The significant technical changes with respect to the previous edition are as follows: Type Minor and editorial changes Extension technical Major changes Complete restructuring and d
Safety principles
Facilities that manage or store flammable materials must be designed, built, operated, and maintained to minimize the release of these substances This approach aims to reduce the size of hazardous areas during both normal and abnormal operations, taking into account the frequency, duration, and quantity of any potential releases.
To reduce the risk of flammable substance releases, it is crucial to analyze the components of process equipment and systems Modifying the design can help minimize both the likelihood and frequency of these releases, as well as control the quantity and rate at which substances are released.
Early-stage design development of any process plant must prioritize these fundamental considerations, which are crucial for conducting a thorough area classification study.
For activities outside of normal operations, such as commissioning or non-routine maintenance, the existing area classification may not apply It is essential that these activities are managed through a safe system of work Additionally, any routine maintenance should be considered in the area classification.
3.6.5 flammable mist droplets of liquid, dispersed in air so as to form an explosive atmosphere
3.6.6 hybrid mixture mixture of a flammable gas or vapour with a dust
Note 1 to entry: According IEC 60079-10-2 the term “dust” is defined as including both combustible dust and combustible flyings
3.6.7 relative density of a gas or a vapour density of a gas or a vapour relative to the density of air at the same pressure and temperature (air is equal to 1,0)
3.6.8 flashpoint lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture
3.6.9 boiling point temperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar)
The initial boiling point for liquid mixtures should reflect the lowest boiling point within the range of present liquids, as determined through standard laboratory distillation without fractionation.
3.6.10 vapour pressure pressure exerted when a solid or liquid is in equilibrium with its own vapour
Note 1 to entry: This is also, the partial pressure of the substance in the atmosphere above the liquid It is a function of the substance and of the temperature
3.6.11 ignition temperature of an explosive gas atmosphere lowest temperature of a heated surface which, under specified conditions (according to
IEC 60079-20-1), will ignite a flammable substance in the form of a gas or vapour mixture with air
3.6.12 lower flammable limit (LFL) the concentration of flammable gas, vapour or mist in air below which an explosive gas atmosphere will not be formed
[SOURCE: IEC 60050-426:2009, 426-02-09, modified (definition in 60050-426 referred to
3.6.13 upper flammable limit (UFL) the concentration of flammable gas, vapour or mist in air above which an explosive gas atmosphere will not be formed
[SOURCE: IEC 60050-426:2009, 426-02-10, modified (definition in 60050-426 referred to
3.7 operation 3.7.1 normal operation situation when the equipment is operating within its designed parameters
Failures resulting from accidents, including issues like pump seal breakdowns, flange gasket failures, or spillages, are not classified as part of normal operations when they necessitate repairs or shutdowns.
Note 2 to entry: Normal operation includes start-up and shut-down conditions and routine maintenance, but excludes initial start up as part of commissioning
3.7.2 routine maintenance action to be performed occasionally or periodically in normal operation to maintain proper performance of equipment
3.7.3 rare malfunction type of malfunction which may happen only in rare instances
Rare malfunctions, as defined by this standard, refer to the failure of independent process controls—whether automated or manual—that could initiate a series of events resulting in a significant release of flammable substances.
Rare malfunctions may encompass unforeseen circumstances not addressed in the plant design, such as unexpected corrosion leading to a release However, if releases due to corrosion or similar conditions are reasonably anticipated as part of normal plant operations, they do not qualify as rare malfunctions.
3.7.4 catastrophic failure an occurrence which exceeds the design parameters of the process plant and control system resulting in a release of flammable substance
Catastrophic failures, as defined by this standard, encompass significant incidents like the rupture of a process vessel and extensive equipment or piping failures, including the complete breakdown of a flange or seal.
Facilities that manage or store flammable materials must be designed, built, operated, and maintained to minimize the release of these substances This approach aims to reduce the size of hazardous areas during both normal and abnormal operations, taking into account the frequency, duration, and quantity of any potential releases.
To reduce the risk of flammable substance releases, it is crucial to analyze the components of process equipment and systems Modifying the design can help minimize both the likelihood and frequency of these releases, as well as control the quantity and rate at which substances are released.
Early-stage design development of any process plant must prioritize these fundamental considerations, which are crucial for conducting a thorough area classification study.
For activities outside of normal operations, such as commissioning or non-routine maintenance, the area classification may not apply It is essential that these activities are managed through a safe system of work Additionally, the area classification must consider any routine maintenance activities.
