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Project number IEC 60079-10-1/Ed2

Title of TC/SC: Classification of hazardous areas and installation requirements

Date of circulation 2013-05-03

Closing date for comments 2013-08-09

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Supersedes document 31J/194/CD and 31J/200A/CC

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Title: IEC 60079-10-1/Ed2: Explosive atmospheres Part 10-1: Classification of areas Explosive gas atmospheres

(Titre) :

Introductory note

Copyright 2013 International Electrotechnical Commission, IEC. All rights reserved. It is permitted to download this electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions. You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without permission in writing from IEC.

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CONTENTS 1

FOREWORD 2

INTRODUCTION 3

1 Scope 4

2 Normative references...9 5

3 Terms and definitions.10 6

4 General.....13 7

4.1 Safety principles...13 8

4.2 Area classification objectives..........14 9

4.3 Competence of personnel.........15 10

5 Area classification methods........15 11

5.1 General.................................................................................................................15 12

5.2 Classification by calculation of sources of release...........15 13

5.3 Use of indusry codes and national standards...............15 14

5.4 Simplified methods........16 15

5.5 Combination of methods........................................................................................16 16

6 Release of flammable material......................................................................................16 17

6.1 General.................................................................................................................16 18

6.2 Sources of release 19

6.3 Forms of release 20

6.3.1 Gaseous release 21

6.3.2 Liquified under pressure 22

6.3.3 Liquified by refrigeration 23

6.3.4 Aerosols 24

6.3.5 Vapours 25

6.3.6 Liquid releases 26

7 Ventilation (air movement) and dispersion 27

7.1 General 28

7.2 Main types of ventilation 29

7.2.1 Natural ventilation 30

7.2.2 Artificial ventilation 31

7.2.2.1 General 32

7.2.2.2 Ventilation considerations 33

7.2.2.3 Examples of artificial ventilation 34

7.2.3 Degree of dilution 35

8 Type of zone 36

8.1 Influence of grade of the source of release 37

8.2 Influence of dilution 38

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8.3 Influence of availability of ventilation 39

9 Extent of zone 40

9.1 Influence of release rate 41

9.2 Influence of form of release 42

9.3 Influence of lower explosive limit (LEL) 43

9.4 Influence of ventilation 44

9.5 Influence of climatic conditions 45

9.6 Influence of topography 46

10 Documentation 47

10.1 General 48

10.2 Drawings, data sheets and tables 49

Annex A (informative) Suggested documentation formats 50

A.1 Hazardous aea suggested shapes 51

Annex B (informative) Examples of sources of release 52

B.1 Process plant 53

B.1.1 Sources giving a continuos grade of release 54

B.1.2 Sources giving a primary grade of release 55

B.1.3 Sources giving a secondary grade of release 56

B.2 Hole size and source radius 57

B.3 Release rate 58

B.3.1 Estimation of release rate 59

B.3.1.1 Release rate of liquids 60

B.3.1.2 Release rate of gas or vapour 61

B.3.1.2.1 Release rate of gas with non choked gas velocity 62

B.3.1.2.2 Release rate of gas with choked gas velocity 63

B.3.2 Release rate of evaporative pools 64

B.4 Openings 65

B.4.1 Openings as possible sources of release 66

B.4.2 Openings classification 67

Annex C (informative) Estimation of types of zones 68

C.1 Introduction 69

C.2 Assesssment of ventilation or dilution and its influence on hazardous area 70

C.2.1 General 71

C.2.1.1 Wind induced ventilation in indoor situations 72

C.2.1.2 Buoyancy induced ventilation 73

C.2.1.3 Combination of the natural ventilation induced by wind and buoyancy 74

C.2.2 Effectiveness of ventilation 75

C.2.2.1 Background concentration and releases in a ventilated room 76

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C.2.3 Criteria for dilution 77

C.2.3.1 Assesment of effecive venilation/air velociy 78

C.2.3.2 Assessment of ventilation/dilution 79

C.2.4 Criteria for availability of ventilation 80

C.2.4.1 Criteria for natural ventilation 81

C.2.4.2 Criteria for artificial ventilation 82

C.3 Practical guide for estimating types of the zones by using Table C.1 83

C.3.1 Area of applicability 84

C.3.2 Assessment of grades of release 85

C.3.3 Assessment of release rate 86

C.3.4 Assesssment of ventilation or dilution 87

C.3.5 Assessment of availability of the ventilation 88

C.4 Examples of ventilation arrangements and assessments 89

C.4.1 Introduction 90

C.4.2 Jet release in a large building 91

C.4.3 Jet release in a small naturally ventilated building 92

C.4.4 Jet release in a small artificially ventilated building 93

C.4.5 Releases with low velocity 94

C.4.6 Fugitive emissions 95

C.4.7 Local ventilation-extracrion 96

Annex D (informative) Estimation of extents of zones 97

Annex E (informative) Examples of hazardous area classification 98

E.1 General 99

E.2 Relevant industry codes 100

E.3 Example case study for area classification 101

Annex F (informative) Schematic approach to classification of hazardous areas - Section F1 102




Annex G (informative) Flammable mist 106

Annex H (informative) Hydrogen 107

Annex I (informative) Hybrid mixtures 108

Annex J (informative) Useful equations in support to hazardous area classification 109

J.1 Introduction 110

J.2 Estimate of lower explosive limit of flammable mixtures 111

J.3 Estimate of vapour pressure 112

J.3.1 Vapour pressure of pure flammable liquid 113

J.3.2 Vapour pressure of flammable liquids mixtures 114

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J.4 Dilution in air of flammable material release 115

J.5 The time required to dilute a flammable material release 116

Bibliography 117

Figure 6.1 - Forms of release 118

Figure A.1 - Preferred symbols for hazardous area zones 119

Figure A.2 - Gas/vapour at low pressure (or at high pressure in case of unknown release 120 direction) 121

Figure A.3 - Gas/vapour at high pressure 122

Figure A.4 - Gas/vapour (liqified under pressure or by refrigeration) 123

Figure A.5 - Flammable liquids (non boiling evaporative pools) 124

Figure B.1 - Volumetric evaporative rate of liquids 125

Figure C.1 - Volumetric flow rate of fresh air 126

Figure C.2 - Example of opposing ventilation driving forces 127

Figure C.3 - Chart for determining the degree of dilution 128

Figure C.4 - Practical guide for using Table C.1 129

Figure C.5 - Self diffusion of an unimpeded high velocity jet release 130

Figure C.6 - 131

Figure C.7 - Supply only ventilation 132

Figure C.8 - Supply and extraction ventilation 133

Figure C.9 - Local extraction ventilation 134

Figure D.1 - Hazardous distances 135

Figure E.1 - Compressor facility 136

Figure E.2 - Example of area classification for a compressor facility handling natural gas 137

Figure E.2a - Example of area classification for a compressor facility handling natural gas 138 (lay out drawing) 139

Table A.1 - Hazardous area classification data sheet - Part I: Flammable material list and 140 characteristics 141

Table A.2 - Hazardous area classification data sheet - Part II: List of sources of release 142

Table B.1 - Hole cross sections 143 Table B.2 - Effect of openings on grade of release 144

Table C.1 - Influence of ventilation on type of zone 145

Table E.1 - Examples of codes and standards 146

147

148

149

150

151

152

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INTERNATIONAL ELECTROTECHNICAL COMMISSION 153

____________ 154 155

EXPLOSIVE ATMOSPHERES 156 157

Part 10-1: Classification of areas Explosive gas atmospheres 158 159 160

FOREWORD 161 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising 162

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote 163 international co-operation on all questions concerning standardization in the electrical and electronic fields. To 164 this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, 165 Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as IEC 166 Publication(s)). Their preparation is entrusted to technical committees; any IEC National Committee interested 167 in the subject dealt with may participate in this preparatory work. International, governmental and non-168 governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely 169 with the International Organization for Standardization (ISO) in accordance with conditions determined by 170 agreement between the two organizations. 171

2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international 172 consensus of opinion on the relevant subjects since each technical committee has representation from all 173 interested IEC National Committees. 174

3) IEC Publications have the form of recommendations for international use and are accepted by IEC National 175 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC 176 Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any 177 misinterpretation by any end user. 178

4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications 179 transparently to the maximum extent possible in their national and regional publications. Any divergence 180 between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in 181 the latter. 182

5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity 183 assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any 184 services carried out by independent certification bodies. 185

5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any 186 equipment declared to be in conformity with an IEC Publication. 187

6) All users should ensure that they have the latest edition of this publication. 188 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and 189

members of its technical committees and IEC National Committees for any personal injury, property damage or 190 other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and 191 expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC 192 Publications. 193

