ISO/IEC 80079-20-1:2017
(Main)Explosive atmospheres — Part 20-1: Material characteristics for gas and vapour classification — Test methods and data
Explosive atmospheres — Part 20-1: Material characteristics for gas and vapour classification — Test methods and data
ISO/IEC 80079-20-1:2017 is published as a dual log standard and provides guidance on classification of gases and vapours. It describes a test method intended for the measurement of the maximum experimental safe gaps (MESG) for gas-air mixtures or vapour-air mixtures under normal conditions of temperature and pressure (20 °C, 101,3 kPa) so as to permit the selection of an appropriate group of equipment. This document also describes a test method intended for use in the determination of the auto-ignition temperature (AIT) of a vapour-air mixture or gas-air mixture at atmospheric pressure, so as to permit the selection of an appropriate temperature class of equipment. Values of chemical properties of materials are provided to assist in the selection of equipment to be used in hazardous areas. Further data may be added as the results of validated tests become available. The materials and the characteristics included in a table (see Annex B) have been selected with particular reference to the use of equipment in hazardous areas. The data in this document have been taken from a number of references which are given in the bibliography. These methods for determining the MESG or the AIT may also be used for gas-air-inert mixtures or vapour-air-inert mixtures. However, data on air-inert mixtures are not tabulated. Keywords: classification of gases and vapours, measurement of the maximum experimental safe gaps (MESG)
Atmosphères explosives — Partie 20-1: Caractéristiques des produits pour le classement des gaz et des vapeurs — Méthodes et données d'essai
ISO/IEC 80079-20-1:2017 est publiée en tant que norme sous double logo et donne des recommandations pour le classement des gaz et des vapeurs. Elle décrit une méthode d'essai destinée à mesurer les interstices expérimentaux maximaux de sécurité (IEMS ou MESG maximum experimental safe gaps) des mélanges gaz-air ou vapeur-air dans des conditions normales de température et de pression (20 °C, 101,3 kPa) afin de pouvoir choisir un groupe approprié d'appareils. Le présent document décrit également une méthode d'essai qui permet d'établir la température dauto-inflammation (TAI ou AIT auto-ignition temperature) d'un mélange vapeur-air ou gaz-air à la pression atmosphérique, afin de pouvoir choisir une classe de température appropriée des appareils. Les valeurs des propriétés chimiques des produits sont fournies pour faciliter le choix des appareils à utiliser dans les emplacements dangereux. Des données supplémentaires peuvent être ajoutées au fur et à mesure de lobtention de résultats dessai validés. Les produits et les caractéristiques indiqués dans un tableau (voir lAnnexe B) ont été choisis tout particulièrement pour l'utilisation d'appareils dans des emplacements dangereux. Les données fournies dans le présent document sont extraites dun certain nombre de documents de référence qui sont cités dans la bibliographie. Ces méthodes de détermination de lIEMS ou détablissement de la TAI peuvent être également appliquées pour les mélanges gaz-air-matière inerte ou les mélanges vapeur-air-matière inerte. Les données relatives aux mélanges air-matière inerte ne sont toutefois pas fournies. Mots clés: classement des gaz et des vapeurs, mesurer les interstices expérimentaux maximaux de sécurité (IEMS ou MESG)
General Information
Standards Content (Sample)
ISO/IEC 80079-20-1
Edition 1.0 2017-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Explosive atmospheres –
Part 20-1: Material characteristics for gas and vapour classification – Test
methods and data
Atmosphères explosives –
Partie 20-1: Caractéristiques des produits pour le classement des gaz et des
vapeurs – Méthodes et données d'essai
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ISO/IEC 80079-20-1
Edition 1.0 2017-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Explosive atmospheres –
Part 20-1: Material characteristics for gas and vapour classification – Test
methods and data
Atmosphères explosives –
Partie 20-1: Caractéristiques des produits pour le classement des gaz et des
vapeurs – Méthodes et données d'essai
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 13.230; 29.260.20 ISBN 978-2-8322-5164-5
− 2 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Classification of gases and vapours . 9
4.1 General . 9
4.2 Classification according to the maximum experimental safe gap (MESG) . 9
4.3 Classification according to the minimum igniting current ratio (MIC ratio) . 10
4.4 Classification according to the similarity of chemical structure . 10
4.5 Classification of mixtures of gases . 10
5 Data for flammable gases and vapours, relating to the use of equipment . 11
5.1 Determination of the properties . 11
5.1.1 General . 11
5.1.2 Equipment group . 11
5.1.3 Flammable limits . 11
5.1.4 Flash point (FP) . 11
5.1.5 Temperature class . 12
5.1.6 Minimum igniting current (MIC) . 12
5.1.7 Auto-ignition temperature (AIT) . 12
5.2 Properties of particular gases and vapours . 12
5.2.1 Coke oven gas . 12
5.2.2 Ethyl nitrite . 12
5.2.3 MESG of carbon monoxide . 12
5.2.4 Methane, Equipment Group IIA . 13
6 Method of test for the maximum experimental safe gap (MESG) . 13
6.1 Outline of method . 13
6.2 Test apparatus . 13
6.2.1 General . 13
6.2.2 Material and mechanical strength . 14
6.2.3 Exterior chamber . 14
6.2.4 Interior chamber . 14
6.2.5 Gap adjustment . 14
6.2.6 Injection of mixture . 14
6.2.7 Position of ignition source . 14
6.3 Procedure . 14
6.3.1 Preparation of gas mixtures . 14
6.3.2 Temperature and pressure . 14
6.3.3 Gap adjustment . 15
6.3.4 Ignition . 15
6.3.5 Observation of the ignition process . 15
6.4 Determination of maximum experimental safe gap (MESG) . 15
6.4.1 General . 15
6.4.2 Preliminary tests . 15
6.4.3 Confirmatory tests . 15
6.4.4 Reproducibility of maximum experimental safe gaps (MESG) . 15
6.4.5 Tabulated values . 16
6.5 Verification of the MESG determination method . 16
7 Method of test for auto-ignition temperature (AIT) . 16
7.1 Outline of method . 16
7.2 Apparatus . 16
7.2.1 General . 16
7.2.2 Test vessel and support . 17
7.2.3 Thermocouples . 17
7.2.4 Oven . 17
7.2.5 Metering devices . 18
7.2.6 Mirror . 18
7.2.7 Timer . 18
7.2.8 Equipment for purging the test vessel with air . 18
7.2.9 Automated apparatus . 18
7.3 Sampling, preparation and preservation of test samples . 19
7.3.1 Sampling . 19
7.3.2 Preparation and preservation . 19