3.6.5 flammable mist droplets of liquid, dispersed in air so as to form an explosive atmosphere
3.6.6 hybrid mixture mixture of a flammable gas or vapour with a dust
Note 1 to entry: According IEC 60079-10-2 the term “dust” is defined as including both combustible dust and combustible flyings
3.6.7 relative density of a gas or a vapour density of a gas or a vapour relative to the density of air at the same pressure and temperature (air is equal to 1,0)
3.6.8 flashpoint lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture
3.6.9 boiling point temperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar)
The initial boiling point for liquid mixtures should reflect the lowest boiling point within the range of present liquids, as determined through standard laboratory distillation without fractionation.
3.6.10 vapour pressure pressure exerted when a solid or liquid is in equilibrium with its own vapour
Note 1 to entry: This is also, the partial pressure of the substance in the atmosphere above the liquid It is a function of the substance and of the temperature
3.6.11 ignition temperature of an explosive gas atmosphere lowest temperature of a heated surface which, under specified conditions (according to
IEC 60079-20-1), will ignite a flammable substance in the form of a gas or vapour mixture with air
3.6.12 lower flammable limit (LFL) the concentration of flammable gas, vapour or mist in air below which an explosive gas atmosphere will not be formed
[SOURCE: IEC 60050-426:2009, 426-02-09, modified (definition in 60050-426 referred to
3.6.13 upper flammable limit (UFL) the concentration of flammable gas, vapour or mist in air above which an explosive gas atmosphere will not be formed
[SOURCE: IEC 60050-426:2009, 426-02-10, modified (definition in 60050-426 referred to
3.7 operation 3.7.1 normal operation situation when the equipment is operating within its designed parameters
Failures resulting from accidents, including the breakdown of pump seals, flange gaskets, or spillages, are not classified as part of normal operations when they necessitate repairs or shutdowns.
Note 2 to entry: Normal operation includes start-up and shut-down conditions and routine maintenance, but excludes initial start up as part of commissioning
3.7.2 routine maintenance action to be performed occasionally or periodically in normal operation to maintain proper performance of equipment
3.7.3 rare malfunction type of malfunction which may happen only in rare instances
Area classification objectives
Area classification is a crucial method for analyzing environments where explosive gas atmospheres may arise, ensuring the safe selection, installation, and operation of equipment This process considers the ignition characteristics of gases or vapors, including ignition energy and temperature The primary goals of area classification are to identify the type of hazardous zones and to define their extent.
NOTE Selected characteristics may be designated for equipment e.g ignition energy and temperature ratings, see IEC 60079-20-1
In many practical scenarios involving flammable substances, it is challenging to completely prevent the occurrence of an explosive gas atmosphere or to eliminate potential ignition sources Consequently, in environments where the risk of an explosive gas atmosphere is significant, it is essential to utilize equipment designed to minimize the chances of ignition Conversely, in situations where the likelihood of such an atmosphere is lower, less stringent equipment standards may be acceptable.
To enhance safety, the number and extent of zone 0 and zone 1 areas in plants should be minimized through careful design and appropriate operating procedures, prioritizing zone 2 or non-hazardous areas In cases where the release of flammable substances is unavoidable, equipment should be limited to those that produce secondary grade releases If primary or continuous grade releases are necessary, they must be restricted to very limited quantities and rates Plant design must prioritize these principles, ensuring that even during abnormal operations, the release of flammable substances into the atmosphere is minimized to reduce the hazardous area.
After classifying a plant and preparing the necessary documentation, it is crucial to consult with the area classification authorities before making any modifications to equipment or operating procedures Any changes to the plant or operations should prompt an update to the classification Regular reviews should be conducted throughout the plant's operational life.
Explosion risk assessment
After completing the area classification, a risk assessment can be conducted to determine if the potential consequences of igniting an explosive atmosphere necessitate the use of equipment with a higher equipment protection level (EPL) or if it may be appropriate to use equipment with a lower EPL than typically required.
A negligible extent (NE) zone can be classified as non-hazardous, indicating that any potential explosion would have minimal consequences This NE zone concept is applicable regardless of other risk assessment adjustments when determining the Equipment Protection Level (EPL).
Zone NE is characterized by a natural gas cloud that has an average concentration of 50% by volume of the Lower Flammable Limit (LFL) and occupies less than 0.1 m³ or 1.0% of the enclosed space, whichever is smaller.
The EPL requirements may be recorded, as appropriate, on the area classification documents and drawings to allow proper selection of equipment
NOTE 2: IEC 60079-0 describes EPLs and IEC 60079-14 defines the application of EPLs to an installation.
Competence of Personnel
Area classification must be conducted by individuals who comprehend the properties of flammable substances, gas/vapor dispersion principles, and are knowledgeable about the relevant processes and equipment Involving professionals from other engineering disciplines, such as electrical and mechanical engineers, as well as safety personnel, can enhance the area classification process The competency of the personnel should align with the specific nature of the plant and the methodology employed Regular continuing education and training are essential for personnel to maintain their qualifications as needed.
