Pressurization and Purging (Ex p) Explained

Pressurized and purged enclosures stay popular because they let standard industrial hardware operate inside hazardous locations with minimal re-design. The Ex p method keeps a protective gas flow through an enclosure, displacing the explosive atmosphere and maintaining a positive pressure to prevent ingress. The concept is simple, but reliable performance depends on careful sizing, monitoring, and maintenance.

Core Principles

Ex p relies on a continuous supply of clean protective gas—typically instrument air or nitrogen—to achieve two things:

• Pre-purge: Flush out the enclosure volume before energizing equipment.
• Pressurization: Maintain positive pressure relative to the hazardous atmosphere while the system operates.

Standards such as IEC 60079-2 and NFPA 496 define the purge timing, minimum overpressure levels, and monitoring requirements based on the zone rating.

Know Your Type

Pressurization comes in three variants. Use the correct one for the risk level and gas group:

• Type px: Highest integrity. Suitable for Zone 1/Division 1, requires pre-purge, alarms, and automatic power shutdown.
• Type py: Similar to px but allows some Ex-certified components inside. Typically Zone 1 applications.
• Type pz: Reduced requirements for Zone 2/Division 2 locations, still needs monitoring and alarms.

Essential Hardware

Keep the design lean. At a minimum every enclosure needs:

• Purging controller with built-in pressure sensing and timing.
• Protective gas supply sized for both purge rates and continuous pressurization.
• Inlet filtration and sometimes dryers to avoid contamination.
• Relief vent or breather sized to handle flow while maintaining pressure.
• Power interlock to shut down energized circuits if pressure is lost.

Sizing the Purge

Calculate purge duration using enclosed volume and required air changes from the governing standard. For px/p y systems that often means at least four volume exchanges before energizing equipment. Set your purge flow high enough to complete the exchange quickly but low enough to avoid over-pressurizing the enclosure.

Commissioning Checklist

Maintenance and Inspection

Schedule quarterly verifications of purge controller calibration, pressure switch response, and alarm circuits. Replace filters and dryers per manufacturer recommendations, and re-run leak checks after any enclosure opening. Keep field log sheets—auditors expect traceable data, especially for px systems in Zone 1.

Common Pitfalls

• Undersized gas supply leading to slow purge times and startup delays.
• Bypassing or jumpering pressure switches to “keep production running.”
• Allowing energized equipment during maintenance while pressure is lost.
• Ignoring condensation or contamination inside the enclosure.

Pressurization rewards teams who design for simplicity and document every step. When you keep purge hardware calibrated and combine it with clear operating procedures, Ex p becomes a reliable bridge between standard industrial hardware and hazardous locations.