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31J/234/CDV
COMMITTEE DRAFT FOR VOTE (CDV) PROJET DE COMITÉ POUR VOTE (CDV)
Project number IEC 60079-10-1/Ed2 Numéro de
projet
IEC/TC or SC: SC 31J
CEI/CE ou SC:
Date of circulation Date de diffusion 2014-07-11
Closing date for voting (Voting mandatory for P-members) Date de
clôture du vote (Vote obligatoire pour les membres (P))
2014-10-17
Also of interest to the following committees Intéresse également
les comités suivants TC 18
Supersedes document Remplace le document 31J/221/CD and
31J/231/CC
Proposed horizontal standard Norme horizontale suggérée
Other TC/SCs are requested to indicate their interest, if any, in
this CDV to the TC/SC secretary Les autres CE/SC sont requis
d’indiquer leur intérêt, si nécessaire, dans ce CDV à l’intention
du secrétaire du CE/SC
Functions concerned Fonctions concernées
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DOCUMENTATION.
Introductory note
ATTENTION IEC – CENELEC
PARALLEL VOTING The attention of IEC National Committees, members
of CENELEC, is drawn to the fact that this Committee Draft for
Vote
(CDV) for an International Standard is submitted for parallel
voting. The CENELEC members are invited to vote through the CENELEC
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4 General 138
5.1 General 1514
5.4 Simplified methods 1617
6 Release of flammable substance 1719
6.1 General 1720
6.3.1 Gaseous release 1823
6.3.4 Aerosols 1926
6.3.5 Vapours 2027
6.5 Main types of ventilation 2130
6.5.1 Natural Ventilation 2131
6.5.2 Artificial Ventilation 2232
7 Type of zone 2334
7.1 Influence of grade of the source of release 2435
7.2 Influence of dilution 2436
7.3 Influence of availability of ventilation 2437
8 Extent of zone 2438
9 Documentation 2539
9.1 General 2540
Annex A (informative) Suggested presentation of hazardous
areas 2742
Annex B (informative) Estimation of sources of release
3244
B.1 Symbols 32 45
B.2.1 Sources giving a continuous grade of release 32 47
B.2.2 Sources giving a primary grade of release 33 48
B.2.3 Sources giving a secondary grade of release 33 49
B.3 Assessment of grades of release 33 50
B.4 Summation of releases 33 51
B.5 Hole size and source radius 34 52
B.6 Forms of release 37 53
B.7 Release rate 38 54
B.7.1 Estimation of Release Rate 38 55
B.7.2 Release rate of evaporative pools 41 56
B.8 Release from openings in buildings 42 57
B.8.1 Openings as possible sources of release 42 58
B.8.2 Openings classification 43 59
Annex C (informative) Ventilation guidance 4560
C.1 Symbols 45 61
C.2 Introduction 46 62
C.3 Assessment of ventilation and dilution and its influence on
hazardous area 46 63
C.3.1 General 46 64
C.3.4 Assessment of ventilation velocity 48 67
C.3.5 Assessment of the degree of dilution 49 68
C.3.6 Dilution in a room 51 69
C.3.7 Criteria for availability of ventilation 52 70
C.4 Examples of ventilation arrangements and assessments
53 71
C.4.1 Introduction 53 72
C.4.2 Jet release in a large building 53 73
C.4.3 Jet release in a small naturally ventilated building
54 74
C.4.4 Jet release in a small artificially ventilated building
55 75
C.4.5 Release with low velocity 56 76
C.4.6 Fugitive emissions 56 77
C.4.7 Local ventilation-extraction 56 78
C.5 Natural Ventilation in buildings 57 79
C.5.1 Wind induced ventilation 57 80
C.5.2 Buoyancy induced ventilation 58 81
C.5.3 Combination of the natural ventilation induced by wind
and82
buoyancy 59 83
D.1 General 61 85
D.3 Estimating the extent of the hazardous zone 62 87
Annex E (Informative) Examples of hazardous area
classification 6488
E.1 General 64 89
E.2 Examples 64 90
Annex F (informative) Schematic approach to classif ication
of hazardous areas - Sect ion91
F.2 8892
Annex F (informative) Schematic approach to classification of
hazardous areas – Section F.3 8993
Annex F (informative) Schematic approach to classification of
hazardous areas – Section F.4 9094
Annex G (informative) Flammable mists 9195
Annex H (informative) Hydrogen 9396
Annex I (informative) Hybrid mixtures 9597
I.1 General 9598
Annex J (informative) Useful equations in support to
hazardous area classification 96101
Annex K (informative) Industry codes and national standards
98102
K.1 General 98 103
Figure A.2 – Gas/vapour at low pressure 30107
Figure A.3 – Gas/vapour at high pressure 30108
Figure A.4.1 – Gas or vapour (liquefied under pressure or
by refrigeration) 31 109
Figure A.4.2 – Gas or vapour (liquefied under pressure or
by refrigeration) with spillage 31110
Figure A.5 – Flammable liquid (non boiling evaporative
pool) 31111
Figure B.1 – Forms of rekease 37112
Figure B.2 – Volumetric evaporation rate of liquids
42113
Figure C.1 – Chart for determining the degree of dilution
50114
Figure C.2 – Self diffusion of an unimpeded high velocity
jet release 54115
Figure C.3 – Supply only ventilation 55116
Figure C.4 – Supply and extraction ventilation
55117
Figure C.5 – Local extraction ventilation 57118
Figure C.6 – Volumetric flow rate of fresh air per m 2
of equivalent effective opening area 59119
Figure C.7 – Example of opposing ventilation driving
forces 60120
Figure D.1 – Chart for estimating hazardous area
distances 62121
Figure E.1 – Compressor facility handling natural gas
78122
Figure E.2 – Example of area classification for a
compressor facility handling natural gas 85123
Figure E.2a – Example of area classification for a
compressor facility handling natural gas 86124
125
Table A.1 Hazardous area classification data
sheet – Part I: Flammable substance list and126
characteristics 28127
Table A.2 Hazardous area classification data sheet Part
II: List of sources of release 29128
Table B.1 – Suggested hole cross sections for secondary
grade of releases 35129
Table B.2 – Effect of hazardous zones on openings as
possible sources of release 44130
Table C.1 – Indicative outdoor ventilation velocities
(uw) 49131
Table D.1 – Zones for grade of release and effectiveness
of ventilation 61132
Table K.1 – Examples of codes and standards 98133
134
135
143
144
FOREWORD145
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International Standards, Technical Specifications,149
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agreement between the two organizations.155
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Publications is accurate, IEC cannot be held responsible for the
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end user.162
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between any IEC Publication and the corresponding national or
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the latter.166
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Independent certification bodies provide conformity167
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6) All users should ensure that they have the latest edition of
this publication.170
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members of its technical committees and IEC National Committees for
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Publications.175
8) Attention is drawn to the Normative references cited in this
publication. Use of the referenced publications is176
indispensable for the correct application of this
publication.177
9) Attention is drawn to the possibility that some of the elements
of this IEC Publication may be the subject of178
patent rights. IEC shall not be held responsible for identifying
any or a ll such patent rights.179
International Standard IEC 60079-10-1 has been prepared by
subcommittee 31J:180
Classification of hazardous areas and installation requirements, of
IEC technical committee181
31: Equipment for explosive atmospheres.182
This second edition of IEC 60079-10-1 cancels and replaces the
first edition, published in183
2009, and constitutes a technical revision.184
The significant technical changes with respect to the previous
edition are as follows:185
a) Restructuring of main body of the text and dividing it into
sections to identify possible186
methodologies for classifying hazardous areas and to provide
further explanation on187
specific assessment factors;188
b) Introducing new terms and the definitions in section 3;189
c) Introducing clauses for alternative methods of area
classification in section 5;190
d) Restructuring of the Annexes including:191
Updating examples for presentation of hazardous area
classification in Annex A;192
Updating calculations for release rate in Annex B;193
60079-10-1/Ed2/CDV © IEC:2014 7
Complete re-write of Annex C for ventilation assessment,
with a new approach based194
upon the degree of dilution instead of the degree of
ventilation;195
Introduction of Annex D for zone extents:196
Update of Annex E with the examples to explain the
methodology set forth in Annexes197
