IS 5572 : 1994(Reaffirmed 1999)

Edition 3.1(2000-03)

Indian Standard

CLASSIFICATION OF HAZARDOUS AREAS (OTHER THAN MINES) HAVING

FLAMMABLE GASES AND VAPOURS FOR ELECTRICAL INSTALLATION

( Second Revision )(Incorporating Amendment No. 1)

UDC 621.316.17 : 614.87

© BIS 2002

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Price Group 8

Electrical Apparatus for Explosive Atmospheres Sectional Committee, ET 22

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized bythe Electrical Apparatus for Explosive Atmospheres Sectional Committee had been approved bythe Electrotechnical Division Council.When electrical equipment is to be installed in or about a hazardous area, it is frequently possibleby care in the layout of the installation to locate much of the equipment in less hazardous or non-hazardous areas, and thus reduce the amount of special equipment required. This standard hasbeen drawn up keeping this as a primary objective.The classification and delineation of any particular hazardous areas shall be based on availableinformation concerning materials and processes to be used, including such evidence as may besupplied by the consumer, and shall take into account factors, such as height, ventilation,standards of maintenance, type of apparatus, operation and competent personnel available forinspection, which may affect the nature and extent of the hazard. Each room, section of area shallbe considered individually in determining its classification and also in relation to others.The classification of areas has been done according to the extent of risk involved so that it providesa guideline to the choice of equipment to be installed in different areas on the basis of the extent ofhazard.This standard was originally proposed to be brought out in following two parts:

Part 1 Areas having flammable gases and vapoursPart 2 Dust laden atmospheres

However in view of separate standards now being brought out for electrical apparatus for use inpresence of combustible dusts, Part 2 of this standard which was envisaged earlier is no longerrequired. Therefore this standard is not being published in parts, and is intended to cover onlychemical industries, hydrocarbon industries and other similar areas where flammable vapours areencountered.Flammable gases and vapours may either be heavier or lighter than air. The area classification inthe original standard was based on the assumption that the flammable vapours encountered wereheavier than air. In the first revision both the possibilities were taken care of in the classificationof areas. This revision takes into account the latest technological developments in the field ofclassification of hazardous areas at the international level. Some typical area classification detailshave also been included in this revision.This standard includes generalized statements and recommendations on matters on which thereare diverse opinions. It is, therefore, important that sound engineering judgement take precedenceover a literal interpretation of text. Good judgement should be exercised without jeopardizing therequirements laid down in this standard.In preparation of this standard assistance has been derived from IEC Pub 79-10 (1986) ‘Electricalapparatus for explosive gas atmospheres — Part 10: Classification of hazardous areas’, issued bythe International Electrotechnical Commission (IEC).This edition 3.1 incorporates Amendment No. 1 (March 2000). Side bar indicates modification ofthe text as the result of incorporation of the amendment.

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Indian Standard

CLASSIFICATION OF HAZARDOUS AREAS (OTHER THAN MINES) HAVING

FLAMMABLE GASES AND VAPOURS FOR ELECTRICAL INSTALLATION

( Second Revision )1 SCOPEThis standard provides guidance on theclassification of areas where flammable gas orvapour risks may arise in order to permit theproper selection of electrical apparatus for usein such areas ( see Note 1 ).It is intended for application in all industrieswhere there may be a risk due to the presenceof flammable gas or vapour, mixed with airunder normal atmospheric conditions ( seeNote 2 ), but does not apply to:

a) mining applications;b) processing and manufacture of explosives;c) areas where risks may arise due to the

presence of ignitable dusts or fibres;d) catastrophic failures, which are beyond

the concept of abnormality dealt with inthis standard ( see Note 4 ); and

e) ignition sources other than thoseassociated with electrical apparatus( see Note 5 ).

f) areas where pyroforic substances arehandled.

This standard does not take into account theeffects of consequential damage.Definitions and explanations of terms are giventogether with the main principles andprocedures relating to area classification.The objective of area classification is thenotional division of a plant into zones withinwhich the likelihood of the existence of anexplosive gas/air mixture is judged to be high,medium, low or so low as to be regarded asnegligible. An area classification established inthis way provides a basis for the selection ofelectrical apparatus that is protected to adegree appropriate to the risk involved. Thetype of protection of the apparatus selected willbe such that the likelihood of it being a sourceof ignition, at the same time as the surroundingatmosphere is explosive, is accepted as beingsmall.

