Hazardous Area Classification
Definition of a Hazardous Area
Any area that could have a potentially explosive atmosphere in quantities such as to require special precautions to protect the safety of workers is defined as a Hazardous Area. And, a potentially explosive atmosphere is said to exist where there is a risk of explosion due to mixtures of gas/air, vapour/air, dust/air or other flammable combinations.
Why are Hazardous Areas Important?
Instrument engineers introduce a potential ignition source into a Hazardous Area when they locate an electrical or electronic instrument, e.g. a guided wavel radar level transmitter or an electrical or electronic final element e.g. shut down valve actuators in that area. Understanding the degree of risk, and the design of the instrument, or final element, allows the instrument engineer to minimise the likelihood of an explosion occuring.
Hazardous Area Assessment for Gases
Process plants, refineries, oil and gas platforms etc are divided into Hazardous Area Zones (European and IEC method) or Divisions (North American method) according to the frequency and duration of a potentially explosive atmosphere being present, e.g.
Zone 0 Hazardous Area
An area in which an explosive mixture is continuously present, or present for long periods, typically for more than 1,000 hours per year.
Zone 1 Hazardous Area
An area in which an explosive mixture is likely to occur in normal operation, typically for more than 10, but less than 1,000 hours per year.
Zone 2 Hazardous Area
An area in which an explosive mixture is not likely to occur in normal operation and if it occurs it will exist only for a short time, typically for less than 10 hours per year, but still sufficiently likely as to require controls over ignition sources.
Hazardous Area Assessment for Dusts
Hazardous area classification for flammable dusts should be undertaken in the same manner as that for flammable gases and vapours. The zone numbers used are 20, 21 and 22, corresponding to 0,1 and 2 used for gases/vapours
International Standards for Hazardous Area Classification
The following international standards are used to define hazardous area classification zones, or their equivalent:
- EN 1127-1:1997; Explosive atmosphere - Explosion prevention and protection. Part 1: Basic concepts and methodology
- EN 60079-10:2003; Electrical apparatus for explosive gas atmospheres. Part 10: Classification of potentially explosive atmospheres
- EN 50281-3:2002; Electrical apparatus for use in the presence of combustible dust. Part 3: Classification of areas where dusts are or may be present
- NFPA 497; Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
- NFPA 70; National Electrical Code Chapter 5 Article 500.
A comparison of Zones to Divisions is shown below:
It is worth noting that many - though by no means all - countries outside of Europe and North America use the IEC Standards as a basis for their own national standards.
Hazardous Area Drawings
Hazardous area zones are usually marked on a drawing of the plant, the drawing being referred to as the Hazardous Area Classifiaction Drawing, or Hazardous Area Chart. These drawings show areas where flammable liquids, gasses or vapors are handed, processed or stored. The hazardous area drawing also includes temperature classification information and gas group information. By using these drawings the engineer can specify instruments and equipment suitable for the area in which it is to be located.
Technical Library
The following pages on Control and Instrumentation.com give more detail on Hazardous Area Classification, the ATEX Directive and other process safety information:
