As 2430-1 Classification of Hazardous Areas

AS 2430.1—1987

Australian Standard  Classification of hazardous areas

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Part 1: Explosive gas atmospheres

This Australian standard was prepared by Committee MS/11, Classification of Hazardous Areas. It was approved on behalf of the Council of the Standards Association of Australia on 9 December 1986 and published on 2 February 1987.

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The following interests are represented on Committee MS/11: Australian Electrical and Electronic Manufacturers Association Australian Gas Association Australian Institute of Petroleum Australian Liquefied Petroleum Gas Association Confederation of Australian Industry Department of Defence Department of Labour, Vic. Department of Industrial Relations, N.S.W. Department of Mines, Qld Electricity Supply Association of Australia Grain Handling authority of N.S.W. Insurance Council of Australia Royal Australian Chemical Institute State electricity regulatory authorities The Pipeline Authority

Review of Australian Standards. To keep abreast of progress in industry, Australian Standards are subject to periodic review and are kept up to date by the issue of amendments or new editions as necessary. It is important therefore that Standards users ensure that they are in possession of the latest ed ition, and any amendments thereto. Full details of all Australian Standards and related publications will be found in the Standards Australia Catalogue of Publications; this information is supplemented each month by the magazine ‘The Australian Standard’, which subscribing members receive, and which gives details of new publications, new editions and amendments, and of withdrawn Standards. Suggestions for improvements to Australian Standards, addressed to the head office of Standards Australia, are welcomed. Notification of any inaccuracy or ambiguity found in an Australian Standard should be made without delay in order that the matter may be investigated and appropriate action taken.

AS 2430.1—1987

Australian Standard  Classification of hazardous areas Part 1: Explosive gas atmospheres

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First published . . . . . . . . . . . . . . . . . . . . . . . . Second edition . . . . . . . . . . . . . . . . . . . . . . . . Third edition . . . . . . . . . . . . . . . . . . . . . . . . . .

1981 1982 1987

PUBLISHED BY STANDARDS AUSTRALIA (STANDARDS ASSOCIATION OF AUSTRALIA) 1 THE CRESCENT, HOMEBUSH, NSW 2140 ISBN 0 7262 4493 7

AS 2430.1—1987

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PREFACE This edition of this standard was prepared by the Association’s Committee on Classification of Hazardous Locations to supersede AS 2430, Part 1—1982. The standard is intended for the guidance of industry and relevant statutory authorities concerned with the classification of hazardous areas. It is Part 1 of a series dealing with specified hazardous atmospheres. This standard deals with the classification of hazardous areas according to the probability of the existence of an explosive mixture of gas, vapour or mist with air in the atmosphere. The content of this standard is solely concerned with the classification of hazardous areas and not with the types of equipment or ignition sources which can be used in such areas. Such types of equipment are covered in standards published by the specific technology group, i.e. electrical, mechanical, gas, chemical. In its terminology, definitions and general treatment of the subject, this standard is similar to the following report issued by the International Electrotechnical Commission: IEC 79 Electrical Apparatus for Explosive Gas Atmospheres Part 10: Classification of Hazardous Areas The major changes between this standard and the previous edition are as follows: (a) Closer alignment with IEC 79-10. (b) The addition of some important clarifications in the Scope and Application Clauses. (c) The elimination of the two ‘time criterion’ methods for determining hazardous areas and distinguishing between Zones 0, 1 and 2. (d) The removal of the specific occupancy Appendix from this standard. (It is now published as AS 2430, Part 3.)

CONTENTS

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Page FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 2 3 4 5 6 7 8 9

SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCED DOCUMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BASIC PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DETERMINATION OF THE TYPE AND EXTENT OF ZONES . . . . . VENTILATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PROPERTIES OF FLAMMABLE LIQUIDS, GASES AND VOLATILE SOLIDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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APPENDIX A. SOURCES OF RELEASE . . . . . . . . . . . . . . . . . . . . . . . .

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AS 2430.1—1987

FOREWORD

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Many gases, vapours and dusts which are generated processed, handled and stored in industry are combustible. When ignited they may burn rapidly and with considerable explosive force if mixed with air in the appropriate proportions. Areas where gases and vapours, dusts, flyings the fibres occur in dangerous quantities are classified as hazardous. This standard deals with the zonal classification of hazardous areas where explosive gas atmospheres are present. For classification of combustible dust areas, refer to AS 2430, Part 2. The word ‘zone’ is internationally accepted as indicating the probability of the presence of a flammable, combustible or explodable material, and the extent, dimension, shape of hazardous area and volume in which the hazardous material can be expected. This standard covers the following zones: ZONE 0: In which an explosive gas atmosphere is present continuously, or is present for long periods. ZONE 1: In which an explosive gas atmosphere is likely to occur in normal operation. ZONE 2: In which an explosive gas atmosphere is likely to occur in normal operation and if it does occur it will exist for a short period only. The area classification should be carried out by those who have full knowledge both of the processes, systems and equipment concerned and of safety and personnel. The agreement reached on the area classification should be formally recorded. The properties (which are relevant to area classification) of all process materials should be listed and should include flashpoint, boiling point, melting point, ignition temperature, ignition energy, vapour pressure, vapour density and flammability limits. The initial step is to decide if a process item contains flammable material and if a release can occur. The procedure outlined herein should be applied to each item of equipment. The type and extent of the zonal areas can then be established.

