Basics of hazardous area classification and protection methods

hazardous area classification
Hazardous locations are places that contains or are expected to contain sufficient quantities of flammable liquids, gases, vapor or combustible dusts to produce an explosion or fire. Industries like oil&gas, plant floor, chemical, refineries, and many others require explosion protected equipment.

Presently there are two systems that classify hazardous areas, the class division system (used in the United States and Canada) and the zone system based on the international electrochemical commission (IEC) and European Committee for Electrochemical Standardization

Class/Division system

Class I areas– Locations where flammable gases or vapors are present in the atmosphere and in sufficient quantities to produce an explosion or fire. Normally class I areas uses division methods, listed below:

Division 1– Locations where the presence of ignitable mixtures of flammable gases or vapors is present because of following reasons
• Occurs under normal conditions
• Occurs due to frequent leakage
• Occurs due to repair or maintenance
• Occurs due to a faulty equipment
Division 2– Locations where presence of ignitable mixtures of flammable gases or vapors are present because of following reasons
Class I areas follows division concept, there are four different groups categorized on liquid and gas nature of flammability. Refer below:

• Group A– Atmosphere with presence of acetylene
• Group B– Flammable gases or vapors in atmosphere with MESG (maximum experimental safe gap) less than or equal to 0.45mm or a minimum ignition current (MIC) ratio less than equal to 040mm.For example, gases like hydrogen, butadiene, propylene oxide etc.
Group C-Flammable gases or vapors in atmosphere having MESG greater than 0.45 and less than 0.75mm or MIC ratio greater than 0.40mm or less than 0.80 mm.

Class II Areas-Locations where combustible dust is present; this can be any solid material 420 microns or less in diameter capable of resulting in a fire or explosion. Class II areas are divided into two different divisions and differentiated on the basis of operation.

Division 1-Location where combustible dust is present in the air
Division 2- Location where combustible dust is present due to abnormal or faulty operations. Also, the quantity of combustible dust has to be sufficient enough to produce fire or explosion.

Class II areas comprise three distinct groups based on physical characteristics of dust

Group E– Atmosphere present with combustible metal dusts like magnesium, aluminum and others.
Group F- Atmosphere with presence of combustible carbonaceous dusts with more than 8% volatiles or other materials capable of resulting in a fire or explosion. For example coal, carbon, charcoal and coke.
Group G- Atmosphere with presence of combustible dusts like flour, grain, wood flour, plastic and other chemicals.

Class III Areas

These are considered as hazardous locations because of the presence of ignitable fibers and flying. However, in these locations ignitable fibers may or may not be present in sufficient quantities to produce an exploration or fire. This area does not have grouping like Class I or II.

Protection methods and techniques

The National Electrical code (NEC), National Fire Protection Association, (NFPA) and OSHA has set up certain benchmarks for hazardous area explosion protections. North America region factory manual, Underwriters Laboratories (UL), and Canadian Standard Association (CSA) are certifying agencies. There are several protection techniques and methods developed to reduce and even eliminate the risks of explosion or fire in hazardous locations. Few of the protections methods for class/division system are listed below:

Explosion Proof- This type of protection technique in electrical equipment have the enclosure capable to withstanding explosive gas or vapor within it and present ignitions of an explosive gas or vapors.
Dust-ignition proof: This type of equipment will exclude dust and present spark and fire on the enclosure.
Hermetically Sealed Equipment– This type of equipment is able to prevent entry of flammable gases or vapors from external atmosphere. It does not allow arcs, sparks or heat inside the enclosure.
Intrinsically safe– This type of protection technique in electrical equipment is incapable of releasing sufficient amount of energy to cause fire or explosion in a hazardous atmosphere
Non Incendive– This type of protection in electrical equipment makes it incapable to cause ignition of specific flammable gas or vapors-in-air mixture.
Purged and pressurized system– A purge and pressurization system supplies one or more enclosures with instrument air or inert gas to keep hazardous gases out of the enclosure for safe use.

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