NOTES1 Flammable materials for the purpose of areaclassification include:

a) Petroleum having flash point below 65°C or anyflammable gas or vapour in a concentrationcapable of ignition.

b) Petroleum or any flammable liquid having flashpoint above 65°C where likely to be refined,blended, handled or stored at or above its flashpoint.

2 For the purposes of this standard an area is a three-dimensional region or space.3 Normal atmospheric conditions include variationsabove and below reference levels of 101.3 kPa (1 013mbar) and 20°C provided the variations have anegligible effect on the explosion properties of theflammable materials.4 Catastrophic failure in this context is applied, forexample, to the rupture of a process vessel or pipeline.5 In any plant installation irrespective of size there maybe numerous sources of ignition apart from thoseassociated with electrical apparatus. Additionalprecautions may be necessary to ensure safety in thisaspect but these are outside the scope of this standard.

2 TERMINOLOGY2.1 Explosive Gas AtmosphereA mixture with air, under normal atmosphericconditions, of flammable materials in the formof gas, vapour, or mist, in which, after ignition,combustion spreads througout the unconsumedmixture.

NOTES1 This definition specifically excludes dusts and fibresin suspension in air.2 Although a mixture which has a concentration abovethe upper explosive limit (UEL) is not an explosive gasatmosphere, in certain cases for area classificationpurposes it is advisable to consider it as an explosive gasatmosphere.

2.2 Hazardous AreaAn area in which an explosive gas atmosphereis present, or likely to be present, in quantitiessuch as to require special precautions for theconstruction, installation and use of electricalapparatus.2.3 Non-hazardous AreaAn area in which an explosive gas atmosphereis not expected to be present in quantities suchas to require special precautions for theconstruction, installation and use of electricalapparatus.2.4 ZonesHazardous areas are classified in zones basedupon the frequency of the appearance and theduration of an explosive gas atmosphere asfollows.

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2.4.1 Zone 0

An area in which an explosive gas atmosphereis present continuously or is present for longperiods.

2.4.2 Zone 1

An area in which an explosive gas atmosphereis likely to occur in normal operation.

2.4.3 Zone 2

An area in which an explosive gas atmosphereis not likely to occur in normal operation and ifit does occur it will exist for a short period only.

2.5 Normal Operation

The situation when the plant equipment isoperating within its design parameters.

Minor releases of flammable material may bepart of normal operation. For example, releasesfrom seals which rely on wetting by the fluidbeing pumped are considered to be minorreleases.

Failures (such as the breakdown of pump seals,flange gaskets or spillages caused by accidents)which involve repair or shut-down are notconsidered to be part of normal operation.

2.6 Explosive Limits

2.6.1 Lower Explosive Limit ( LEL )

The concentration of flammable gas, vapour ormist in air, below which an explosive gasatmosphere will not be formed.

2.6.2 Upper Explosive Limit ( UEL )

The concentration of flammable gas, vapour ormist in air, above which an explosive gasatmosphere will not be formed.

2.7 Relative Density of a Gas or a Vapour

The density of a gas or a vapour relative to thedensity of air at the same pressure and at thesame temperature. (Air is equal to 1.0.)

2.8 Flammable Material

Material consisting of flammable gas, vapour,liquid and/or mist, see 5.

2.9 Flammable Gas or Vapour

Gas or vapour which, when mixed with air incertain proportions, will form an explosive gasatmosphere.

2.10 Flammable Liquid

A liquid capable of producing a flammablevapour or mist under any forseeable operatingconditions.

2.11 Flammable Mist

Droplets of flammable liquid, dispersed in air,so as to form an explosive atmosphere.

2.12 Flash Point

The temperature at which the liquid gives somuch vapour that this vapour, when mixedwith air, forms an ignitable mixture and gives amonetary flash on application of a small pilotflame under specified conditions of test.2.13 Boiling Point

The temperature of a liquid boiling at anambient pressure of 101.3 kPa (1 013 mbar).

NOTE — For liquid mixtures the initial boiling pointshould be used.

‘Initial boiling point’ is used for liquid mixtures toindicate the lowest value of the boiling point for therange of liquids present.