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STANDARDS ASSOCIATION OF AUSTRALIA Australian Standard for CLASSIFICATION OF HAZARDOUS AREAS PART 1

EXPLOSIVE GAS ATMOSPHERES

1 SCOPE. This standard provides guidance for the classification of hazardous areas according to the existence of or likelihood of the existence of an explosive gas/air mixture. NOTE: Areas classified as Zones 0, 1 or 2 in accordance with this standard are sometimes referred to as Class I areas.

It applies to open and enclosed areas, in all above-ground situations, where there may be a risk due to the presence of flammable gas or vapour mixed with air under normal atmosphere conditions (see Note 1). This standard does not apply to — (a) underground workings including coal mining; (b) processing and manufacture of explosives; (c) areas which may be affected by oxygen enrichment; (d) areas where risks may arise due to the presence of combustible dusts (see Note 3); or (e) potential catastrophic failures such as the bursting of pressure vessels, pipelines or tanks. NOTES: 1. Normal atmospheric conditions include variations above and below reference levels of 101.3 kPa and 20°C provided the variations have a negligible effect on the explosion properties of the flammable materials. 2. The design, construction and installation of plant and equipment have a significant bearing on classification. However, this standard does not specify any requirements for such design, construction or installation. 3. The classification of hazardous areas where combustible dusts are present (Class II areas) is covered in AS 2430, Part 2.

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2 APPLICATION. This standard is intended for application by occupiers of premises and sites for the purpose of recognizing and classifying hazardous areas. NOTES: 1. Responsibility for the recognition and classification of hazardous areas is normally vested in the occupier. 2. Where a statutory authority has jurisdiction, it may be necessary to have the classification arrived at by the occupier ratified by the statutory authority. 3. If necessary, the occupier should seek assistance from those competent in classification principles and procedure. Statutory authorities normally do not perform this function.

3 REFERENCED DOCUMENTS. The following documents are referred to in this standard: AS 1940 SAA Flammable and Combustible Liquids Code AS 2106 Methods for the Determination of the Flashpoint of Flammable Liquids (Closed Cup) AS 2430 Classification of Hazardous Areas Part 2 — Combustible Dusts Part 3 — Specific Occupancies

NFPA 325M

Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids

4 DEFINITIONS. For the purpose of this standard, the following definitions apply: 4.1 Area, hazardous - an area in which an explosive atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of potential ignition sources. NOTES: 1. For the purpose of this standard, an area is regarded as a three-dimensional region or space. 2. Examples of potential ignition sources are electrical equipment, naked flames, sparks from grinding and welding operations, and hot surfaces.

4.2 Area, non-hazardous — an area in which an explosive atmosphere is not expected to be present in quantities such as to require special precautions for the construction, installation and use of potential ignition sources. 4.3 Authority, Statutory — the person or body responsible for the implementation of legislation relating to the handling, processing or storage of materials constituting a hazard. 4.4 Boiling point — the temperature of a liquid boiling at an ambient pressure of 101.3 kPa. NOTE: For liquid mixtures, the initial boiling point should be used. ‘Initial boiling point’ in respect of liquid mixtures indicates the lowest value of the boiling point for the range of liquids present.

4.5 Explosive gas atmosphere — mixture with air, under normal atmospheric conditions (see Clause 1, Note 1), of flammable materials in the form of gas, vapour, or mist, in which, after ignition, combustion spreads throughout the unconsumed mixture. NOTES: 1. This definition specifically excludes dusts and fibres in suspension in air. 2. Although a mixture which has a concentration above the upper explosive level (UEL) is not an explosive gas atmosphere, in certain cases for area classification purposes it is advisable to consider it as an explosive gas atmosphere.

4.6 Explosive limit, lower (LEL) — the concentration of flammable gas, vapour or mist in air, below which an explosive gas atmosphere will not be formed. 4.7 Explosive limit, upper (UEL) — the concentration of flammable gas, vapour or mist in air, above which an explosive gas atmosphere will not be formed. 4.8 Flammable gas or vapour - gas or vapour which, when mixed with air in certain proportions, will form an explosive gas atmosphere.

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NOTE: All vapours of flammable liquids are flammable vapours.

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4.9 Flammable liquid — any Class 3.1 (Class A) or Class 3.2 (Class B) liquid having a flashpoint of not more than 61°C. NOTES: 1. AS 1940 defines a Class A flammable liquid as a flammable liquid having a flashpoint up to but not including 23°C, and a Class B flammable liquid as a flammable liquid having a flashpoint not less than 23°C and up to and including 61°C. 2. In accordance with the above definition, kerosine and white spirit are flammable liquids whereas diesel fuel (distillate) is not.