7.4 Procedure . 19
7.4.1 General . 19
7.4.2 Sample injection . 20
7.4.3 Determination of the auto-ignition temperature (AIT) . 20
7.5 Auto-ignition temperature (AIT) . 21
7.6 Validity of results . 21
7.6.1 Repeatability . 21
7.6.2 Reproducibility . 21
7.7 Data . 22
7.8 Verification of the auto-ignition temperature determination method . 22
Annex A (normative) Ovens of test apparatus for the tests of auto-ignition temperature . 23
A.1 General . 23
A.2 “IEC oven” . 23
A.3 “DIN oven” . 23
Annex B (informative) Tabulated values . 30
Annex C (informative) Determination of cool flames . 84
Annex D (informative) Volume dependence of auto-ignition temperature . 86
Bibliography . 87
Figure 1 − Test apparatus . 13
Figure A.1 − Test apparatus: assembly . 24
Figure A.2 − Section A-A (flask omitted) . 25
Figure A.3 − Base heater (board made of refractory material) . 25
Figure A.4 − Flask guide ring (board made of refractory material) . 26
Figure A.5 − Neck heater (board made of refractory material) . 26
Figure A.6 − Oven . 27
Figure A.7 − Lid of steel cylinder . 28
Figure A.8 − Lid of steel cylinder . 29
Figure A.9 − Injection of gaseous sample. 29
Figure C.1 − Additional thermocouple to detect cool flames . 84
− 4 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
Figure C.2 − ‘Negative temperature coefficient’ shown for butyl butyrate as an example . 85
Figure D.1 − Volume dependence of auto-ignition temperature . 86
Table 1 − Classification of temperature class and range of auto-ignition temperatures . 12
Table 2 − Values for verification of the apparatus . 16
Table 3 − Values for verification of the apparatus . 22
Table B.1 − Material data . 32
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 20-1: Material characteristics for gas and vapour classification –
Test methods and data
FOREWORD
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard ISO/IEC 80079-20-1 has been prepared by subcommittee 31M: Non-
electrical equipment and protective systems for explosive atmospheres, of IEC technical
committee 31: Equipment for explosive atmospheres.
This first edition of ISO/IEC 80079-20-1 cancels and replaces IEC 60079-20-1:2010. It
constitutes a technical revision. No significant changes were made with respect to
IEC 60079-20-1:2010.
It is published as a double logo standard.
− 6 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
The text of this standard is based on the following documents:
FDIS Report on voting
31M/122/FDIS 31M/126/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60079 series, under the general title: Explosive atmospheres, as
well as the International Standard 80079 series, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
EXPLOSIVE ATMOSPHERES –
Part 20-1: Material characteristics for gas and vapour classification –
Test methods and data
1 Scope
This part of ISO/IEC 80079 provides guidance on classification of gases and vapours. It
describes a test method intended for the measurement of the maximum experimental safe
gaps (MESG) for gas-air mixtures or vapour-air mixtures under normal conditions of
temperature and pressure (20 °C, 101,3 kPa) so as to permit the selection of an appropriate
group of equipment. This document also describes a test method intended for use in the
determination of the auto-ignition temperature (AIT) of a vapour-air mixture or gas-air mixture
at atmospheric pressure, so as to permit the selection of an appropriate temperature class of
equipment.
Values of chemical properties of materials are provided to assist in the selection of equipment
to be used in hazardous areas. Further data may be added as the results of validated tests
become available.
The materials and the characteristics included in a table (see Annex B) have been selected
with particular reference to the use of equipment in hazardous areas. The data in this
document have been taken from a number of references which are given in the bibliography.
These methods for determining the MESG or the AIT may also be used for gas-air-inert
mixtures or vapour-air-inert mixtures. However, data on air-inert mixtures are not tabulated.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60050-426, International Electrotechnical Vocabulary − Part 426: Electrical apparatus for
explosive atmospheres (available at http://www.electropedia.org/)
IEC 60079-11, Explosive atmospheres − Part 11: Equipment protection by intrinsic safety "i"
IEC 60079-14, Explosive atmospheres − Part 14: Electrical installations design, selection and
erection
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-426 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
− 8 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
3.1
auto-ignition
reaction which is evidenced by a clearly perceptible flame and/or explosion, and for which the
ignition delay time does not exceed 5 min
Note 1 to entry: See 7.2.2 for a test method.
3.2
ignition delay time
time between the completed injection of the flammable material and the ignition
3.3
auto-ignition temperature
AIT
lowest temperature (of a surface) at which under specified test conditions an ignition of a
flammable gas or vapour in mixture with air or air-inert gas occurs
Note 1 to entry: See Clause 7 for a test method.
3.4
maximum experimental safe gap
MESG
maximum gap of a joint of 25 mm in width which prevents any transmission of an explosion
during tests made under the conditions specified in this document
Note 1 to entry: See Clause 6 for a test method.
3.5
minimum ignition current
MIC
minimum current in a specified test circuit that causes the ignition of the explosive test
mixture in the spark test apparatus according to IEC 60079-11
Note 1 to entry: See 5.1.6 for the test circuit.
3.6
flammable limits
lower flammable limit (LFL) and upper flammable limit (UFL) of gas in a gas-air mixture,
between which a flammable mixture is formed
Note 1 to entry: The term “explosive limits” is used especially in European standardization and regulations
interchangeably to describe these limits.
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.6.1
lower flammable limit
LFL
concentration of flammable gas or vapour in air, below which an explosive gas atmosphere
does not form
Note 1 to entry: For the purposes of Ex Equipment, this was previously referred to as the lower explosive limit
(LEL).
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.6.2
upper flammable limit
UFL
concentration of flammable gas or vapour in air, above which an explosive gas atmosphere
does not form
Note 1 to entry: For the purposes of Ex Equipment, this was previously referred to as the upper explosive limit
(UEL).