8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is 194 indispensable for the correct application of this publication. 195

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of 196 patent rights. IEC shall not be held responsible for identifying any or all such patent rights. 197

International Standard IEC 60079-10-1 has been prepared by subcommittee 31J: 198 Classification of hazardous areas and installation requirements, of IEC technical committee 199 31: Equipment for explosive atmospheres. 200

This second edition of IEC 60079-10-1 cancels and replaces the first edition, published in 201 2009, and constitutes a technical revision. 202

The significant technical changes with respect to the previous edition are as follows: 203

a) Restructuring of main body of the text and dividing it into sections to identify possible 204 methodologies for classifying hazardous areas and to provide further explanation on 205 specific assessment factors; 206

b) Introducing new terms and the definitions in section 3; 207 c) Introducing clauses for alternative methods of area classification in section 5; 208 d) Restructuring of the Annexes including: 209

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Updating calculations for release rate in Annex B; 210 Updating examples for hazardous area classification; 211 Complete re-write of Annex C for ventilation assessment, with a new approach based 212

upon the degree of dilution instead of the degree of ventilation; 213 Introduction of Annex D for zone extents: 214 Update for Annex E to reference to national and industry codes for specific examples 215

of hazardous area classification; 216 Introduction of Annex H for hydrogen; 217 Introduction of Annex I for hybrid mixtures. 218

The text of this standard is based on the following documents: 219

FDIS Report on voting

31J/xxx/FDIS 31J/xxx/RVD

220 Full information on the voting for the approval of this standard can be found in the report on 221 voting indicated in the above table. 222

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. 223

A list of all parts of the IEC 60079 series, under the general title Explosive atmospheres, can 224 be found on the IEC website. 225

The committee has decided that the contents of this publication will remain unchanged until 226 the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data 227 related to the specific publication. At this date, the publication will be 228

reconfirmed, 229 withdrawn, 230 replaced by a revised edition, or 231 amended. 232 233

The National Committees are requested to note that for this publication the stability date 234 is .... 235

THIS TEXT IS INCLUDED FOR THE INFORMATION OF THE NATIONAL COMMITTEES AND WILL BE 236 DELETED AT THE PUBLICATION STAGE. 237

238

The committee has decided that the contents of this publication will remain unchanged until 239 the maintenance result date1 indicated on the IEC web site under "http://webstore.iec.ch" in 240 the data related to the specific publication. At this date, the publication will be 241

reconfirmed; 242 withdrawn; 243 replaced by a revised edition, or 244 amended. 245

246

1 The National Committees are requested to note that for this publication the maintenance result date is 2014.

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INTRODUCTION 247

In areas where dangerous quantities and concentrations of flammable gas or vapour may 248 arise, protective measures are to be applied in order to reduce the risk of explosions. This 249 part of IEC 60079 sets out the essential criteria against which the ignition hazards can be 250 assessed, and gives guidance on the design and control parameters which can be used in 251 order to reduce such hazards. 252

253

254

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EXPLOSIVE ATMOSPHERES 255

Part 10-1: Classification of areas Explosive gas atmospheres 256

1 Scope 257

This part of IEC 60079 is concerned with the classification of areas where flammable gas or 258 vapour hazards may arise and may then be used as a basis to support the proper selection 259 and installation of equipment for use in hazardous areas. 260

It is intended to be applied where there may be an ignition hazard due to the presence of 261 flammable gas or vapour, mixed with air (see note 4), but it does not apply to: 262 a) mines susceptible to firedamp; 263 b) the processing and manufacture of explosives; 264 c) areas where a hazard may arise due to the presence of combustible dusts or fibres (refer 265

IEC 60079-10-2; 266 d) catastrophic failures or rare malfunctions which are beyond the concept of abnormality 267

dealt with in this standard (see 3.34 and 3.35); 268 e) rooms used for medical purposes; 269 f) commercial and industrial applications where only low pressure fuel gas is used for 270

appliances e.g. for cooking, water heating and similar uses, where the installation is 271 compliant with relevant gas codes 272

g) domestic premises. 273

This standard does not take into account the consequences of ignition of an explosive 274 atmosphere. 275 For the purpose of this standard, an area is a three-dimensional region or space. 276

Flammable mists may form or be present at the same time as flammable vapours. Liquids not 277 considered to be hazardous in terms of this standard (due to the flash point), when released 278 under pressure may also generate flammable mists. In such cases, the strict application of 279 area classification for gases and vapours may not be appropriate as the basis for selection of 280 equipment. Information on flammable mists is provided in Annex F. 281

The use of IEC 60079-14 for selection of equipment and installations is not required for mist 282 hazards. 283

For the purpose of this standard, an area is a three-dimensional region or space. 284

Atmospheric conditions include variations above and below reference levels of 101,3 kPa (1 285 013 mbar) and 20 C (293 K), provided that the variations have a negligible effect on the 286 explosion properties of the flammable materials. 287

In any process plant, irrespective of size, there may be numerous sources of ignition apart 288 from those associated with equipment. Appropriate precautions will be necessary to ensure 289 safety in this context. This standard may be used with judgement for other ignition sources. 290

2 Normative references 291

The following referenced documents are indispensable for the application of this document. 292 For dated references, only the edition cited applies. For undated references, the latest edition 293 of the referenced document (including any amendments) applies. 294 IEC 60050-426, International Electrotechnical Vocabulary (IEV) Part 426: Equipment for 295 explosive atmospheres 296 IEC 60079-0, Explosive atmospheres Part 0: Equipment General requirements 297 IEC 60079-20-1, Explosive atmospheres Part 20-1: Material characteristics for gas and 298 vapour classification Test methods and data 299

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3 Terms and definitions 300

For the purposes of this document, the terms and definitions given in IEC 60079-0 and the 301 following apply. 302

NOTE Additional definitions applicable to explosive atmospheres can be found in the IEC 60050-426. 303

3.1 Explosive atmosphere 304 Mixture with air, under atmospheric conditions, of flammable substances in the form of gas, 305 vapour, dust, fibres, or flyings, which, after ignition, permits self-sustaining flame propagation. 306 [IEC 60079-0] 307

3.2 Explosive gas atmosphere 308 Mixture with air, under atmospheric conditions, of flammable substances in the form of gas or 309 vapour, which, after ignition, permits self-sustaining flame propagation. 310 [IEC 60079-0] 311

NOTE 1 Although a mixture which has a concentration above the upper explosive limit (UEL) is not an explosive 312 gas atmosphere, it can readily become so and, generally for area classification purposes, it is advisable to 313 consider it as an explosive gas atmosphere. 314

NOTE 2 There are some gases which are explosive with the concentration of 100% (e.g. acetylene, C2H2; vinyl 315 acetylene, C4H4; propyl nitrate vapour, CH3 (CH2)2 NO3; iso-propyl nitrate vapour, (CH3)2 CH ONO2; ethylene oxide 316 vapour, (CH2)2 O; hydrazine, N2H4 vapour). 317

3.3 Hazardous area (on account of explosive gas atmospheres) 318 An area in which an explosive gas atmosphere is or may be expected to be present, in 319 quantities such as to require special precautions for the construction, installation and use of 320 equipment. 321

NOTE The interior of many items of process equipment is commonly considered as a hazardous area even though 322 a flammable atmosphere may not normally be present to account for the possibility of air entering the equipment. 323 Where specific controls such as inerting are used the interior of process equipment may not need to be classified 324 as hazardous areas. 325

3.4 Non-hazardous area (on account of explosive gas atmospheres) 326 An area in which an explosive gas atmosphere is not expected to be present in quantities 327 such as to require special precautions for the construction, installation and use of equipment. 328

3.5 Zones 329 Hazardous areas are classified into zones based upon the frequency of the occurrence and 330 duration of an explosive gas atmosphere, as follows: 331

3.6 Zone 0 332 An area in which an explosive gas atmosphere is present continuously or for long periods or 333 frequently. 334

NOTE Both long and frequently are the terms which are intended to describe a very high likelihood of potentially 335 explosive atmosphere to be present in the area. In that respect, those terms do not necessarily need to be 336 quantified. 337

3.7 Zone 1 338 An area in which an explosive gas atmosphere is likely to occur periodically or occasionally in 339 normal operation. 340

3.8 Zone 2 341 An area in which an explosive gas atmosphere is not likely to occur in normal operation but, if 342 it does occur, it will exist for a short period only. 343 [IEC 60050-426] 344

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NOTE Indications of the frequency of the occurrence and duration may be taken from codes relating to specific 345 industries or applications. 346

