Hazardous Area Classification Zones (0, 1, 2) for Gases, Vapours, Mists & Dusts

Hazardous Area Classification Zones (0, 1, 2) for Gases, Vapours, Mists & Dusts

Hazardous Area Electrical, Process & Instrumentation Equipment

T&D are specialist distributors of Electrical, Mechanical, Process & Instrumentation Equipment for hazardous areas.

The dangers of installing electrical apparatus in areas where explosive mixtures of gases and air could occur was first recognised in the mining industry early in the 20th century.

Awareness spread quickly to the chemical and petrochemical industries who recognised that, unlike the coal mining industry where the release of firedamp is unpredictable in quantity and duration, the occurrence of flammable atmospheres was due to mechanical and process failures or to deliberate situations created by personnel.

This meant that the presence and persistence of such situations could be predicted to a level much higher than was the case in mining. For an ignition to take place, a source of ignition must coincide with an explosive atmosphere.

Hazardous area zoning therefore applies to non-mining situations only.

Industrial sites handling flammable materials are divided up according to the probability of a flammable atmosphere occurring into three zones of risk, based upon frequency and persistence.


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What Is A Flammable Atmosphere?
A flammable atmosphere must contain fuel, plus oxygen to allow the fuel to burn and release energy. The only source of oxygen considered is air, which is approximatly one fifth oxygen. The fuel must be finely divided form - the ultimate state of division is the individual molecule of fuel, such as occurs in flammable gases and vapours. Thus, gases and vapours can, under ideal conditions, mix intimately with the oxygen molecules and, if the mixture is within the flammable range, introducing an ignition source will cause the entire mixture to burn rapidly, usually within a fraction of a second. If this occurs outdoors, the result is likley to be a flash fire. If indoors, an explosion could result.  Anyone undertaking a hazardous area classification exercise should take a special care to identify and deal with any areas where an explosion risk is present.

Hazardous Area Zones for Gases, Vapours and Mists
Gas/Vapour/Mist Zones

  • Zone 0 Hazardous Area - Flammable atmosphere present continuosly or for long periods or frequently. Typically 10-1000 hr/year
  • Zone 1 Hazardous Area - Flammable atmosphere likley in normal operation occasionally. Typically 10-1000 hr/year
  • Zone 2 Hazardous Area - Flammable atmosphere unlikley in normal operation and, if it occurs, will exist only for a short time. Typically 1-10 hr/year

Zones for Flammable Dusts (IEC 60079-10-2:2009) - Dust Zones

  • Zone 20 Hazardous Area - Flammable atmosphere present continuously or for long periods or frequently - Typically 1000 hr/year
  • Zone 21 Hazardous Area - Flammable atmosphere likley in normal operation occasionally - Typically 10-1000 hr/year
  • Zone 22 Hazardous Area - Flammable atmosphere unlikley in normal operation and, if it occurs, will exist only for a short time - Typically 10hr / year, max 1hr / release

Hazardous Areas

Zone 0 Hazardous Area
Releases causing a flammable atmospheres that are part of the normal running of the plant may occur with some regularity. Examples would be releases as a result of sampling operations or the opening of a manway to allow more reagent to be added to a process. Zones 1 may occur on parts of plant where people may work but are associated with smaller releases that create only small localised areas, so it is less likley that personnel will have significant exposure to them. (Although the toxic nature of chemicals is completely divorced from area classification, it is true that exposure to many flammables also has health implications).

An example of Zone 1 is the tank which was used as the Zone 0 example. When this tank is filling, the atmosphere in its vapour space will be expelled. The zone extent, typically 3m from the vent, represents the distance for the vapour to dilute to a fraction of its LEL.

Again as a rule of thumb, it can be said that if a potentially explosive atmosphere exists for between 10 and 1000 hours a year, it is classified Zone 1.

As a result of the difficulty of adequately assessing the effectivness of ventilation in indoor plant areas, many of these are given a blanket Zone 1 classification, even when only secondary grade releases are present. Large Zones 1 in outdoor plant should normally be avoided, however.

Outdoor Zones 1 are relatively rare compared to Zones 2 and usually small.


Indoor process areas with poor ventilation are often Zone 1 throughout.
Zone 2 Hazardous Area
By definition this is the least hazardous of zoned areas. On a normal outdoor process plant it is usually the case that the great majority of areas will be Zone 2, because of its very nature, Zone 2 is the zone created by the unlikley condition. It is not intended to cover conditions of catastrophic process failure which are so unlikley as to be almost unforeseeable. Because it is the abnormal occurrence that results in Zone 2, it is also likely to involve the largest quantity of flammable material released and thus the largest hazardous area results. Typical of the Zone 2 situation is the area in bund of the vented tank. This takes account of possible over-filling of the tank  (an abnormal situation), resulting in a small quantity of liquid running down the side of the tank and collecting in the bund. Note that the example in this particular code of practise does not cover the situation where the entire contents of the tank are emptied into the bund: this is considered a 'catastrophic' failure of the control system.

Most outdoor hazardous areas are Zone 2. Indoors, zone 2 is only possible with adequate ventilation (say 12 air changes/hr), otherwise Zone 1 is appropriate.



Safe Working with Flammable Substances in Hazardous Areas - HSE

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