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ExplosiveAtmospheres
Overview
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Two types of explosions
Deflagration: Burns with great heat and light.
Flame front speed 1 m/sec
Speed of sound 330 m/sec
Detonation: A violent explosion
Flame front speed 2000-8000 m/sec
Speed of sound 330 m/sec
Deflagration and Detonation
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Explosion Properties
Fuel
Oxygen
Ignition source
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Explosion Properties
All flammable gases, vapours and mists require to be
mixed with oxygen to make them burn. There is about20-21% of oxygen in the air we breath. Mixtures of aflammable gas and certain percentages of air will burn ifignited.
Too much or too little oxygen, the mixture will not ignite.The upper and lower concentrations of gas in
atmospheric air, by volume, are known as theirflammability or explosive limit.
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Flammable Range All dangerous substances have a flammable
range
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Flammable Range
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Examples of Flammable Range(PD IEC 60079-20)
774Hydrogen1002.3Acetylene
362.3Ethylene111.7Propane
174.4Methane
UEL (%)LEL (%)Substance
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Explosion Properties
For any mixture of a
combustible gas orvapour with an oxidizer(air) there is a criticalignition energy.
Release less than thecritical amount ofenergy into the mixture
and there will not be anexplosion.
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At a critical concentration called the mosteasily ignited concentration (MEIC), theamount of energy required to cause ignition isminimal.
The critical energy at the MEIC is calledminimum ignition energy (MIE).
Explosion Properties
MEIC
MIE
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477
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Explosion Properties
The LEL (LFL) and UEL (UFL) are not inherent
properties of a combustible mixture. Theirvalues depend on the nature of the experimentby which they are determined.
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Historical Background
The first hazardous area was discovered in the
coal mines.
This area held a double hazard: methane gas
(firedamp) and coal dust.
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Historical Background
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Historical Background
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In 1815 Sir HumphreyDavy invented theDavy lamp, whichwas a kerosene lamp
with a fine brassmesh. This was usedto indicate methane
concentration in themine atmosphere.
Historical Background
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Electrical Equipment
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This method of
protection lead tothe development ofthe Flameproof
(FLP) concept ofprotection
Ex d.
Historical Background
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Historical BackgroundHistorical BackgroundMine explosion in South Wales 1912.
Cause- a spark generated by the signalling system
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Historical BackgroundHistorical Background
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Research at the Mines Research Establishment,Buxton, England that these low voltage circuitswere capable of igniting mine gases, it also ledto new circuit designs in which the stored energywas reduced to a non ignition capable level.
Historical BackgroundHistorical Background
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This technique was labelled intrinsic safety Ex
i and it was the beginning of a new era insafety methods for explosive hazardous areas.
More on the Ex i concept later.
Historical BackgroundHistorical Background
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Gas Group 1(I)
Explosive Atmosphere
Gas Group 2 (II)
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Group I is for mining and not covered further in
this presentation.
Group II is for surface industries
Group II gases is further divided into 3 sub-divisions.
Explosive Atmosphere
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IIA, for atmospheres containing propane orgases of an equivalent hazard.
IIB, for atmospheres containing ethylene orgases of an equivalent hazard.
IIC, for atmospheres containing hydrogen orgases of an equivalent hazard.
Explosive Atmosphere
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Examples of Flammable Range(PD IEC 60079-20*)
77 IIC4Hydrogen
100 IIC2.3Acetylene
36 IIB2.3Ethylene11 IIA1.7Propane
17 IIA4.4Methane
UEL (%) GGLEL (%)Substance
* There are 299 substances listed in PD IEC 60079-20 (Flammable Gas Data).
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Temperature class
Equipment is identified with a temperature class
Identified by a T rating (or by a temperature indegrees C).
The temperature class identifies the hottest
temperature that the equipment can obtain innormal operation.
This can be either the inside or the outside of the
equipment depending on the protection concept.
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Temperature ClassThe full list of temperature codes are
85T6
100T5
135T4
200T3
300T2
450T1
Max Temp limit
(C)
Temperature
Class
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The temperature codes for the substanceslisted previously (temperature
classification) are shown below:
T1Hydrogen
T2Acetylene
T2Ethylene
T1Propane
T1Methane
Temp ClassSubstance
Temperature Class
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T1 IICHydrogen
T2 IICAcetylene
T2 IIBEthylene
T1 IIAPropane
T1 IIAMethane
Temp Class Gas GroupSubstance
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Equipment Marking
Example
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Any Questions