What is a hazardous area? Area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus.
Why? To enable selection of suitable electrical and instrumentation equipment
Fire Triangle (Fig. 1):
- Fuel: This is what burns
- Oxygen: Required to support fire
- Ignition: Heat energy required to start fire
Fire / explosion can happen only if all three are present together in appropriate proposition.
Flashpoint- 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 flammable vapour.
Lower Explosive Level (LEL): Concentration of flammable gas or vapour in air, below which the gas atmosphere is not explosive.
Upper Explosive Level (UEL): Concentration of flammable gas or vapour in 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): Lowest temperature at which flammable gas or vapour 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 release:
- Continuous grade: Release which is continuous or is expected to occur frequently or for long periods (>1000 hours per year). Eg: Area inside 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 – Zones (Fig.3):
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.
Extent of 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
Hazardous area classification:
- 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.
Hazardous area – Gas group:
- 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. Maximum surface temperature of selected equipment not to exceed the limiting value.
Area with H2S (Hydrogen Sulphide) concentration above 50ppm. H2S is highly toxic even in very low concentration
- LEL – 4% (40,000ppm)
- UEL – 46%
- Auto ignition temperature – 260degC
- Gas group IIB
- Sour areas with H2S concentration below 4% in process stream need not be classified as hazardous area
- IEC 60079 series
- IP 15
- API 505