4.10 Flammable material — material consisting of flammable gas, vapour, liquid and/or mist. 4.11 Flammable mist — droplets of flammable or combustible liquid, dispersed in air, so as to form an explosive atmosphere. 4.12 Flashpoint — the lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in quantity such as to be capable of forming an ignitable vapour/air mixture. NOTES: 1. Flashpoint data are normally associated with liquids, though they are also relevant to solids which sublime. It is important to note that care must be taken in the use of flashpoint data in applications where the ignition source may itself raise the temperature of the combustible material. 2. The value for the flashpoint depends to some extent on the method of test. For the purpose of this standard, flashpoints are determined in accordance with the relevant requirements of AS 2106.

4.13 Normal operation — the situation when the plant equipment is operating within its design parameters. NOTES: 1. Minor releases of flammable material may be part of normal operation. For example, releases from seals which rely on wetting by the fluid being pumped are considered to be minor releases. 2. Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by accidents) which involve repair or shut down are not considered to be part of normal operation.

4.14 Relative density (of a gas or vapour) — the density of a gas or a vapour relative to the density of air at the same pressure and at the same temperature. NOTE: The relative density of air is 1.0.

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4.15 Source of release — a point or location from which a gas, vapour, mist or liquid may be released into the atmosphere so that an explosive gas atmosphere could be formed. 4.16 Ventilation, general artificial — movement of air and its replacement with fresh air by artificial means (e.g. fans) and applied to a general area. 4.17 Ventilation, local artificial — movement of air and its replacement with fresh air by artificial means (usually extraction) applied to a particular source of release or local area. 4.18 Ventilation, natural — movement of air and its replacement with fresh air due to the effects of wind and/or buoyancy forces. 4.19 Ventilation, no — exists where no arrangements have been made to cause air replacement with fresh air. 4.20 Zones, hazardous - the zones into which hazardous areas are classified based upon the frequency of the appearance and the duration of an explosive gas atmosphere.

AS 2430.1—1987

4.21 Zone 0 - an area in which an explosive gas atmosphere is present continuously or is present for long periods. 4.22 Zone 1 - an area in which an explosive gas atmosphere is likely to occur in normal operation. 4.23 Zone 2 - an area in which an explosive gas atmosphere is not likely to occur in normal operation and if it does occur it will exist for a short period only. NOTE: This zone applies to areas where flammable materials are used but where an explosive gas atmosphere would occur only in the event of an accident or some unusual operating condition. This zoning may also apply to the interface between a Zone 1 area and a non-hazardous area.

5 GENERAL. Area classification is a method of analyzing and classifying the environment where explosive gas atmospheres may occur to allow the proper selection of equipment, particularly electrical equipment, to be installed in that environment. The object of the classification procedure is to enable the equipment to be operated safely in these environments. Where it is necessary to use equipment in an environment in which there may be an explosive gas atmosphere and it is not possible to: (a) eliminate the likelihood of an explosive gas atmosphere occurring around the source of ignition, or (b) eliminate the source of ignition; then measures should aim at reducing the likelihood of the occurrence of either or both of the above factors so that the likelihood of coincidence is so small as to be acceptable. In most practical situations where flammable materials are used it is difficult to ensure that an explosive gas atmosphere will never occur. It may also be difficult to ensure that equipment will never give rise to a source of ignition. Therefore in situations where an explosive atmosphere has a high likelihood of occurring, reliance is placed on using equipment which has an extremely low likelihood of creating a source of ignition. Conversely where the likelihood of an explosive atmosphere occurring is reduced, equipment which has an increased likelihood of becoming a source of ignition may be used. To apply this approach the first step is to assess the likelihood of an explosive atmosphere occurring in accordance with the definitions of Zone 0, Zone 1 and Zone 2. The following clauses give guidance on this first step, viz: on the classification of areas in which there may be an explosive gas atmosphere into Zones 0, 1 and 2. 6 BASIC PRINCIPLES. 6.1 Fundamental safety concepts. It is recommended that plants and installations in which flammable materials are handled or stored be so designed that hazardous areas are kept to a minimum; in particular Zone 0 and Zone 1 areas should be a minimum in both number and extent. In other words the hazardous areas should be mainly Zone 2. Where release of flammable material is unavoidable, plant equipment items should be mainly limited to secondary grade sources of release, or failing this (that is where primary or continuous grade sources of release are unavoidable) the sources of release should be such as to have a very limited quantity or rate of release to atmosphere. In carrying

