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Application of IEC 60079-10-1 edition 2.0 for hazardous area classification - PDF Free Download
Application of IEC 60079-10-1 edition 2.0 for hazardous area classification
This document provides guidance on the application IEC 60079-10-1 Edition 2.0: Explosive Atmospheres - Part 10-1: Classification of areas - Explosive ... IEC Standards - Hazardous areas - Ventilation - Shape - Atmosphere - Liquids - explosives - hazardous areas - IEC standards - hazardous area classification - design approach - hazardous locations - flammable gas - application guidance - explosive gas atmospheres - application IEC 60079-10 edition 2.0 - IEC 60079-10-1 - Hazardous Area Classification - Explosive gas atmospheres
Author: Allan Bozek
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APPLICATION OF IEC 60079-10-1 EDITION 2.0 FOR HAZARDOUS AREA CLASSIFICATION Copyright Material IEEE Paper No. 3&,& Allan Bozek, P.Eng. MBA Senior Member, IEEE EngWorks Inc. 1620 49th Avenue S.W. Calgary, AB T2T 2T7 Canada [email protected] their development given the opportunity to participate. IEC National Committees apply IEC publications to the maximum extent possible within the context their national and regional standards publications. The development of IEC 60079-10-1 is the result of the efforts of the TC 31J committee responsible for the preparation and maintenance of IEC standards relating to the classification of hazardous areas and their installation requirements. Edition 1.0 of IEC 60079-10-1 [2] evolved from IEC 79-1 [3] first published in 1972. The original publication consisted of 13 pages of guidance addressing the classification of locations where flammable gas/vapour atmosphere may exist. In 2008, the flammable gas/vapour and combustible dust standards were amalgamated under the 60079 series of standards. The subsequent revision of the IEC 60079-10 standard was renumbered to IEC 60079-10-1 Ed.1.0 to address the classification of locations where flammable gas/vapour hazards may exist. The former IEC 61241-10 standard was renumbered to IEC 60079-10-2 to address combustible dust hazards. The IEC 60079-10-1 document is considered an objective based standard meaning that the requirements and objectives that are important to safety, health and the technical integrity of a design are addressed. Every requirement is related to at least one of the standards stated objectives. The standard will state why an objective is important and it may provide guidance on how to achieve an objective but how a solution is implemented is left up to the user. For this reason, the IEC standards for hazardous area classification do not incorporate application diagrams to define a hazardous location. This is in contrast to such publications as API RP 505 [4], NFPA 497 [5] and EI15 [6] where “classification by example” diagrams are often used to designate the degree and extent of a hazardous area classification. There was general industry consensus that the edition 1.0 of IEC-60079-10-1 required a revision to address several shortcomings. While the body of the document addressed the objectives required of a hazardous area classification, the guidance provided in the supporting annexes did not always result in an acceptable solution. This prompted the development of a new approach supported by scientific research and experimental data. The calculation methods were incorporated into Edition 2.0 of IEC 60079-10-1
Abstract – This document provides guidance on the application IEC 60079-10-1 Edition 2.0: Explosive Atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres. The IEC 60079-10-1 Ed. 2.0 document incorporates significant revisions from previous editions in both technical content and design approach to classifying hazardous locations where flammable gas or vapours may be present. The design concepts incorporated into the document are introduced with application guidance provided in the context of real world examples. Index Terms — IEC 60079-10-1, Hazardous Area Classification, Explosive gas atmospheres.
IEC 60079-10-1 [1] is the core document used within the IEC system of standards for classifying locations where flammable gas or vapour hazards may be present. The document supports the proper selection and installation of equipment using the “zone” method of hazardous area classification. Edition 2.0 of IEC 60079-10-1 incorporates significant changes to address the shortcomings of previous editions and provides several new evaluation methods for determining the degree and the extent of a hazardous location. This paper describes the most significant changes incorporated into IEC 60079-10-1 Ed. 2.0 and provides guidance on how the document may be used to classify a location. Several examples are presented with a discussion on how the results compare with API and NFPA recommended practices.