A, B, C and D;198
Update of the flow chart in Annex F illustrating the area
classification procedure by199
dividing it in four (4) sections;200
Introduction of Annex H for hydrogen;201
Introduction of Annex I for hybrid mixtures;202
Introduction of Annex J for supplementary
equations;203
Introduction of Annex K for reference to national and
industry codes with specific204
examples of hazardous area classification;205
The text of this standard is based on the following
documents:206
FDIS Report on voting
207
Full information on the voting for the approval of this standard
can be found in the report on208
voting indicated in the above table.209
This publication has been drafted in accordance with the ISO/IEC
Directives, Part 2.210
A lis t of all parts of the IEC 60079 series, under the
general tit le Explosive atmospheres, can211
be found on the IEC website.212
The committee has decided that the contents of this publication
will remain unchanged until213
the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data214
related to the specific publication. At this date, the publication
will be215
reconfirmed,216
withdrawn,217
amended.219
220
The National Committees are requested to note that for this
publication the stability date221
is 2020222
THIS TEXT IS INCLUDED FOR THE INFORMATION OF THE NATIONAL
COMMITTEES AND WILL BE223
DELETED AT THE PUBLICATION STAGE.224
225
INTRODUCTION226
In areas where dangerous quantities and concentrations of flammable
gas or vapour may227
arise, protective measures are to be applied in order to reduce the
risk of explosions. This228
part of IEC 60079 sets out the essential criteria against which the
ignition hazards can be229
assessed, and gives guidance on the design and control parameters
which can be used in230
order to reduce such hazards.231
232
233
1 Scope236
This part of IEC 60079 is concerned with the classification of
areas where flammable gas or237
vapour hazards may arise and may then be used as a basis to support
the proper selection238
and installation of equipment for use in hazardous areas.239
It is intended to be applied where there may be an ignition hazard
due to the presence of240
flammable gas or vapour, mixed with air, but it does not apply
to:241
a) mines susceptible to firedamp;242
b) the processing and manufacture of explosives;243
c) areas where a hazard may arise due to the presence of
combustible dusts or fibres (refer244
IEC 60079-10-2;245
d) catastrophic failures or rare malfunctions which are beyond the
concept of abnormality246
dealt with in this standard (see 3.7.3 and 3.7.4);247
e) rooms used for medical purposes;248
f) commercial and industrial applications where only low pressure
fuel gas is used for249
appliances e.g. for cooking, water heating and similar uses, where
the installation is250
compliant with relevant gas codes;251
g) domestic premises.252
This standard does not take into account the consequences of
ignition of an explosive253
atmosphere.254
Flammable mists may form or be present at the same time as
flammable vapours. Liquids not255
considered to be hazardous in terms of this standard (due to the
flash point), when released256
under pressure may also generate flammable mists. In such cases,
the strict application of257
area classification for gases and vapours may not be appropriate as
the basis for selection of258
equipment. Information on flammable mists is provided in Annex
G.259
For the purpose of this standard, an area is a three-dimensional
region or space.260
Atmospheric conditions inc lude var iations above and below
reference levels of 101,3 kPa261
(1 013 mbar) and 20 °C (293 K), provided that the variations have a
negligible effect on the262
explosion properties of the flammable substances.263
In any process plant, irrespective of size, there may be numerous
sources of ignition apart264
from those associated with equipment. Appropriate precautions will
be necessary to ensure265
safety in this context. This standard may be used with judgement
for other ignition sources.266
2 Normative references267
The following referenced documents are indispensable for the
application of this document.268
For dated references, only the edition cited applies. For undated
references, the latest edit ion269
of the referenced document (including any amendments)
applies.270
IEC 60050-426, International Electrotechnical Vocabulary
(IEV) – Part 426: Equipment for271
explosive atmospheres272
vapour classification – Test methods and data275
IEC 60079-10-2, Explosive atmospheres – Part 10-2;
Classification of areas – Combustible276
dust atmospheres277
erection279
3 Terms and definitions280
For the purposes of this document, the terms and definitions given
in IEC 60079-0 and the281
following apply.282
NOTE 1 to entry; Additional definitions applicable to explosive
atmospheres can be found in the IEC 60050-426.283
3.1 Explosive atmosphere284
Mixture with air, under atmospheric conditions, of flammable
substances in the form of gas,285
vapour, dust, fibres, or flyings, which, after ignition, permits
self-sustaining flame propagation.286
IEC 60079-0 287
3.2 Explosive gas atmosphere288
Mixture with air, under atmospheric conditions, of flammable
substances in the form of gas or289
vapour, which, after ignition, permits self-sustaining flame
propagation.290
IEC 60079-0 291
NOTE 1 to entry;
Althou gh a mixture which has a concent ration above the
upper flammable lim it (UEL) is not an292
explosive gas atmosphere, it can readily become so and, generally
for area classification purposes, it is advisable293
to consider it as an explosive gas atmosphere.294
NOTE 2 to entry; There are some gases which are explosive with the
concentration of 100% (e.g. acetylene, C 2H2;295
vinyl acetylene, C4H4; propyl nitrate vapour, CH3 (CH2)2 NO3;
iso-propyl nitrate vapour, (CH3)2 CH ONO2; ethylene296 oxide
vapour, (CH2)2 O; hydrazine, N2H4 vapour).297
3.3 Hazardous areas and zones298
3.3.1 Hazardous area (on account of explosive gas
atmospheres)299
An area in which an explosive gas atmosphere is or may be
expected to be present, in300
quantities such as to require special precautions for the
construction, installation and use of301
equipment.302
NOTE 1 to entry; The interior of many items of process equipment
are commonly considered as a hazardous area303
even though a flammable atmosphere may not normally be present to
account for the possibility of air entering the304 equipment. Where
specific controls such as inerting are used the interior of process
equipment may not need to be305
classified as a hazardous area.306
3.3.2 Non-hazardous area (on account of explosive gas
atmospheres)307
An area in which an explosive gas atmosphere is not expected
to be present in quantities308
such as to require special precautions for the construction,
installation and use of equipment.309
3.3.3 Zones310
Hazardous Area classification based upon the frequency of the
occurrence and duration of an311
explosive atmosphere.312
3.3.4 Zone 0313
An area in which an explosive gas atmosphere is present
continuously or for long periods or314
frequently.315
NOTE 1 to entry; Both “long” and “frequently are the terms which
are intended to describe a very high likelihood of316
a potentially explosive atmosphere to be present in the area. In
that respect, those terms do not necessarily need317
to be quantified.318
3.3.5 Zone 1319
An area in which an explosive gas atmosphere is likely to
occur periodica lly or occasionally in320
normal operation.321
3.3.6 Zone 2322
An area in which an explosive gas atmosphere is not likely to
occur in normal operation but, if323
it does occur, it will exist for a short period only.324
IEC 60050-426 325
60079-10-1/Ed2/CDV © IEC:2014 11
NOTE 1 to entry; Indications of the frequency of the occurrence and
duration may be taken from codes relating to326
specific industries or applications.327
3.3.7 Extent of zone328
Distance in any direction from the source of release where a
gas/air mixture has been diluted329
by air to a concentration below the lower flammable limit.330
3.4 Releases331
3.4.1 Source of release332
A point or location from which a gas, vapour, mist or liquid
may be released into the333
atmosphere so that an explosive gas atmosphere could be
formed.334
IEC 60050-426 335
3.4.2 Continuous grade of release336
Release which is continuous or is expected to occur f requently
or for long periods.337
NOTE 1 to entry; Both “long” and “frequently are the terms which
are intended to describe a very high likelihood of338
a potential release . In that respect, those terms do not
necessarily need to be quantified.339
3.4.3 Primary grade of release340
Release which can be expected to occur periodically or occasionally
during normal operation.341
