Hazardous Areas

A “Hazardous area” can be

defined as an area or location wherethe atmosphere contains, or maycontain the quantities of flammableor explosive gases, dusts orvapours. In such an atmosphere afire or explosion is possible.

Classification of Hazardous area

• Presently there are two systemsused for classification of thesehazardous areas;– The Class/Division system,

– The Zone system

• The Class/Division system is used inthe United States and Canada,whereas the rest of the worldgenerally uses the Zone system.

Referred Std - BS EN 60079/10; BS EN 61241/3

Classification• Class I—Locations in which

flammable gases or vaporsmay or may not be insufficient quantities to produceexplosive or ignitablemixtures.

• Class II—Locations in whichcombustible dusts (either insuspension, intermittently, orperiodically) may or may notbe in sufficient quantities toproduce explosive or ignitablemixtures.

• Class III—Locations in whichignitable fibers may or maynot be in sufficient quantitiesto produce explosive orignitable mixtures

• Division 1 - indicates that thehazardous material has a highprobability of producing anexplosive or ignitable mixturedue to it being presentcontinuously, intermittently, orPeriodically or from theequipment itself under normaloperating conditions.

• Division 2 - indicates that thehazardous material has a lowprobability of producing anexplosive or ignitable mixtureand is present only duringabnormal conditions for a shortperiod of time.

Group-The Group defines the type of hazardous material in the

surrounding atmosphere.

– Group A—Atmospheres containing acetylene.

– Group B—Atmospheres containing a flammable gas, flammable liquid-produced vapor,

or combustible liquid-produced vapor whose MESG is less than 0.45 mm or MIC ratio is

less than 0.40. Typical gases include hydrogen, butadiene, ethylene oxide, propylene

oxide, and acrolein

– Group C—Atmospheres containing a flammable gas, flammable liquid-produced vapor,

or combustible liquid-produced vapor whose MESG is greater than 0.45 mm but less

than 0.75 mm or MIC ratio is greater than 0.40 but less than 0.80. Typical gases

include ethyl either, ethylene, acetaldehyde, and cyclopropane.

– Group D—Atmospheres containing a flammable gas, flammable liquid-produced vapor,

or combustible liquid-produced vapor whose MESE is greater than 0.75 mm or MIC

ration is greater than 0.80. Typical gases include acetone, ammonia, benzene, butane,

ethanol, gasoline, methane, natural gas, naphtha, and propane.

– Group E—Atmospheres containing combustible metal dusts such as aluminum,

magnesium, and their commercial alloys.

– Group F—Atmospheres containing combustible carbonaceous dusts with 8% or more

trapped volatiles such as carbon black, coal, or coke dust.

– Group G—Atmospheres containing combustible dusts not included in Group E or Group

F. Typical dusts include flour, starch, grain, wood, plastic, and chemicals

Zone SystemZone—The Zone defines the probability of the hazardous material, gas or dust, being

present in sufficient quantities to produce explosive or ignitable mixtures.

GasZone 0—Ignitableconcentrations of flammablegases or vapors which arepresent continuously or for longperiods of time.Zone 1—Ignitableconcentrations of flammablegases or vapors which are likelyto occur under normal operatingconditions.Zone 2—Ignitableconcentrations of flammablegases or vapors which are notlikely to occur under normaloperating conditions and do soonly for a short period of time.

Dust

Zone 20—An area where

combustible dusts or ignitablefibers and flyings are presentcontinuously or for long periodsof time.

Zone 21—An area where

combustible dusts or ignitablefibers and flyings are likely tooccur under normal operatingconditions.

Zone 22—An area wherecombustible dusts or ignitable

fibers and flyings are not likely tooccur under normal operatingconditions and do so only for ashort period of time.

Group I

Equipment used in mines with atmospheres containing methane or

gases and vapors of equivalent

hazard

Group IIIIA - Atmospheres containing

propane, or gases and vapors of equivalent hazard.

IIB -Atmospheres containing ethylene, or gases and

vapors of equivalent hazard.

IIC - Atmospheres containing acetylene or hydrogen, or gases and

vapors of equivalent hazard.

Sub Division of Gas Group

Temperature Code (T Code)A mixture of hazardous gases and air may be ignited by

coming into contact with a hot surface. The conditions underwhich a hot surface will ignite a gas depends on surface area,temperature, and the concentration of the gas. The same canbe said about combustible dusts.

Group II Temperature Class

T Code Maximum Surface Temperature Ignition temperature

T1 450 0C > 450 0C

T2 300 0C > 3000C ≤ 450 0C

T3 200 0C > 200 0C ≤ 300 0C

T4 130 0C > 130 0C ≤ 200 0C

T5 100 0C > 100 0C ≤ 135 0C

T6 85 0C > 85 0C ≤ 100 0C

The T code of a product denotes the maximum surface temperature that a given product will not exceed under a specified ambient temperature. For example, a product with a T code of T3 means that its maximum surface temperature will not exceed 2000C provided it is operated in a ambient temperature defined by the manufacturer

Protection Techniques and Methods

• Class/Division system

– Explosion-proof

– Intrinsically Safe

– Dust Ignition-proof

• Zone system

– Flame-proof

– Intrinsically Safe

Gas Dust Hazardous Area Characteristics

Zone 0 Zone 20

A hazardous atmosphere is highly likely to be present and may be present for long periods of time (>1000 hours per year) or even continuously

Zone 1 Zone 21 A hazardous atmosphere is possible but unlikely to be present for long periods of time (>10 <1000 hours per year

Zone 2 Zone 22

A hazardous atmosphere is not likely to be present in normal operation or infrequently and for short periods of time (<10 hours per year)

Hazardous Area Characteristics