2.14 Ignition Temperature

The lowest temperature at which ignitionoccurs in a mixture of explosive gas and airwhen the method of testing ignitiontemperatures specified in relevant IndianStandard is followed.2.15 Source of Release

A source of release is a point or location fromwhich a gas, vapour, mist or liquid may bereleased into the atmosphere so that ahazardous atmosphere could be formed.2.16 Adequate Ventilation

Adequate ventilation is that which is sufficientto prevent accumulations of significantquantities of gas-air mixtures in concentrationover one-fourth of the lower flammable limit.Adequately ventilated area could be naturallyventilated or artificially ventilated.2.17 Protected Fired Vessel

Any fired vessel that is provided withequipment, such as flame arrestors, stacktemperatures shutdowns, forced draft burnerswith safety controls and spark arrestors,designed to eliminate the air intake andexhaust as sources of ignition.2.18 Pressurized Room

A room which has been made safe bypressurizing or purging with a plenum of safeatmosphere by keeping minimum 25 Pa morepressure than that of surrounding atmospherewith all doors and windows closed.

3 GENERAL

Area classification is a method of analyzing andclassifying the environment where explosivegas atmospheres may occur to allow the properselection of electrical apparatus to be installedin that environment.

The object of the classification procedure is toenable electrical apparatus to be operatedsafely in these environments.

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Where it is necessary to use electricalapparatus in an environment in which theremay be an explosive gas atmosphere and it isnot possible to:

a) eliminate the likelihood of an explosivegas atmosphere occurring around thesource of ignition, or

b) eliminate the source of ignition;then measures should aim at reducing thelikelihood of the occurrence of either or both ofthe above factors so that the likelihood ofcoincidence is so small as to be acceptable.In most practical situations where flammablematerials are used it is difficult to ensure thatan explosive gas atmosphere will never occur. Itmay also be difficult to ensure that electricalapparatus will never give rise to a source ofignition. Therefore in situations where anexplosive gas atmosphere has a high likelihoodof occurring, reliance is placed on usingelectrical apparatus which has an extremelylow likelihood of creating a source of ignition.Conversely where the likelihood of an explosivegas atmosphere occurring is reduced, electricalapparatus which has an increased likelihood ofbecoming a source of ignition may be used. Toapply this approach the first step is to assessthe likelihood of an explosive gas atmosphereoccurring in accordance with the definitions ofZone 0, Zone 1 and Zone 2. The followingclauses give guidance of this first step, namelyon the classification of areas in which theremay be an explosive gas atmosphere intoZones 0, 1 and 2.

4 CLASSIFICATION OF HAZARDOUS AREA4.1 To determine the type of electricalinstallation appropriate to a particularsituation, the hazardous areas have beendivided into three zones, namely, Zone 0,Zone 1 and Zone 2, according to the degree ofprobability of the presence of hazardousatmosphere. Typical examples of suchclassifications are given below.4.1.1 Zone 0 AreasExamples are vapour space above closedprocess vessels, storage tanks or closedcontainers, areas containing open tanks ofvolatile, flammable liquid.4.1.2 Zone 1 AreasZone 1 locations may be distinguished whenany of the following conditions exits:

— Flammable gas or vapour concentration islikely to exist in the air under normaloperating conditions.

— Flammable atmospheric concentration islikely to occur frequently because ofmaintenance, repairs or leakage.

— Failure of process, storage or otherequipment is likely to cause an electricalsystem failure simultaneously with therelease of flammable gas or liquid.

— Flammable liquid or vapour pipingsystem (containing valves, meters orscrewed or flanged fittings) is in aninadequately ventilated area.

— The area below the surrounding elevationor grade is such that flammable liquids orvapours may accumulate therein.

This classification typically includes:— Imperfectly fitting peripheral seals on

floating roof tanks.— Inadequately ventilated pump rooms for

flammable gas or for volatile, flammableliquids.

— Interiors of refrigerators and freezers inwhich volatile flammable materials arestored in lightly stoppered or easilyruptured containers.

— API separators.— Oily waste water sewer/basins.

4.1.3 Zone 2 AreasZone 2 locations may be distinguished whenany one of the following conditions exist:

— The system handling flammable liquid orvapour is in an adequately ventilated areaand is so designed and operated that theexplosive or ignitable liquids, vapours orgases will normally be confined withinclosed containers or closed systems fromwhich they can escape only duringabnormal conditions such as accidentalfailure of a gasket or packing.

— The flammable vapours can be conductedto the location as through trenches, pipesor ducts.

— Locations adjacent to Zone 1 areas.— In case of use of positive mechanical

ventilation, as the failure or abnormaloperation of ventilating equipment canpermit atmospheric vapour mixtures tobuild up to flammable concentrations.