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out area classification, these principles should receive prime consideration and where necessary, the design, operation or location of process equipment should be modified to meet these requirements. Similarly, consideration should be given to the design and operation of process equipment to ensure that even when it is operating abnormally the amount of flammable material released to the atmosphere is minimized in order to reduce the extent of the area (Zone 2) made hazardous. Where the situation is covered in AS 2430, Part 3, the classification given therein should be used, provided a careful check is made to ensure that the specific occupancy specified is appropriate to the case under consideration. Where the occupancy is not specified in AS 2430, Part 3, or where unusual circumstances apply, the procedure illustrated in Fig. 1 may be used. Once a plant has been classified and any necessary records made it is important that no modification to equipment or operating procedures is made without discussion with those responsible for the area classification. Unauthorized action may invalidate the area classification. It is necessary to ensure that process equipment which has been subjected to maintenance shall be carefully checked during and after re-assembly to ensure that the integrity of the original design as it affects safety, has been maintained before it is returned to service. It should be recognized that the opening of parts of closed process systems (e.g. filter changing, batch filling) should also be considered as sources of release in developing the area classification. 6.2 Factors which determine type and extent of zones. 6.2.1 Type of zone. The likelihood of the presence of an explosive gas atmosphere and hence the type of zone depends mainly on the grade of source of release. In some cases the ventilation and other factors may also affect the type of zone. To facilitate the classification of areas, this standard gives some typical and specific examples (see Figs 5 and 6) which may be used where they are applicable to the situations under consideration. Where the examples do not strictly apply a classification may be arrived at by adjusting those examples to the actual conditions and using the logic typified in Fig. 1. Where the actual conditions are not related to the examples, additional considerations should be introduced in the classification procedure. For open air unrestricted ventilated situations, Fig. 1 may be used. Depending on conditions of release and subsequent propagation, it may be necessary in some cases to require a less hazardous zone surrounding a more hazardous zone. 6.2.2 Extent of zone. The extent of zones depends mainly on the following parameters: (a) Release rate of flammable material. The extent may increase with increasing release rate (but see release velocity). (b) Release velocity. Due to an improved dilution for release of flammable gases, vapours and/or mists in air, the extent of a hazardous area may decrease if, with constant release rate, the release velocity increases above that which causes turbulent flow. (c) Concentration of flammable gases, vapours and/or mists in the released mixture. The extent may increase with increasing concentration at the release source. (d) Ventilation. With an increased rate of ventilation, the

extent of a hazardous area may be reduced. The extent may also be reduced by an improved arrangement of the ventilation system. (e) Obstacles. Obstacles may impede the ventilation and thus may enlarge the extent of the zone. On the other hand some obstacles, for example dikes and walls, may limit the movement of a cloud of an explosive gas atmosphere and thus may reduce the extent. (f) Boiling point of flammable liquids (initial boiling point for liquid mixtures). For flammable liquids, the concentration of the released vapour is related to the vapour pressure at the relevant maximum liquid temperature. The lower the initial boiling point, the greater the vapour pressure for a given liquid temperature and hence the greater the extent of the hazardous area. (g) Lower explosive limit. The lower the LEL, the larger may be the extent of the hazardous area. (h) Flashpoint. An explosive gas atmosphere cannot exist if the flashpoint is significantly above the relevant maximum temperature of the flammable liquid. The lower the flashpoint, the larger may be the extent of the hazardous area. Some liquids (such as certain halogenated hydrocarbons) do not possess a flashpoint although they are capable of producing an explosive gas atmosphere; in these cases the equilibrium liquid temperature corresponding to saturated concentration at LEL should be compared with the relevant maximum liquid temperature. NOTE In special conditions the mist of a flammable or combustible liquid may be released at a temperature below the flashpoint and may therefore produce an explosive gas atmosphere.

(j)

Relative density. The horizontal extent of a hazardous area may increase with increasing relative density. Where the relative density is less than unity, the vertical extent may increase with a decrease in relative density. (k) Liquid temperature. The extent of a hazardous area may increase with increasing temperature of process liquid provided the temperature is above the flashpoint. It should be noted that the liquid or vapour temperature after the release may be increased or decreased by the ambient temperature or other factors (e.g. a hot surface). 6.3 Grading of source of release (see also Appendix A). Each item of process equipment (e.g. tank, pump, pipeline, vessel etc.) should be considered as a potential source of release of flammable material. If the item cannot contain flammable material it will clearly not give rise to a hazardous area around it. The same will apply if the item contains a flammable material but cannot release it to atmosphere (e.g. an all-welded pipeline is not a source of release). If it is established that the item may release flammable material to the atmosphere, (as will be the case with most items) it is necessary, first of all, to determine the grade of the source of release. By means of this procedure each item will be graded either ‘Continuous’, ‘Primary’, ‘Secondary’, or ‘Multi-grade’ . The source grade should be estimated by considering operational openings to atmosphere and the possibility of releases under all (normal and abnormal) operating conditions of the plant, installation or process.