NOTE Competency can be demonstrated in accordance with a training and assessment framework relevant to national regulations or standards or user requirements
General
Determining which components of a plant correspond to the three zonal definitions (zones 0, 1, and 2) cannot typically be achieved through a simple inspection A comprehensive analysis is essential, focusing on the fundamental potential for the presence of an explosive gas atmosphere.
To assess the potential for a release of flammable gas or vapor, it is essential to evaluate the likelihood and duration of the release based on the definitions of continuous, primary, and secondary grades By analyzing the grade of release, along with factors such as release rate, concentration, velocity, and ventilation, one can establish a solid foundation for determining the possible presence of an explosive gas atmosphere in nearby areas and identifying the type and extent of hazardous zones.
This method necessitates a thorough evaluation of every piece of process equipment that either contains a flammable substance or can become flammable under certain process conditions, as these could potentially lead to a release.
Subclauses 5.3 to 5.6 provide guidance on classifying areas that may have an explosive gas atmosphere, with a schematic example of hazardous area classification included in Annex F.
The area classification should be carried out when the initial process and instrumentation line diagrams and initial layout plans are available, and should be confirmed before plant start-up
When assessing hazardous areas, it is crucial to evaluate the type, number, and location of potential release points to assign appropriate zone and boundary conditions Implementing control systems that adhere to Functional Safety standards can minimize both the likelihood and volume of releases, such as through batch sequence controls and inerting systems These controls should be considered in relation to the classification of hazardous areas.
When classifying areas, it is essential to evaluate prior experiences with similar installations Simply identifying a potential source of flammable substances is insufficient; a thorough assessment is necessary to accurately define the extent of Zone 1.
To prevent the occurrence of an explosive gas atmosphere, it is essential to either eliminate the potential for such an atmosphere around ignition sources or to remove the ignition source itself.
When direct prevention is not feasible, it is essential to implement protective measures, process equipment, systems, and procedures that minimize the likelihood of simultaneous occurrences of risks a) and b) to an acceptable level These safety measures can be employed individually, provided they are deemed highly reliable, or in combination to ensure the necessary safety standards are met.
Area classification is a crucial process for analyzing environments where explosive gas atmospheres may arise, ensuring the safe selection, installation, and operation of equipment This method considers the ignition characteristics of gases or vapors, including ignition energy and temperature The primary goals of area classification are to identify the type of hazardous zones and to define their extent.
NOTE Selected characteristics may be designated for equipment e.g ignition energy and temperature ratings, see IEC 60079-20-1
In many practical scenarios involving flammable substances, it is challenging to completely prevent the occurrence of an explosive gas atmosphere or to eliminate potential ignition sources Consequently, in environments where the risk of an explosive gas atmosphere is significant, it is essential to utilize equipment designed to minimize the chances of ignition Conversely, in situations where the likelihood of such an atmosphere is lower, less stringent equipment standards may be acceptable.
To enhance safety, the number and extent of zone 0 and zone 1 areas in plants should be minimized through careful design and appropriate operating procedures, prioritizing zone 2 or non-hazardous areas In cases where the release of flammable substances is unavoidable, equipment should be limited to those that produce secondary grade releases If primary or continuous grade releases are necessary, they must be restricted to very limited quantities and rates Plant design must prioritize these principles, ensuring that even during abnormal operations, the release of flammable substances into the atmosphere is minimized to reduce the hazardous area.
After classifying a plant and preparing the necessary documentation, it is crucial to consult with the area classification authorities before making any modifications to equipment or operating procedures Any changes to the plant or operations should prompt an update to the classification Regular reviews should be conducted throughout the plant's operational life.
After completing the area classification, a risk assessment can be conducted to determine if the potential consequences of igniting an explosive atmosphere necessitate the use of equipment with a higher equipment protection level (EPL) or if it may be appropriate to use equipment with a lower EPL than typically required.
A negligible extent (NE) zone can be classified as non-hazardous, indicating that any potential explosion would have minimal consequences This NE zone concept is applicable regardless of other risk assessment adjustments when determining the Equipment Protection Level (EPL).
Zone NE is characterized by a natural gas cloud that has an average concentration of 50% by volume of the Lower Flammable Limit (LFL) and occupies less than 0.1 m³ or 1.0% of the enclosed space, whichever is smaller.