3.4.4 Secondary grade of release342
Release which is not expected to occur in normal operation and, if
it does occur, is likely to do343
so only infrequently and for short periods.344
3.4.5 Release rate345
Quantity of flammable gas, vapour or mist emitted per unit time
from the source of release.346
3.5 Ventilation and dilution347
3.5.1 Ventilation348
Movement of air and its replacement with fresh air due to the
effects of wind, temperature349
gradients, or artificial means (for example, fans or
extractors).350
3.5.2 Dilution351
The mixing of flammable vapour or gas with air which, over time,
will reduce the flammable352
concentration. 353
3.5.3 Dilution volume354
The volume in the vicinity of a source of release where the
concentration of flammable gas or355
vapour is not diluted to a safe level.356
NOTE to entry; In certain instances, the volumes under 3.5.3 and
3.5.5 could be the same.357
3.5.4 Background concentration358
The mean concentration of flammable substance within the volume
under consideration359
outside of the release plume or jet.360
3.5.5 Volume under consideration361
The volume served by the actual ventilation in the vicinity of the
release being considered.362
NOTE to entry; For an enclosed space this could be an entire room
or part of a larger space where the considered363
ventilation will dilute the gas or vapour from a given source of
release. Outdoors, this is the volume around a364
source of release where an explosive mixture could form. In
congested outdoor places this volume could be365
3.6.1 Flammable substance368
Substance which is itself flammable, or is capable of producing a
flammable gas, vapour or369
mist.370
3.6.2 Flammable liquid371
Liquid capable of producing a flammable vapour under any
foreseeable operating conditions.372
NOTE 1 to entry; An example of a foreseeable operating condition is
one in which the flammable liquid is handled373
at temperatures close to or above its flash point.374
NOTE 2 to entry; This definition is used for the classification of
hazardous areas and may be different from the375
definition of flammable liquids used for other purposes e.g. codes
for classification of flammable liquids for376
transport.377
3.6.3 Liquefied flammable gas378
Flammable substance which is stored or handled as a liquid and
which at ambient379
temperature and atmospheric pressure is a flammable gas.380
3.6.4 Flammable gas or vapour381
Gas or vapour which, when mixed with air in certain proportions,
will form an explosive gas382
atmosphere.383
3.6.5 Flammable mist384
Droplets of liquid, dispersed in air so as to form an explosive
atmosphere.385
3.6.6 Relative density of a gas or a vapour386
Density of a gas or a vapour relative to the density of air at the
same pressure and387
temperature (air is equal to 1,0).388
3.6.7 Flashpoint389
Lowest liquid temperature at which, under certain standardized
conditions, a liquid gives off390
vapours in a quantity such as to be capable of forming an ignitable
vapour/air mixture.391
3.6.8 Boiling point392
Temperature of a liquid boiling at an ambient pressure of 101,3 kPa
(1 013 mbar).393
NOTE 1 to entry; The initial boiling point that should be used
for liquid mixtures is to indicate the lowest value of394
the boiling point for the range of liquids present, as determined
in a standard laboratory distillation without395
fractionation.396
3.6.9 Vapour pressure397
Pressure exerted when a solid or liquid is in equilibrium with its
own vapour398
NOTE 1 to entry; This is also, the partial pressure of the
substance in the atmosphere above the liquid. It is
a399
function of the substance and of the temperature.400
3.6.10 Ignition temperature of an explosive gas atmosphere401
Lowest temperature of a heated surface which, under specified
conditions (according to IEC402
80079-20-1), will ignite a flammable substance in the form of a gas
or vapour mixture with air.403
IEC 60079-0]404
3.6.11 Lower flammable limit (LFL)405
The concentration of flammable gas, vapour or mist in air below
which an explosive gas406
atmosphere will not be formed.407
IEC 60050-426 408
3.6.12 Upper flammable limit (UFL)409
The concentration of flammable gas, vapour or mist in air above
which an explosive gas410
atmosphere will not be formed.411
IEC 60050-426 412
3.7.1 Normal operation414
Situation when the equipment is operating within its designed
parameters .415
NOTE 1 to entry; Minor releases of flammable substance may be part
of normal operation. For example, releases416 from seals which rely
on wetting by the fluid being pumped are considered to be minor
relea ses.417
NOTE 2 to entry; Failures (such as the breakdown of pump seals,
flange gaskets or spillages caused by accidents418
which involve repair or shut-down are not considered to be part of
normal operation.419
NOTE 3 to entry; Normal operation includes start-up and shut-down
conditions and routine maintenance.420
3.7.2 Routine maintenance421
Act ion recommended by the manufacturer to be performed occas
ionally or periodically in422
normal operation to maintain proper performance of
equipment.423
3.7.3 Rare malfunction424
Type of malfunction which may happen only in rare instances.
425
NOTE 1 to entry; Rare malfunctions in the context of this standard
include failure of separate and independent426
process controls, that may be either automated or manual, that
could trigger a chain of events that would lead to427 major release
of flammable substance. `428
NOTE 2 to entry; Rare malfunctions could also include unanticipated
conditions that are not covered by the plant429
design such as unexpected corrosion that results in a release.
Where releases due to corrosio n or similar430
conditions may or could reasonably be expected as part of the plant
operations then this is not considered as a431
rare malfunction.432
3.7.4 Catastrophic failure433
An occurrence which exceeds the design parameters of the
process plant and control system434
resulting in a major release of flammable substance.
435
NOTE 1 to entry; Catastrophic failures in the context of this
standard include, for example, major accidents such as436
the rupture of a process vessel, or large scale failures of
equipment or piping such as total breakdown of a flange437
or seal.438
4 General439
4.1 Safety principles440
Installations in which flammable substances are handled or stored
should be designed,441
constructed, operated and maintained so that any releases of
flammable substance, and442
consequently the extent of hazardous areas, are kept to a minimum,
whether in normal443
operation or otherwise, with regard to frequency, duration and
quantity of a release.444
It is important to examine those parts of process equipment and
systems from which a release445
of flammable substance may arise and to consider modifying the
design to minimize the446
likelihood and frequency of such releases and the quantity and rate
of release of substance.447
These fundamental considerations should be examined at an early
stage of the design448
development of any process plant and should also receive prime
attention in carrying out the449
area classification study.450
In the case of activities other than those of normal operation,
e.g. commissioning or non-451
routine maintenance, the area classification may not be valid. It
is expected that this would be452
dealt with by a safe system of work.The area classification should
take into account any453
routine maintenance.454
60079-10-1/Ed2/CDV © IEC:2014 14
In a situation in which there may be an explosive gas atmosphere,
the following steps should455
be taken:456
a) eliminate the likelihood of an explosive gas atmosphere
occurring around the source of457
ignition, or458
b) eliminate the source of ignition.459
Where this is not possible, protective measures, process equipment,
systems and procedures460
should be selected and prepared so the likelihood of the
coincidence of a) and b) is so small461
as to be accepted as low as reasonably practicable. Such measures
may be used singularly,462
if they are recognized as being highly reliable or in combination
to achieve the required level463
of safety.464
4.2 Area classification objectives465
Area classification is a method of ana lys ing and class
ifying the environment where explosive466
gas atmospheres may occur, so as to facilitate the proper selection
and installation of467
equipment to be used safely in that environment. The classification
also takes into account468
the ignition characteristics of the gas or vapour such as ignition
energy and ignition469
temperature. Area classification has two components, the
determination of the type of any470
hazardous zone, and the extent of the zone (see 7, 8 and
9).471
NOTE Selected characteristics may be designated for equipment e.g.
