asphyxiation warning; IEC 60079-2 asks the manufacturer to consider a means of flushing out the inert gas before doors are opened, and IEC 60079-14 (2007 text) recommends always purging it again after a pressure loss.

Air or nitrogen: what each is used for

The protective gas should normally be no warmer than 40 °C at the enclosure inlet, unless the manufacturer specifies otherwise and marks it.

When an inert gas is required

Air works by keeping the outside gas out and diluting it. Inert gas works by keeping oxygen out, which matters where flammable material can be released inside the enclosure, typically from an analyser's sample lines. IEC 60079-2 tabulates the permitted protective gas against the type of internal release:

• Liquid release: inert gas only, and never a release in normal operation.
• Gas release, leakage compensation: inert gas only, and only where the release is limited to abnormal conditions (no normal release), the substance contains no more than 2 % oxygen by volume and its upper flammable limit (UFL) does not exceed 80 %.
• Gas release, continuous dilution: air or inert gas where the UFL is below 80 %; with air, the release must be diluted to no more than 25 % of the LFL.
• Gas with a UFL above 80 %, such as substances that can burn with little or no oxygen: inert gas cannot protect it, so continuous dilution with air is required.
• Static pressurisation (enclosure filled once in a safe area and sealed): inert gas only, with no internal release.

For enclosures without an internal release, air and inert gas are both acceptable where the instructions permit them.

Managing the asphyxiation hazard

Nitrogen has no smell and gives no warning. The US Chemical Safety Board identified 85 nitrogen asphyxiation incidents in US workplaces from 1992 to 2002, with 80 deaths and 50 injuries; a person breathing an oxygen-deficient atmosphere cannot sense that anything is wrong.

For a nitrogen-pressurised enclosure, IEC 60079-2 requires a warning that the enclosure contains inert gas and may be an asphyxiation hazard, and that the flammable substance inside may be within its flammable limits when exposed to air. In practice:

• Before opening: isolate the electrical supply, flush the enclosure with air if a means is provided, and keep the face away from the opening.
• Vent and exhaust points: route the leakage and purge exhaust to a place where nitrogen cannot collect where people work. Several continuously leaking enclosures in a small analyser house or room can lower its oxygen level.
• Monitoring: where enclosures share a room or shelter with people, consider fixed oxygen monitoring as part of the risk assessment.
• Supply integrity: a nitrogen bottle that runs empty stops the protection. Size the supply for the leakage rate and include low-pressure alarms.

Common mistakes

• Using plant air from a compressor whose intake is in or near the hazardous area. The protective gas must not bring flammable gas into the enclosure.
• Substituting nitrogen for air on an analyser enclosure designed for continuous air dilution, or the reverse, without checking the certificate: the protection concept depends on the gas.
• Feeding oxygen-enriched air or a breathing air mixture into the enclosure; the oxygen content must not exceed that of air.
• Opening a nitrogen-filled cabinet and leaning in to look at a fault.

Related on excodex

• Pressurisation (Ex p): px, py and pz, purging and protective gas
• Oxygen deficiency: nitrogen, argon and other inert gas hazards
• Pressurised rooms and analyser houses: making an area non-hazardous by ventilation
• How long must a pressurised enclosure be purged before switching on?
• Gas cards: nitrogen; standards space IEC 60079-2

Standards and further reading

• IEC 60079-2, equipment protection by pressurised enclosure "p" (protective gas, internal release, inert gas marking); current edition 6.0 (2014).
• IEC 60079-14, design, selection and installation (protective gas quality and source, inert gas precautions).
• IEC 60079-2:2014 text: law.resource.org
• IEC 60079-14:2007 as AS/NZS 60079.14:2009: archive.org
• US Chemical Safety Board, safety bulletin on hazards of nitrogen asphyxiation (2003): csb.gov

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.