EVOLUTION OF IEC 60079-10-1
The IEC is a world organization that publishes international standards for electrical, electronic and related technologies. IEC standards publications form the basis of recommendations for international use and to promote international uniformity. The standards are developed on a consensus bases with all member countries interested in
1 978-1-5090-5877-8/17/$31.00 © 2017 IEEE
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providing an improved method of assessment that better reflects real world applications.
hazardous area influenced primarily by the release velocity as illustrated in Fig. 1. The suggested shape of the hazardous area under such conditions would be as illustrated in Fig. 2. A low pressure gas release in contrast will be influenced more by material vapour density and atmospheric conditions. The shape of the hazardous area would likely be as illustrated in Fig. 3. In contrast, a liquid hydrocarbon release would likely form a pool near the vicinity of the release. The extent of the hazardous area and its shape will be influenced by vapour pressure of the flammable fluid as it evaporates under ambient conditions. The shape of the hazardous area would be as illustrated in Fig 4. The standard also provides recommended hazardous area shapes for liquefied flammable gas release scenarios and discusses how aerosol and hybrid mixtures incorporating flammable gas and combustible dusts may be handled.
III. TECHNICAL REVISIONS INCORPORATED INTO IEC 60079-10-1 ED. 2.0 The significant technical changes incorporated into IEC 60079-10-1 Ed. 2.0 with respect previous editions include: A.
Recognition of Alternative Area Classification Standards and Recommended Practices
IEC 60079-10-1 Ed. 2.0 recognizes the use of other standards and recommended practices where they provide guidance or examples appropriate to the application and comply with general principles of the IEC standard [1]. Historically, the IEC standard has relied on “source of release” evaluation methodologies where release scenarios were modelled using calculations to assess a situation and determine an appropriate classification. The standard now recognizes the use of “simplified methods” where the zone classification and extent are determined using typical diagrams sourced from a variety of publications including API RP 505, NFPA 497 and EI15. IEC 60079-10-1 cautions Users that where a standard is selected as a preferred base for a site or application, examples from another standard should not be selected to achieve a less rigorous classification without due justification. An extensive list of industry codes and national standards for hazardous area classification are included for reference in Annex K of the document. Where such industry codes or national standards are used, they shall be quoted as the basis for classification and not IEC 60079-10-1. The IEC standard describes a “combination of methods” approach where “simplified methods” are used to classify facilities in the early stages of a design and then later optimized using “source of release” methods when detailed process information becomes available as the project evolves. The “source of release” methodology is described in detail in a series of schematic flowcharts incorporated into Annex F of the document. The standard also recognizes the value of prior experience when classifying facilities. Clause 5.5.4 from NFPA 497 was paraphrased within the standard allowing for the evaluation of same or similar installations to be used as a basis for classifying new facilities. It also implies that existing facilities may be reclassified based on operating experience. This allows for experience and documented evidence to be incorporated into a hazardous area classification design with proper justification. B.
Fig. 1 High Velocity Jet Release
Fig. 2 High Pressure Gas/Vapour Jet Release Hazardous Area Shape
IEC 60079-10-1 Ed. 2.0 incorporates a detailed discussion of material properties as they apply to a potential source of release and how it influences a hazardous area classification design. The characteristics of a gaseous release, a gas liquefied by pressure or by temperature or a liquid pool release are discussed in the context of release behavior and their influence on the shape of hazardous area. For example, a high pressure gaseous release may be selfdiluting under certain conditions with the shape of the
Fig. 3 Low Pressure Gas/Vapour Release Hazardous Area Shape
ventilation within an area. Using these parameters, Table D.1 (Fig. 5) of the standard then suggests an appropriate zone classification for the location. It should be cautioned that the zone classification suggested by the table should be evaluated against the formal definitions for a zone classification. For example, the standard defines a zone 2 location as one where a flammable gas atmosphere is “not likely to occur” and if it does, “exists for a short time only”. If the “short time only” criteria cannot be achieved, which may the case in remote unattended or unmonitored facilities; the assignment of the zone classification may warrant further evaluation.