3.9 Extent of zone 347 Distance in any direction from the source of release where a gas/air mixture has been diluted 348 by air to a concentration below the lower explosive limit. 349

NOTE The extent of a zone should also consider appropriate safety factors and the level of uncertainty in the 350 assessment 351

3.10 Source of release 352 A point or location from which a gas, vapour, mist or liquid may be released into the 353 atmosphere so that an explosive gas atmosphere could be formed. 354 [IEC 60050-426] 355

3.11 Grades of release 356 There are three basic grades of release, as listed below in order of decreasing frequency of 357 occurrence and/or duration of release of flammable material: 358 a) continuous grade; 359 b) primary grade; 360 c) secondary grade. 361 A source of release may give rise to any one of these grades of release, or to a combination 362 of more than one. 363

3.12 Continuous grade of release 364 Release which is continuous or is expected to occur frequently or for long periods. 365

NOTE Both long and frequently are the terms which are intended to describe a very high likelihood of a potential 366 release . In that respect, those terms do not necessarily need to be quantified. 367

3.13 Primary grade of release 368 Release which can be expected to occur periodically or occasionally during normal operation. 369

3.14 Secondary grade of release 370 Release which is not expected to occur in normal operation and, if it does occur, is likely to do 371 so only infrequently and for short periods. 372

3.15 Release rate 373 Quantity of flammable gas, vapour or mist emitted per unit time from the source of release. 374

3.16 Normal operation 375 Situation when the equipment is operating within its designed parameters 376

NOTE 1 Minor releases of flammable material may be part of normal operation. For example, releases from seals 377 which rely on wetting by the fluid being pumped are considered to be minor releases. 378

NOTE 2 Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by accidents) which 379 involve repair or shut-down are not considered to be part of normal operation. 380

NOTE 3 Normal operation includes start-up and shut-down conditions and routine maintenance. 381

3.17 Ventilation 382 Movement of air and its replacement with fresh air due to the effects of wind, temperature 383 gradients, or artificial means (for example, fans or extractors). 384

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3.18 Dilution 385 The mixing of flammable vapour or gas with air which, over time, will reduce the flammable 386 concentration to a safe level. 387

3.19 Volume under consideration 388 The volume served by the actual ventilation in the vicinity of the release being considered. 389

NOTE For an enclosed space this may be an entire room or part of a larger space where the considered 390 ventilation will dilute the gas or vapour from a given source of release. Outdoors, this is the volume around a 391 source of release where an explosive mixture could form. In congested outdoor places this volume may be dictated 392 by the partial enclosure provided by the surrounding process plant. 393

3.20 Dilution volume 394 The Volume in the vicinity of a source of release where the concentration of flammable gas or 395 vapour is not diluted to a safe concentration 396

NOTE 1 The dilution volume is mathematically equal to the hazardous volume but the boundary of the hazardous 397 area should additionally take into account possible movement of the release due to the direction and velocity of the 398 release and of the surrounding volume of air. 399

NOTE 2 In certain instances, the volumes under 3.19 and 3.20 may be the same. 400

3.203.21 Background concentration 401 The mean concentration of flammable material within the volume under consideration outside 402 of the release plume or jet. 403

3.213.22 Lower explosive limit (LEL) 404 The concentration of flammable gas, vapour or mist in air below which an explosive gas 405 atmosphere will not be formed. 406 [IEC 60050-426] 407

3.223.23 Upper explosive limit (UEL) 408 The concentration of flammable gas, vapour or mist in air above which an explosive gas 409 atmosphere will not be formed. 410 [IEC 60050-426] 411

3.233.24 Relative density of a gas or a vapour 412 Density of a gas or a vapour relative to the density of air at the same pressure and 413 temperature (air is equal to 1,0). 414

3.243.25 Flammable material (flammable substance) 415 Material which is itself flammable, or is capable of producing a flammable gas, vapour or mist. 416

3.253.26 Flammable liquid 417 Liquid capable of producing a flammable vapour under any foreseeable operating conditions. 418

NOTE 1 An example of a foreseeable operating condition is one in which the flammable liquid is handled at 419 temperatures close to or above its flash point. 420

NOTE 2 This definition is used for the classification of hazardous areas and may be different from the definition of 421 flammable liquids used for other purposes e.g. codes for classification of flammable liquids for transport. 422

3.263.27 Flammable gas or vapour 423 Gas or vapour which, when mixed with air in certain proportions, will form an explosive gas 424 atmosphere. 425

3.273.28 Flammable mist 426 Droplets of liquid, dispersed in air so as to form an explosive atmosphere. 427

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3.283.29 Flashpoint 428 Lowest liquid temperature at which, under certain standardized conditions, a liquid gives off 429 vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture. 430

3.293.30 Boiling point 431 Temperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar). 432

NOTE The initial boiling point that should be used for liquid mixtures is to indicate the lowest value of the boiling 433 point for the range of liquids present, as determined in a standard laboratory distillation without fractionation. 434

3.303.31 Vapour pressure 435 Pressure exerted when a solid or liquid is in equilibrium with its own vapour 436

NOTE This is also, the partial pressure of the substance in the atmosphere above the liquid. It is a function of the 437 substance and of the temperature. 438

3.313.32 Ignition temperature of an explosive gas atmosphere 439 Lowest temperature of a heated surface which, under specified conditions (according to IEC 440 80079-20-1), will ignite a flammable substance in the form of a gas or vapour mixture with air. 441 [IEC 60079-0] 442

3.323.33 Liquefied flammable gas 443 Flammable material which is stored or handled as a liquid and which at ambient temperature 444 and atmospheric pressure is a flammable gas. 445

3.34 Catastrophic failure 446 An occurrence which exceeds the design parameters of the process plant and control system 447 resulting in a major release of flammable material. 448

NOTE Catastrophic failures in the context of this standard include, for example, major accidents such as the 449 rupture of a process vessel, or large scale failures of equipment or piping such as total breakdown of a flange or 450 seal. 451

3.333.35 Rare malfunction 452 Type of malfunction which may happen only in rare instances. 453

NOTE 1 Rare malfunctions in the context of this standard include failure of separate and independent process 454 controls, that may be either automated or manual, that could trigger a chain of events that would lead to major 455 release of flammable material. ` 456

NOTE 2 Rare malfunctions could also include unanticipated conditions that are not covered by the plant design 457 such as unexpected corrosion that results in a release. Where releases due to corrosion or similar conditions may 458 be expected as part of the plant operations then this is not considered as a rare malfunction. 459

4 General 460

3.344.1 Safety principles 461 Installations in which flammable materials are handled or stored should be designed, 462 operated and maintained so that any releases of flammable material, and consequently the 463 extent of hazardous areas, are kept to a minimum, whether in normal operation or otherwise, 464 with regard to frequency, duration and quantity of a release. 465

It is important to examine those parts of process equipment and systems from which a release 466 of flammable material may arise and to consider modifying the design to minimize the 467 likelihood and frequency of such releases and the quantity and rate of release of material. 468

These fundamental considerations should be examined at an early stage of the design 469 development of any process plant and should also receive prime attention in carrying out the 470 area classification study. 471

In the case of activities other than those of normal operation, e.g. commissioning or non-472 routine maintenance, the area classification may not be valid. It is expected that this would be 473

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dealt with by a safe system of work.The area classification should take into account any 474 routine maintenance. 475

In a situation in which there may be an explosive gas atmosphere, the following steps should 476 be taken: 477 a) eliminate the likelihood of an explosive gas atmosphere occurring around the source of 478

ignition, or 479 b) eliminate the source of ignition. 480

Where this is not possible, protective measures, process equipment, systems and procedures 481 should be selected and prepared so the likelihood of the coincidence of a) and b) is so small 482 as to be acceptable. Such measures may be used singularly, if they are recognized as being 483 highly reliable or in combination to achieve the required level of safety. 484

3.354.2 Area classification objectives 485 Area classification is a method of analysing and classifying the environment where explosive 486 gas atmospheres may occur so as to facilitate the proper selection and installation of 487 equipment to be used safely in that environment. The classification also takes into account 488 the ignition characteristics of the gas or vapour such as ignition energy (gas group) and 489 ignition temperature (temperature class). 490

In most practical situations where flammable materials are used, it is difficult to ensure that an 491 explosive gas atmosphere will never occur. It may also be difficult to ensure that equipment 492 will never give rise to a source of ignition. Therefore, in situations where an explosive gas 493 atmosphere has a high likelihood of occurring, reliance is placed on using equipment which 494 has a low likelihood of creating a source of ignition. Conversely, where the likelihood of an 495 explosive gas atmosphere occurring is reduced, equipment constructed with less rigorous 496 requirements may be used. 497