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.7
equipment grouping
classification system of equipment related to the explosive atmosphere for which they are
intended to be used
Note 1 to entry: IEC 60079-0 identifies three equipment groups:
Group I − equipment for mines susceptible to fire damp;
Group II, which is sub-divided into groups IIA, IIB and IIC − equipment for all places with an explosive gas
atmosphere other than mines susceptible to fire damp;
Group III, which is sub-divided into groups IIIA, IIIB and IIIC − equipment for all places with an explosive dust
atmosphere other than mines susceptible to fire damp.
3.8
flash point
FP
lowest liquid temperature at which, under specified test conditions, a liquid gives off vapours
in quantity such as to be capable of forming an ignitable vapour-air mixture
3.9
gas
gaseous phase of a substance that cannot reach equilibrium with its liquid or solid state in the
temperature and pressure range of interest
Note 1 to entry: This is a simplification of the scientific definition, and merely requires that the substance is above
its boiling point or sublimation point at the ambient temperature and pressure.
3.10
vapour
gaseous phase of a substance that can reach equilibrium with its liquid or solid state in the
temperature and pressure range of interest
Note 1 to entry: This is a simplification of the scientific definition, and merely requires that the substance is below
its boiling point or sublimation point at the ambient temperature and pressure.
4 Classification of gases and vapours
4.1 General
Equipment Group I addresses mines susceptible to firedamp.
NOTE Firedamp consists mainly of methane, but always contains small quantities of other gases, such as
nitrogen, carbon dioxide, and hydrogen, and sometimes ethane and carbon monoxide. The terms firedamp and
methane are used frequently in mining practice as synonyms.
Equipment Group II addresses flammable gases and vapours other than in mines susceptible
to firedamp. Equipment Group II gases and vapours are classified according to their MESG
and/or MIC ratio into equipment groups IIA, IIB and IIC.
All flammable materials are classified according to their AIT into temperature classes.
4.2 Classification according to the maximum experimental safe gap (MESG)
Gases and vapours may be classified according to their MESG into Equipment Groups IIA, IIB
or IIC, based on the determination method described in this document. In order to ensure
standardized results the MESG apparatus is dimensioned to avoid the possible effects of
obstruction on the safe gaps.
− 10 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
NOTE 1 The standard method for determining MESG is described in 6.2, but where determinations have been
undertaken only in an 8 l spherical vessel with ignition close to the flange gap these can be accepted provisionally.
NOTE 2 The design of the test apparatus for safe gap determination, other than that used for selecting the
appropriate equipment group of enclosure for a particular gas, may need to be different to the one described in this
document. For example, the volume of the enclosure, flange width, gas concentrations and the distance between
the flanges and any external wall or obstruction may have to be varied. As the design depends on the particular
investigation which is to be undertaken, it is impracticable to recommend specific design requirements, but for most
applications the general principles and precautions indicated in this document will still apply.
NOTE 3 In IEC 60079-14 minimum distances of obstruction from the flameproof flange joints related to the
equipment group of the hazardous area are given.
For the purpose of classification the MESG limits are:
Equipment Group IIA: MESG ≥ 0,90 mm.
Equipment Group IIB: 0,50 mm < MESG < 0,90 mm.
Equipment Group IIC: MESG ≤ 0,50 mm.
Determination of both the MESG and MIC ratio is required when 0,50 < MESG < 0,55. Then
the equipment group is determined by MIC ratio,
NOTE 4 For gases and highly volatile liquids, the MESG is determined at 20 °C.
NOTE 5 If it was necessary to do the MESG determination at temperatures higher than ambient temperature a
temperature 5 K above that needed to give the necessary vapour pressure or 50 K above the flash point is used
and this value of MESG is given in the table and the classification of the equipment group is based on this result.
4.3 Classification according to the minimum igniting current ratio (MIC ratio)
Gases and vapours may be classified according to the ratio of their minimum igniting currents
(MIC) to the ignition current of laboratory methane into Equipment Groups IIA, IIB or IIC. The
purity of laboratory methane shall be not less than 99,9 % by volume.
NOTE The standard method of determining MIC ratios is with the apparatus described in IEC 60079-11, but where
determinations have been undertaken in other apparatus these can be accepted provisionally.
For the purpose of classification the MIC ratios are:
Equipment Group IIA: MIC > 0,80.
Equipment Group IIB: 0,45 ≤ MIC ≤ 0,80.
Equipment Group IIC: MIC < 0,45.
Determination of both the MESG and MIC ratio is required when 0,70 < MIC < 0,90 or
0,40 < MIC < 0,50. Then the equipment group is determined by MESG.
4.4 Classification according to the similarity of chemical structure
When a gas or vapour is a member of a homologous series of compounds, the classification
of the gas or vapour can provisionally be inferred from the data of the neighbouring members
of the series.
The classification according to the similarity of chemical structure is not allowed if the
classification of one of the neighbouring members is based on MESG and the other on MIC
ratio.
4.5 Classification of mixtures of gases
Mixtures of gases should generally be allocated to an equipment group only after a special
determination of MESG or MIC ratio. One method to estimate the equipment group is to
determine the MESG of the mixture by applying a form of Le Châtelier’s principle:
MESG =
mix
X
i
∑
MESG
i
i
Where X is the percentage by volume of material i and MESG is the MESG of material i.
i i
This method should not be applied in case of exceptions to the Le Châtelier’s principle and to
mixtures and/or streams that have:
a) acetylene or its equivalent hazard (e.g. self-decomposition properties);
b) oxygen or other strong oxidizer as one of the components;
c) large concentrations (over 5 % by volume) of carbon monoxide. Because unrealistically
high MESG values may result, caution should be exercised with two component mixtures
where one of the components is an inert, such as nitrogen.
For mixtures containing an inert such as nitrogen in concentrations less than 5 % by volume,
use an MESG of infinity. For mixtures containing an inert such as nitrogen in concentrations
5 % by volume and greater, use an MESG of 2.
NOTE An alternate method that includes stoichiometric ratios is presented in the essay “Maximum experimental
safe gap of binary and ternary mixtures,” by Brandes and Redeker.