ignition energy and temperature ratings,472
see IEC 80079-20-1473
In most practical situations where flammable substances are used,
it is difficult to ensure that474
an explosive gas atmosphere will never occur. It may also be
difficult to ensure that475
equipment will never give rise to a source of ignition. Therefore,
in situations where an476
explosive gas atmosphere has a high likelihood of occurring,
reliance is placed on using477
equipment which has a low likelihood of creating a source of
ignition. Conversely, where the478
likelihood of an explosive gas atmosphere occurring is reduced,
equipment constructed with479
less rigorous requirements may be used.480
In particular, zone 0 or zone 1 areas should be minimized in number
and extent by design or481
suitable operating procedures. In other words, plants and
installations should be mainly482
zone 2 or non-hazardous. Where release of flammable substance is
unavoidable, process483
equipment items should be limited to those which give secondary
grade releases or, failing484
this (that is where primary or continuous grade releases are
unavoidable), the releases should485
be of very limited quantity and rate. In carrying out area
classification, these principles should486
receive prime consideration. Where necessary, the design, operation
and location of process487
equipment should ensure that, even when it is operating abnormally,
the amount of flammable488
substance released into the atmosphere is minimized, so as to
reduce the extent of the489
hazardous area.490
Once a plant has been classified and all necessary records made, it
is important that no491
modification to equipment or operating procedures is made without
reference to those492
responsible for the area classification. A description should be
added to update the493
classification for any plant or operational changes. Reviews should
be carried out during the494
life of the plant.495
4.3 Explosion risk assessment496
Subsequent to the completion of the area classification, a risk
assessment may be carried out497
to assess whether the consequences of ignition of an explosive
atmosphere requires the use498
of equipment of a higher equipment protection level (EPL) or may
justify the use of equipment499
with a lower equipment protection level than normally
required.500
In some cases a zone of negligible extent (NE) may arise and may be
treated as non501
hazardous. Such a zone implies that an explosion, if it takes
place, will have negligible502
consequences. Experiments have shown that a flammable cloud that is
less than 0,1 m 3 or503
1,0% of the enclosed space concerned (whichever is smaller) could
be considered as zone504
NE. The zone NE concept can be applied irrespective of any other
adjustments for risk505
assessment to determine EPL.506
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The EPL requirements may be recorded, as appropriate, on the area
classification documents507
and drawings to allow proper selection of equipment.508
NOTE IEC 60079-0 describes EPLs and IEC 60079-14 defines the
application of EPLs to an installation .509
4.4 Competence of Personnel510
The area classification should be carried out by those who
understand the relevance and511
significance of the properties of the flammable materials,
principles of gas/vapour dispersion512
and those who are familiar with the process and the equipment. It
may be beneficial for other513
engineering disciplines, e.g. electrical and mechanical engineers,
and personnel with specific514
responsibility for safety to be part of and have an input to the
area classification process. The515
competency of the person shall be relevant to the nature of the
plant and methodology used516
for carrying out the area classification. Appropriate continuing
education or training should be517
undertaken by personnel on a regular basis where required.518
NOTE Competency can be demonstrated in accordance with a training
and assessment framework relevant to519
national regulations or standards or user requirements.520
5 Area classification methodology521
It is rarely possible by a simple examination of a plant or plant
design to decide which parts of522
the plant can be equated to the three zonal definitions (zones 0, 1
and 2). A more detailed523
approach is therefore necessary and this involves the analysis of
the basic possibility of an524
explosive gas atmosphere occurring.525
In determining where a release of flammable gas or vapour may
occur, the likelihood and526
duration of the release should be assessed in accordance with the
definitions of continuous,527
primary and secondary grades of release. Once the grade of release,
the release rate,528
concentration, velocity, ventilation and other factors are assessed
there is then a firm basis529
on which to assess the likely presence of an explosive gas
atmosphere in the surrounding530
areas and determine the type and/or extent of the hazardous
zones.531
This approach therefore requires detailed consideration to be given
to each item of process532
equipment which contains a substance flammable by itself or due to
process conditions, and533
which could therefore be a source of release.534
For more details, see section 6, 7 and 8.535
5.1 General536
The following sub clauses give guidance on options for classifying
areas in which there may537
be an explosive gas atmosphere. An example of a schematic approach
to the classification of538
hazardous areas is given in Annex F.539
The area classification should be carried out when the initial
process and instrumentation line540
diagrams and initial layout plans are available, and should be
confirmed before plant start-up.541
Consideration should always be given to the type, number and
location of various potential542
points of release so that relevant zone and boundary conditions are
assigned in the overall543
assessment. Control systems designed and installed to a Functional
Safety standard may544
reduce the potential for a source of release and/or the quantity of
a release (e.g. batch545
sequence controls, inerting systems). Such controls may therefore
be considered where546
relevant to the hazardous area classification.547
When classifying buildings, consideration should also be given to a
careful evaluation of prior548
experience with the same or similar installations. It is not enough
to identify only a potential549
source of the combustible or flammable material within the building
and proceed immediately550
to defining the extent of Zone 1 or Zone 2 classified areas. Where
experience indicates that a551
particular design concept is sound, a more hazardous classification
for similar installations552
60079-10-1/Ed2/CDV © IEC:2014 16
When classifying hazardous areas, consideration should also be
given to a careful evaluation555
of prior experience with the same or similar installations. Where
documented evidence556
indicates that a particular design concept is sound this experience
may be used to support the557
classification chosen.558
5.2 Classification by calculation of sources of releases559
Classification may be approached by calculation, considering
appropriate statistical and560
numerical assessments for the factors concerned, for each source of
release.561
Formulae relevant to determining the release rates under specified
conditions can be found in562
Annex B. These formulae are generally accepted as providing a
good basis for calculating563
release rates for the conditions provided.564
Guidance on the assessment of ventilation and dispersion is
provided in Annex C. This565
guidance is not intended to be universally applicable and may not
be reliable in some566
situations.567
Other forms of assessment, e.g. computational fluid dynamics (CFD),
may be used and may568
provide a good basis for assessment in some situations. Computer
modelling is also an569
appropriate tool when assessing the interaction of multiple
factors.570
In all cases the assessment method and tools used should be
validated as suitable or used571
with appropriate caution. Those carrying out the assessment should
also understand the572
limitations or requirements of any tools and adjust the input
conditions or results accordingly573
to ensure appropriate conclusions.574
5.3 Use of industry codes and national standards575
Industry codes and national standards may be used where they
provide guidance or examples576
appropriate to the application and comply with the general
principles of this standard.577
Annex K identif ies some relevant industry codes and national
standards that may provide578
further detail as well as examples.579
5.4 Simplified methods580
Where it is not practicable to make required assessments from
individual sources of release,581
a simplified method may be used.582
Simplified methods shall identify sources for each of the zone
types, zone 0, 1 and 2 that are583
suitably conservative to allow for potential sources of release
without individual detail. The584
judgement is best made by reference to a set of cri teria
based on indus try ex per ience and585
appropriate to the particular plant.586
It is not necessary to carry out a detailed assessment of all items
in a plant where an587
assessment for one item or condition would be adequate to provide a
conservative588
classification for all other similar items or conditions on the
plant.589
Larger zone areas are characteristic of simplified methods,
stemming from the approach and590
the necessity to apply more conservative zonal classification where
doubt exists as to the591
hazards involved. This approach shall err on the side of
safety.592
To arrive at less conservative or more accurate figures of the
boundaries of the classified593
area, reference to illustrative examples or more detailed
assessment of point sources of594
release, as applicable should be used.595
5.5 Combination of methods596
The use of different methods may be appropriate for classification
of a plant at various stages597
of its development or for various parts of the plant.598
For example, at the initial conceptual stage of a plant the
simplified method may be599
appropriate to set out the equipment separations, plant layout and
plant boundaries. This600
60079-10-1/Ed2/CDV © IEC:2014 17
release. As the plant design proceeds and detailed data is
available on the potent ial sources602
of release, the classification should be upgraded using method more
detailed assessment.603
In some cases the simplified method can be applied to a group of
similar equipment in604
sections of plant (e.g. sections of piping with flanges, such as
pipe racks) while applying a605
more detailed assessment to the more significant potential sources
of release (e.g. relief606
valves, vents, gas compressors, pumps and the like).607
In many cases the classification examples provided in relevant
national or industry cod es can,608
where appropriate, be used to classify some components of larger
plants.609
6 Release of flammable substance610
6.1 General611
The release rate of flammable substance is the most important
factor that affects the extent of612
a zone. The higher the release rate the larger the extent of a
zone.613
For a given release volume, the lower the LFL the
greater will be the extent of the zone.614
NOTE Experience has shown that a release of ammonia, with a
LFL of 15 % by volume, will often dissipate
rapidly615
in the open air, so an explosive gas atmosphere will, in most cases
be of negligible extent.616
An int roduction to the nature of releases that should be
considered when approaching617
classification of potentially explosive areas is provided in the
following clauses.618
6.2 Sources of release619
The basic elements for establishing the hazardous zone types are
the identification of the620
source of release and the determination of the grade or grades of
the release.621
Since an explosive gas atmosphere can exist only if a flammable gas
or vapour is present622
with air, it is necessary to decide if any flammable substances can
exist in the area623
concerned. Generally speaking, such gases and vapours (and
flammable liquids or solids624
which may give rise to them) are contained within process equipment
that may or may not be625
totally enclosed. It is necessary to identify where a flammable
atmosphere can exist inside626
process equipment, or where a release of flammable substances can
create a flammable627
atmosphere outside process equipment.628
Each item of process equipment (for example, tank, pump, pipeline,
vessel, etc.) should be629
considered as a potential source of release of flammable substance.