Fig. 4 Flammable Liquid Pool Release Hazardous Area Shape C.
Source of Release Calculation Methods
Annex B of IEC 60079-10-1 Ed. 2.0 provides calculation methods for evaluating a source of release based on its form of release. 1) Release rates of gas or vapours: The standard provides a method for determining the release rate of low pressure subsonic releases and high pressure sonic releases. The calculation methods for gas releases are very similar to the methods described in 60079-10-1 Ed. 1.0 with the addition of a coefficient of discharge factor added to the equations which serves to model the viscosity properties of a liquid or gas with respect to a release opening. The standard provides some guidance on selecting an appropriate coefficient of discharge factor if no other information is available to the User. The standard also incorporates a new table in Annex B that recommends suitable hole cross sectional diameters for secondary grades of release. Previous editions of the standard omitted this information and it was left up to the user to determine an appropriate hole size for a given situation. The hole size had a large influence on the calculation resulting in a wide variation of potential solutions. The new table provides consistency in modelling similar situations which should help to achieve more uniform design solutions. 2) Release rate of liquids: The formula from 60079-101 Ed. 1.0 with the addition of coefficient of discharge factor (Cd) has been incorporated into the standard. The intent is to use the liquids release calculation to determine an appropriate pool size for determining the release rate of an evaporative pools. 3) Release rates of evaporative pools: A new calculation model for evaluating the evaporation rate of a pool release is provided. This is a very common scenario in many facilities handling flammable liquids where a release scenario usually results in a pool formation adjacent the leak source. The standard also provides a table to assist in determining the volumetric evaporation rate of a pool release. D.
Fig. 5 IEC 60079-10-1 Ed 2.0 Table D.1 – Zones for Grade of Release and Effectiveness of Ventilation E.
Introducing the Concept of Dilution for Ventilation Assessment
Previous editions of IEC 60079-10 evaluated enclosed locations based on the hypothetical volume Vz. Vz was defined as a volume in which a gas/vapour concentration was equal to a certain Lower Flammable Limit (LFL) safety threshold depending on the intended zone classification. A Zone 2 classification incorporated a 50% LFL (2X safety factor) safety threshold. A Zone 0 or 1 location incorporated a 25% LFL (4X safety factor) threshold. The Vz calculation was used to differentiate between “high”, “medium” and “low” ventilation in enclosed spaces which subsequently influenced the zone classification for the area. The use of Vz as a basis for determining a zone classification was controversial as there was no scientific basis for the formulae. The calculation of Vz often resulted in very large volumes up to 3 orders of magnitude greater than what was observed through Computational Fluid Dynamic (CFD) modelling and real world experimental testing [7][8][9]. There was general consensus among users that a new scientifically validated approach was needed to better reflect reality. IEC 60079-10-1 Ed. 2.0 replaces Vz with the concept of dilution. Dilution is a measure of the ability of ventilation or atmospheric conditions to dilute a release to a safe level. It is influenced by the release rate of the flammable material and ventilation velocity. Ventilation velocity is a measure of turbulence which is necessary to dilute a gas or vapour release. Air movement is required to promote a turbulent boundary layer between the release source and surrounding
Determining the Zone Classification of a Location
Annex D of the standard provides a structured method for selecting the appropriate zone classification for both indoor and open air locations based on the “grade of release”, “availability” of ventilation and the “effectiveness” of
atmosphere. This allows air to mix with the release reducing the LFL of the gas/air mixture and transporting it away. Ventilation turbulence may be caused by momentum of the gas/vapour leak itself, by buoyancy of the release in air or by wind flow interacting with the release. For indoor situations, ventilation velocity is calculated by determining the volumetric flow of the ventilation system accounting for any release sources and dividing the value by the cross sectional area perpendicular to the flow. This results in an average flow velocity (Uw) that can be used for assessing the ability of a ventilation system to control a release. To perform an assessment, the “ventilation velocity” is determined by the design of the ventilation system or by outdoor atmospheric conditions and plotted against the “release characteristic” which describes the flammable source of release in the context of the size and rate of release. The standard provides a method for determining the ventilation velocity of open areas using Fig. 6. The release characteristic of a source of release may be calculated using the following formula: Wg (m3/sec) (ʌg x k x LFL)
stopped. A low dilution environment typically leads to a Zone 1 or even Zone 0 classification based on Table D.1 (Fig. 5).