5 Data for flammable gases and vapours, relating to the use of equipment
5.1 Determination of the properties
5.1.1 General
The compounds listed in this document are in accordance with Clause 4, or have physical
properties similar to those of other compounds in that list.
5.1.2 Equipment group
The equipment groups are the result of MESG or MIC ratio determination except where there
is no value listed for MESG or MIC ratio. For these, the equipment group is based on
chemical similarity (see Clause 4).
NOTE If it was necessary to do the MESG determination at temperatures higher than ambient temperature, a
temperature 5 K above that needed to give the necessary vapour pressure or 50 K above the Flash Point is used.
This value of MESG is given in Table B.1 and the classification of the equipment group is based on this result.
5.1.3 Flammable limits
Determinations have been made by a number of different methods, but the preferred method
is with a low energy ignition at the bottom of a vertical tube. The values (in percentage by
volume and mass per volume) are listed in Table B.1.
If the flash point is high, the compound does not form a flammable vapour air/mixture at
normal condition of temperature (20 °C). Where flammability data are presented for such
compounds the determinations have been made at a temperature sufficiently elevated to allow
the vapour to form a flammable mixture with air.
5.1.4 Flash point (FP)
The value given in Table B.1 is the “closed cup” measurement. When this data was not
available, the “open cup” value is quoted and indicated by (oc). The symbol < (less than),
indicates that the flash point is below the value (in degree Celsius) stated, this probably being
the limit of the apparatus used.
− 12 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
5.1.5 Temperature class
The temperature class of a gas or vapour is given according to IEC 60079-14, as shown in
Table 1:
Table 1 − Classification of temperature class
and range of auto-ignition temperatures
Temperature class Range of auto-ignition temperature (AIT)
°C
T1 > 450
T2
300 < AIT ≤ 450
T3 200 < AIT ≤ 300
T4 135 < AIT ≤ 200
T5
100 < AIT ≤ 135
T6
85 < AIT ≤ 100
5.1.6 Minimum igniting current (MIC)
The apparatus for the determination of minimum igniting current is defined in IEC 60079-11.
The test apparatus shall be operated in a 24 V d.c. circuit containing a (95 ± 5) mH air-cored
coil. The current in this circuit is varied to a minimum value until ignition of the most easily
ignited concentration of the specific gas or vapour in air is obtained.
5.1.7 Auto-ignition temperature (AIT)
The value of auto-ignition temperature depends on the method of testing. The preferred
method and data obtained is given in Clause 7 and in Annex A.
If the compound is not included in these data, the data obtained in similar apparatus, such as
the apparatus described by ASTM E659, is listed.
NOTE Results from using the apparatus described in ASTM D2155 (now replaced by ASTM E659) were reported
by C.J. Hilado and S.W. Clark. The apparatus is similar to the one used by Zabetakis. If there is no determination
by either the IEC apparatus, nor similar apparatus, the lowest value obtained in other apparatus is listed. A more
comprehensive list of data for auto-ignition temperature, with the reference to sources, is given by Hilado and
Clark.
5.2 Properties of particular gases and vapours
5.2.1 Coke oven gas
Coke oven gas is a mixture of hydrogen, carbon monoxide and methane. If the sum of the
concentrations (Vol. %) of hydrogen and carbon monoxide is less than 75 % by volume of the
total, flameproof equipment of Equipment Group IIB is recommended; otherwise, equipment of
Equipment Group IIC is recommended.
5.2.2 Ethyl nitrite
The auto-ignition temperature of ethyl nitrite is 95 °C, above which the gas suffers explosive
decomposition.
NOTE Ethyl nitrite is not be confused with its isomer, nitroethane.
5.2.3 MESG of carbon monoxide
The MESG for carbon monoxide relates to a mixture with air saturated with moisture at normal
ambient temperature. This determination indicates the use of Equipment Group IIB equipment
in the presence of carbon monoxide. A larger MESG may be observed with less moisture. The
lowest MESG (0,65 mm) is observed for a mixture of CO/H O near 7:1 mole ratio. Small
quantities of hydrocarbon in the carbon monoxide-air mixture have a similar effect in reducing
the MESG so that Equipment Group IIB equipment is required.
5.2.4 Methane, Equipment Group IIA
Industrial methane, such as natural gas, is classified as Equipment Group IIA, provided it
does not contain more than 25 % by volume of hydrogen. A mixture of methane with other
compounds from Equipment Group IIA, in any proportion, is classified as Equipment
Group IIA.
6 Method of test for the maximum experimental safe gap (MESG)
6.1 Outline of method
The interior and exterior chambers of the test apparatus are filled with a known mixture of the
gas or vapour in air, under normal conditions of temperature and pressure (20 °C, 101,3 kPa)
and with the circumferential gap between the two chambers accurately adjusted to the
desired value. The internal mixture is ignited and the flame propagation, if any, is observed
through the windows in the external chamber. The maximum experimental safe gap for the
gas or vapour is determined by adjusting the gap in small steps to find the maximum value
of gap which prevents ignition of the external mixture, for any concentration of the gas or
vapour in air.
NOTE An exception is made for substances with vapour pressures which are too low to permit mixtures of the
required concentrations to be prepared at normal ambient temperatures. For these substances, a temperature
5 K above that needed to give the necessary vapour pressure or 50 K above the flash point is used.
6.2 Test apparatus
6.2.1 General
The apparatus is described in the following subclauses and is shown schematically in
Figure 1. It is also possible to use an automatic set-up when it is proven that the same results
are obtained as with a manual apparatus.
c
d
b
i
a
g
f
h
+
–
e
IEC
Key
a f
interior spherical chamber observation windows
b g
exterior cylindrical enclosure spark electrode
c h
adjustable part lower gap plate, fixed
d i
outlet of mixture upper gap plate, adjustable
e
inlet of mixture
Figure 1 − Test apparatus
− 14 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
6.2.2 Material and mechanical strength
The whole apparatus is constructed to withstand a maximum pressure of 1 500 kPa without
significant expansion of the gap, so that no such expansion of the gap will occur during an
explosion.