If the item cannot630
foreseeably contain flammable substance, it will clearly not give
rise to a hazardous area631
around it. The same will apply if the item contains a flammable
substance but cannot release632
it into the atmosphere (for example, a full y welded pipeline is
not considered to be a source of633
release).634
If it is established that the item may release flammable substance
into the atmosphere, it is635
necessary, first of all, to determine the grade or grades of
release in accordance with the636
definitions, by establishing the likely frequency and duration of
the release. It should be637
recognized that the opening-up of parts of enclosed process systems
(for example, during638
filter changing or batch filling) should also be considered as
sources of release when639
developing the area classification. By means of this procedure,
each release will be graded640
either ‘continuous’, ‘primary’ or ’secondary’. 641
NOTE 1 Releases may form part of process, e.g. taking samples, or
may occur as part of a routine maintenance642
procedure. These forms of release are generally classified as
continuous or primary grades of release. Other643 accidental
releases are generally classified as a secondary grade of
release.644
NOTE 2 One item may give rise to more than one grade of release.
For example, there may be a small primary645
grade release, but a larger release could occur under abnormal
operation; thus giving rise to a secondary grade646
release. In this situation, both release conditions (both grades of
release) need full consideration as described in647
this standard.648
Having established the grade or grades of the release, it is
necessary to determine the649
60079-10-1/Ed2/CDV © IEC:2014 18
If the total quantity of flammable substance available for release
is ‘small’, for example, labo -651
ratory use, whilst a potential explosion condition may exist, it
may not be appropriate to use652
this area classification procedure. In such cases, account shall be
taken of the particular653
factors involved.654
The area classification of process equipment in which flammable
substance is burned, for655
example, fired heaters, furnaces, boilers, gas turbines etc.,
should take into account purge656
cycle, start-up and shut-down conditions.657
In some cases the construction of closed systems where specific
construction codes are met658
can be accepted as effectively preventing and/or limiting releases
of flammable substances to659
a negligible leakage hazard. The hazardous area classification of
such equipment or660
installations requires a complete assessment to verify the full
compliance of the installation to661
the relevant constructional and operating standards. Verification
of compliance should662
consider design, installation, operation, maintenance and
monitoring activities.663
Mists which form through leaks of pressurized liquid can be
flammable even though the liquid664
temperature is below the flash point. It is important therefore to
ensure that clouds of mist do665
not occur (see Annex G).666
6.3 Forms of release667
The characteristic of any release depends upon the physical state
of the flammable668
substance, its temperature and pressure. The physical states
include:669
a gas, which may be at an elevated temperature or
pressure;670
a gas liquefied by the application of pressure, e.g.
LPG;671
a gas which can only be liquefied by refrigeration, e.g.
methane;672
a liquid with an associated release of flammable
vapour.673
Releases from such plant items as pipe connections, pumps and
compressor seals and valve674
packings often start with a low flow rate. However, if the release
is not stopped erosion of the675
source of the release can greatly increase the rate of release and
hence the extent of the676
hazard.677
A release of flammable substance above its flashpoint wil l
give rise to a flammable vapour or678
gas cloud which may initially be less or more dense than the
surrounding air or may be679
neutrally buoyant. The forms of release and the pattern of
behaviour at various conditions are680
displayed as a flow chart in Figure B.1.681
Every form of release will eventually end as a gaseous or vapour
release and the gas or682
vapour may appear as buoyant, neutrally buoyant or heavy. This
characteristics will affect the683
extent of the zone generated by a particular form of release (see
Figure B.1).684
The horizontal extent of the zone at ground level will generally
increase with increasing685
relative density and the vertical extent above the source will
generally increase with686
decreasing relative density.687
6.3.1 Gaseous release688
A gas release wil l produce a gas jet or plume at the release
source depending on the689
pressure at the point of release, e.g. pump seal, pipe connection
or evaporative pool area.690
The relative density of the gas, the degree of turbulent mixing and
the prevailing air691
movement will all influence the subsequent movement of any gas
cloud.692
In calm conditions low velocity releases of a gas that is
significantly less dense than air will693
tend to move upwards, e.g. hydrogen and methane. Conversely, a gas
that is significantly694
denser than air will tend to accumulate at ground level or in any
pits or depressions, e.g.695
60079-10-1/Ed2/CDV © IEC:2014 19
with air and become neutrally buoyant. A gas or vapour with density
that is not significantly697
different to air is regarded as neutrally buoyant.698
Higher pressure releases will initially produce jets of released
gas which will mix turbulently699
with the surrounding air and entrain a ir in the jet.700
At high pressures , a thermodynamic effect due to expans ion
can come into play. As the gas701
escapes, it expands and cools down and may initially behave as
heavier than air. However,702
the cooling due to the Joule-Thomson effect is eventually offset by
the heat supplied by the703
air. The resulting gas cloud will eventually become neutrally
buoyant. The transition from704
heavier than air to neutrally bouyant behaviour may occur at any
time depending on the705
nature of the release and may occur after the cloud has been
diluted to below the LFL .706
NOTE Hydrogen demonstrates a reverse Joule-Thomson effect.707
6.3.2 Liquefied under pressure708
Some gases can be liquefied by the application of pressure alone,
e.g. propane and butane,709
and are usually stored and transported in this form.710
When a pressurized liquefied gas leaks from its containment the
most likely scenario is that711
the substance will escape as a gas from any vapour space or gas
lines. The rap id evaporation712
produces significant cooling at the point of release and icing due
to the condensation of water713
vapour from the atmosphere may occur.714
A liquid leak wil l par tiall y evaporate at the point of
release. This is known as flash715
evaporation. The evaporating liquid pulls energy from itself and
the surrounding atmosphere716
and in turn cools down the leaking fluid. The cooling of the fluid
prevents total evaporation717
and therefore an aerosol is produced. If the leak is large enough
then cold pools of fluid can718
accumulate on the ground which will evaporate over time to add to
the gas release.719
The cold aerosol cloud will act like a dense gas. A pressurized
liquid release can often be720
seen as the cooling effect of evaporation will condense ambient
humidity to produce a vapour721
cloud.722
6.3.3 Liquefied by refrigeration723
Other gases, the so-called permanent gases, can only be liquefied
by refrigeration e.g.724
methane and hydrogen. Small leaks of refrigerated gas will
evaporate quickly without forming725
a pool of liquid by drawing heat from the environment. If the leak
is large a cold pool of liquid726
may form.727
As the cold liquid pulls energy f rom the ground and
surrounding atmosphere the liquid wil l boi l728
generating a cold dense gas cloud. As with liquids, dikes or bund
walls can be used to direct729
or hold the flow of leakages.730
NOTE Care needs to be taken when classifying areas containing
cryogenic flammable gases such as liquefied731
natural gas. Vapours emitted will generally be heavier than air at
low temperatures but will become neutrally732
buoyant on approaching ambient temperature.733
6.3.4 Aerosols734
An aerosol is not a gas , but consists of small droplets of
liquid suspended in air. The droplets735
are formed from vapours or gases under certain thermodynamic
conditions or by flash736
evaporation of pressurized liquids. The scattering of light within
an aerosol cloud frequently737
makes the cloud visible to the naked eye. The dispersion of an
aerosol may vary between the738
behaviour of a dense gas or a neutrally buoyant gas. Aerosol
droplets can coalesce and rain739
60079-10-1/Ed2/CDV © IEC:2014 20
surrounding environment, evaporate and add to the gas/vapour cloud
(for more details see741
Annex G).742
6.3.5 Vapours743
Liquids at equilibrium with their environment will generate a layer
of vapour above their744
surface. The pressure this vapour exerts in a closed system is
known as the vapour pressure,745
which increases in a non-linear function with temperature.746
The process of evaporation uses energy which may come from a
variety of sources, for747
example from the liquid or the surrounding environment. The
evaporation process may748
decrease the temperature of the liquid and may tend to balance the
heat input to the liquid to749
limit temperature rise. However, changes in liquid temperature due
to increased evaporation750
from normal environmental conditions are considered too marginal to
affect the hazardous751
area classification. The concentration of the generated vapour is
not easy to predict as it is a752
function of the evaporation rate, temperature of the liquid and the
surrounding air flow.753
6.3.6 Liquid releases754
The release of flammable liquids will normally form a pool on the
ground, with a vapour cloud755
at the liquid’s surface unless the surface is absorbent. The size
of the vapour cloud will756
depend on the properties of the substance and its vapour pressure
at the ambient757
temperature (see B.7.2).758
NOTE The vapour pressure is an indication of a liquid's evaporation
rate. A substance with a high vapour759
pressure at normal temperatures is often referred to as volatile.