Fig. 7 IEC 60079-10-1 Ed 2.0 Figure C.1 – Chart for Assessing the Degree of Dilution
(1) For enclosed areas, both the ventilation velocity and the background concentration of flammables entrained in the air must be assessed. Dilution in an enclosed area may result from the exchange of fresh air from outside the enclosure or by the enclosure itself having sufficient volume to allow the release to disperse. This makes it possible for large enclosed areas to have minimal interior/exterior air exchange rates while still maintaining sufficient ventilation effectiveness to disperse a release. To assess the background concentration of an enclosed location, the flow rate from the flammable release source must be compared to the fresh air introduction rate accounting for mixing inefficiencies. The standard incorporates a calculation formula to estimate the background concentration as follows: f x Qg (vol/vol) (2) Xb = Qg + Q1
where Wg ʌg k LFL
mass release rate of flammable substance (kg/s) Density of gas or vapour (kg/m3) Safety factor attributed to LFL Lower flammable limit (vol/vol)
where Xb Qg
Fig. 6 IEC 60079-10-1 Ed 2.0 Table C.1 – Indicative Outdoor Ventilation Velocities
The degree of dilution may then be determined using Fig. 7. “High dilution” refers to situations where the concentration near the source of release can be quickly reduced and there will be no persistence after the release is stopped. Under the appropriate conditions, this will permit a “NE” negligible extent that may be used as a basis to designate an area “non-hazardous”. “Medium dilution” applies to situations where the concentration of the release is controlled resulting in a stable boundary when the release is in progress and the explosive gas atmosphere does not persist after the release has stopped. For most secondary grade source release applications, a medium dilution environment will lead to a Zone 2 classification. “Low dilution” applies to situations where there is a significant concentration while the release is in progress and the flammable atmosphere will persists after the release is
Background concentration (vol/vol) Volumetric flow of flammable gas from the source of release (m3/sec) Volumetric flow rate of air entering the room through aperatures (m3/sec) Degree to which the air inside the enclosure is well mixed f = 1 where the background concentration is uniform thoughout the enclosure f ग़ 1 where inefficient mixing inside the enclosure results in gradients of background concentration
The criteria for assessment is Xb << Xcrit where Xcrit is the maximum acceptable gas concentration determined by the user. Xcrit would normally be the LFL alarm setpoint for gas detectors in the area.
For artificially ventilated enclosures, the ventilation velocity used for evaluation is the average flow velocity caused by the ventilation system accounting for any inefficiencies or flow obstructions. For naturally ventilated enclosures, the ventilation velocity will be a function of the thermal stack effect, wind effects on the enclosure and the combination effects of both. The concept of dilution and background concentration is further developed in the context of several application scenarios in Annex C of the standard.
• properties of flammable materials • identification and location of sources of release Area classification design documentation should also include plans, elevations or three dimensional models that indicate the type and extent of zones as well as the appropriate group classification, ignition temperature and/or temperature class. The standard also discusses the option to indicate equipment protection levels (EPLs) on drawings to assist with the selection of equipment in hazardous locations. The standard provides examples of data sheets that may be used for documenting flammable material substances, sources of release and hatching symbols for designating hazardous locations on drawings.