The main parts of the test apparatus, and in particular the walls and flanges of the inner
chamber and the electrodes of the spark-gap, are normally of stainless steel. Other materials
may have to be used with some gases or vapours, however, in order to avoid corrosion or
other chemical affects. Light alloys should not be used for the spark-gap electrodes.
6.2.3 Exterior chamber
The exterior cylindrical enclosure "b" has a diameter of 200 mm and a height of 75 mm.
6.2.4 Interior chamber
The interior chamber "a" is a sphere with a volume measuring 20 cm . The interior chamber is
placed in the centre of the exterior chamber.
6.2.5 Gap adjustment
The two parts "i" and "h" of the internal chamber are so arranged that an adjustable 25 mm
gap can be set up between the plane parallel faces of the opposing rims. The exact width of
the gap can be adjusted by means of the micrometer (part "c").
6.2.6 Injection of mixture
The internal chamber is filled with the gas-air or vapour-air mixture through an inlet ("e"). The
exterior chamber is filled with the mixture via the gap. The inlet and outlet should be protected
by flame arresters.
6.2.7 Position of ignition source
The electrode "g" shall be mounted in such a way that the spark occurs in the centre of the
internal chamber.
6.3 Procedure
6.3.1 Preparation of gas mixtures
As the consistency of the mixture concentration, for a particular test series, has a pronounced
effect on the dispersion of the test results, it has to be carefully controlled. The flow of the
mixture through the chamber is therefore maintained until the inlet and outlet concentrations
are the same, or a method of equivalent reliability shall be used.
For the classification according to this document, the moisture content of the air used for the
preparation of the mixture should not exceed 10 % relative humidity. Higher values of
humidity could result for some substances in lower MESG values.
6.3.2 Temperature and pr
...
ISO/IEC 80079-20-1
Edition 1.0 2017-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Explosive atmospheres –
Part 20-1: Material characteristics for gas and vapour classification – Test
methods and data
Atmosphères explosives –
Partie 20-1: Caractéristiques des produits pour le classement des gaz et des
vapeurs – Méthodes et données d'essai
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ISO/IEC 80079-20-1
Edition 1.0 2017-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Explosive atmospheres –
Part 20-1: Material characteristics for gas and vapour classification – Test
methods and data
Atmosphères explosives –
Partie 20-1: Caractéristiques des produits pour le classement des gaz et des
vapeurs – Méthodes et données d'essai
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 13.230; 29.260.20 ISBN 978-2-8322-5164-5
− 2 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Classification of gases and vapours . 9
4.1 General . 9
4.2 Classification according to the maximum experimental safe gap (MESG) . 9
4.3 Classification according to the minimum igniting current ratio (MIC ratio) . 10
4.4 Classification according to the similarity of chemical structure . 10
4.5 Classification of mixtures of gases . 10
5 Data for flammable gases and vapours, relating to the use of equipment . 11
5.1 Determination of the properties . 11
5.1.1 General . 11
5.1.2 Equipment group . 11
5.1.3 Flammable limits . 11
5.1.4 Flash point (FP) . 11
5.1.5 Temperature class . 12
5.1.6 Minimum igniting current (MIC) . 12
5.1.7 Auto-ignition temperature (AIT) . 12
5.2 Properties of particular gases and vapours . 12
5.2.1 Coke oven gas . 12
5.2.2 Ethyl nitrite . 12
5.2.3 MESG of carbon monoxide . 12
5.2.4 Methane, Equipment Group IIA . 13
6 Method of test for the maximum experimental safe gap (MESG) . 13
6.1 Outline of method . 13
6.2 Test apparatus . 13
6.2.1 General . 13
6.2.2 Material and mechanical strength . 14
6.2.3 Exterior chamber . 14
6.2.4 Interior chamber . 14
6.2.5 Gap adjustment . 14
6.2.6 Injection of mixture . 14
6.2.7 Position of ignition source . 14
6.3 Procedure . 14
6.3.1 Preparation of gas mixtures . 14
6.3.2 Temperature and pressure . 14
6.3.3 Gap adjustment . 15
6.3.4 Ignition . 15
6.3.5 Observation of the ignition process . 15
6.4 Determination of maximum experimental safe gap (MESG) . 15
6.4.1 General . 15
6.4.2 Preliminary tests . 15
6.4.3 Confirmatory tests . 15
6.4.4 Reproducibility of maximum experimental safe gaps (MESG) . 15
6.4.5 Tabulated values . 16
6.5 Verification of the MESG determination method . 16
7 Method of test for auto-ignition temperature (AIT) . 16
7.1 Outline of method . 16
7.2 Apparatus . 16
7.2.1 General . 16
7.2.2 Test vessel and support . 17
7.2.3 Thermocouples . 17
7.2.4 Oven . 17
7.2.5 Metering devices . 18
7.2.6 Mirror . 18
7.2.7 Timer . 18
7.2.8 Equipment for purging the test vessel with air . 18
7.2.9 Automated apparatus . 18
7.3 Sampling, preparation and preservation of test samples . 19
7.3.1 Sampling . 19
7.3.2 Preparation and preservation . 19
7.4 Procedure . 19
7.4.1 General . 19
7.4.2 Sample injection . 20
7.4.3 Determination of the auto-ignition temperature (AIT) . 20
7.5 Auto-ignition temperature (AIT) . 21
7.6 Validity of results . 21
7.6.1 Repeatability . 21
7.6.2 Reproducibility . 21
7.7 Data . 22
7.8 Verification of the auto-ignition temperature determination method . 22
Annex A (normative) Ovens of test apparatus for the tests of auto-ignition temperature . 23
A.1 General . 23
A.2 “IEC oven” . 23
A.3 “DIN oven” . 23
Annex B (informative) Tabulated values . 30
Annex C (informative) Determination of cool flames . 84
Annex D (informative) Volume dependence of auto-ignition temperature . 86
Bibliography . 87
Figure 1 − Test apparatus . 13
Figure A.1 − Test apparatus: assembly . 24
Figure A.2 − Section A-A (flask omitted) . 25
Figure A.3 − Base heater (board made of refractory material) . 25
Figure A.4 − Flask guide ring (board made of refractory material) . 26
Figure A.5 − Neck heater (board made of refractory material) . 26
Figure A.6 − Oven . 27
Figure A.7 − Lid of steel cylinder . 28
Figure A.8 − Lid of steel cylinder . 29
Figure A.9 − Injection of gaseous sample. 29
Figure C.1 − Additional thermocouple to detect cool flames . 84
− 4 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
Figure C.2 − ‘Negative temperature coefficient’ shown for butyl butyrate as an example . 85
Figure D.1 − Volume dependence of auto-ignition temperature . 86
Table 1 − Classification of temperature class and range of auto-ignition temperatures . 12
Table 2 − Values for verification of the apparatus . 16
Table 3 − Values for verification of the apparatus . 22
Table B.1 − Material data . 32
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 20-1: Material characteristics for gas and vapour classification –
Test methods and data
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard ISO/IEC 80079-20-1 has been prepared by subcommittee 31M: Non-
electrical equipment and protective systems for explosive atmospheres, of IEC technical
committee 31: Equipment for explosive atmospheres.
