Equipment
Groups
Explosive atmosphere equipment is specified in terms of the types of gases or dust present.
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 Groups
Different explosive atmospheres (gases, vapours and dusts) will have different properties including flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. These properties will determine the likelihood and severity of an explosion. In order to clearly identify these properties testing is performed on the atmosphere to determine critical properties like maximum experimental safe gap, minimum ignition current, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. These values help to classify the substance so that they can be grouped in order to select the right equipment for explosive areas.
The groupings are:
| Gas Groups | Definitions |
|---|---|
| I | Firedamp (underground coal mining) |
| IIA | Propane family |
| IIB | Ethylene family |
| IIIC | Hydrogen and Acetylene family |
| Gas Zones | Definitions |
|---|---|
|
Zone 0 Ma Ga |
According to IEC 60079-10-1 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. |
|
Zone 1 Mb Gb |
According to IEC 60079-10-1 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. |
|
Zone 2 Gc |
According to IEC 60079-10-1 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 combined group of IIB+H2 is often used for equipment which is designed for Group IIB gasses but can also be used when Hydrogen is also available.
Group IIC is the most volatile group and the gasses in this group can be ignited quite easily. Any equipment which is marked as IIC can be used in atmospheres that contain Group IIB and Group IIA gasses. Conversely equipment marked IIA cannot be used in IIB or IIC explosive environments.
Dust Groups
Like gas, dusts are grouped in order to effectively identify the risks of ignition and to identify the severity of explosion. The size of the dust particles is 500 micro meter or less. Dust can remain suspended in air and may settle due to its own weight.
The groupings are:
| Dust Groups | Definitions |
|---|---|
| IIIA | Combustible Fibres and Flyings |
| IIIB | Group IIIA dusts plus, Non-Conductive Dusts |
| IIIC | Groups IIIA and IIIB dusts plus, Conductive Dusts |
| Dust Zones | Definitions |
|---|---|
|
Zone 20 Da |
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. |
|
Zone 21 Db |
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. |
|
Zone 22 Dc |
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. |
Just like equipment in gas, the equipment marked IIIC can be used for IIIB and IIIA atmospheres.
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