Estimating the Extent of a Hazardous Zone Annex D of IEC 60079-10-1 Ed. 2.0 incorporates a structured method for determining the extent of a hazardous zone in outdoor locations. This is a welcome addition to the standard as previous editions did not provide a means of assessing appropriate extents. The hazardous distances suggested are based on release characteristic formulae (1) discussed earlier. Fig. 8 provides a graphical means of determining an appropriate extent based on the type of release expected. Release behavior is characterized as a heavy gas release, typical of a liquid pool release, a diffusive release resulting from a low velocity gas/vapour release or a jet release characteristic resulting from a high velocity gas release. As always, the distances calculated must be evaluated using engineering judgement and an appropriate safety factor applied to account for facility layout and site conditions. The method described is for open air situations and does not apply to indoor low dilution applications.
Annex E of the standard provides several examples illustrating the use of concepts described. They are not intended to be used as a basis for design but to illustrate the means of assessment as described in the annex sections of the standard. Examples include: • pump application in open air • pump application within an enclosed location • process vessel in open air • control valve in a congested location • process piping in an enclosed location • compressor facility handing natural gas The compressor facility example is fully documented to illustrate the level of documentation expected for a given application. It should be noted that the examples are intended to show application of the evaluation methods and are not intended to be used as representative examples for classification purposes.
IV. APPLICATION OF IEC 60079-10-1 ED. 2.0 IEC 60079-10-1 Ed. 2.0 is intended to be used by competent personnel who are well versed in the properties of flammable materials (chemistry knowledge), able to identify potential release sources (process and mechanical knowledge), assess ventilation requirements (mechanical knowledge) as well as understand the implications of a classification design as it applies to the selection of electrical equipment (electrical knowledge). The standard encourages the use of a multidisciplinary team who possess competency in each of these areas to participate in the design process. The methodologies described in the Annex sections of IEC 60079-10-1 Ed. 2.0 require detailed process information to perform an assessment. Often, this information is not available in the early stages of a project when preliminary area classification design information is needed for long lead item purchases. This is where the use of other recognized documents such as API RP 505, NFPA 497 and EI15 may have value. The documents provide conservative classification by example diagrams that may be used to determine the degree and extent of a classification based on preliminary process information. Engineering judgement must always be used when employing such diagrams to ensure they represent the true nature of the hazard. The properties of the flammable materials handled must be considered to ensure the diagram used is appropriate for the application.
Fig. 8 IEC 60079-10-1 Ed 2.0 Figure D.1 – Chart for Estimating Hazardous Area Distances G. Documentation IEC 60079-10-1 Ed. 2 recommends all area classification designs be fully documented. This is important in capturing the rationale used to classify a facility and to maintaining the integrity of the design over the life of the facility. Information to be incorporated into a documented design includes: • sources of information used (code, national standard or calculation) • gas a vapour dispersion calculations • study of ventilation characteristics with consideration given to position of openings in buildings for ventilation
As additional process information becomes available, the hazardous area classification may be optimized using the calculation methods described in the Annex sections of IEC 60079-10-1 Ed. 2.0. This provides the opportunity for the hazardous area classification design to be optimized to reflect the true nature of the hazard.
Using the process information provided, the calculated release rate would be 1.6 X 10-3 kg/s. 2) Zone Classification: To determine the appropriate zone classification, the ventilation velocity and the release characteristic must be determined. The ventilation velocity can be determined from Table C.1 of IEC 60079-10-1 (Fig. 6). For a lighter than air release in an unobstructed area, a ventilation velocity of 0.5 m/s at grade would be appropriate. The release characteristic would then be determined using formula (1). This requires the LFL of natural gas be determined as well as an appropriate safety factor applied based on the LFL. Given that the LFL of natural gas (methane) is relatively high at 5%, a k = 1.0 safety factor would be appropriate. In other situations where the LFL is less than 5%, a k safety factor of between 0.5 and 1.0 would be appropriate. Based on the information provided, the characteristic of release would be 0.042. Applying the results of the release characteristic calculation to the expected ventilation velocity as illustrated in Fig. 9, the application results in a medium dilution situation.
