Explosive Atmosphere Zone Classifications
In an industrial plant such as a refinery or chemical process plant, handling of large quantities of flammable liquids and gases creates a risk of leaks. In some cases the gas, ignitable vapor or dust is present all the time or for long periods. Other areas would have a dangerous concentration of flammable substances only during process upsets, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical plants are accordingly divided into areas of risk of release of gas, vapor or dust known as divisions or zones.
The process of determining the type and size of these explosive areas is called area classification. Guidance on assessing the extent of the hazards gas zones is given in the current edition of IEC 60079-10-1. For explosive dusts, the guiding standard is IEC 60079-10-2.
Gas Zone Definitions
Zones 0, 1 & 2: Atmospheres with explosive gases & vapours (AS2380; AS/NZS/IEC 60079)
Typical gas hazards are from hydrocarbon compounds, but hydrogen and ammonia are common industrial gases that are flammable.
In the mining regulations for the Australian states of Queensland and New South Wales coal mines these zones are defined in terms which are equivalent but are dynamic and allows the zone to change depending on environment conditions. For example a Zone which, according to IEC60079-10-1, may be identified as a Zone 1 might change to a Zone 0 in the case where ventilation to the mine fails or if a methane detector identifies the presence of gas above a specified level.
Zone 0 Area
The definition according to IEC60079-10-1 is “an area in which an explosive gas atmosphere is present continuously or for long periods or frequently”. As a guide for Zone 0, this can be defined as over 1000 hours/year or >10% of the time. A potential few examples of this are:
- The air space inside a tank of petrol
- An area adjacent to the face of a seam of coal
- The area around a vent which is venting explosive gas
Zone 1 Area
The definition according to IEC60079-10-1 is “an area in which an explosive gas atmosphere is likely to occur in normal operation occasionally”. This includes areas in which the explosive mixture may exist frequently because of leakage. As a guide for Zone 1, this can be defined as 10–1000 hours/year or 0.1–10% of the time. A few potential zone 1 examples are:
- Outside of an unvented petrol tank
- In ventilated coal mine shafts
- An area of a petro-chemical plant which is poorly ventilated
Zone 2 Area
The definition according to IEC60079-10-1 is “an area in which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur it will exist for a short period only”. As a general guide for Zone 2, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time. A few examples of this kind of zone 2 are:
- Open walkways in LNG Plants
- The petrol filling station
- 1m from a fuel truck
Dust Zone Definitions
Zones 20, 21 & 22: Hazardous Areas due to the presence of combustible dusts, fibres or flyings (AS/NZS/IEC 60079-31)
Some historically associated standards that have been superseded include AS 2236 and AS/NZS 61241. These standards relate to flammable dusts which, when suspended in air can explode. Some dry powders such as flour, cocoa powder and baby formula powder can explode violently when suspended in the air and ignited. Typically dusts are not an explosive hazard when they are settled, however often there is also dust suspended in a cloud. If the dust which is suspended ignites it can set off a chain reaction which will cause the settled dust to lift off and disperse which in turn adds fuel to the explosion.
An area in which combustible dust, as a cloud, is present continuously or frequently, during normal operation, in sufficient quantity to be capable of producing an explosive concentration of combustible dust mixed with air, and/or where layers of dust of uncontrollable and excessive thickness can be formed.
An area not classified as zone 20 in which combustible dust, as a cloud, is likely to occur during abnormal operation, in sufficient quantities to be capable of producing an explosive concentration of dust mixed with air.
Areas not classified as zone 21 in which combustible dust clouds may occur infrequently, and persist for only short periods, or in which accumulations or layers of combustible dust in air may be present only rarely. Where, following an abnormal condition, the removal of dust accumulations or layers cannot be assured, then the area is to be classified zone 21.