4.2 Areas not Classified

In general, the following locations areconsidered safe from the point of view ofelectrical installation:

a) Areas where the piping system is withoutvalves, fittings, flanges or similarappurtenances.

b) Areas where flammable liquids or vapoursare transported only in suitablecontainers or vessels.

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c) Areas where permanent ignition sourcesare present like area where combustiongases are present, for example flare tips,flare pits, other open flames and hotsurfaces.

d) Enclosed premises in which a plenum orpurging stream of safe atmosphere iscontinuously maintained, so that noopening therein may be a point of ingressof gases or vapours coming from anexternal source of hazard.

e) Gas turbine installation meetingrequirements of Annex A.

f) Diesel Generator room/shed havingadequate ventilation.

g) Oil/gas fired boilers installations.Consideration should be given, however,to potential leak sources in pumps, valves,etc, or in waste product and fuel linesfeeding flame or heat producingequipment to avoid installing electricaldevices which could then become primaryignition sources for such leaks.

NOTES1 A protected fired vessel is not considered a source ofignition and the surrounding area is classified the sameas for a hydrocarbon pressure vessel.2 The area around the fired components and exhaustoutlets of unprotected fired vessels need not beclassified from the standpoint of installation of electricalequipment.3 Lack of classification around unprotected fired vesselsdoes not imply the safe placement of fired vessels in theproximity to other production equipment becauseunprotected fired vessels are themselves sources ofignition.4 The area around a flare tip or flare pit need not beclassified from the stand point of installation ofelectrical equipment.5 For example, electrical equipment may be exposed toflammable gas during a purge cycle of a fired heater orfurnace. The lack of classification around unprotectedfired vessels and flare tips does not imply the safeplacement of fired vessels and flare tips in the proximityto other equipment because unprotected fired vesselsand flare tips are themselves sources of ignition.

5 PROPERTIES OF FLAMMABLE MATERIAL5.1 Flammable substances, the potentialrelease of which shall be considered in areaclassification for electrical installations, includenon-liquefiable gases (permanent gases or fixedgases), liquefied petroleum gases and vapoursof flammable liquids.5.1.1 Non-liquefiable gases commonlyencountered include methane and its mixtureswith small quantities of thelow-molecular-weight hydro-carbons, themixtures being generally lighter than air.Hydrogen, because of its properties, shall begiven special consideration.5.1.1.1 Non-liquefiable gases released from anopening of given size will dissipate rapidly

because of their low relative density and willnot usually affect as wide an area as theliquefied petroleum gases.5.1.2 Liquefied petroleum gases includepropane, butane and mixtures thereof havingdensities from 1.5 to approximately 2.0 timesthat of air. Vapour pressures exceed2.81 kg/cm2 at 37.8°C.5.1.2.1 These gases in their liquefied state arehighly volatile and have low boilingtemperature so that they readily pick up heatcreating large volumes of vapour. They shouldbe treated very conservatively in consideringthe extent of areas affected, especially since theheavy vapours travel along the ground for longdistances if air currents do not assist diffusion.5.1.3 Flammable liquids vary in volatility andhave a flash point below 93°C and a vapourpressure not exceeding 2.81 kg/cm2 at 37.8°C.These are divided into three classes, as follows:

Densities of the saturated vapours of theseflammable liquids at ordinary atmospherictemperatures are generally less than 1.5 timesthat of air.5.1.3.1 Class A liquids may produce largevolumes of vapour when released inappreciable quantities to the open.5.1.3.2 Class B liquids are heavier and lessvolatile than gasoline, but have flash point ator slightly below normal ambient airtemperatures. Few commercial products are inthis class, although in a refinery some stocks inthe process of refining will be of Class B. Atnormal storage temperature such oils releasevapour slowly and are hazardous only near thesurface of the liquid. At elevated temperaturesClass B liquids approach the characteristics ofClass A liquids in vapour release.5.1.3.3 Class C includes a broad range fromcleaners’ solvent to heavy fuel oil in commercialgrades. The degree of hazard is low because therate of vapour release is nil at normal ambienttemperatures of handling and storage. Whenvapours from heated Class C products inprocess are released to the atmosphere, thechance of ignition by electrical equipment is notas great as in case of Class A or Class B liquidsbecause vapour either condense rapidly orignite spontaneously.5.1.3.4 Normally, Class A and Class B liquidswill produce vapours considered to be in the

Class A Flammable liquids having flashpoint below 23°C

Class B Flammable liquids having flashpoint 23°C and above butbelow 65°C

Class C Flammable liquids having flashpoint 65°C and above butbelow 93°C

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flammable range for electrical design purposes.Class C liquids having a flash point below 93°Cshould be considered as producing flammablevapours when handled, processed or storedunder such conditions that the temperature ofthe liquid, when released to the atmosphere,would exceed its flash point.