This first edition of ISO/IEC 80079-20-1 cancels and replaces IEC 60079-20-1:2010. It
constitutes a technical revision. No significant changes were made with respect to
IEC 60079-20-1:2010.
It is published as a double logo standard.
− 6 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
The text of this standard is based on the following documents:
FDIS Report on voting
31M/122/FDIS 31M/126/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60079 series, under the general title: Explosive atmospheres, as
well as the International Standard 80079 series, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
EXPLOSIVE ATMOSPHERES –
Part 20-1: Material characteristics for gas and vapour classification –
Test methods and data
1 Scope
This part of ISO/IEC 80079 provides guidance on classification of gases and vapours. It
describes a test method intended for the measurement of the maximum experimental safe
gaps (MESG) for gas-air mixtures or vapour-air mixtures under normal conditions of
temperature and pressure (20 °C, 101,3 kPa) so as to permit the selection of an appropriate
group of equipment. This document also describes a test method intended for use in the
determination of the auto-ignition temperature (AIT) of a vapour-air mixture or gas-air mixture
at atmospheric pressure, so as to permit the selection of an appropriate temperature class of
equipment.
Values of chemical properties of materials are provided to assist in the selection of equipment
to be used in hazardous areas. Further data may be added as the results of validated tests
become available.
The materials and the characteristics included in a table (see Annex B) have been selected
with particular reference to the use of equipment in hazardous areas. The data in this
document have been taken from a number of references which are given in the bibliography.
These methods for determining the MESG or the AIT may also be used for gas-air-inert
mixtures or vapour-air-inert mixtures. However, data on air-inert mixtures are not tabulated.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60050-426, International Electrotechnical Vocabulary − Part 426: Electrical apparatus for
explosive atmospheres (available at http://www.electropedia.org/)
IEC 60079-11, Explosive atmospheres − Part 11: Equipment protection by intrinsic safety "i"
IEC 60079-14, Explosive atmospheres − Part 14: Electrical installations design, selection and
erection
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-426 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
− 8 − ISO/IEC 80079-20-1:2017 © ISO/IEC 2017
3.1
auto-ignition
reaction which is evidenced by a clearly perceptible flame and/or explosion, and for which the
ignition delay time does not exceed 5 min
Note 1 to entry: See 7.2.2 for a test method.
3.2
ignition delay time
time between the completed injection of the flammable material and the ignition
3.3
auto-ignition temperature
AIT
lowest temperature (of a surface) at which under specified test conditions an ignition of a
flammable gas or vapour in mixture with air or air-inert gas occurs
Note 1 to entry: See Clause 7 for a test method.
3.4
maximum experimental safe gap
MESG
maximum gap of a joint of 25 mm in width which prevents any transmission of an explosion
during tests made under the conditions specified in this document
Note 1 to entry: See Clause 6 for a test method.
3.5
minimum ignition current
MIC
minimum current in a specified test circuit that causes the ignition of the explosive test
mixture in the spark test apparatus according to IEC 60079-11
Note 1 to entry: See 5.1.6 for the test circuit.
3.6
flammable limits
lower flammable limit (LFL) and upper flammable limit (UFL) of gas in a gas-air mixture,
between which a flammable mixture is formed
Note 1 to entry: The term “explosive limits” is used especially in European standardization and regulations
interchangeably to describe these limits.
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.6.1
lower flammable limit
LFL
concentration of flammable gas or vapour in air, below which an explosive gas atmosphere
does not form
Note 1 to entry: For the purposes of Ex Equipment, this was previously referred to as the lower explosive limit
(LEL).
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.6.2
upper flammable limit
UFL
concentration of flammable gas or vapour in air, above which an explosive gas atmosphere
does not form
Note 1 to entry: For the purposes of Ex Equipment, this was previously referred to as the upper explosive limit
(UEL).
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.7
equipment grouping
classification system of equipment related to the explosive atmosphere for which they are
intended to be used
Note 1 to entry: IEC 60079-0 identifies three equipment groups:
Group I − equipment for mines susceptible to fire damp;
Group II, which is sub-divided into groups IIA, IIB and IIC − equipment for all places with an explosive gas
atmosphere other than mines susceptible to fire damp;
Group III, which is sub-divided into groups IIIA, IIIB and IIIC − equipment for all places with an explosive dust
atmosphere other than mines susceptible to fire damp.
3.8
flash point
FP
lowest liquid temperature at which, under specified test conditions, a liquid gives off vapours
in quantity such as to be capable of forming an ignitable vapour-air mixture
3.9
gas
gaseous phase of a substance that cannot reach equilibrium with its liquid or solid state in the
temperature and pressure range of interest
Note 1 to entry: This is a simplification of the scientific definition, and merely requires that the substance is above
its boiling point or sublimation point at the ambient temperature and pressure.
3.10
vapour
gaseous phase of a substance that can reach equilibrium with its liquid or solid state in the
temperature and pressure range of interest
Note 1 to entry: This is a simplification of the scientific definition, and merely requires that the substance is below
its boiling point or sublimation point at the ambient temperature and pressure.
4 Classification of gases and vapours
4.1 General
Equipment Group I addresses mines susceptible to firedamp.
NOTE Firedamp consists mainly of methane, but always contains small quantities of other gases, such as
nitrogen, carbon dioxide, and hydrogen, and sometimes ethane and carbon monoxide. The terms firedamp and
methane are used frequently in mining practice as synonyms.