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7 DETERMINATION OF THE TYPE AND EXTENT OF ZONES. 7.1 General. Having established the grade of the release source it is necessary to determine the release rate from the item and, furthermore, the other necessary factors which may influence the type and extent of the hazardous area, as mentioned in Clause 6.2 (and illustrated in Figs 5 and 6). It is rarely possible by cursory examination of a plant or plant design to decide which parts of the plant can be equated to the three zonal definitions (Zones 0, 1 and 2). A more objective approach is therefore necessary and involves the analysis of the basic possibility of an explosive gas atmosphere occurring. Since an explosive gas atmosphere can exist only if a flammable gas, vapour or mist is present with air, it is necessary to decide if any of these flammable materials can exist in the area concerned. Generally speaking such materials (and also flammable liquids and solids which may give rise to them) are contained within process equipment which may or may not provide a totally enclosed containment. To discover the cause of an explosive gas atmosphere existing in an area it is necessary therefore to determine how the items of process equipment containing flammable materials can release them to atmosphere. Once the likely frequency and duration of release (and hence the grade of source of release), the release rate, concentration, velocity, ventilation and other factors which affect the type of zone and/or extent of the hazardous area have been determined, there is then a firm basis on which to determine the likely presence of an explosive gas atmosphere in the surrounding areas. This approach therefore requires detailed consideration to be given to each item of process equipment which contains a flammable material, and which could therefore be a source of release. In some cases there may be other considerations (e.g. consequential damage) which may call for a different classification but these considerations are outside the scope of this Standard. It is often convenient to carry out an area classification study in tabular form. An example of such a table is shown in Fig. 2, which should also include details of apparatus group and temperature classification. When the extent of each zone surrounding each source of release has been determined, the composite area classification drawing can be produced. An example of such a drawing is shown in Fig. 3. The preferred way of indicating the zones is shown in Fig. 4. NOTE: Consideration should be given to the possibility that an explosive gas atmosphere may flow into areas in the soil below ground level such as cavities, pits and depressions.

7.2 Open air unrestricted ventilated situations. 7.2.1 Procedure. A procedure which may be adopted in determining the type and extent of zones around a particular process equipment item is illustrated in Fig. 1. NOTE: The following text is given in explanation of the steps in the figure and should be read in conjunction with it. The numbers which appear in the text are the box numbers in the figure.

7.2.2 Quantity of flammable material. It is first necessary to determine whether the process equipment item contains more than a specified minimum quantity of flammable material specified by relevant regulation or standard (e.g. AS 1940). If there is no relevant regulation or standard, refer to Fig. 1. (1 and 2). If it does not, the area around it will be non-hazardous (32). Should it contain more than the specified minimum it is necessary to determine (3) if release of the flammable material is possible (neglecting

AS 2430.1—1987

catastrophic events such as the bursting of process vessels, large pipelines or tanks). If it is not possible, the area around the process equipment item will be non-hazardous (32). Should release be possible the grade of source of release should be determined in accordance with the definitions in Appendix A. It should be noted that the grade of source of release is dependent only on the frequency and duration of release. 7.2.3 Sources of release graded ‘continuous’. Where the process equipment item is graded as ‘continuous’ (5) consideration should be given to improving the process equipment item to reduce the frequency and/or duration of release of the flammable material (6). Where significant improvement is possible the process equipment item may be regraded as a primary (18) or a secondary (27) grade source of release. Where such improvement is not possible the release rate, concentration, velocity, ventilation and the other factors which affect the type and extent of zone should be determined (7) and, where possible, modified so as to reduce the extent. When these factors have been modified as much as possible, the distance from the process equipment item to the point at which the vapour/air mixture is below the lower explosive limit* should be determined taking into account the release rate, concentration, velocity, ventilation and the other necessary factors, basing the distance on a chosen low wind speed (8) for the district concerned. The distance so determined is the extent of the Zone 0 from the source (9) (see Note at the end of Clause 7.2.5). It is then necessary to determine whether release from the same process equipment item as a primary grade source of release is likely (10). This is of consequence only if the release rate, concentration or manner of release is such as to result in a distance to the LEL greater than that in the continuous mode for the same chosen low wind speed. If this is the case the distance from the process equipment item to the lower explosive limit (12) should be determined. This distance is the extent of the Zone 1 from the source (13) (see Note at the end of Clause 7.2.5). Where the distance is not greater than in the continuous mode there will be no resultant Zone 1(11). Consideration should then be given to the likelihood of release from the same process equipment item as a secondary grade (14) source of release. This is of consequence only where the release rate, concentration or manner of release is such as to result in a distance to the LEL greater than that in the primary grade mode (12) for the same chosen low wind speed. If this is the case the distance to the lower explosive limit (16) should be determined. This distance is the extent of the Zone 2 from the source (17). Where the distance is not greater than in the primary grade mode there will be no resultant Zone 2 (15). An example of a process equipment item graded as continuous is an open vessel of volatile liquid which releases a small rate of vapour continuously (continuous grade) giving rise to a small Zone 0 (9) but where the liquid is subject to frequent rises in temperature which * Consideration should be given to a need for a safety factor on LEL (e.g. a value lower than LEL).