6 EXTENT OF HAZARDOUS AREA

6.1 General Consideration

6.1.1 In the absence of wall, enclosures, orother barriers, and in the absence of aircurrents or similar disturbing forces, gases andvapours disperse in all directions as governedby the vapour density and velocity, for example,heavier-than-air vapour disperse principallydownward and outward and lighter-than-airvapours principally upward and outward. Thus,if the source of hazard were a single point, thehorizontal area covered by the vapour would bea circle.6.1.2 For vapours released at or near groundlevel, the areas where potentially hazardousconcentrations are most likely to be found arebelow ground, those at ground are next mostlikely, and, as the height above groundincreases, the potential hazard decreases.

NOTE — For lighter-than-air gases the opposite is true,there being little or no potential hazard at and belowground and greater potential hazard above ground.

6.1.3 Elevated or depressed sources of releasewill alter the areas of potential hazard.6.1.4 Effect of Air CurrentAir currents may substantially alter the outlineof the limits of potential hazard. A very mildbreeze may serve to extend the area in thosedirections to which vapours might normally becarried. However, a stronger breeze may soaccelerate the dispersion of vapours that theextent of potentially hazardous area would begreatly reduced.6.2 Heavier-than-Air Gases and Vapours

6.2.1 Open-Air Situationsa) Zone 2 classification Figures 1 and 2

illustrate the situation when a source ofhazard which may give rise to a hazardousatmosphere only under abnormalconditions is located in the open air. Thehazardous area should in this case beclassified as Zone 2. For heavier-than-airgases and vapours the classifiedhazardous area shall extend vertically 8 mabove the source of hazard andhorizontally 16 m in all direction from thesource of hazard. Beyond 8 m from thesource of the hazard in the horizontalplane the vertical extent of the Zone 2area may be reduced to 8 m above groundlevel.

If there is a possibility of large release ofvolatile products, the Zone 2 area may befurther extended horizontally beyond 16m up to 32 m with a height of 0.63 m.In case of petroleum pipelines where well-maintained valves, fittings, and meters ofa pipeline system transporting petroleum(crude oil, products, and gases) areinstalled in well-ventilated situations orin a pit, the extent of the Zone 2 areaabove ground may be reduced to 4 m in alldirections from the possible source ofhazard, although the pit itself should beclassified as Zone 1 area.

NOTE — Any trench of pit below ground level andlocated within the area defined above should beclassified as Zone 1 area.

b) Zone 1 classification If the source ofhazard gives rise to a hazardousatmosphere under normal operatingconditions, the area described in 6.2.1 (a)as Zone 2 should be classified as Zone 1.In the event of an enclosed premises notcontaining a source of hazard but situatedwithin either a Zone 1 or Zone 2 area theinside of the premises should be classifiedas Zone 1 unless separated from theoutside hazardous area by a fire wall.

6.2.2 Enclosed Premises and SurroundingAreas

a) Zone 2 classification Figure 3 illustratesthe situation when a source of hazardwhich may give rise to a hazardousatmosphere under abnormal conditions islocated within enclosed premises. Thewhole of the inside of the building shouldbe classified as Zone 1, as rapid dispersalof hazardous atmosphere may not beexpected due to lack of ventilation.The Zone 2 area shall extend 16 mhorizontally from the source of hazard and8 m vertically from the ground level. If thearea covered above does not contain thearea 3 m past the perimeter of thebuilding, the Zone 2 area shall beextended in such a way that this area iscovered. In case of unpierced vapour-tightwalls, the area 3 m past perimeter neednot be considered within Zone 2 area.If there is a possibility of large release ofvolatile products, Zone 2 area may befurther extended horizontally beyond16 m up to 32 m with height of 0.63 m.

NOTE — Any trench or pit below ground level andlocated within the area defined should be classified asZone 1 area.

b) Zone 1 classification If the source ofhazard within the enclosed premises gives

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rise to hazardous atmosphere under normalconditions, the area within the buildingshould be classified as Zone 1 and the areadescribed in 6.2.2 (a) as Zone 2 should beclassified as Zone 1.

When the building has a ridge type roof withventilators and not a sealed roof as shown inFig. 3, special consideration is necessary inconnection with the vertical extent of thehazardous area.