Subsequent to the completion of the area classification, a risk assessment may be carried out 498 to assess whether the consequences of ignition of an explosive atmosphere requires the use 499 of equipment of a higher equipment protection level (EPL) or may justify the use of equipment 500 with a lower equipment protection level than normally required. The risk assessment may also 501 result in modification of the extent of any area where equipment with a relevant EPL should 502 be applied. 503

The EPL requirements may be recorded, as appropriate, on the area classification documents 504 and drawings to allow proper selection of equipment. 505

It is rarely possible by a simple examination of a plant or plant design to decide which parts of 506 the plant can be equated to the three zonal definitions (zones 0, 1 and 2). A more detailed 507 approach is therefore necessary and this involves the analysis of the basic possibility of an 508 explosive gas atmosphere occurring. 509

In determining where a release of flammable gas or vapour may occur the likelihood and 510 duration of the release should be assessed in accordance with the definitions of continuous, 511 primary and secondary grades of release. Once the grade of release, the release rate, 512 concentration, velocity, ventilation and other factors are assessed there is then a firm basis 513 on which to assess the likely presence of an explosive gas atmosphere in the surrounding 514 areas and determine the type and/or extent of the hazardous zones,. 515

This approach therefore requires detailed consideration to be given to each item of process 516 equipment which contains a flammable material, and which could therefore be a source of 517 release. 518

In particular, zone 0 or zone 1 areas should be minimized in number and extent by design or 519 suitable operating procedures. In other words, plants and installations should be mainly 520 zone 2 or non-hazardous. Where release of flammable material is unavoidable, process 521 equipment items should be limited to those which give secondary grade releases or, failing 522 this (that is where primary or continuous grade releases are unavoidable), the releases should 523 be of very limited quantity and rate. In carrying out area classification, these principles should 524

60079-10-1/Ed2/CD IEC (E) 15

receive prime consideration. Where necessary, the design, operation and location of process 525 equipment should ensure that, even when it is operating abnormally, the amount of flammable 526 material released into the atmosphere is minimized, so as to reduce the extent of the 527 hazardous area. 528

Once a plant has been classified and all necessary records made, it is important that no 529 modification to equipment or operating procedures is made without discussion with those 530 responsible for the area classification. Unauthorized action may invalidate the area 531 classification. It is necessary to ensure that all equipment affecting the area classification 532 which has been subjected to maintenance is carefully checked during and after re-assembly 533 to ensure that the integrity of the original design, as it affects safety, has been maintained 534 before it is returned to service. 535

3.364.3 Competence of Personnel 536 The area classification should be carried out by those who understand the relevance and 537 significance of the properties of the flammable materials and those who are familiar with the 538 process and the equipment along with safety, electrical, mechanical and other qualified 539 engineering personnel. 540

45 Area classification methods 541

4.15.1 General 542 The following sub clauses give guidance on options for classifying areas in which there may 543 be an explosive gas atmosphere. An example of a schematic approach to the classification of 544 hazardous areas is given in Annex F. 545

The area classification should be carried out when the initial process and instrumentation line 546 diagrams and initial layout plans are available and should be confirmed before plant start-up. 547 Reviews should be carried out during the life of the plant. 548

Consideration should always be given to the type, number and location of various potential 549 points of release so that relevant zone and boundary conditions are assigned in the overall 550 assessment. Control systems designed and installed to a Functional Safety standard may 551 reduce the potential for a source of release and/or the volume of a release. Such controls may 552 therefore be considered where relevant to the hazardous area classification. 553

4.25.2 Classification by calculation of sources of releases 554 Classification may be approached by calculation, considering appropriate statistical and 555 numerical assessments for the factors concerned, for each source of release. 556

Formulae relevant to determining the release rates under specified conditions can be found in 557 Annex B. These formulae are generally accepted as providing a good basis for calculating 558 release rates for the conditions provided. 559

Guidance on the assessment of ventilation and dispersion is provided in Annex C.This 560 guidance is not intended to be universally applicable and may not be reliable in some 561 situations. 562

Other forms of assessment, e.g. computational fluid dynamics (CFD), may be used and may 563 provide a good basis for assessment in some situations. Computer modelling is also an 564 appropriate tool when assessing the interaction of multiple factors. 565

In all cases the assessment method and tools used should be validated as suitable or used 566 with appropriate caution. Those carrying out the assessment should also understand the 567 limitations or requirements of any tools and adjust the input conditions or results accordingly 568 to ensure appropriate conclusions. 569

4.35.3 Use of industry codes and national standards 570 Industry codes and national standards may be used where they provide guidance or examples 571 appropriate to the application and comply with the general principles of this standard. 572

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Annex E identifies some relevant industry codes and national standards that may provide 573 further detail as well as examples. 574

4.45.4 Simplified methods 575 Where it is not practicable to assess distances from individual sources of release, a simplified 576 method may be used. 577

Simplified methods shall identify sources for each of the zone types, zone 0, 1 and 2 that are 578 suitably conservative to allow for potential sources of release without individual detail. The 579 judgement is best made by reference to a set of criteria based on industry experience and 580 appropriate to the particular plant. 581

Larger zone areas are characteristic of simplified methods, stemming from the approach and 582 the necessity to apply more conservative zonal classification where doubt exists as to the 583 hazards involved. This approach shall err on the side of safety. 584

To arrive at less conservative or a more accurate figures of the boundaries of the classified 585 area, reference to illustrative examples or more detailed assessment of point sources of 586 release, as applicable could be used. 587

4.55.5 Combination of methods 588 The use of different methods may be appropriate for classification of a plant at various stages 589 of its development or for various parts of the plant. 590

For example, at the initial conceptual stage of a plant the simplified method may be 591 appropriate to set out the equipment separations, plant layout and plant boundaries. This 592 might be the only method that could be applied due to lack of detailed data on sources of 593 release. As the plant design proceeds and detailed data is available on the potential sources 594 of release, the classification should be upgraded using method more detailed assessment. 595

In some cases the simplified method can be applied to a group of similar equipment in 596 sections of plant (e.g. sections of piping with flanges, such as pipe racks) while applying a 597 more detailed assessment to the more significant potential sources of release (e.g. relief 598 valves, vents, gas compressors, pumps and the like). 599

In many cases the classification examples provided in relevant national or industry codes can, 600 where appropriate, be used to classify some components of larger plants. 601

56 Release of flammable material 602

5.16.1 General 603 An introduction to the nature of releases that should be considered when approaching 604 classification of potentially explosive areas is provided in the following clauses. 605

5.26.2 Sources of release 606 The basic elements for establishing the hazardous zone types are the identification of the 607 source of release and the determination of the grade or grades of the release. 608

Since an explosive gas atmosphere can exist only if a flammable gas or vapour is present 609 with air, it is necessary to decide if any flammable materials can exist in the area concerned. 610 Generally speaking, such gases and vapours (and flammable liquids or solids which may give 611 rise to them) are contained within process equipment that may or may not be totally enclosed. 612 It is necessary to identify where a flammable atmosphere can exist inside process equipment, 613 or where a release of flammable materials can create a flammable atmosphere outside 614 process equipment. 615

Each item of process equipment (for example, tank, pump, pipeline, vessel, etc.) should be 616 considered as a potential source of release of flammable material. If the item cannot 617 foreseeably contain flammable material, it will clearly not give rise to a hazardous area 618 around it. The same will apply if the item contains a flammable material but cannot release it 619

60079-10-1/Ed2/CD IEC (E) 17

into the atmosphere (for example, a fully welded pipeline is not considered to be a source of 620 release). 621

If it is established that the item may release flammable material into the atmosphere, it is 622 necessary, first of all, to determine the grade or grades of release in accordance with the 623 definitions, by establishing the likely frequency and duration of the release. It should be 624 recognized that the opening-up of parts of enclosed process systems (for example, during 625 filter changing or batch filling) should also be considered as sources of release when 626 developing the area classification. By means of this procedure, each release will be graded 627 either continuous, primary or secondary. 628

NOTE Releases may form part of process, e.g. taking samples, or may occur as part of a regular maintenance 629 procedure. These forms of release are generally classified as continuous or primary grades of release. Other 630 accidental releases are generally classified as a secondary grade of release. 631

Having established the grade or grades of the release, it is necessary to determine the 632 release rate and other factors that may influence the type and extent of the zone. 633

If the total quantity of flammable material available for release is small, for example, labo-634 ratory use, whilst a potential explosion condition may exist, it may not be appropriate to use 635 this area classification procedure. In such cases, account shall be taken of the particular 636 factors involved. 637