Equipment Group II addresses flammable gases and vapours other than in mines susceptible
to firedamp. Equipment Group II gases and vapours are classified according to their MESG
and/or MIC ratio into equipment groups IIA, IIB and IIC.
All flammable materials are classified according to their AIT into temperature classes.
4.2 Classification according to the maximum experimental safe gap (MESG)
Gases and vapours may be classified according to their MESG into Equipment Groups IIA, IIB
or IIC, based on the determination method described in this document. In order to ensure
standardized results the MESG apparatus is dimensioned to avoid the possible effects of
obstruction on the safe gaps.
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NOTE 1 The standard method for determining MESG is described in 6.2, but where determinations have been
undertaken only in an 8 l spherical vessel with ignition close to the flange gap these can be accepted provisionally.
NOTE 2 The design of the test apparatus for safe gap determination, other than that used for selecting the
appropriate equipment group of enclosure for a particular gas, may need to be different to the one described in this
document. For example, the volume of the enclosure, flange width, gas concentrations and the distance between
the flanges and any external wall or obstruction may have to be varied. As the design depends on the particular
investigation which is to be undertaken, it is impracticable to recommend specific design requirements, but for most
applications the general principles and precautions indicated in this document will still apply.
NOTE 3 In IEC 60079-14 minimum distances of obstruction from the flameproof flange joints related to the
equipment group of the hazardous area are given.
For the purpose of classification the MESG limits are:
Equipment Group IIA: MESG ≥ 0,90 mm.
Equipment Group IIB: 0,50 mm < MESG < 0,90 mm.
Equipment Group IIC: MESG ≤ 0,50 mm.
Determination of both the MESG and MIC ratio is required when 0,50 < MESG < 0,55. Then
the equipment group is determined by MIC ratio,
NOTE 4 For gases and highly volatile liquids, the MESG is determined at 20 °C.
NOTE 5 If it was necessary to do the MESG determination at temperatures higher than ambient temperature a
temperature 5 K above that needed to give the necessary vapour pressure or 50 K above the flash point is used
and this value of MESG is given in the table and the classification of the equipment group is based on this result.
4.3 Classification according to the minimum igniting current ratio (MIC ratio)
Gases and vapours may be classified according to the ratio of their minimum igniting currents
(MIC) to the ignition current of laboratory methane into Equipment Groups IIA, IIB or IIC. The
purity of laboratory methane shall be not less than 99,9 % by volume.
NOTE The standard method of determining MIC ratios is with the apparatus described in IEC 60079-11, but where
determinations have been undertaken in other apparatus these can be accepted provisionally.
For the purpose of classification the MIC ratios are:
Equipment Group IIA: MIC > 0,80.
Equipment Group IIB: 0,45 ≤ MIC ≤ 0,80.
Equipment Group IIC: MIC < 0,45.
Determination of both the MESG and MIC ratio is required when 0,70 < MIC < 0,90 or
0,40 < MIC < 0,50. Then the equipment group is determined by MESG.
4.4 Classification according to the similarity of chemical structure
When a gas or vapour is a member of a homologous series of compounds, the classification
of the gas or vapour can provisionally be inferred from the data of the neighbouring members
of the series.
The classification according to the similarity of chemical structure is not allowed if the
classification of one of the neighbouring members is based on MESG and the other on MIC
ratio.
4.5 Classification of mixtures of gases
Mixtures of gases should generally be allocated to an equipment group only after a special
determination of MESG or MIC ratio. One method to estimate the equipment group is to
determine the MESG of the mixture by applying a form of Le Châtelier’s principle:
MESG =
mix
X
i
∑
MESG
i
i
Where X is the percentage by volume of material i and MESG is the MESG of material i.
i i
This method should not be applied in case of exceptions to the Le Châtelier’s principle and to
mixtures and/or streams that have:
a) acetylene or its equivalent hazard (e.g. self-decomposition properties);
b) oxygen or other strong oxidizer as one of the components;
c) large concentrations (over 5 % by volume) of carbon monoxide. Because unrealistically
high MESG values may result, caution should be exercised with two component mixtures
where one of the components is an inert, such as nitrogen.
For mixtures containing an inert such as nitrogen in concentrations less than 5 % by volume,
use an MESG of infinity. For mixtures containing an inert such as nitrogen in concentrations
5 % by volume and greater, use an MESG of 2.
NOTE An alternate method that includes stoichiometric ratios is presented in the essay “Maximum experimental
safe gap of binary and ternary mixtures,” by Brandes and Redeker.
5 Data for flammable gases and vapours, relating to the use of equipment
5.1 Determination of the properties
5.1.1 General
The compounds listed in this document are in accordance with Clause 4, or have physical
properties similar to those of other compounds in that list.
5.1.2 Equipment group
The equipment groups are the result of MESG or MIC ratio determination except where there
is no value listed for MESG or MIC ratio. For these, the equipment group is based on
chemical similarity (see Clause 4).
NOTE If it was necessary to do the MESG determination at temperatures higher than ambient temperature, a
temperature 5 K above that needed to give the necessary vapour pressure or 50 K above the Flash Point is used.
This value of MESG is given in Table B.1 and the classification of the equipment group is based on this result.
5.1.3 Flammable limits
Determinations have been made by a number of different methods, but the preferred method
is with a low energy ignition at the bottom of a vertical tube. The values (in percentage by
volume and mass per volume) are listed in Table B.1.
If the flash point is high, the compound does not form a flammable vapour air/mixture at
normal condition of temperature (20 °C). Where flammability data are presented for such
compounds the determinations have been made at a temperature sufficiently elevated to allow
the vapour to form a flammable mixture with air.
5.1.4 Flash point (FP)
The value given in Table B.1 is the “closed cup” measurement. When this data was not
available, the “open cup” value is quoted and indicated by (oc). The symbol < (less than),
indicates that the flash point is below the value (in degree Celsius) stated, this probably being
the limit of the apparatus used.