6.2.3 Storage Tanks

Figure 4 illustrates the classification of thearea surrounding a floating-roof tank undernormal operating conditions. The space above

the roof and within the shell or the tank shouldbe classified as Zone 1.The area surrounding the tank should beclassified as Zone 2. It shall extend vertically3 m above the tank or shall be horizontally 3 mfrom it. If there is a dike on one side of thetank, the Zone 2 area should extend up to thedike, the vertical extension from the groundlevel being the same as the height of the dike.

In the case of fixed-roof tanks, the areasurrounding the tank shown in Fig. 4 as Zone 2should be classified as Zone 1. However, in caseof fixed roof tanks which breathe into closedsystem and not to atmosphere (during

FIG. 1 FREELY VENTILATED PROCESS AREA (SOURCE OF HAZARD LOCATED NEAR GROUND LEVEL)

FIG. 2 FREELY VENTILATED PROCESS AREA (SOURCE OF HAZARD LOCATED ABOVE GROUND LEVEL)

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filling/emptying) area surrounding the tankshall be classified as Zone 2 as shown in Fig. 4.

NOTE — Any trench or pit below ground level andlocated within the area defined should be classified as

Zone 1 area.

6.2.4 Figures 5 to 11 give some typical areaclassification details.

FIG. 3 PROCESS AREA WITH RESTRICTED VENTILATION

FIG. 4 TANK WITH FLOATING ROOF

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FIG. 5 PRESSURE STORAGE TANK

FIG. 6 COOLING TOWER-HANDLING PROCESS COOLING WATER

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FIG. 7A TANK CAR/TANK TRUCK LOADING AND UNLOADING VIACLOSED SYSTEM. BOTTOM PRODUCT TRANSFER ONLY

FIG. 7B TANK CAR/TANK TRUCK LOADING AND UNLOADING VIAOPEN SYSTEM. BOTTOM PRODUCT TRANSFER ONLY

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FIG. 8A TANK CAR/TANK TRUCK LOADING AND UNLOADING VIACLOSED SYSTEM. TRANSFER THROUGH DOME ONLY

FIG. 8B TANK CAR/TANK TRUCK LOADING AND UNLOADING VIACLOSED SYSTEM. TRANSFER THROUGH DOME ONLY

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FIG. 9 TYPICAL DRUM DISPENSING INSTALLATION

FIG. 10 TYPICAL VENT INSTALLATION

FIG. 11 BALL OR PIG LAUNCHING OR RECEIVING INSTALLATION INA NON-ENCLOSED ADEQUATELY VENTILATED AREA

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6.3 Lighter-than-Air Gases and Vapours

6.3.1 Definition of Lighter-than-Air Gases andVapoursWhere a substantial volume of gas or vapour isreleased into the atmosphere from a localizedsource, a vapour density less than one, that is,lighter-than-air, for the combustible indicatesthe gas or vapour will rise in a comparativelystill atmosphere. A vapour density greater thanone, that is heavier-than-air, indicates the gasor vapour will tend to sink, and may therebyspread some distance horizontally and at a lowlevel. The latter effects will increase withcompounds of greater vapour density.In process industries, the boundary betweencompounds which may be consideredlighter-than-air is set at a vapour density of0.75. This limit is chosen so as to provide afactor of safety for these compounds whosedensities are close to that of air, and wheremovement may not therefore, be predictedwithout a detailed assessment of localconditions.6.3.2 Basic Extent of Hazardous Area for aSource of Hazard Located in Open Air6.3.2.1 The hazardous area surrounding a pointof hazard located in the air is a cylinder withradius 4.5 m whose axis passes verticallythrough the source of hazard. Its height is 8 mabove and 4.5 m below the source of hazard. Ifthe source of hazard is situated at a height lessthan 4.5 m above ground, the height of thehazardous area extends below the source ofhazard up to ground level. This is illustrated inFig. 12. The entire area is Zone 2.6.3.2.2 Source of hazard located inside enclosedpremises with restricted ventilation — Openingson top and bottomIn this case the entire area within the enclosedpremises is a Zone 1 area and the extremities ofthe openings are considered as source ofhazard. The hazardous areas are shown inFig. 13.6.3.2.3 Source of hazard located inside enclosedpremises with restricted ventilation — Openingson sidesIn this case the entire area within the enclosedpremises is a Zone 1 area and the extremities ofthe openings are considered as sources of

hazard. The hazardous areas are shown inFig. 14.6.3.2.4 Source of hazard located outsideenclosure with one side openIn this case the area within the enclosedpremises above the opening is Zone 1 whereasthe rest of the enclosed premises form a Zone 2area. The area outside the enclosure isclassified as stated in 6.3.2.1. The hazardousarea is shown in Fig. 15.

