What is a hazardous area?
A hazardous area is an area in which an explosive gas atmosphere (flammable gases or vapors, combustible dust, flammable liquids, ignitable fibers, etc.) is present, or maybe expected to be present, in quantities such as to require special precautions for the construction, installation, and use of apparatus. The term hazardous area is associated with the installation of electrical and instrumentation equipment so that special design consideration is applied to meet special requirements considering the safety of the operating personnel.
What is Hazardous Area Classification?
Hazardous area classification is the scientific evaluation of facilities where the explosive environment is present and classify them following scientific and engineering principles. To ensure process safety, Hazardous area classification is of utmost importance. Normally, a hazardous area classification is presented on a plan view of plant drawings. These are also known as area classification drawings. To reduce the risk of fire and explosion, the electrical and electronic equipment is installed following the guidelines of hazardous area classification drawings.
Purpose of Hazardous area classification
To enable the selection of suitable electrical and instrumentation equipment to ensure a safe work environment.
In recent times, it is mandatory for chemical plants, oil refineries, LNG plants, sewerage treatment plants, paint manufacturers, distilling, offshore drilling rigs, Spray Booths, Petrochemical complexes, Laboratory and Fume Cupboards to prepare hazardous area classification drawings as hazardous gas vapors are normally present in all such industries.
Three elements are required to be present together to cause a fire or explosion. These are
- Fuel: This is what burns
- Oxygen: Required to support fire
- Ignition: Heat energy required to start a fire
Refer to Fig. 1 that shows these elements.
Fire/explosion can happen only if all three are present together in an appropriate proposition.
Flashpoint- The lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture. It is the vapour that mixes with air to form a flammable vapour.
Lower Explosive Level (LEL): Concentration of flammable gas or vapor in the air, below which the gas atmosphere is not explosive.
Upper Explosive Level (UEL): Concentration of flammable gas or vapor in the air, above which the gas atmosphere is not explosive.
Ignition energy (Fig. 2): Minimum energy of a spark that can ignite a flammable gas or vapour.
Ignition temperature (Fig. 2): The lowest temperature at which flammable gas or vapor gets ignited by itself.
Vapour density (Fig. 2): Density of a vapour or gas relative to the density of air, at same temperature and pressure.
Grades of hazardous release
The release of hazardous elements are grouped as follows:
- Continuous grade: Release which is continuous or is expected to occur frequently or for long periods (>1000 hours per year). Eg: Area inside a tank, sump, etc.
- Primary grade: Release which can be expected to occur periodically or occasionally during normal operation ( > 10 hours per year > 1000 hours per year). Eg: Sampling points, equipment nozzle
- Secondary grade: Release which is not expected to occur in normal operation and, if it does occur, is likely to do so only infrequently and for short periods (<10 hours per year). Eg: Piping flanges
Hazardous area classification guide
Two widely used systems are followed in industries for hazardous area classification.
- the Class/Division system and
- the Zone system
While Canada and the United States predominantly use the class/division system, other parts of the world use the zone system of Hazardous area classification. In the below paragraphs, we will explore the hazardous area zone classification.
Hazardous area Zone Classification
The Zone system of hazardous area classification, defines the probability of the hazardous material, gas or dust, being present in sufficient quantities that can generate explosive or ignitable mixtures. Refer to Fig.3 that shows the hazardous area zone classification based on hazardous gas release grade. There are three zones, Zone 0, Zone 1, and Zone 2.
Zone classification will be influenced by ventilation also. IEC 60079-10 categorises ventilation degree as High, medium and low. Poor ventilation may push the zone higher by one level.
Depending on the presence of combustible dusts or ignitable fibers and flyings, the hazardous area is classified in three zones: Zone 20, Zone 21 and Zone 22.
In both the above zone classification the probability of explosion severity reduces when we move from zone 0 (or zone 20) to zone 2 (zone 22).
The extent of the Hazardous area zone
Distance in any direction from the source of release to the point where the gas/air mixture has been diluted by air to a value below the lower explosive limit. Refer to Fig. 3 above that shows a typical example of a hazardous area zone extents.
- Pressure breathing valve (Fig. 3) in the open air, from process vessel.
- A fixed process mixing vessel (Fig. 3); liquids are piped into and out of the vessel through all-welded pipework flanged at the vessel.
Gas groups in Hazardous Area Classification
There are two groups as listed below:
- Group I – For use in mines (Methane)
- Group II – Other than mines
Sub-divisions in group II based on ignition energy requirement
- IIA – Atmospheres containing acetone, ammonia, ethyl, alcohol, gasoline, methane, propane or similar gases
- IIB – Atmospheres containing ethylene, acetaldehyde or similar gases
- IIC – Atmospheres containing acetylene, hydrogen or similar gases
Hazardous area – Temperature class
Classification based on ignition temperature (Fig. 4 (a)) of gas or vapour. The maximum surface temperature of selected equipment not to exceed the limiting value.
The area with H2S (Hydrogen Sulphide) concentration above 50 ppm. H2S is highly toxic even in very low concentration
- LEL – 4% (40,000ppm)
- UEL – 46%
- Autoignition temperature – 260degC
- Gas group IIB
- Sour areas with H2S concentration below 4% in process stream need not be classified as a hazardous area
Codes and Standards for Hazardous Area Classification
The following codes and standards provide detailed guidelines for Hazardous Area Classification for industries
- IEC 60079 series
- IP 15
- API 505
References and Further Reading for Hazardous Area Classification Guide
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