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cause it to release as a primary grade source at a greater rate giving rise to a larger Zone 1(13) and where infrequent spillage of the liquid causes it to act as a secondary grade source giving rise to a larger Zone 2 (17). The process equipment item is therefore behaving as a multigrade source of release giving rise to the three zonal classifications based on different release rates and the same chosen low wind speed (see Note at the end of Clause 7.2.5).

The following main types of ventilation are recognized: (a) Natural ventilation. (b) General artificial ventilation. (c) Local artificial ventilation.

7.2.4 Sources of release graded ‘primary’. Should the item be graded as primary (18), a procedure similar to that described above and illustrated in boxes (19) to (26) should be followed. An example of a process equipment item graded as primary is the seal of a pump handling a volatile liquid where the conditions are such that it releases a small rate of liquid or vapour frequently (primary grade) giving rise to a small Zone 1(22) but where failure of the pump seal occurs infrequently (secondary grade) releasing liquid or vapour at a higher rate thus giving rise to a larger Zone 2 (26). The process equipment item is therefore behaving as a multigrade source of release giving rise to two zonal classifications based on different release rates and the same chosen low wind speed (see Note at the end of Clause 7.2.5).

8.2 Natural ventilation. Examples are: (a) Open air situations typical of those in the chemical and petroleum industries which comprise open structures, pipe racks, pump bays and the like. (b) An open building which, having regard to the relative density of the gases and/or vapours involved, has openings in the walls and/or roof so dimensioned and located that the ventilation inside the building for the purpose of area classification, can be regarded as equivalent to that in an open air situation. (c) A building which is not an open building but which has natural ventilation (generally less than that of an open building) provided by permanent openings made for ventilation purposes.

7.2.5 Sources of release graded ‘secondary’. Should the process equipment item be graded secondary (27), a procedure similar to that described above and illustrated in boxes (28) to (31) should be followed. An example of a process equipment item graded as secondary is the seal of a pump or pipe flange where there is no frequent release but where failure of the seal or gasket will occur infrequently (secondary grade) giving rise to a Zone 2 (31) around the process equipment item based on the release rate and the same chosen low wind speed.

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NOTE: This explanation is based on the single low wind speed concept. Where it is considered that lower wind speeds prevail for sufficiently high percentages of the total time in a year, distances based on this lower wind speed may also be calculated. The distances so determined will give rise to zones of higher number but of greater extent than those using the original wind speed.

7.3 Situations other than those in the open air. A similar procedure to that described in Clause 7.2 may be used for determining the type and extent of zones in other than open air situations. It is not considered practical to include standard figures to cover these situations but it is suggested that for particular situations Fig. 1 can be used as a basis provided modifications are introduced to cover special features such as differing degrees of natural or artificial ventilation, and the presence of obstructions. Reference should be made to Clause 6.2 and Clause 8. 8 VENTILATION. 8.1 General. Gas or vapour released to the atmosphere can be diluted by dispersion or diffusion in the air until its concentration is below the lower explosive limit. Ventilation, that is air movement, will promote dispersion. The degree of ventilation, for example number of air changes per hour, may affect the type and/or extent of zones. The most important factor is that the design of ventilation is related to the source of release in order to achieve optimal ventilation conditions in the hazardous area. In considering the effect of ventilation, the relative density of the gases or vapours is important and should receive special consideration in determining the ventilation arrangements.

NOTE: It should be noted that the above terms are related to the type and not to the degree of ventilation.

It is also necessary to recognize: (d) No ventilation.

8.3 General artificial ventilation. Examples are: (a) A building which is provided with fans in the walls and/or in the roof to improve the general ventilation in the building. (b) An open air situation provided with suitably located fans to improve the general ventilation of the area. 8.4 Local artificial ventilation. Examples are: (a) An air/vapour extraction system applied to a process machine or vessel which continuously or periodically releases flammable vapour. (b) A forced or extract ventilation system applied to a small inadequately ventilated local area where it is expected that an explosive atmosphere may otherwise occur. 8.5 No ventilation. An example of an area with no ventilation is an enclosure or room having no permanent opening. NOTE: In a large building, the volume of which is large compared with the hazardous area around the source of release, that area need not necessarily be considered to have ‘no ventilation’.

8.6 Relationships between grades of source of release, ventilation and type and extent of zone. 8.6.1 Natural and general artificial ventilation. A continuous grade source of release may lead to a Zone 0, a primary grade to a Zone 1 and a secondary grade to a Zone 2. In some cases, however, the degree of ventilation may be so good that the extent of the zone may be so small as to be negligible or the zone may have a higher number or in the end may become non-hazardous. On the other hand, the degree of ventilation may be so poor that the zonal classification will have a larger extent and in some cases a lower zone number (see Clause 8.6.5). NOTE: For open air situations, the zone number and extent can be determined from Fig. 1.