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5.1.5 Temperature class
The temperature class of a gas or vapour is given according to IEC 60079-14, as shown in
Table 1:
Table 1 − Classification of temperature class
and range of auto-ignition temperatures
Temperature class Range of auto-ignition temperature (AIT)
°C
T1 > 450
T2
300 < AIT ≤ 450
T3 200 < AIT ≤ 300
T4 135 < AIT ≤ 200
T5
100 < AIT ≤ 135
T6
85 < AIT ≤ 100
5.1.6 Minimum igniting current (MIC)
The apparatus for the determination of minimum igniting current is defined in IEC 60079-11.
The test apparatus shall be operated in a 24 V d.c. circuit containing a (95 ± 5) mH air-cored
coil. The current in this circuit is varied to a minimum value until ignition of the most easily
ignited concentration of the specific gas or vapour in air is obtained.
5.1.7 Auto-ignition temperature (AIT)
The value of auto-ignition temperature depends on the method of testing. The preferred
method and data obtained is given in Clause 7 and in Annex A.
If the compound is not included in these data, the data obtained in similar apparatus, such as
the apparatus described by ASTM E659, is listed.
NOTE Results from using the apparatus described in ASTM D2155 (now replaced by ASTM E659) were reported
by C.J. Hilado and S.W. Clark. The apparatus is similar to the one used by Zabetakis. If there is no determination
by either the IEC apparatus, nor similar apparatus, the lowest value obtained in other apparatus is listed. A more
comprehensive list of data for auto-ignition temperature, with the reference to sources, is given by Hilado and
Clark.
5.2 Properties of particular gases and vapours
5.2.1 Coke oven gas
Coke oven gas is a mixture of hydrogen, carbon monoxide and methane. If the sum of the
concentrations (Vol. %) of hydrogen and carbon monoxide is less than 75 % by volume of the
total, flameproof equipment of Equipment Group IIB is recommended; otherwise, equipment of
Equipment Group IIC is recommended.
5.2.2 Ethyl nitrite
The auto-ignition temperature of ethyl nitrite is 95 °C, above which the gas suffers explosive
decomposition.
NOTE Ethyl nitrite is not be confused with its isomer, nitroethane.
5.2.3 MESG of carbon monoxide
The MESG for carbon monoxide relates to a mixture with air saturated with moisture at normal
ambient temperature. This determination indicates the use of Equipment Group IIB equipment
in the presence of carbon monoxide. A larger MESG may be observed with less moisture. The
lowest MESG (0,65 mm) is observed for a mixture of CO/H O near 7:1 mole ratio. Small
quantities of hydrocarbon in the carbon monoxide-air mixture have a similar effect in reducing
the MESG so that Equipment Group IIB equipment is required.
5.2.4 Methane, Equipment Group IIA
Industrial methane, such as natural gas, is classified as Equipment Group IIA, provided it
does not contain more than 25 % by volume of hydrogen. A mixture of methane with other
compounds from Equipment Group IIA, in any proportion, is classified as Equipment
Group IIA.
6 Method of test for the maximum experimental safe gap (MESG)
6.1 Outline of method
The interior and exterior chambers of the test apparatus are filled with a known mixture of the
gas or vapour in air, under normal conditions of temperature and pressure (20 °C, 101,3 kPa)
and with the circumferential gap between the two chambers accurately adjusted to the
desired value. The internal mixture is ignited and the flame propagation, if any, is observed
through the windows in the external chamber. The maximum experimental safe gap for the
gas or vapour is determined by adjusting the gap in small steps to find the maximum value
of gap which prevents ignition of the external mixture, for any concentration of the gas or
vapour in air.
NOTE An exception is made for substances with vapour pressures which are too low to permit mixtures of the
required concentrations to be prepared at normal ambient temperatures. For these substances, a temperature
5 K above that needed to give the necessary vapour pressure or 50 K above the flash point is used.
6.2 Test apparatus
6.2.1 General
The apparatus is described in the following subclauses and is shown schematically in
Figure 1. It is also possible to use an automatic set-up when it is proven that the same results
are obtained as with a manual apparatus.
c
d
b
i
a
g
f
h
+
–
e
IEC
Key
a f
interior spherical chamber observation windows
b g
exterior cylindrical enclosure spark electrode
c h
adjustable part lower gap plate, fixed
d i
outlet of mixture upper gap plate, adjustable
e
inlet of mixture
Figure 1 − Test apparatus
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6.2.2 Material and mechanical strength
The whole apparatus is constructed to withstand a maximum pressure of 1 500 kPa without
significant expansion of the gap, so that no such expansion of the gap will occur during an
explosion.
The main parts of the test apparatus, and in particular the walls and flanges of the inner
chamber and the electrodes of the spark-gap, are normally of stainless steel. Other materials
may have to be used with some gases or vapours, however, in order to avoid corrosion or
other chemical affects. Light alloys should not be used for the spark-gap electrodes.
6.2.3 Exterior chamber
The exterior cylindrical enclosure "b" has a diameter of 200 mm and a height of 75 mm.
6.2.4 Interior chamber
The interior chamber "a" is a sphere with a volume measuring 20 cm . The interior chamber is
placed in the centre of the exterior chamber.
6.2.5 Gap adjustment
The two parts "i" and "h" of the internal chamber are so arranged that an adjustable 25 mm
gap can be set up between the plane parallel faces of the opposing rims. The exact width of
the gap can be adjusted by means of the micrometer (part "c").
6.2.6 Injection of mixture
The internal chamber is filled with the gas-air or vapour-air mixture through an inlet ("e"). The
exterior chamber is filled with the mixture via the gap. The inlet and outlet should be protected
by flame arresters.
6.2.7 Position of ignition source
The electrode "g" shall be mounted in such a way that the spark occurs in the centre of the
internal chamber.
6.3 Procedure
6.3.1 Preparation of gas mixtures
As the consistency of the mixture concentration, for a particular test series, has a pronounced
effect on the dispersion of the test results, it has to be carefully controlled. The flow of the
mixture through the chamber is therefore maintained until the inlet and outlet concentrations
are the same, or a method of equivalent reliability shall be used.
For the classification according to this document, the moisture content of the air used for the
preparation of the mixture should not exceed 10 % relative humidity. Higher values of
humidity could result for some substances in lower MESG values.
6.3.2 Temperature and pr
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