The area classification of process equipment in which flammable material is burned, for 638 example, fired heaters, furnaces, boilers, gas turbines etc., should take into account purge 639 cycle, start-up and shut-down conditions. 640

In some cases the construction of closed systems where specific construction codes are met 641 can be accepted as effectively preventing and/or limiting releases of flammable materials to a 642 negligible leakage hazard. The hazardous area classification of such equipment or 643 installations requires a complete assessment to verify the full compliance of the installation to 644 the relevant constructional and operating standards. Verification of compliance should 645 consider design, installation, operation, maintenance and monitoring activities. 646

Mists which form through leaks of pressurized liquid can be flammable even though the liquid 647 temperature is below the flash point. It is important therefore to ensure that clouds of mist do 648 not occur (see Annex G). 649

5.36.3 Forms of release 650 The characteristics of any release depends upon the physical state of the flammable material, 651 its temperature and pressure. The physical states include: 652

a gas, which may be at an elevated temperature or pressure; 653 a gas liquefied by the application of pressure, e.g. LPG; 654 a gas which can only be liquefied by refrigeration, e.g. methane; 655 a liquid with an associated release of flammable vapour. 656

Releases from such plant items as pipe connections, pumps and compressor seals and valve 657 packings often start with a low flow rate. However, if the release is not stopped erosion of the 658 source of the release can greatly increase the rate of release and hence the extent of the 659 hazard. 660

A release of flammable material above its flashpoint will give rise to a flammable vapour or 661 gas cloud which may initially be less or more dense than the surrounding air or may be 662 neutrally buoyant. The forms of release and the pattern of behaviour at various conditions are 663 displayed as a flow chart in Figure 6.1. 664

5.3.16.3.1 Gaseous release 665

60079-10-1/Ed2/CD IEC (E) 18

A gas release will produce a gas jet or plume at the release source depending on the 666 pressure at the point of release, e.g. pump seal, pipe connection or evaporative pool area. 667 The relative density of the gas, the degree of turbulent mixing and the prevailing air 668 movement will all influence the subsequent movement of any gas cloud. 669

In calm conditions low velocity releases of a gas that is significantly less dense than air will 670 tend to move upwards, e.g. hydrogen and methane. Conversely, a gas that is significantly 671 denser than air will tend to accumulate at ground level or in any pits or depressions, e.g. 672 butane and propane. Over time, atmospheric turbulence will cause the released gas to mix 673 with air and become neutrally buoyant. A gas or vapour with density that is not significantly 674 different to air is regarded as neutrally buoyant. 675

Higher pressure releases will initially produce jets of released gas which will mix turbulently 676 with the surrounding air and entrain air in the jet. 677

At high pressures, a thermodynamic effect due to expansion can come into play. As the gas 678 escapes, it expands and cools down and may initially behave as heavier than air. However, 679 the cooling due to the Joule-Thomson effect is eventually offset by the heat supplied by the 680 air. The resulting gas cloud will eventually become neutrally buoyant. The transition from 681 heavier than air to neutrally bouyant behaviour may occur at any time depending on the 682 nature of the release and may occur after the cloud has been diluted to below the LEL. 683

NOTE Hydrogen demonstrates a reverse Joule-Thomson effect. 684

6.3.2 Liquefied under pressure 685

Some gases can be liquefied by the application of pressure alone, e.g. propane and butane, 686 and are usually stored and transported in this form. 687

When a pressurized liquefied gas leaks from its containment the most likely scenario is that 688 the material will escape as a gas from any vapour space or gas lines. The rapid evaporation 689 produces significant cooling at the point of release and icing due to the condensation of water 690 vapour from the atmosphere may occur. 691

A liquid leak will partially evaporate at the point of release. This is known as flash 692 evaporation. The evaporating liquid pulls energy from itself and the surrounding atmosphere 693 and in turn cools down the leaking fluid. The cooling of the fluid prevents total evaporation 694 and therefore an aerosol is produced. If the leak is large enough then cold pools of fluid can 695 accumulate on the ground which will evaporate over time to add to the gas release. 696

The cold aerosol cloud will act like a dense gas. A pressurized liquid release can often be 697 seen as the cooling effect of evaporation will condense ambient humidity to produce a vapour 698 cloud. 699

6.3.3 Liquefied by refrigeration 700

Other gases, the so-called permanent gases, can only be liquefied by refrigeration e.g. 701 methane and hydrogen. Small leaks of refrigerated gas will evaporate quickly without forming 702 a pool of liquid by drawing heat from the environment. If the leak is large a cold pool of liquid 703 may form. 704

As the cold liquid pulls energy from the ground and surrounding atmosphere the liquid will boil 705 generating a cold dense gas cloud. As with liquids, dikes or bund walls can be used to direct 706 or hold the flow of leakages. 707

NOTE Care needs to be taken when classifying areas containing cryogenic flammable gases such as liquefied 708 natural gas. Vapours emitted will generally be heavier than air at low temperatures but will become neutrally 709 buoyant on approaching ambient temperature. 710

60079-10-1/Ed2/CD IEC (E) 19

6.3.4 Aerosols 711

An aerosol is not a gas, but consists of small droplets of liquid suspended in air. The droplets 712 are formed from vapours or gases under certain thermodynamic conditions or by flash 713 evaporation of pressurized liquids. The scattering of light within an aerosol cloud frequently 714 makes the cloud visible to the naked eye. The dispersion of an aerosol may vary between the 715 behaviour of a dense gas or a neutrally buoyant gas. Aerosol droplets can coalesce and rain 716 out of the plume or cloud. More usually they absorb heat from the surrounding environment, 717 evaporate and add to the a gas/vapour cloud (for more details see Annex G). 718

6.3.5 Vapours 719

Liquids at equilibrium with their environment will generate a layer of vapour above their 720 surface. The pressure this vapour exerts in a closed system is known as the vapour pressure, 721 which increases in a non-linear function with temperature. 722

The process of evaporation uses energy which normally comes from the liquid and the 723 surrounding environment. The evaporation process may decrease the temperature of the 724 liquid and may tend to balance the heat input to the liquid to limit temperature rise. Changes 725 in temperature due to inceased evaporation from normal environmental conditions is 726 considered too marginal to affect the hazardous area classifcation. The concentration of the 727 generated vapour is not easy to predict as it is a function of the evaporation rate, temperature 728 of the liquid and the surrounding air flow (see B.3.3). 729

6.3.6 Liquid releases 730

The release of flammable liquids will normally form a pool on the ground, with a vapour cloud 731 at the liquids surface unless the surface is absorbent. The size of the vapour cloud will 732 depend on the properties of the material and its vapour pressure at the ambient temperature 733 (see B.3.3). 734

NOTE The vapour pressure is an indication of a liquid's evaporation rate. A substance with a high vapour 735 pressure at normal temperatures is often referred to as volatile. As a general rule, vapour pressure of liquid at 736 ambient temperatures increases with decreasing boiling point. As the temperature rises so does the vapour 737 pressure. 738

Release may also occur on water. Many flammable liquids are less dense than water and and 739 are often not miscible. Such liquids will spread on the surface of water, whether it is on the 740 ground, in plant drains, pipe trenches or on open waters (sea, lake or river), forming a thin 741 film and increasing the evaporation rate due to the increased surface area. In these 742 circumstances the calculations in Annex B may not be suitable. 743

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744

Source of

release

Heavy

Consider larger zone

Flash evapo ration

Initially heavy

Hot gas Initially buoyant

Heavy

Neutral

Buoyant

Heavy

Heavy

Gases and

vapours

Flamm able

liquids Boiling pool

See eq. B.6

Non boiling evaporative

pools See eq. B.6

Aerosol

Cold gas

Neutrally buoyant

gas

Aerosol

Cold gas

Self diluted See Figure

C.5

Cold gas

Dense gas

(heavier than air)

Conden sation

Heat evapor ation

Heat evapor ation

Fuming

Sonic gas jet

Buoyant gas

(lighter than air)

At low pressure

See eq. B.3

Gases liquefied

by refrigera

tion

At high pressure

Gases liquefied

under pressure

Flammable or combustible liquids

Possibility of mists

Vapour

Obstructed release

See eq. B.4

Any (depending on gas

conditions and nature of release)

60079-10-1/Ed2/CD IEC (E) 21

Figure 6.1 Forms of release 745

67 Ventilation (or air movement) and dispersion 746

6.17.1 General 747 Gas or vapour released into the atmosphere will dilute through turbulent mixing with air, and 748 to a lesser extent by diffusion driven by concentration gradients, until the gas disperses 749 completely and the concentration is essentially zero. In buildings air movement due to natural 750 or artificial ventilation will promote dispersion. Increased air movement may also increase the 751 rate of release of vapour due to increased evaporation on an open liquid surface. 752