As a general rule, vapour pressure of liquid at760
ambient temperatures increases with decreasing boiling point. As
the temperature rises so does the vapour761
pressure.762
Release may also occur on water. Many flammable liquids are less
dense than water and are763
often not miscible. Such liquids will spread on the surface of
water, whether it is on the764
ground, in plant drains, pipe trenches or on open waters (sea, lake
or river), forming a thin765
film and increasing the evaporation rate due to the increased
surface area. In these766
circumstances the calculations in Annex B may not be
suitable.767
6.4 Ventilation (or air movement) and dilution768
Gas or vapour released into the atmosphere may dilute through
turbulent mixing with air, and769
to a lesser extent by diffusion driven by concentration gradients,
until the gas disperses770
completely and the concentration is essentially zero. Air movement
due to natural or artificial771
ventilation will promote dispersion. Increased air movement may
also increase the rate of772
release of vapour due to increased evaporation on an open liquid
surface.773
Suitable ventilation rates can reduce persistence of an explosive
gas atmosphere thus774
influencing the type of zone.775
A structure with suff icient openings to allow free passage
of air through all parts of the776
building is considered in many cases to be well ventilated and
should be treated as an open777
air area, e.g. a shelter with open sides and rooftop ventilation
openings.778
Dispersion or diffusion of a gas or vapour into the atmosphere is a
key factor in reducing the779
concentration of the gas or vapour to below the lower flammable
limit.780
Ventilation and air movement has two basic functions:781
a) To increase the rate of dilution and promote dispersion to limit
the extent of a zone;782
b) To avoid the persistence of an explosive atmosphere that may
influence the type of a783
zone.784
With increased ventilation or air movement the extent of a zone
will normally be reduced.785
60079-10-1/Ed2/CDV © IEC:2014 21
On the other hand, some obstacles, for example, dykes, walls and
ceilings, which limit the787
extent of vapour or gas movement, may also limit the extent of the
zone.788
NOTE Increased air movement may also increase the release rate of
vapour due to increased evaporation from789
open liquid surfaces. However the benefits of increased air
movement normally outweigh the increase in release790
rate.791
For low velocity releases the rate of gas or vapour dispersion in
the atmosphere increases792
with wind speed, but in stable atmospheric conditions layering of
the gas or vapour may occur793
and the distance for safe dispersal can be greatly
increased.794
NOTE 1 In plant areas with large vessels and structures, even at
low wind speeds eddies may be formed behind795
vessels and structures thus forming pockets of gas or vapour,
despite sufficient turbulence that could otherwise796
promote dispersion.797
NOTE 2 In normal practice, the tendency of layering is not
taken into account in area classification because the798
conditions which give rise to this effect are rare and occur for
short periods only. However, if prolonged periods of799
low wind speed are expected for the specific circumstance then the
extent of the zone should take account of the800
additional distance required to achieve dispersion.801
6.5 Main types of ventilation802
The two main types of ventilation are:803
a) natural ventilation;804
b) artificial (or forced) ventilation, either general to the area
or local to the source of release.805
6.5.1 Natural Ventilation806
Natural ventilation in buildings arises from pressure differences
induced by the wind and/or by807
temperature gradients (buoyancy induced ventilation). Natural
ventilation may be effective in808
certain indoor situations (for example, where a building has
openings in its walls and/or roof)809
to dilute releases safely.810
Examples of natural ventilation:811
an open building which, having regard to the relative
density of the gases and/or vapours812
involved, has openings in the walls and/or roof so dimensioned and
located that the813
ventilation inside the building, for the purpose of area
classification, can be regarded as814
equivalent to that in an open-air situation;815
a building which is not an open building but which has
natural ventilation (generally less816
than that of an open building) provided by permanent openings made
for ventilation817
purposes.818
Consideration of natural ventilation in buildings should recognise
that gas or vapour buoyancy819
may be a significant factor and should be arranged to promote
dispersion and dilution.820
Ventilation rates arising from natural ventilation are inherently
very variable. Where dilution of821
releases is by natural ventilation, the worst case scenario shall
preferably be considered to822
determine the degree of ventilation. Such a scenario will then lead
to a higher level of823
availability even though the degree of the ventilation is reduced.
Generally, with any natural824
ventilation, a lower degree of ventilation leads to a higher level
of availability and vice versa825
which will compensate for overly optimistic assumptions made in
estimating the degree of826
ventilation.827
There are some situations which require special care. This is
particularly the case where the828
ventilation openings are limited to mainly one side of the
enclosure. Under certain829
unfavourable ambient conditions, such as windy days when the wind
is blowing onto the830
ventilated face of the enclosure, the external air movement may
prevent the operation of the831
thermal buoyancy mechanism. Under these circumstances the level of
ventilation and the832
6.5.2.1 General835
Air movement required for ventilation may also be provided by
artif icial means, for example,836
fans or extractors. Although artificial ventilation is mainly
applied inside a room or enclosed837
space, it can also be applied to situations in the open air to
compensate for restricted or838
impeded air movement due to obstacles.839
The artificial ventilation may be either general (e.g. a whole
room) or local (e.g. extraction840
near a point of release) and for both of these, differing degrees
of air movement and841
replacement can be appropriate.842
With the use of artificial ventilation it is sometimes possible to
achieve:843
reduction in the type and/or extent of zones;844
shortening of the time of persistence of an explosive gas
atmosphere;845
prevention of the generation of an explosive gas atmosphere
.846
6.5.2.2 Ventilation considerations847
Artif icial venti lat ion can provide an effective and
reliable venti lation sys tem in an indoor848
situation. The following considerations should be included for
artificial ventilation systems:849
a) classification of the inside of the extraction system
and immediately outside the extraction850
system discharge point and other openings of the extraction
system;851
b) for ventilation of a hazardous area the ventilation air should
normally be drawn from a852
non-hazardous area taking into account the suction effects on the
surrounding area;853
c) before determining the dimensions and design of the ventilation
system, the location,854
grade of release, release velocity and release rate should be
defined.855
In addition, the following factors will influence the quality of an
artificial ventilation system:856
a) flammable gases and vapours usually have densities other than
that of air, thus they may857
accumulate near to either the floor or ceiling of an enclosed area,
where air movement is858
likely to be reduced;859
b) proximity of the artificial ventilation to the source of
release; artificial ventilation close to860
the source of release will normally be more effective and may be
needed to adequately861
control gas or vapour movement;862
c) changes in gas density with temperature;863
d) impediments and obstacles may cause reduced, or even no, air
movement, i.e. no864
ventilation in certain parts of the area;865
e) turbulence and circulating air patterns.866
For more details see Annex C.867
Consideration should be given to the possibility or need for
recirculation of air in the868
ventilation arrangement. This may impact the background
concentration and effectiveness of869
the ventilation system in reducing the hazardous area. In such
cases the classification of the870
hazardous area may need to be modified accordingly. Recirculation
of air may also be871
necessary in some applications e.g. for some processes or to
provide for the needs of872
personnel or equipment in high or low ambient temperatures where
supplemental cooling or873
heating of the air is required. Where recirculation of air is
needed then additional controls for874
safety may also be required. e.g. a gas analyzer with dampers
controlling fresh air intake.875
6.5.2.3 Examples of artificial ventilation876
General artificial ventilation may include a building which is
provided with fans in the walls877
and/or in the roof to improve the general ventilation in the
building.878
The role of fans may be twofold. They can increase the air flow
through a building, helping to879
60079-10-1/Ed2/CDV © IEC:2014 23
dilution of a cloud which is much smaller than the room which
contains it, even if no gas is881
transported out of the room. Fans may also enhance dilution by
increasing turbulence in some882
outdoor situations.883
Local artificial ventilation may be:884
a) An air/vapour extraction system applied to an item of process
equipment which885
continuously or periodically releases flammable vapour.886
b) A forced or extraction ventilation system applied to a local
area where it is expected that887
an explosive gas atmosphere may otherwise occur.888
For more details see C.4.889
6.5.3 Degree of dilution890
The effectiveness of the ventilation in controlling dispersion and
persistence of the explosive891
atmosphere will depend upon the ‘degree of dilution’, the
availability of ventilation and the892
design of the system. For example, ventilation may not be
sufficient to prevent the formation893
of an explosive atmosphere but may be sufficient to avoid its
persistence.894
The degree of dilution is defined to correspond with the ability of
a given release to dilute895
down to a safe level within defined ventilation or atmospheric
conditions. Therefore a larger896
release corresponds with a lower degree of dilution for a given set
of ventilation / atmospheric897
conditions, and a lower ventilation rate corresponds with a lower
degree of dilution for a given898
size of release.899
If other forms of ventilation, e.g. cooling fans are taken into
account, then care should be900
exercised as to ventilation availability. Ventilation for other
purposes may also affect dilution901
in either a positive or negative manner.902
The degree of dilution will also affect the dilution volume.