Natural Gas Release A hazardous area classification is required for a pressure vessel located in an outdoor location in an upstream gas processing facility. The vessel handles natural gas at a pressure of 4500 kPA. Fig. 48 of API RP 505 recommends a Zone 2 classification extent of 3 meters as illustrated in Fig. 8.
Fig. 8 API RP 505 Figure 48 To assess this situation using the methods incorporated into the annex sections of IEC 60079-10-1 Ed. 2.0 requires several steps as follows: 1) Calculation of the release rate: The first step is to model a typical release scenario. This would be done by applying the appropriate formulae provided in Annex B of the standard. For this particular example, a choked gas velocity (sonic gas) release scenario using the following formulae would be applicable.
0.042 Fig. 9 Degree of Dilution The zone classification for the application may then be determined from table D.1 from IEC 60079-10-1 based on a secondary grade of release, good availability of ventilation (typical of most outdoor locations) and medium degree of dilution. Fig. 10 indicates a Zone 2 classification would be appropriate. Typically, most secondary release applications in open areas result in a Zone 2 classification unless there are significant impediments to the flow of natural ventilation in the area. Once the appropriate zone classification has been defined, the extent of the Zone 2 classification may then be determined. Given that the release is sonic, a jet release would be most likely warrant a 1 meter Zone 2 classification as illustrated in Fig. 11. The results of the IEC source of release calculation would indicate that a 1 meter Zone 2 classification surrounding the vessel flange connections would be appropriate for the application. This compares to a 3 meter classification extent recommended by API RP 505. The difference can be attributed to the nature of the release. The behaviour of a natural gas jet release is such
(3) Table B.1 from the standard recommends a hole cross sectional area of 0.25mm2 be applied. Additional process information required to complete the calculation include: • • • • • • • • • •
Operating pressure: P = 4500 kPA Process temperature: T = 25°C = 303.15K Mole weight of Natural gas: 19 kg/kmole Specific heat at constant pressure: Cȡ = 2.22 (kJ/kg K) Polytropic Index of expansion : ѫ = 1.32 Compressibility Factor : Z = 1.0 Coefficient of Discharge; Cd = 0.75 Atmospheric Pressure; pa = 101kPA Universal Gas Constant; R = 8314 Gas density, ȡg = 0.764 kg/m3
that the momentum of the gas release will mix turbulently with the surrounding air as illustrated in Fig. 1. This would limit the extent of the hazardous radii to a fairly short distance. From an applications perspective, electrical and instrumentation equipment mounted on the vessel would need to be certified for a Zone 2 hazardous location. Electrical and instrumentation equipment on process equipment adjacent the vessel may not require a hazardous location certification if they do not handle flammable materials and are outside of the 1 meter zone classification.