Suitable ventilation rates can reduce persistence of an explosive gas atmosphere thus 753 influencing the type of zone. Increased ventilation will normally reduce the extent of the zone. 754 Obstacles which impede the flow of air and hence reduce the local ventilation rate may 755 increase the extent of the zone. On the other hand, some obstacles, for example, dykes, walls 756 or ceilings, may limit the extent of the zone. 757

A well ventilated shelter e.g. a shelter with a large roof-ventilator and with the sides open 758 sufficient to allow free passage of air through all parts of the building is considered in many 759 cases to be well ventilated and should be treated as an open air area (i.e. medium degree 760 and good availability). 761

7.2 Main types of ventilation 762 The two main types of ventilation are: 763 a) natural ventilation; 764 b) artificial (or forced) ventilation, either general to the area or local to the source of release. 765

6.1.17.2.1 Natural Ventilation 766 Natural ventilation in buildings arises from pressure differences induced by the wind and/or by 767 temperature gradients (buoyancy induced ventilation). Natural ventilation may be effective in 768 certain indoor situations (for example, where a building has openings in its walls and/or roof) 769 to dilute releases safely. 770

Examples of natural ventilation: 771 an open building which, having regard to the relative density of the gases and/or vapours 772

involved, has openings in the walls and/or roof so dimensioned and located that the 773 ventilation inside the building, for the purpose of area classification, can be regarded as 774 equivalent to that in an open-air situation; 775

a building which is not an open building but which has natural ventilation (generally less 776 than that of an open building) provided by permanent openings made for ventilation 777 purposes. 778

Consideration of natural ventilation in buildings should recognise that gas or vapour buoyancy 779 may be a significant factor and should be arranged to promote dispersion and dilution. 780

Ventilation rates arising from natural ventilation are inherently very variable. Where dilution of 781 releases is by natural ventilation, the worst case scenario shall preferably be considered to 782 determine the degree of ventilation. Such a scenario will then lead to a higher level of 783 availability even though the degree of the ventilation is reduced. Generally, with any natural 784 ventilation, a lower degree of ventilation leads to a higher level of availability and vice versa 785 which will compensate for overly optimistic assumptions made in estimating the degree of 786 ventilation. 787

There are some situations which require special care. This is particularly the case where the 788 ventilation openings are limited to mainly one side of the enclosure. Under certain 789 unfavourable ambient conditions, such as windy days when the wind is blowing onto the 790 ventilated face of the enclosure, the external air movement may prevent the operation of the 791 thermal buoyancy mechanism. Under these circumstances the level of ventilation and the 792 availability will both be poor resulting in a more rigorous classification. 793

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6.1.27.2.2 Artificial Ventilation 794 6.1.2.17.2.2.1 General 795 Air movement required for ventilation may also be provided by artificial means, for example, 796 fans or extractors. Although artificial ventilation is mainly applied inside a room or enclosed 797 space, it can also be applied to situations in the open air to compensate for restricted or 798 impeded air movement due to obstacles. 799

The artificial ventilation of an area may be either general or local and, for both of these, 800 differing degrees of air movement and replacement can be appropriate. 801

With the use of artificial ventilation it is sometimes possible to achieve: 802 reduction in the type and/or extent of zones; 803 shortening of the time of persistence of an explosive gas atmosphere; 804 prevention of the generation of an explosive gas atmosphere. 805

6.1.2.27.2.2.2 Ventilation considerations 806 Artificial ventilation can provide an effective and reliable ventilation system in an indoor 807 situation. The following considerations should be included for artificial ventilation systems: 808

a) classification of the inside of the extraction system and immediately outside the extraction 809 system discharge point and other openings of the extraction system; 810

b) for ventilation of a hazardous area the ventilation air should normally be drawn from a 811 non-hazardous area taking into account the suction effects on the surrounding area; 812

c) before determining the dimensions and design of the ventilation system, the location, 813 grade of release, release velocity and release rate should be defined. 814

In addition, the following factors will influence the quality of an artificial ventilation system: 815 a) flammable gases and vapours usually have densities other than that of air, thus they may 816

accumulate near to either the floor or ceiling of an enclosed area, where air movement is 817 likely to be reduced (see 6.3.1); 818

b) proximity of the artificial ventilation to the source of release; artificial ventilation close to 819 the source of release will normally be more effective and may be needed to adequately 820 control gas or vapour movement; 821

c) changes in gas density with temperature; 822 d) impediments and obstacles may cause reduced, or even no, air movement, i.e. no 823

ventilation in certain parts of the area; 824 e) turbulence and circulating air patterns. 825

For more details see C.6. 826

Consideration should be given to the possibility or need for recirculation of air in the 827 ventilation arrangement. This may impact the background concentration and effectiveness of 828 the ventilation system in reducing the hazardous area. In such cases the classification of the 829 hazardous area may need to be modified accordingly. Recirculation of air may also be 830 necessary in some applications e.g. for some processes or to provide for the needs of 831 personnel or equipment in high or low ambient temperatures where supplemental cooling or 832 heating of the air is required. Where recirculation of air is needed then additional controls for 833 safety may also be required. 834

6.1.2.37.2.2.3 Examples of artificial ventilation 835 General artificial ventilation may include a building which is provided with fans in the walls 836 and/or in the roof to improve the general ventilation in the building. 837

The role of fans may be twofold. They can increase the air flow though a building, helping to 838 remove gas from the building. Fans within a building can also increase turbulence and aid the 839 dilution of a cloud which is much smaller than the room which contains it, even if no gas is 840

60079-10-1/Ed2/CD IEC (E) 23

transported out of the room. Fans may also enhance dilution by increasing turbulence in some 841 outdoor situations. 842

Local artificial ventilation may be: 843 a) An air/vapour extraction system applied to an item of process equipment which 844

continuously or periodically releases flammable vapour. 845 b) A forced or extraction ventilation system applied to a local area where it is expected that 846

an explosive gas atmosphere may otherwise occur. 847

For more details see C.4. 848

6.1.37.2.3 Degree of Dilution 849 The effectiveness of the ventilation in controlling dispersion and persistence of the explosive 850 atmosphere will depend upon the degree of dilution, the availability of ventilation and the 851 design of the system. For example, ventilation may not be sufficient to prevent the formation 852 of an explosive atmosphere but may be sufficient to avoid its persistence. 853

The degree of dilution is defined to correspond with the ability of a given release to dilute 854 down to a safe level within defined ventilation or atmospheric conditions. Therefore a larger 855 release corresponds with a lower degree of dilution for a given set of ventilation / atmospheric 856 conditions, and a lower ventilation rate corresponds with a lower degree of dilution for a given 857 size of release. 858

If other forms of ventilation, e.g. cooling fans are taken into account, then care should be 859 exercised as to ventilation availability. 860

Degrees of dilution depend not only on the ventilation, but also on the nature and the type of 861 the expected release of gas. Some releases will be amenable to mitigation by enhanced 862 ventilation with others much less so. 863

The following three degrees of dilution are recognized: 864

a) High dilution 865 The concentration near the source of release reduces quickly. 866 b) Medium dilution 867 The concentration is controlled resulting in a stable zone boundary, whilst the release is in 868 progress and the explosive gas atmosphere does not persist unduly after the release has 869 stopped. The type and extent of zone are limited to the design parameters. 870 c) Low dilution 871 There is significant concentration whilst release is in progress and/or significant persistence 872 of a flammable atmosphere after the release has stopped. 873

8 Type of zone 874

The likelihood of the presence of an explosive gas atmosphere depends mainly on the grade 875 of release and the ventilation. This is identified as a zone. Zones are recognized as: zone 0, 876 zone 1, zone 2 and the non-hazardous area. 877

Where zones created by adjacent sources of release overlap and are of different zonal 878 classification, the higher classification criteria will apply in the area of overlap. Where 879 overlapping zones are of the same classification, this common classification will normally 880 apply. 881

8.1 Influence of grade of the source of release 882

The grade of release generally determines type of the zone. A continuous grade of release 883 generally leads to a zone 0 classification, a primary grade to zone 1 and a secondary grade to 884 zone 2. This general rule may be modified by considering the degree of dilution and 885 availability of ventilation which may require a more severe classification or allow a less severe 886 classification (see Annex C). 887