8.6.2 Local artificial ventilation. Local artificial ventilation will usually be more effective than natural and general artificial ventilation in diluting explosive

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gas-air mixtures. As a consequence, the extent of the zones will be reduced and in some cases will be so small as to be negligible or the zone will have a higher number or may become non-hazardous (see Clause 8.6.5). 8.6.3 No ventilation. Where a source of release is within an area with no ventilation, a continuous grade will, and a primary grade may, lead to a Zone 0 and a secondary grade may lead to a Zone 1. Where there are special factors, for example very infrequent release and/or monitoring of the release, higher zone numbers may apply. 8.6.4 Ventilation restricted by obstacles. Some obstacles which exist in the area may impede the ventilation local to the obstacles and may thus require a larger extent and/or a lesser zone number in the local area. In considering the effect of obstacles, especially in the case of pits and pockets some of which may be inverted, particular attention should be paid to the relative density of the gases and vapours involved (see item(e) of Clause 6.2.2).

likely frequency and duration should be established together with the extent of spread of explosive gas atmosphere in the absence of artificial ventilation. This extent of spread may be greater (see Note) than that of the area classification extent already determined with the ventilation operating. The zone number(s) of the whole area under consideration with ventilation off will depend on the likely frequency and duration of ventilation failure and on the classification determined with the ventilation on. Where the ventilation is likely to fail infrequently and for short periods, the additional area (see Note) due to ventilation failure need have a zone number no less than 2. If provision is made to prevent release of flammable material when the ventilation has failed (e.g. by automatically closing down the process) the classification determined with the ventilation operating need not be modified. NOTE: The electrical installation in the additional area need not be suitable for the zone of this area if provision is made to switch off such electrical installation on ventilation failure.

9 PROPERTIES OF FLAMMABLE LIQUIDS, GASES AND VOLATILE SOLIDS. This standard does not contain information on the properties of flammable liquids, gases and volatile solids. However, information may be obtained from NFPA 325M.

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8.6.5 Consequences of artificial ventilation failure. The area classification should be established assuming that the ventilation is in operation since this will be the normal condition. The risk of ventilation failure should then be established. If the risk of failure is negligible (e.g. because an automatic independent stand-by system is provided) the area classification determined with the ventilation operating will not need modification. However should there be a risk of failure of the ventilation, the

AS 2430.1—1987

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NOTE: Copies of NFPA 325M are available for examination at SAA libraries in Sydney and Melbourne. Copies of NFPA 325M are also available for purchase from the SAA sales office in Sydney.

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APPENDIX A

SOURCES OF RELEASE A1 SCOPE. This Appendix describes the grades of sources of release and gives examples. A2 GRADES OF SOURCES OF RELEASE. A2.1 General. There are three basic grades of source of release as listed below in order of decreasing likelihood to the release occurring: (a) Continuous grade. (b) Primary grade. (c) Secondary grade. A2.2 Continuous grade source of release. A source which will release continuously or is expected to release for long periods or for short periods which occur frequently. A2.3 Primary grade source of release. A source which can be expected to release periodically or occasionally during normal operation. A2.4 Secondary grade source of release. A source which is not expected to release in normal operation and if it releases is likely to do so only infrequently and for short periods. NOTE: A source of release which is a combination of two or three of the above grades, is graded continuous or primary.

A source of release which is basically graded continuous may in addition be graded primary if the rate of release of flammable material, for the primary grade frequency and/or duration, exceeds that for the continuous grade. It may, additionally or alternatively to the primary grade, also be graded secondary if the rate of release of flammable material, for the secondary grade frequency and/or duration, exceeds that for the continuous and, if applicable, the primary grade. Similarly a source of release which is basically graded primary may in addition be graded secondary if the rate of release of flammable material for the secondary grade frequency and/or duration exceeds that for the primary grade. A3 EXAMPLES OF SOURCES OF RELEASE. A3.1 General. The following are examples of the grading of some sources of release. The examples are not intended to be rigidly applied and may need to be varied to suit particular equipment, situations and national practices.

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A3.2 Continuous grades of release. (a) The surface of a flammable liquid in a non-inerted fixed-roof tank. (b) The surface of a flammable liquid which is open to the atmosphere. (May apply to oil-water separators.) (c) Free vents and other openings which release flammable gas or vapour to atmosphere frequently or for long periods. A3.3 Primary grades of release. (a) Seals of pumps, compressors and valves if release during normal operation of the equipment is expected. (b) Process equipment water drains located on tanks containing flammable liquids which may release flammable material to atmosphere when draining off water during normal operation. (c) Sample points which are expected to release flammable material to atmosphere during normal operation. A3.4 Secondary grades of release. (a) Seals of pumps, compressors and valves where release during normal operation of the equipment is not expected to occur. (b) Flanges, connections and pipe fittings. (c) Relief valves, vents and other openings which are not expected to release flammable gas or vapour to atmosphere during normal operation. (d) Sample points which are not expected to release flammable material to atmosphere during normal operation. COPYRIGHT

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Fig. 1. THEORETICAL APPROACH TO THE DETERMINATION OF TYPE AND EXTENT OF ZONES FOR OPEN AIR UNRESTRICTED VENTILATION SITUATIONS

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AS 2430.1—1987

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12 Process Flammable temperature material and Location pressure

Process equipment item No.

