Introduction
A1
A technical seminar
on
Intrinsic Safety
Introduction
A2Section A
The Flammable Facts
Gas, Temperature and Area Classification
Introduction
A3
Spark
Explosion Triangle
Source ofIgnition
Hazardouslocation
AirGas
SparkHeat
Introduction
A4Requirement for plant
classification
Plant and Installations are classified according to:-
The nature of the Hazardous Atmosphere
The probability that the Hazardous Atmosphere will be present
- Gas Classification- Ignition Temperature
- Area Classification
Introduction
A5Requirement for
equipment
classification
- Apparatus Group
- Temperature Classification
Equipment is classified according to:-
The maximum spark energy it can produce
Its maximum surface temperature
Introduction
A6Spark Ignition Characteristic
Lower Explosive Limit (LEL) (UEL) Upper Explosive Limit
Minimum Ignition Energy (MIE)
Ignition Energy(milli Joules)
Volume concentration (%)
1.0
0.1
0.01 0 10 20 30 40 50 60 70 80 90 100
Hydrogen-air(1 atmos.)
Flammable Range
Propane-air (1 atmos.)Ethylene-air(1 atmos.)
Introduction
A7Explosive limits
for material
s
*
Flammable Range Gas/air mixtures are ignitable over a range of concentrations called the
Flammable Range Material Lower explosive limit Upper explosive limit Minimum
LEL UEL ignition energy
propane 2 % 9.5 % 180µJ ethylene 2.7 % 34 %
60µJ hydrogen 4 % 76 % 20µJ * of most easily ignitable mixture
Introduction
A8Flammable Range
Methane
Propane
Ethylene
Hydrogen
GAS 0 10 20 30 40 50 60 70 80 90 100
EXPLOSIVE LIMITS (% gas in air by volume)
Introduction
A9Gas classification: the two main systems
Representative(test) gas
Gas classificationIgnitability
IEC countries(including Europe) USA & Canada
AcetyleneHydrogenEthylenePropane
Methane
Metal dustCarbon dustFlour, starch,grain
Fibres & flyings
Group IICGroup IICGroup IIBGroup IIA
Group I (mining)
(Dusts notclassified)Discussions anddraft documentsin circulation
Class I, Group AClass I, Group BClass I, Group CClass I, Group D
(No classification)
Class II, Group EClass II, Group FClass II, Group G
Class III
Moreeasilyignited
Introduction
A10Temperature
classification
Flammable materials have a temperature at which ignition will take place, even in the absence of an external source
This temperature is called the
Ignition Temperature of that material
Apparatus must be selected such that it will not expose the gas-air mixture to a temperature exceeding the ignition
temperature
Introduction
A11Temperature
classification
(IEC/European)
T1T2T3T4T5T6
450 C 300 C200 C135 C100 C 85 C
BetterApparatus
Temperatureclassification
Maximumsurface
temperature
o
o
o
o
o
o
Tamb = 40oCunless otherwise
stated
Operation abovestated Tamb will
require re-assessment
Introduction
A12Temperature
classification in North
America
T1T2T2AT2BT2CT2DT3T3AT3BT3CT4T4AT5T6
450300280260230215200180165160135120100 85
T class
Apparatus is marked with either T rating or maximum surface temperature
User has responsibility to ensure that the T-rating is below the Spontaneous
Ignition Temperature ( SIT )
° C
Tamb = 40oC
Introduction
A13Gas/equipment compatibility
Ammonia 630 Hydrogen 560 Methane 537
Propane 470 Ethylene 425 Butane 365
Gas Ignition Temperature
700
600
500
400
300
200
100
T1
T2
T3
T6 T4
T5
ApparatusTemperatureClassification
Cyclohexane 259
Diethyl Ether 170
Carbon Disulphide 95
Introduction
A14Gas Ignition Temperature
T4 for apparatus is usually OK Exception:
Carbon Disulphide,CS2 requiring T6
Don't confuse T Class with maximum operatingtemperature!
Introduction
A15Flashpoint Definition
The flashpoint of a flammable liquid is defined as the temperature at which the free
surface of the liquid emits sufficient vapour to be ignited by a small flame
Flashpoint
Note: Don’t confuse Flashpoint with Ignition Temperature(eg for kerosene, Flashpoint = 38°C; Ignition Temp. = 210°C)
Introduction
A16Properties of Gases
Extract from BS.5345 Part 1
Compound Vapour LFL UFL Ignition T Apparatus Density Temp C Class Group
Taken from IEC 79-10:1995
AcetoneAmmoniaButaneCarbon DisulphideCyclohexaneDiethyl EtherEthyleneHydrogenKeroseneMethanePropane
2.000.592.052.642.902.550.970.07
0.551.56
2.0151.51.01.21.72.74.00.75.02.0
13288.5607.8363475.65159.5
535630372 95259160425560210537470
T1T1T2T6T3T4T2T1T3T1T1
IIAIIAIIAIICIIAIIBIIBIICIIAI / IIAIIA
Introduction
A17Why Area Classify?