60079-10-1/Ed2/CD IEC (E) 24

8.2 Influence of dilution 888

The effectiveness of ventilation or degree of dilution must be considered when estimating the 889 type of zone classification. A medium degree of dilution will generally result in the 890 predetermined types of the zones based upon the types of the sources of release. A high 891 degree of dilution will allow a less severe classification, e.g. zone 1 instead of zone 0, zone 2 892 instead of zone 1 and even zone NE in some cases. On the other hand a low degree of 893 dilution (DL) will require a more severe classification (see Annex C). 894

8.3 Influence of availability of ventilation 895

The availability of ventilation has an influence on the presence or formation of an explosive 896 gas atmosphere and thus also on the type of zone. As availability, or reliability, of the 897 ventilation decreases, the likelihood of not dispersing flammable atmospheres increases. The 898 zone classification will tend to be more severe, i.e. a zone 2 may change to a zone 1 or even 899 zone 0. Guidance on availability is given in Annex C. 900

NOTE Combining the concepts of the efficiency of ventilation and the availability of ventilation results in a 901 qualitative method for the evaluation of the zone type. This is further explained in Annex C. 902

9 Extent of zone 903

The extent of the zone depends on the estimated or calculated distance over which an 904 explosive atmosphere exists before it disperses to a concentration in air below its lower 905 explosive limit. An appropriate safety factor should be selected based on any uncertainties 906 and the assessment methodology. When assessing the area of spread of gas or vapour 907 before dilution to below its lower explosive limit, expert advice should be sought. 908

Consideration should always be given to the possibility that a gas which is heavier than air 909 may flow into areas below ground level (for example, pits or depressions) and that a gas 910 which is lighter than air may be retained at high level (for example, in a roof space). 911

Where the source of release is situated outside an area or in an adjoining area, the pene-912 tration of a significant quantity of flammable gas or vapour into the area can be prevented by 913 suitable means such as: 914 a) physical barriers; 915 b) maintaining a sufficient overpressure in the area relative to the adjacent hazardous 916

areas, so preventing the ingress of the explosive gas atmosphere; 917 c) purging the area with sufficient flow of fresh air, so ensuring that the air escapes from all 918

openings where the flammable gas or vapour may enter. 919

The extent of the zone is mainly affected by a number of physical and chemical parameters, 920 some of which are intrinsic properties of the flammable material; others are specific to the 921 process. For simplicity, the effect of each parameter listed in the following clauses assumes 922 that the other parameters remain unchanged. 923

9.1 Influence of release rate 924

The release rate of flammable material is the most important factor that affects the extent of a 925 zone. The higher the release rate the larger the extent of a zone. 926

9.2 Influence of form of release 927

Every form of release will eventually end as a gaseous or vapour release and the gas or 928 vapour may appear as buoyant, neutrally buoyant or heavy. This characteristic will affect the 929 extent of the zone generated by a particular form of release (see Figure 6.1). 930

The horizontal extent of the zone at ground level will generally increase with increasing 931 relative density and the vertical extent above the source will generally increase with 932 decreasing relative density. 933

60079-10-1/Ed2/CD IEC (E) 25

9.3 Influence of lower explosive limit 934

For a given release volume, the lower the LEL the greater will be the extent of the zone. 935

NOTE Experience has shown that a release of ammonia, with an LEL of 15 % by volume, will often dissipate 936 rapidly in the open air, so an explosive gas atmosphere will, in most cases be of negligible extent. 937

9.4 Influence of ventilation 938

Dispersion or diffusion of a gas or vapour into the atmosphere is a key factor in reducing the 939 concentration of the gas or vapour to below the lower explosive limit. 940

Ventilation and air movement has two basic functions: 941 a) To increase the rate of dilution and promote dispersion to limit the extent of a zone; 942 b) To avoid the persistence of an explosive atmosphere that may influence the type of a 943

zone. 944

With increased ventilation or air movement the extent of a zone will normally be reduced. 945 Obstacles which impede the ventilation or air movement may increase the extent of a zone. 946 On the other hand, some obstacles, for example, dykes, walls and ceilings, which limit the 947 extent of vapour or gas movement, may also limit the extent of the zone. 948

NOTE Increased air movement may also increase the release rate of vapour due to increased evaporation from 949 open liquid surfaces. However the benefits of increased air movement normally outweigh the increase in release 950 rate. 951

9.5 Influence of climatic conditions 952

For low velocity releases the rate of gas or vapour dispersion in the atmosphere increases 953 with wind speed, but in stable atmospheric conditions layering of the gas or vapour may occur 954 and the distance for safe dispersal can be greatly increased. 955

NOTE 1 In plant areas with large vessels and structures even at low wind speeds eddies may be formed behind 956 vessels and structures thus forming pockets of gas or vapour, despite sufficient turbulence that could otherwise 957 promote dispersion. 958

NOTE 2 In normal practice, the tendency of layering is not taken into account in area classification because the 959 conditions which give rise to this effect are rare and occur for short periods only. However, if prolonged periods of 960 low wind speed are expected for the specific circumstance then the extent of the zone should take account of the 961 additional distance required to achieve dispersion. 962

9.6 Influence of topography 963

Under some conditions dense gases and vapour can behave like a spilled liquid spreading 964 down terrain slopes, through plant drains or pipe trenches and can be ignited at a point 965 remote from the original leakage, therefore putting at risk a large area of plant (see 6.3.4). 966 The layout of the plant, where possible, should be designed to aid the rapid dispersal of 967 explosive gas atmospheres. 968

An area with restricted ventilation (for example, in pits or trenches) that would otherwise be 969 zone 2 may require zone 1 classification; on the other hand, wide shallow depressions used 970 for pumping complexes or pipe reservations may not require such rigorous treatment. 971

710 Documentation 972

7.110.1 General 973

It is recommended that the steps taken to carry out area classification and the information and 974 assumptions used are fully documented. The area classification document should be a living 975 document and should include the method used for area classification and should be revised 976 during any plant changes. All relevant information used should be referred to. Examples of 977 such information, or of a method used, would be: 978 a) recommendations from relevant codes and standards; 979 b) gas and vapour dispersion characteristics and calculations; 980

60079-10-1/Ed2/CD IEC (E) 26

c) a study of ventilation characteristics in relation to flammable material release parameters 981

so that the effectiveness of the ventilation can be evaluated. 982 d) the properties of all process materials used on the plant (see IEC 60079-20-1), which may 983

include: 984 molar mass 985 flash point 986 boiling point 987 minimum ignition temperature 988 vapour pressure 989 vapour density 990 flammability limits 991 gas group and temperature class 992

A suggested format for the materials listing is given in Table A.1 and a format for recording 993 the results of the area classification study and any subsequent alterations is given in Table 994 A.2. 995

The source of information (code, national standard, calculation) needs to be recorded so that, 996 at subsequent reviews, the philosophy which was adopted is clear to the area classification 997 team. 998

7.210.2 Drawings, data sheets and tables 999 Area classification documents may be in hard copy or electronic form and should include 1000 plans and elevations or three dimensional models, as appropriate, which show both the type 1001 and extent of zones, gas group, ignition temperature and/or temperature class. 1002

Where the topography of an area influences the extent of the zones, this should be 1003 documented. 1004

The documents should also include other relevant information such as: 1005 a) The location and identification of sources of release. For large and complex plants or 1006

process areas, it may be helpful to itemize or number the sources of release so as to 1007 facilitate cross-referencing between the area classification data sheets and the drawings; 1008

b) The position of openings in buildings (for example, doors, windows and inlets and outlets 1009 of air for ventilation). 1010

The area classification symbols which are shown in Figure A.1 are the preferred ones. A 1011 symbol key shall always be provided on each drawing. Different symbols may be necessary 1012 where multiple equipment groups and/or temperature classes are required within the same 1013 type of zone (for example, zone 2 IIC T1 and zone 2 IIA T3). 1014

1015

1016

1017

1018

1019

1020

60079-10-1/Ed2/CD IEC (E) 27

Annex A 1021

(informative) 1022 Suggested presentation of hazardous areas 1023

1024

1025

1026

Zone 0

Zone 1

Zone 2IEC 1253/02 1027

1028

1029

Figure A.1 Preferred symbols for hazardous area zones 1030

60079-10-1/Ed2/CD IEC (E) 28

Table A.1 Hazardous area classification data sheet Part I: Flammable material list and characteristics 1031

Plant: Reference drawing:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Flammable material Volatilitya LEL Ex characteristics

No. Name Composi- tion

Molar mass

(kg/ kmol)

Rela tive

density gas/air

Polytropic index of adiabatic

expansion

Flash point

(C)

Ignition temp.