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Source of release Ventilation Description

Grade

Horizontal distance from source to*

Code reference Boundary Boundary Boundary of Zone 0 of Zone 1 of Zone 2 Column 10 Column 11 Column 12 Column 13 — — —m XYZ National Code, page XY

Column 1 Column 2 C 52 Hydrogen vessel

Column 3 Column 4 Area 2 in Hydrogen open air

Column 5 30°C 2 500 kPa

Column 6 Closed system with valves, relief valve to flare

Column 7 Natural (open air)

Column 8 Flanges and valve seals (see col. 14)

Column 9 Secondary

J 29

Area 5 in Xylene open air

80°C 300 kPa

Closed system with valves and drains. Mechanical seals and throttle bush

Natural (open air)

Flanges and valve seals (see col. 14)

Secondary

—

—

—m

XYZ National Code, page XX

Mechanical seal (see col. 14)

Primary/ secondary (multigrade)

—

—m

—m

XYZ National Code, page YX

Flanges, glands and valve seals (see col. 14) Vents and drain points (see col. 14)

Secondary

—

—

—m

XYZ National Code, page XZ

Primary/ secondary (multigrade)

—

—m

—m

XYZ National Code, page ZX

Tank vent (see col. 14)

Continuous/ primary/ secondary (multigrade)

—m

—m

XYZ National Code, page XZ

J 94

J 32

*

Description

Description of flammable material containment

Xylene pump

Ethylene compressor (reciprocating)

Fixed roof tank

Area 4 in Ethylene open building

Area 3 in Gasoline open air

70°C 2 000 kPa

Ambient

Closed system with glands, vents and cooler drain points

Closed system except for pressure/vacuum valve

Natural (equivalent to open air)

Natural (open air)

Zone 0 in vapour space

Vertical distances may also be recorded.

NOTE: Other information such as temperature class and apparatus group may need to be included.

Fig. 2 EXAMPLE OF TABLE FOR USE DURING AREA CLASSIFICATION STUDY

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Remarks Column 14 Release due to flange gasket or valve seal failures (abnormal) Release due to flange gasket or valve seal failures (abnormal) Small release in normal operation. Larger release on seal failure (abnormal) Releases due to flange gasket, gland or valve seal failures (abnormal) Small releases in normal operation. Larger release may occur due to incorrect operation (abnormal) Vapour vented during normal filling. Possibility of overfilling (abnormal)

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NOTES: 1. All dimensions are measured from sources of release. 2. In some cases it may be more practical to establish the actual boundary of a classified area at more convenient geographical limits. 3. Details to be added where necessary regarding: (a) Localized Zones 0 or 1. (b) Vertical distances of Zones 0, 1 and 2. In some cases it will be necessary to draw sections to clarify vertical distances. (c) The title of the code which has been used as a basis for extent of areas. (d) Apparatus group and temperature class for selection of electrical apparatus.

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Symbol A B C E F G H J K L N M

Description Control room, pressurized Switch house, pressurized Workshop Vessel Distillation column Drain Pump (seals unlikely to release in normal operation) Pump (seals likely to release in normal operation) Pump (seals likely to release in normal operation) Reciprocating compressor Compressor house (open building) Vents (high or low level)

NOTE: The above table is given for explanation of this drawing only. It will not appear on the completed area classification drawing.

Fig. 3. EXAMPLE OF AN AREA CLASSIFICATION DRAWING

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Fig. 4. PREFERRED SYMBOLS FOR THE ZONES OF A HAZARDOUS AREA

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NOTES: 1. The diagrams relate to— (a) open air situations; and (b) sources of release near to ground level. 2. The shapes and sizes of the zones will depend on many factors (see Clause 6.2.2) 3. Zone 1

Zone 2

Fig. 5. EXAMPLE ILLUSTRATING AREA CLASSIFICATION AROUND VARIOUS SOURCES OF RELEASE FOR HEAVIER-THAN-AIR GASES OR VAPOURS

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AS 2430.1—1987

NOTES: 1. The diagrams relate to— (a) open air situations; (b) sources of release above ground level; and. (c) gases which are significantly lighter than air. 2. The shapes and sizes of the zones will depend on many factors (see Clause 6.2.2) 3. Zone 1

Zone 2

Fig. 6. EXAMPLE ILLUSTRATING AREA CLASSIFICATION AROUND VARIOUS SOURCES OF RELEASE FOR LIGHTER-THAN-AIR GASES

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