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Key takeaways
- Hazardous area classification divides a plant into zones by how likely a flammable atmosphere is: Zone 0, Zone 1 and Zone 2.
- Piping creates many of the release sources that set those zones: flanges, valve stems, pump seals, vents, drains and sample points.
- Fewer leak points, smart vent routing and separation from ignition sources make zones smaller and layouts safer and cheaper.
What zoning is
Hazardous area classification identifies where an explosive gas atmosphere might occur, so that electrical and mechanical equipment can be chosen not to ignite it. Under the IEC system used in most of the world:
- Zone 0: an explosive atmosphere is present continuously or for long periods (inside a tank vapour space, for example).
- Zone 1: it is likely to occur in normal operation.
- Zone 2: it is not likely in normal operation and, if it occurs, lasts only a short time.
North American projects may use the Class I, Division 1 and 2 system of API RP 500 instead, or the zone system of API RP 505.
Why piping engineers care
The area classification engineer starts with a list of sources of release, and most of them are piping items:
- flanged joints and instrument connections,
- valve stem packings and control valves,
- pump and compressor seals,
- vents, drains and sample points,
- relief valve outlets and atmospheric vents.
Each source gets a zone of a certain extent around it. More sources, closer together, means larger zones and more expensive certified electrical equipment.
Layout moves that shrink zones
- Weld instead of flange where maintenance doesn’t need a break.
- Route vents to a safe location, away from air intakes, hot surfaces and accommodation.
- Group sources of release so zones overlap instead of spreading across the unit.
- Keep ignition sources apart: fired heaters, gas turbines and non-certified electrical equipment go on the upwind side or behind barriers.
- Use closed drains for hydrocarbon draining instead of open funnels.
Onshore
Onshore, distance is the main tool: put fired heaters and control rooms well away from release sources, usually upwind of the prevailing wind.
Offshore
Offshore, distance is scarce, so separation relies on fire and blast walls, ventilation and strict control of leak points. Accommodation and the temporary refuge are placed as far as practical from process areas.
Code reference
IEC 60079-10-1 Classification of areas: explosive gas atmospheres.
API RP 505 / API RP 500 Area classification at petroleum facilities (zone and division systems).
API RP 14J Design and hazards analysis for offshore production facilities.
What to take back to your desk
Ask for the hazardous area drawings early and overlay them on your model. Every flange you remove from a congested area is one less leak point and can shrink a zone.
Educational content. Always design to your project specification.