(C)

Melting point

(C)

Boiling point

(C)

Vapour pressure at 200C

(kPa)

vol

(%)

(kg/m3) Group Temp. class

Any other relevant

information and remarks

60079-10-1/Ed2/CD IEC (E) 29

a Normally, the value of vapour pressure is given, but in the absence of that, boiling point can be used.

1032

Table A.2 Hazardous area classification data sheet Part II: List of sources of release 1033

Plant:

Area:

Reference drawing:

1 2 3 4 5 6 7 8 9 10 11 12 13

Source of release Flammable material Ventilation Hazardous area

No. Description Location Grade of releasea

Refer-enceb

Operating temperature

and pressure

Statec Typed Degreee Avail-abilitye

Zone type

0-1-2

Zone extent

(m)

Referencef

Any other relevant information and/or

remarks

(C) (kPa) Verti-cal

Hori-zontal

60079-10-1/Ed2/CD IEC (E) 30

a C Continuous; S Secondary; P Primary

b Quote the number of list in Part I

c G Gas; L Liquid; LG Liquefied gas; S Solid

d N Natural; AG Arificial General; AL Artificial Local.

e See IEC 60079-10-1 Annex C

f Indicate code reference if used, or calculation reference

60079-10-1/Ed2/CD IEC (E) 31

A.1 Hazardous area suggested shapes 1034

The following figures show some suggested hazardous area shapesbased on the form of 1035 release described in 6.3, which may be useful in the preparation of hazardous area 1036 classification drawings. The effects of impingement of the release on obstacles and the 1037 influence of topography are not considered. The hazardous area generated by a release may 1038 result in the combination of different shapes. 1039

Key 1040 SR Source of release 1041 r Main extension of hazardous area to be defined taking into consideration the estimated 1042

hazardous distance (see Annex D) 1043 r, r Secondary extension(s) of hazardous area to be defined taking into account release 1044

behaviour 1045 h Distance between source of release and ground level or surface below the release 1046

or

1047 Figure A.2 Gas/vapour at low pressure 1048

(or at high pressure in case of unknown release direction) 1049

1050

or

1051 Figure A.3 Gas/vapour at high pressure 1052

60079-10-1/Ed2/CD IEC (E) 32

1053 Figure A.4.1 Gas or vapour (liquefied under pressure or by refrigeration) 1054

NOTE Liquid pool should not be formed in case of dripping. 1055

1056

Figure A.4.2 Gas or vapour (liquefied under pressure or by refrigeration) with spillage 1057

NOTE Liquid pool could be formed in case of spillage. In this case, an additional source of release should be 1058 considered 1059

1060 Figure A.5 Flammable liquid (non boiling evaporative pool) 1061

NOTE Source of spillage of flammable material is not indicated. 1062

1063

60079-10-1/Ed2/CD IEC (E) 33

Annex B 1064 (informative) 1065

Estimation of sources of release 1066

1067 B.1 Examples of grade of release 1068

The following examples are not intended to be rigidly applied and may need to be varied to 1069 suit particular process equipment and the situation. It needs to be recognised that some 1070 equipment may exhibit more than one grade of release. 1071

B.1.1 Sources giving a continuous grade of release 1072 a) The surface of a flammable liquid in a fixed roof tank, with a permanent vent to the atmo-1073

sphere. 1074 b) The surface of a flammable liquid which is open to the atmosphere continuously or for 1075

long periods. 1076

B.1.2 Sources giving a primary grade of release 1077 a) Seals of pumps, compressors or valves if release of flammable material during normal 1078

operation is expected. 1079 b) Water drainage points on vessels which contain flammable liquids, which may release 1080

flammable material into the atmosphere while draining off water during normal operation. 1081 c) Sample points which are expected to release flammable material into the atmosphere 1082

during normal operation. 1083 d) Relief valves, vents and other openings which are expected to release flammable 1084

material into the atmosphere during normal operation 1085

B.1.3 Sources giving a secondary grade of release 1086 a) Seals of pumps, compressors and valves where release of flammable material during 1087

normal operation of the equipment is not expected. 1088 b) Flanges, connections and pipe fittings, where release of flammable material is not 1089

expected during normal operation. 1090 c) Sample points which are not expected to release flammable material during normal 1091

operation. 1092 d) Relief valves, vents and other openings which are not expected to release flammable 1093

material into the atmosphere during normal operation. 1094

B.2 Hole size and source radius 1095

The most significant factor to be estimated in a system is the hole size. It determines the 1096 release rate of the flammable material and thus eventually the type of zone and the extent of 1097 the zone. 1098

Release rate is proportional to the square of the hole radius. A modest underestimate of the 1099 hole size will therefore lead to a gross underestimate of the calculated value for release rate, 1100 which should be avoided. Overestimate of the hole size will lead to a conservative calculation 1101 which is acceptable for safety reasons, however, the degree of conservatism should also be 1102 limited because it eventually results with overlarge zone extents. A carefully balanced 1103 approach is therefore needed when estimating the hole size. 1104

NOTE While the term hole radius is used, most unintended holes are not round. In such cases the coefficient of 1105 discharge is used as a compensating term to reduce the release rate given a hole of equivalent area. 1106

For continuous and primary grades of release the holes sizes are defined by the size and the 1107 shape of the release orifice, e.g. various vents and breather valves where the gas is released 1108 under relatively predictable conditions. 1109

60079-10-1/Ed2/CD IEC (E) 34

A guide to hole sizes that may be considered is included in Table B.1. 1110

Table B.1 Secondary Grade of Releases Suggested Hole Cross Sections 1111

Type of item Item

Leak Considerations

Typical values for the conditions at which the release opening will not

expand

Typical values for the conditions at which the release

opening may expand, e.g

erosion

Typical values for the conditions at which the release

opening may expand up to a severe

failure, e.g blow out

S (mm2) S (mm2) S (mm2)

Sealing elements on fixed

parts

Flanges with compressed fibre gasket

or similar

0,025 up to 0,25 > 0,25 up to 2,5

(sector between two bolts)

(gasket thickness)

usually 1 mm

Flanges with spiral wound

gasket or similar

0,025 0,25

(sector between two bolts)

(gasket thickness)

usually 0,5 mm

Ring type joint

connections 0,1 0,25 0,5

Small bore connections up to 50 mm

(1)

0,025 up to 0,1 > 0,1 up to 0,25 1,0

Sealing elements

on moving parts at low

speed

Valve stem packings 0,25 2,5

To be defined according to Equipment Manufacturers

Data but not less than 2,5 mm2 (5)

Pressure relief valves

(2)

0,1 (orifice section)

NA NA

Sealing elements

on moving parts at

high speed

Pumps and compressors

(3) NA 1 up to 5

To be defined according to Equipment Manufacturers Data and/or Process Unit Configuration but not less

than 5 mm2

(4 and 5)

60079-10-1/Ed2/CD IEC (E) 35

Lower values in a range should be selected for ideal conditions where the likelihood of failure 1112 is low, e.g. operating at well below design ratings. Higher values in a range should be 1113 selected where operating conditions are close to design ratings and where adverse conditions 1114 such as vibrations, temperature dilatations, poor environmental conditions or contamination of 1115 gases may increase the likelihood of failure. Generally unattended installations require 1116 special considerations to avoid severe failure scenarios. The basis for selection of a hole size 1117 should be properly documented. 1118

B.3 Release rate 1119

The release rate depends itself on other parameters, namely: 1120

a) Nature and type of release 1121 This is related to the physical characteristics of the source of release, for example, an open 1122 surface, leaking flange, etc. 1123

b) Release velocity 1124 For a given source of release, the release rate increases with the release pressure. For a 1125 subsonic release, the release velocity is related to the process pressure. The size of a cloud 1126 of flammable gas or vapour is determined by the rate of flammable vapour release and the 1127 rate of dilution. Gas and vapour flowing from a leak at high velocity will entrain air and may be 1128 self-diluting. The extent of the explosive gas atmosphere may be almost independent of air 1129 flow. If the material is released at low velocity or if its velocity is reduced by impingement on a 1130 solid object, it will be carried by the air flow and its dilution and extent will depend on air flow. 1131

c) Concentration 1132 The mass of flammable material released increases with the concentration of flammable 1133 vapour or gas in the released mixture. 1134

d) Volatility of a flammable liquid 1135 This is related principally to the vapour pressure, and the enthalpy (heat) of vaporization. If 1136 the vapour pressure is not known, the boiling point and flashpoint can be used as a guide. 1137

An explosive gas atmosphere cannot exist if the flashpoint is above the relevant maximum 1138 temperature of the flammable liquid. The lower the flashpoint, the greater may be the extent 1139 of the zone. However, if a flammable material is released in a way that forms a mist (for 1140 example, b

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