FIG. 12 POINT OF HAZARD IN THE AIR

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FIG. 13 SOURCE OF HAZARD LOCATED INSIDE ENCLOSED PREMISES WITH RESTRICTED VENTILATION (OPENINGS ON TOP AND BOTTOM)

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FIG. 14 SOURCE OF HAZARD LOCATED INSIDE ENCLOSED PREMISES WITH RESTRICTED VENTILATION (OPENINGS ON SIDES)

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FIG. 15 SOURCE OF HAZARD LOCATED OUTSIDE THE ENCLOSURE WITH ONE SIDE OPEN

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ANNEX A[ Clause 4.2(e) ]

GAS TURBINE INSTALLATIONS

A-1 INTRODUCTION

A-1.1 This Annex applies to gas fired turbineinstallations.A-1.2 If the turbine is equipped with anacoustic hood or other enclosure containingparts of the fuel gas system, this enclosureshould be defined as the turbine hood.A-1.3 If the turbine, with or without a turbinehood, is located in an enclosed area, this areashould be defined as the turbine room.

A-2 AREA CLASSIFICATION OF THE TURBINE ROOM

A-2.1 In order to classify the turbine room asnon-hazardous, the following requirementsshould both be fulfilled:

a) The turbine room should be adequatelyventilated that is at least 12 air changesper hour with proper ventilation patterns.The ventilation system should bearranged so that an over-pressure of atleast 50 Pa (0.5 mbar) is maintained in theturbine room with respect to the inside ofthe turbine hood and any surroundingclassified areas with openings to theturbine room. A pressure switch should beinstalled in order to give an alarm if thedifferential pressure drops below 50 Pa(0.5 mbar).

b) The fuel gas pipe to each turbine hoodshould have no more than one pair offlanges inside the turbine room. All otherequipment as valves, connections, filters,drip pot, etc, have to be located either:— outside the turbine room— inside an enclosure separately

ventilated— inside the turbine hood provided a

special ventilation of turbine hood— inside turbine hood, provided a special

fuel gas supply arrangement asdescribed in A-4

A-2.2 The turbine room may be classified asZone 1 or Zone 2 if the arrangement is not incompliance with the requirements statedin A-2.1 or due to other sources of hazardoutside the turbine hood. The turbine or anyassociated equipment including exhaust piping,should not have a surface temperature above200°C or above 80 percent of the ignitiontemperature for the actual gas/air mixture inthe classified area without special precautions.

A-3 VENTILATION OF THE TURBINE ROOM

A-3.1 The turbine hood for a gas fired turbineshould be adequately ventilated with respect to

FIG. 16 GAS TURBINE ARRANGEMENT

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the removal of heat from the machinery anddilution of flammable gas. The air should betaken from non-hazardous area.A-3.2 If the area outside the turbine hood isclassified as non-hazardous, the ventilationsystem should be arranged so that anunder-pressure of at least 50 Pa (0.5 mbar) ismaintained inside the turbine hood withrespect to the outside. This differentialpressure may be the combined effect of theunder-pressure inside the turbine hood and theover-pressure in the turbine room.A-3.3 If the area outside the turbine hood isZone 2 and the turbine hood contains anysource of ignition such as a surface withtemperature above 200°C, or above 80 percentof the ignition temperature for the actualgas/air mixture the ventilation system of theturbine hood should be arranged so that anover-pressure of at least 50 Pa (0.5 mbar) ismaintained inside the hood with respect to theoutside.A-3.4 In both situations described above apressure switch should be installed in order togive an alarm and shutdown after time delay ifthe differential pressure drops below 50 Pa(0.5 mbar).A-3.5 The number of leakage sources under theturbine hood should be kept to a minimum.However, a manufacturer may require someleak-prone equipment to be located inside theturbine hood. The number of air changesrequired depends upon the probable sources ofleakage, the surface temperature of themachine, etc. Examples are given in A-4.A-3.6 As an alternative to ventilation of theturbine-hood during shutdown of the turbine,Halon may be injected.A-3.7 Provided electrical equipment inside thehood which does not meet zone requirement,the turbine hood should be pre-purged with atleast 5 air changes before starting the turbineor energizing any electrical equipment notsuitable for Zone 1 area.A-3.8 The fan used for pre-purging should meetZone 1 and should be equipped with a startersuitable for Zone 1 or a starter located in anarea remaining non-hazardous duringshutdown.