The dilution volume is903
mathematically equal to the hazardous volume but the boundary of
the hazardous area904
additionally takes into account other factors such as possible
movement of the release due to905
the direction and velocity of the release and of the surrounding
volume of air.906
‘Degrees of dilution’ depend not only on the ventilation, but also
on the nature and the type of907
the expected release of gas. Some releases, e.g. release with low
velocity, will be amenable908
to mitigation by enhanced ventilation with others much less so,
e.g. release with high velocity.909
The following three degrees of dilution are recognized:910
a) High dilution911
b) Medium dilution913
The concentration is controlled resulting in a stable zone
boundary, whilst the release is in914
progress and the explosive gas atmosphere does not persist unduly
after the release has915
stopped. The type and extent of zone are limited to the design
parameters.916
c) Low dilution917
There is significant concentration whilst release is in progress
and/or significant persistence918
of a flammable atmosphere after the release has stopped.919
7 Type of zone920
The likelihood of the presence of an explosive gas atmosphere
depends mainly on the grade921
of release and the ventilation. This is identified as a zone. Zones
are recognized as: zone 0,922
zone 1, zone 2 and the non-hazardous area.923
Where zones created by adjacent sources of release overlap and are
of different zonal924
classification, the more severe classification criteria will apply
in the area of overlap. Where925
overlapping zones are of the same classification, this common
classification will normally926
apply.927
7.1 Influence of grade of the source of release928
There are three basic grades of release, as listed below in order
of decreasing frequency of929
occurrence and/or duration of release of flammable
substance:930
a) continuous grade;931
b) primary grade;932
c) secondary grade.933
A source of re lease may give ri se to any one of these
grades of re lease, or to a combination934
of more than one.935
The grade of release generally determines type of the zone. In an
adequately ventilated area936
(typical open air plant) a continuous grade of release generally
leads to a zone 0937
classification, a primary grade to zone 1 and a secondary grade to
zone 2. This general rule938
may be modified by considering the degree of dilution and
availability of ventilation which may939
result in a more or less severe classification (refer to 7.2 and
7.3).940
7.2 Influence of dilution941
The effectiveness of ventilation or degree of dilution shall be
considered when estimating the942
type of zone classification. A medium degree of dilution will
generally result in the943
predetermined types of the zones based upon the types of the
sources of release. A high944
degree of dilution will allow a less severe classification, e.g.
zone 1 instead of zone 0, zone 2945
instead of zone 1 and even zone of negligible extent in some cases.
On the other hand a low946
degree of dilution will require a more severe classification (see
Annex D).947
7.3 Influence of availability of ventilation948
The availability of ventilation has an influence on the presence or
formation of an explosive949
gas atmosphere and thus also on the type of zone. As availability,
or reliability, of the950
ventilation decreases, the likelihood of not dispersing flammable
atmospheres increases. The951
zone classification will tend to be more severe, i.e. a zone 2 may
change to a zone 1 or even952
zone 0. Guidance on availability is given in Annex D.953
NOTE Combining the concepts of the efficiency of ventilation and
the availability of ventilation results in a954
qualitative method for the evaluation of the zone type. This is
further explained in Annex D.955
8 Extent of zone956
The extent of the zone depends on the estimated or calculated
distance over which an957
explosive atmosphere exists before it disperses to a concentration
in air below its lower958
flammable limit. The extent of the zone should consider the level
of uncertainty in the959
assessment by the application of a safety factor. When assessing
the area of spread of gas or960
vapour before dilution to below its lower flammable limit, expert
advice should be sought.961
Consideration should always be given to the possibility that a gas
which is heavier than air962
may flow into areas below ground level (for example, pits or
depressions) and that a gas963
which is lighter than air may be retained at high level (for
example, in a roof space).964
Where the source of release is situated outside an area or in an
adjoining area, the pene-965
tration of a significant quantity of flammable gas or vapour into
the area can be prevented by966
suitable means such as:967
a) physical barriers;968
b) maintaining a sufficient overpressure in the area relative to
the adjacent hazardous969
areas, so preventing the ingress of the explosive gas
atmosphere;970
c) purging the area with sufficient flow of fresh air, so ensuring
that the air escapes from all971
openings where the flammable gas or vapour may enter.972
The extent of the zone requires assessment of a number of physical
and chemical973
60079-10-1/Ed2/CDV © IEC:2014 25
For releases of limited volume a lesser distance may be accepted to
an on-going release.976
Under some conditions heavier than air gases and vapours can behave
like a spilled liquid spreading 977
down terrain slopes, through plant drains or pipe trenches and can
be ignited at a point remote from978
the original leakage, therefore putting at risk a large area of
plant (see B.6). The layout of the plant,979
where possible, should be designed to aid the rapid dispersal of
explosive gas atmospheres. 980
An area with restricted ventilation (for example, in pits or
trenches) that would otherwise be zone 2981
may require zone 1 classification; on the other hand, wide shallow
depressions used for pumping982
complexes or pipe reservations may not require such rigorous
treatment.983
9 Documentation984
9.1 General985
It is recommended that the steps taken to carry out area
classification and t he information and986
assumptions used are fully documented. The area classification
document should be a living987
document and should include the method used for area classification
and should be revised988
during any plant changes. All relevant information used should be
referenced. Examples of989
such information, or of a method used, would be:990
a) recommendations from relevant codes and standards;991
b) gas and vapour dispersion characteristics and
calculations;992
c) a study of ventilation characteristics in relation to flammable
substance release993
parameters so that the effectiveness of the ventilation can be
evaluated.994
d) the properties of all process substances used on the plant (see
IEC 60079-20-1), which995
may include:996
molar mass997
flash point998
boiling point999
gas group and temperature class1004
A suggested format for the substances lis ting is given in
Table A.1 and a format for recording1005
the results of the area classification study and any subsequent
alterations is given in Table1006
A.2. 1007
The source of information (code, national standard, calculation)
needs to be recorded so that,1008
at subsequent reviews, the philosophy which was adopted is clear to
the area classification1009
team.1010
9.2 Drawings, data sheets and tables1011
Area class ification documents may be in hard copy or
electronic form and should inc lude1012
plans and elevations or three dimensional models, as appropriate,
which show both the type1013
and extent of zones, gas group, ignition temperature and/or
temperature class.1014
Where the topography of an area influences the extent of the zones,
this should be1015
documented.1016
The documents should also include other relevant information such
as:1017
a) The location and identification of sources of release. For large
and complex plants or1018
process areas, it may be helpful to itemize or number the sources
of release so as to1019
60079-10-1/Ed2/CDV © IEC:2014 26
b) The position of openings in buildings (for example, doors,
windows and inlets and outlets1021
of air for ventilation).1022
The area classification symbols which are shown in Figure A.1 are
the preferred ones. A1023
symbol key shall always be provided on each drawing. Different
symbols may be necessary1024
where multiple equipment groups and/or temperature classes are
required within the same1025
1031
1032
1033
60079-10-1/Ed2/CDV © IEC:2014 28
Table A.1 Hazardous area classification data sheet – Part
I: Flammable substance list and characteristics1037
Plant: Reference
drawing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Flammable substance Volatility a LFL Ex
characteristics
Name Composi
and remarks
60079-10-1/Ed2/CDV © IEC:2014 29
Table A.2 Hazardous area classification data sheet Part II: List of
sources of release1038
Plant:
Area:
Refer ence drawing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Source of release Flammable substance Ventilation Hazardous
area
Descrip
tion
Loca
tion
b Quote the number of list in Part I
c G – Gas; L – Liquid; LG
– Liquefied gas; S – Solid
d N – Natural; AG – Artificial General;
AL – Artificial Local.