Fig. 12 Mechanically Ventilated Building handing Natural Gas API addresses such situations based on the presence of “adequately ventilation” which is defined as “Ventilation (natural or artificial) that is sufficient to prevent the accumulation of significant quantities of vapour-air or gas-air mixtures in concentrations above 25% of the LFL”. API also recognizes that locations ventilated at 6 air changes per hour may be designated “adequately ventilated”. The ventilation fan provides approximately 4 ACPH of ventilation which is below the 6ACPH default API adequate ventilation criteria. To determine if the building is adequately ventilated in accordance with API recommendations, a fugitive emission calculation as described in Appendix B of API RP 505 would be required. The IEC standard evaluates the zone classification based on Fig. 5 by evaluating the grade of release, the availability of ventilation and the effectiveness of the ventilation system. Given that the application incorporates a secondary grade of release, the zone classification will be determined by the availability and the effectiveness of the ventilation system. The availability of the ventilation system is categorized using the following criteria from the standard: Good: ventilation that is present continuously. Artificially ventilated locations with power systems redundancy would meet these criteria. Fair: ventilation that is expected to be present during normal operation with discontinuities occurring infrequently and for short periods. Poor: ventilation that does not meet the standard of fair or good, but discontinuities are not expected to occur for long periods. The application was assessed to meet the criteria of “fair” ventilation. The effectiveness of the ventilation system requires evaluation of the “degree of dilution” and determination of the expected background concentration for the application. The assessment of the degree of dilution is based on Fig. 7. The ventilation flow velocity may be calculated based on the volumetric flow of the gas/air mixture divided by the cross sectional area perpendicular to the flow using the following formula:
Fig. 10 Zone Classification
1.0m 0.042 Fig. 11 Extent of Zone Classification – Gas Release
Natural Gas Release in an Enclosed Location
Qa Uw = (L L x H
The same natural gas release scenario is evaluated for a 6m long x 4m wide x 3.5m high enclosed location as illustrated in Fig. 12. The location is mechanically ventilated at 0.09 m3/sec (200 CFM). What would be an appropriate zone classification for the enclosed area?
where Uw Qa L H
Ventilation velocity (m/sec) Air flow rate (m3/sec) Length of the enclosed area (m) Height of the enclosed area (m)
for the application would consist of a small amount of leakage from the pump seal under normal operating conditions. The pump is surrounded by a 2m x 3m fluid containment dyke designed to contain any leakage. The standard API diagram used to classify refinery sources of release is illustrated in Fig. 15. Given that pentane has a vapour density of 2.5 as compared to air, the application requires a transient vapor zone. A Zone 2 classification extending 15m in addition to a 15m transient vapour zone would apply.
The calculated ventilation velocity was determined to be 0.004 m/sec. Applying the characteristic of release and the ventilation velocity information to Figure C.1 from the IEC standard results in a medium degree of dilution as illustrated in Fig. 13.
0.004m/s
Fig. 15 API RP 505 Figure 20
Fig. 13 Enclosed Location Degree of Dilution
Assessing this situation using the IEC source of release calculation method requires examining the nature of a hypothetical release under normal conditions. A pentane release from a pump seal would likely result in some flammable material flashing to atmosphere with the bulk of the release collecting in a pool formation within the containment dyke. The pentane fluid within the dyke would then flash to atmosphere at a rate determined by the ambient air temperature and the vapour pressure of the pentane fluid. The release rate from an evaporative pool may be determined by calculation or by Figure B.2 of IEC 60079-10-1 as illustrated in Fig. 16. The values obtained from the figure are based on the assumption that the liquid temperature of the fluid is equal to the ambient temperature with a wind speed of 0.5 m/sec. Pentane has a vapour pressure of 57 kPA at 20°C and a mole weight of 72 kg/kmole. Referencing the chart, the volumetric evaporation rate would be approximately 0.5 x 10-3 m3/sec for a pool surface area of 1.0 m2. Given that the dimensions of the containment dyke in the application is 6.0 m2 the value obtained from the chart must be multiplied by the actual surface area yielding an estimated volumetric evaporation rate of 3.0 x 10-3 m3/sec. Calculating the release characteristic using formula (1) yields a value of 0.4 based on a k safety factor of 0.5 (Pentane has a LFL of 1.5%) and a wind speed of 0.25 m/sec sourced from table C.1 (Fig. 17) for a heavier than air release in an obstructed area at ground level. Using Figure D.1 from the standard (Fig. 18) for a heavy gas release yields a hazardous location extent of approximately 6 meters. This is in contrast the to the 30m extent recommended by API. The extent determined by the IEC calculations is a starting point and should be viewed with engineering judgement. Other factors such as below grade locations within the classified area and other natural obstacles may also influence the extent of the classification.