Probability of
Gas/Air Mixture
being present
Probability of
Source of
IgnitionX =
Acceptable
Risk
Reasoning : The higher the probability of the gas/air mixture being present demands a lower probability of having a source of ignition and vice versa. Hence in Zone 0 we use IS; in Zone 2 we perhaps use Type N or Exq,Exo,Exm, etc
Introduction
A18Area classifica
tion in IEC/Euro
pean countries
Areas are classified with regard to the probability of a potentiallyexplosive atmosphere being present and the length of time
for which it is likely to exist
Zone 0:
Zone 1:
Zone 2:
in which an explosive gas-air mixture is continuously present or present for long periods
in which an explosive gas-air mixture is likely to occur in normal operation
in which an explosive gas-air mixture is not likely occur in normal operation, and if it occurs it will exist only for a short time
Introduction
A19Area classification
in North America
Division 1Hazardous atmosphere is likely to be present in normal operation
Division 2Hazardous atmosphere is unlikely to be present in normal operation
Introduction
A20
Why area Classify ?• Why go to all the trouble with areas & equipment?• If we know the probability of hazardous area & probability
of source of ignition , we know whether we are taking an acceptable risk.
• Eg. Probability of flammable atmosphere = high• If apparatus insures never sparks = acceptable risk
Introduction
A21Personnel Hazards
For Hydrogen Sulphide, H2S:a very common danger when dealing with oil products
Property VolumeConcentration
Smell detection level 0.1 ppmSafe working level 10Serious health danger level 50Lower Flammable Limit 40,000 (4%)Upper Flammable Limit 460,000 (46%)
Note! Figures for safe working levels vary, the above are from a Middle East location.
Introduction
A22Area Classification Example
Bund wall
Tank vents
Liquid surface
Area Classificationfor a fixed-roof tankwith heavier than air
vapourZone 1
Zone 0
Zone 2
Key
Introduction
A23Thermocouple in a hazardous area
Inside of pipe orprocess vessel
Zone 0Zone 1Outside of pipe
Zone 2
Division 2
Division 1
Introduction
A24Which documents
Within UK, BS EN60079-10 : 1996 IEC79 - 10 : 1995
Book; -
Classification of hazardous locationsby A.W. Cox, F.P. Lees, and M.L. Ang
Published by IChemE ISBN 0 85295 2589
Introduction
A25Explosion protection for dusts
About 70% of dusts occuring in industry are flammable.
Dusts and powders typically require mJ of spark energy for ignition,so a more likely mechanism is hot surface ignition.
Whereas the majority of flammablegases have ignition temperatures above 350 C, some dusts ignite at150 - 200 C.
Ingress protection alone for equipment is not enough: must protectagainst ignition by raised temperatures of enclosures.
Is the dust flammable?
What is its ignitiontemperature?
Dusts need milliJoulesof spark energy to ignite
Ingress protection aloneis not enough
oo
Introduction
A26Ignition Properties of Dusts
Type of Dust Ignition Temperatureof dust cloud (oC)
Minimum Spark Energyrequired for ignition
of cloud ( milli-Joules )
Minimum Explosiveconcentration
( gms / cu.metre )
Aluminium, milledZincPolystyreneUrea resinCocoaCoffeeCotton seedGrain dustSugarCoalCorkSulphurWood flour
550
600490450420410470430350610470190430
35
3851255357045453045303035
15
650158010016080303060451520
Introduction
A27
Following the introduction of new European Directives a group of newStandards are being prepared.
Two standards have been published, they are;
EN 50281-1-1 : 1998 Electrical apparatus protected by enclosures - Construction and testing
EN 50281-2-1 : 1998 Test methods for determining the minimum ignition temperatures of dust
In draft stage are;
prEN 50281-1-3 Classification of areas where combustible dust are or may be present
IEC are working alongside the European Standards committee
IEC/European Dust Standards
Introduction
A28Comparison of dust classification systems
Grade of release BS6467 IEC 1241 DIN VDE 1988 ATEX 137 0165: 1991
Continuous 20
Primary Z 21 10
Secondary Y 22 11
Area classification of dusts and powders must be treated differentlyfrom gases and vapours:
Dusts don’t disperse with timeVentilation can convert layers into clouds
Introduction
A29Area Classification - Dusts
Zone Z An area in which combustible dust is, or may be, present as a cloud handling or cleaningoperations in sufficient quantity to be capable of producing an explosive concentration of combustible or ignitable dustin a mixture with air.
Zone Y An area not classified as Zone Z, in which accumulationsor layers of combustible or ignitable dust may be presentunder abnormal conditions and give rise to ignitable mixtures of dust and air.
Definitions taken fromBritish Standard 6467: Part 2: 1988
during normal processing, handling or cleaning
under abnormal conditions
Introduction
A30Dust Hazards
Relevant Standards for Dusts
BS 6467 "Electrical Apparatus with protection by enclosure for use in the presence of combustible dusts" Part 1 (1985) "Specification for apparatus" Part 2 " Guide to selection, installation and maintenance"
ISA-S12.10 Area classification in hazardous dust locations
BS 7535 : 1992 Guide to the use of electrical apparatus complying with BS 5501 or BS 6941 in the presence of combustible dusts
Introduction
A31Intrinsic safety and dust hazards
Hazardous Area Suitable apparatus
Zone Y
Zone Z
Intrinsically safe withdegree of protectionat least IP5X *
Intrinsically safe withdegree of protectionat least IP6X *
Source: BS 7535 : 1992; Clause 2.7
* Note: do not confuse with NEMA categories
Introduction
A32
Spark
Explosion Triangle
Source ofIgnition
Hazardouslocation
AirGas
SparkHeat