A-4 AREA CLASSIFICATION OF THE TURBINE HOODA-4.1 The combination of ventilation, fuel gassystem arrangement, temperature on exposedsurfaces, electrical equipment inside theturbine hood, etc, should be considered toevaluate the safety of the turbine hood. Thesafety principles will be elucidated by some ofthe most common turbine/turbine hood designs.A-4.2 No Exposed Surface of the TurbineInside the Hood will have a TemperatureAbove 200°C During OperationProvided the ventilation system provides atleast 12 air changes per hour, the hood shouldbe considered as adequately ventilated. The

area inside the hood will be regarded as zone 2area and accordingly all equipment inside thehood have to meet Zone 2 requirement. Theequipment which has to be alive after a shutdown or stop of ventilation of the hood, shouldmeet Zone 1 requirement. This for instanceapplies to trace heater, post lubrication pumps,etc. The post lubrication pumps should besupplied from emergency power sources tooperate after a shutdown in order to preventoverheating of the bearings. Overheating mayignite flammable vapour or gas inside the hood.A-4.3 The Turbine has Exposed Surfaceswith Temperatures Above 200°CIf the actual flammable gas ignitiontemperature can be tested and a statement canbe made that the surface temperature of theturbine will not exceed 80 percent of theignition temperature, the same situation asdescribed in A-4.2 above exists.A-4.4 The Turbine has exposed Surfaceswith Temperature above 80 Percent of theIgnition Temperature of the ActualFlammable Gas or the ElectricalEquipment inside the Hood which will beAlive as the Turbine is Running does notMeet Zone 2 RequirementThe hood then should be ventilated withsufficient number of air changes per hour tomake a highly efficient dilution of anyhazardous gas leakage inside the hood. Therequired ventilation rate depends on theleakage sources inside the hood and should besufficient to keep the internal atmospherebelow an average of 20 percent of the lowerexplosion limit. Ninety air changes per hour isregarded as a minimum. In addition to thenormal ventilation system a 100 percent sparestand-by fan supplied from a continuous powersource should be provided. If the ordinaryventilation fails the spare fan should beautomatically activated and an alarm be givenin the control room. As an alternative toventilation of the turbine-hood duringshutdown of the turbine, HALON may beinjected.A-4.5 During a shut-down, the turbine hoodmay be classified as non-hazardous due tospecial arrangement of the fuel gas supplysystem. A system called “Block and Bleed” isdescribed on Fig. 17. A shut down signal willclose valve No. 1 and open valve No. 2. Thethree way valve will open from the gasdistribution manifold to the flare. The fuel gaslines within the turbine room and the turbinehood will then be depressurized. Theprobability of gas escape inside the hood maythen be regarded as minor. In case of a leakonly small quantities of gas will escape. Thisarrangement does not reduce the requirementsto ventilation while the turbine is running.

A-5 DETECTION OF ESCAPED GASA-5.1 Gas detectors should be installed insidethe turbine hood. Normally, the turbine shouldshut down if gas is detected inside the hood.

IS 5572 : 1994

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A-5.2 The location of the detectors should bechosen with special care being aware of possiblegas pockets, air flow patterns, etc.

A-5.3 Concerning ventilation arrangement,several alternatives exist in case of a shut downdue to gas detection inside the hood.

a) The ventilation of the turbine hoodcontinues until hot surfaces have beencooled to a temperature below 80 percentof the ignition temperature of the gas-airmixture which is present. The ventilationsystem that will be in operation after ashut down should be supplied from anemergency power source with sufficientcapacity.

b) The ventilation stops and inert gas (forexample Halon) is released upon detectionof gas inside the hood.

Other alternatives may be considereddepending on the actual installation.

A-6 ADDITIONAL RECOMMENDATIONS

A-6.1 The shut down of ventilation systemshould correspond to the fire and gas detectionsystem and the fire extinction system installedin the turbine hood and turbine room.Accordingly, other arrangements than thosedescribed in this standard may give anequivalent level of safety.A-6.2 Ventilation and combustion air should betaken from non-hazardous areas.

FIG. 17 BLOCK AND BLEED SYSTEM

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This Indian Standard has been developed from Doc : No. ET 22 (3194)

Amendments Issued Since Publication

Amend No. Date of Issue

Amd. No. 1 March 2000

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