e See IEC 60079-10-1 Annex C
f Indicate code reference if used, or calculation
reference
A.1 Hazardous area suggested shapes1039
The following figures show some suggested hazardous area shapes
based on the form of1040
release described in B.6, which may be useful in the preparation of
hazardous area1041
classification drawings. The effects of impingement of the release
on obstacles and the1042
influence of topography are not considered. The hazardous area
generated by a release may1043
result in the combination of different shapes.1044
Key1045
SR Source of release1046
r Main extent of the hazardous area to be defined taking into
consideration the estimated1047
hazardous distance1048
r ’ , r” Secondary extents of the hazardous area to be
defined taking into account release1049
behaviour1050
h Distances between the source of release and ground level or
surface below the release1051
or
1052
(or at high pressure in case of unknown release
direction)1054
1055
or
1056
1058
Figure A.4.1 – Gas or vapour (liquefied under pressure or
by refrigeration)1059
NOTE Liquid pool should not be formed in case of
dripping.1060
1061
Figure A.4.2 – Gas or vapour (liquefied under pressure or
by refrigeration) with spillage1062
NOTE Liquid pool could be formed in case of spillage. In this case,
an additional source of release could be1063
considered1064
1065
1072
d C
polytropic index of adiabatic expansion or ratio of
specific heats;1077
M molar mass of gas or vapour
(kg/kmol);1078
p pressure inside the container (Pa);1079
p
a p atmospheric pressure (Pa);1081
c p critical pressure (Pa);1082
v p vapour pressure of the liquid at temperature T
(kPa);1083
R universal gas constant (8314 J/kmol K);1084
liquid density (kg/m 3 );1085
g gas or vapour density (kg/m
3 );
1086
S cross section of the opening (hole), through which
the fluid is released (m 2 );1087
T absolute temperature of the fluid, gas or liquid
(K);1088
aT absolute ambient temperature (K);1089
wu wind speed over the liquid pool surface (m/s);1090
W release rate of liquid (mass per time,
kg/s);1091
eW evaporation rate of liquid (kg/s);1092
g W mass release rate of gas
(kg/s);1093
Z compressibility factor.1094
B.2 Examples of grade of release1095
The following examples are not intended to be rigidly applied and
may need to be varied to1096
suit particular process equipment and the situation. It needs to be
recognised that some1097
equipment may exhibit more than one grade of release.1098
B.2.1 Sources giving a continuous grade of release1099
a) The surface of a flammable liquid in a fixed roof tank, with a
permanent vent to the atmo -1100
sphere.1101
b) The surface of a flammable liquid which is open to the
atmosphere continuously or for1102
long periods.1103
B.2.2 Sources giving a primary grade of release1105
a) Seals of pumps, compressors or valves if release of flammable
substance during normal1106
operation is expected.1107
b) Water drainage points on vessels which contain flammable gases
or liquids, which may1108
release flammable substance into the atmosphere while draining off
water during normal1109
operation.1110
c) Sample points which are expected to release flammable substance
into the atmosphere1111
during normal operation.1112
d) Relief valves, vents and other openings which are expected to
release flammable1113
substance into the atmosphere during normal operation
1114
B.2.3 Sources giving a secondary grade of release1115
a) Seals of pumps, compressors and valves where release of
flammable substance during1116
normal operation of the equipment is not expected.1117
b) Flanges, connections and pipe fittings, where release of
flammable substance is not1118
expected during normal operation.1119
c) Sample points which are not expected to release flammable
substance during normal1120
operation.1121
d) Relief valves, vents and other openings which are not expected
to release flammable1122
substance into the atmosphere during normal operation.1123
B.3 Assessment of grades of release1124
A wrong assessment of grades of release may compromise the
outcome of the whole1125
procedure. Although the grades of release are defined (see 3.4.2,
3.4.3 and 3.4.4), in practice1126
it is not always easy to distinguish one grade of release from the
other.1127
For example, it is usually considered that every release that does
not occur in normal1128
operation is a secondary release and the anticipated duration of
the release is usually1129
neglected. However, the concept of a secondary grade of release is
also based upon the1130
assumption that the release will only last for short periods. This
implies that a potentially1131
ongoing release will be detected soon after the beginning of the
release and that remedial1132
action will be taken as soon as possible. Such assumption leads to
the issue of regular1133
monitoring and maintenance of the equipment and
installation.1134
Obviously, if there is no regular monitoring and the maintenance is
poor, the releases may1135
last for hours if not days before being detected. Such delay in
detection does not mean that1136
the sources of the release should therefore be declared as primary
or continuous. There are1137
many unattended remote installations where a release may occur
without being noticed for1138
long time, but even such installations should be monitored and
inspected on a reasonably1139
regular basis. So, any assessment of the release grade must be
based upon careful1140
considerations and the assumption that monitoring and inspection of
the equipment and1141
installations will be performed in a reasonable way according to
the any manufacturer’s1142
instructions, relevant regulations and protocols and sound
engineering practice. Area1143
classification should not be a cover for a poor maintenance
practice but the user must be1144
aware that poor practices may compromise the established basis for
area classification.1145
There are many cases of release which may apparently fit
comfortably with the definition of a1146
primary grade of release. However when scrutinizing the nature of
the release it may be1147
revealed that the release could happen so frequently and so
unpredictably that one cannot be1148
reasonably assured that an explosive atmosphere will not exist near
the source of release. In1149
such cases the definition of continuous grade of release may be
more suitable. Therefore the1150
definition of a continuous grade of release implies not only
continuous releases but releases1151
with a high frequency as well (see 3.4.2).1152
B.4 Summation of releases1153
In indoor areas with more than one source of release, in order to
determine the type and1154
background concentration is determined. Since continuous grade
releases, by definition, can1156
be expected to be releasing most if not all of the time, then all
continuous grade releases1157
should be included.1158
Primary grade releases occur in normal operation but it is unlikely
that all of these sources1159
will be releasing at the same time. Knowledge and experience of the
installation should be1160
used to determine the maximum number of primary grade releases that
may release1161
simultaneously under worst conditions.1162
Secondary grade releases are not expected to release in normal
operation so, given that it is1163
unlikely that more than one secondary source would release at any
one time, only the largest1164
secondary release should be considered.1165
The summation of sources of release with regular (i.e. well
predictable) activity should be1166
based on detailed analysis of operating procedures. In the
determination of the summated1167
releases (both mass and volumetric):1168
the overall continuous release is the sum of all the
individual continuo us releases,1169
the overall primary release is the sum of some of the
individual primary releases1170
combined with the overall continuous release,1171
the overall secondary release is the largest individual
secondary release combined with1172
the overall primary release.1173
Where the same flammable substance is released from all of the
release sources then the1174
release rates (both mass and volumetric) can be summated directly.
However, when the1175
releases are of different flammable substances, the situation is
more complex. In the1176
determination of the degree of dilution (see Figure C.1), the
release characteristics need to be1177
determined for each flammable substance before any summation takes
place. The secondary1178
release with the highest value should be used.1179
In the determination of the background concentration (see equation
C.1) the volumetric1180
release rates can be summated directly. The critical concentration
with which the background1181
concentration is compared is a proportion of the
LFL (typically 20%). Since there are a number1182
of different flammable substances being released the combined LFL
should be used as the1183
comparator.1184
In general, continuous and primary sources of release should
preferably not be located in1185
areas with a low degree of dilution. Either sources of release
should be relocated, ventilation1186
should be improved or the grade of release should be
reduced.1187
B.5 Hole size and source radius1188
The most significant factor to be estimated in a system is the hole
size. It determines the1189
release rate of the flammable substance and thus eventually the
type of zone and the extent1190
of the zone.1191
Release rate is proportional to the square of the hole radius. A
modest underestimate of the1192
hole size will therefore lead to a gross underestimate of the
calculated value for release rate,1193
which should be avoided. Overestimate of the hole size will lead to
a conservative calculation1194
which is acceptable for safety reasons, however, the degree of
conservatism should also be1195
limited because it eventually results with overlarge zone extents.
A carefully balanced1196
approach is therefore needed when estimating the hole
size.1197
NOTE While the term ‘hole radius’ is used, most unintended holes
are not round. In such cases the coefficient of1198
di