Applying the information derived from the calculations to Fig. 14 suggests a Zone 2 classification for the building. This however requires that the background concentration be checked using formulae (3). To do so, a suitable value of f is required which is a safety factor applied to account the degree of inefficiency of air mixing due to equipment congestion and variable air flow patterns. Since the building layout is relatively open, an f factor of 2 was selected. The background concentration Xb was then determined to be 4.5% LFL which is much less than the Xcrit value defined as the 20% LFL gas detection alarm setpoint. Based on this result, a Zone 2 classification would be appropriate for the application as illustrated in Fig. 14.
Fig. 14 Zone Classification for Enclosed Building C.
Pentane Fluid Release A pump in a refinery process handles pentane at 3000 kPA at an operating temperature of 20°C. A typical leak scenario
V. CONCLUSIONS The IEC 60079-10-1 Edition 2.0 standard is a significant update to previous editions. It addresses many of the shortcomings of previous editions and the rationale and design approaches are based on scientific analysis supported by testing and verification. The new document acknowledges the use of alternative publications including API RP 505, NFPA 497, EI 15 to classify locations and recognizes the value in observing the behavior of existing facilities when classifying new or re-classifying existing facilities. The classification methods described in the annexes of IEC 60079-10-1 Ed. 2.0 standard require a high level of competency to perform a hazardous area classification. The calculations require detailed process information to perform an analysis which may not available in the early stages of a project. While employing the methods described will help to achieve an optimized design, the results should always be applied with good engineering judgement taking into account the nature of the flammable release, the working environment and the potential consequences of an ignition event.
Fig. 16 Volumetric Evaporation Rate Estimate
VI. REFERENCES [1] IEC 60079-10-1/Ed:2, 2015, Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres, International Electrotechnical Commission, Geneva, Switzerland. [2] IEC 60079-10-1/Ed:1, 2009, Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres, International Electrotechnical Commission, Geneva, Switzerland. [3] IEC 79-10, 1972, Electrical Apparatus for Explosive Gas Atmospheres – Part 10 Classification of Hazardous Areas, International Electrotechnical Commission, Geneva, Switzerland. [4] ANSI/API RP 505, 1997, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2, American Petroleum Institute, Washington, DC. [5] EI 15, 2015 Model code of safe practice Part 15: Area classification code for installations handling flammable fluids, Energy Institute, London, UK. [6] ANSI/NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, National Fire Protection Association, Quincy, MA. [7] D.M. Webber, M.J. Ivings and R.C. Santon, “Ventilation theory and dispersion modelling applied to hazardous area classification”, Journal of Loss Prevention in the Process Industries”, September 2011, 612-621. [8] HSL Research Report RR630, “Area classification for secondary releases from low pressure natural gas systems”, Health and Safety Executive (HSE) UK 2008. [9] P Persic, “Hypothetical Volume of Potentially Explosive Atmosphere in the Context of IEC Standard 60079-101, Ex-Bulletin, Croatia 2012. Vol 40, 1-2.
Fig. 17 Outdoor Ventilation Velocity for a Pentane Pool Release at Ground Level
~6.0m
0.4 Fig. 18 Extent of Zone Classification – Pentane Release
[10] Rangel Jr., Estellito, Luiz, Aurélio M. and Madureira Jr., Hilton – “Area classification is not a copy-and-paste process: performing reliable hazardous area classification studies”. IEEE IAS Industry Applications Magazine, Jan/Feb 2016, p. 38 – 49.
VII. VITAE Allan Bozek, P.Eng., MBA, graduated from the University of Waterloo in 1986 with BSc in Systems Design Engineering and a MBA from the University of Calgary in 1999. He is a Principal with EngWorks Inc. providing hazardous location consulting services to industry. He is a registered professional engineer in the provinces of Alberta, Ontario, Saskatchewan and British Columbia, Canada and has been a member of the IEEE since 1989. Allan’s areas of expertise include hazardous area classification design, application of hazardous location codes and standards to facilities and the design and certification of equipment in hazardous locations.
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