The EPL requirements may be recorded, as appropriate, on the area classification documents and drawings to allow proper selection of equipment
NOTE 2: IEC 60079-0 describes EPLs and IEC 60079-14 defines the application of EPLs to an installation
Classification by sources of release method
Classification may be approached by calculation, considering appropriate statistical and numerical assessments for the factors concerned, for each source of release
Refer Annex F The source of release approach can be summarized as follows:
• Determine the release rate and grade of release for each source based on likely frequency and duration of release;
• Assess ventilation or dilution conditions and effectiveness;
• Determine zone type based on grade of release and ventilation or dilution effectiveness;
Formulae relevant to determining the release rates under specified conditions can be found in
Annex B These formulae are generally accepted as providing a good basis for calculating release rates for the conditions provided
Annex C offers guidance on evaluating ventilation and dispersion, while alternative assessment methods, such as computational fluid dynamics (CFD), can be effective in certain scenarios Additionally, computer modeling serves as a valuable tool for analyzing the interplay of various factors.
The assessment methods and tools must be validated for suitability and used with caution Assessors should be aware of the limitations and requirements of these tools, adjusting input conditions or results as necessary to draw accurate conclusions.
Use of industry codes and national standards
Industry codes and national standards may be used where they provide guidance or examples appropriate to the application and comply with the general principles of this standard
Annex K identifies some relevant industry codes and national standards that may provide further detail as well as examples.
Simplified methods
Where it is not practicable to make required assessments from individual sources of release, a simplified method may be used
Simplified methods will identify sources for zone types 0, 1, and 2, ensuring a conservative approach to potential release sources without requiring individual details This judgment should be guided by a set of criteria rooted in industry experience and tailored to the specific plant.
A comprehensive evaluation of every item in a plant is unnecessary; assessing just one item or condition can suffice to establish a conservative classification for all similar items or conditions within the facility.
Simplified methods often lead to larger zone areas due to the need for more conservative zonal classification in situations where hazards are uncertain This approach prioritizes safety by erring on the side of caution.
To achieve more precise estimates of the boundaries of the classified area, it is essential to refer to illustrative examples or conduct a more detailed assessment of point sources of release, as appropriate.
Combination of methods
The use of different methods may be appropriate for classification of a plant at various stages of its development or for various parts of the plant
In the early conceptual phase of a plant, a simplified method is suitable for determining equipment separations, layout, and boundaries, especially when detailed data on release sources is unavailable As the design progresses and more specific data on potential release sources becomes accessible, it is essential to enhance the classification using a more comprehensive assessment method.
In certain situations, a simplified assessment can be utilized for groups of similar equipment, such as piping sections with flanges, while a more thorough evaluation is reserved for critical potential release sources like relief valves, vents, gas compressors, and pumps.
In many cases the classification examples provided in relevant national or industry codes can, where appropriate, be used to classify some components of larger plants
General
The release rate of a flammable substance is the most important factor that affects the extent of a zone
Generally, the higher the release rate the larger the extent of the zone
Experience indicates that ammonia, with a lower flammable limit (LFL) of 15% by volume, typically disperses quickly in open air, resulting in an explosive gas atmosphere that is usually minimal.
An introduction to the nature of releases that should be considered when approaching classification of potentially explosive areas is provided in the 6.2 to 6.5.
Sources of release
The basic elements for establishing the hazardous zone types are the identification of the source of release and the determination of the grade or grades of the release
To determine the presence of an explosive gas atmosphere, it is essential to assess whether flammable gases or vapours are present alongside air Typically, these flammable substances, including gases, vapours, and potentially flammable liquids or solids, are found within process equipment, which may not always be fully enclosed Identifying potential locations for flammable atmospheres within process equipment and recognizing areas where the release of flammable substances could lead to an external flammable atmosphere is crucial for safety.
Each item of process equipment (for example, tank, pump, pipeline, vessel, etc.) should be considered as a potential source of release of a flammable substance If the item cannot
– 18 – IEC 60079-10-1:2015 © IEC 2015 zone 2 classified areas Where experience or documented evidence indicates that a particular plant design and operations are sound this may be used to support the classification chosen
Furthermore, it is conceivable that an area could be reclassified based on industry experience or new evidence
5.2 Classification by sources of release method
Classification may be approached by calculation, considering appropriate statistical and numerical assessments for the factors concerned, for each source of release
Refer Annex F The source of release approach can be summarized as follows:
• Determine the release rate and grade of release for each source based on likely frequency and duration of release;
• Assess ventilation or dilution conditions and effectiveness;
• Determine zone type based on grade of release and ventilation or dilution effectiveness;
Formulae relevant to determining the release rates under specified conditions can be found in
Annex B These formulae are generally accepted as providing a good basis for calculating release rates for the conditions provided
Annex C offers guidance on evaluating ventilation and dispersion, while alternative assessment methods, such as computational fluid dynamics (CFD), can be effective in certain scenarios Additionally, computer modeling serves as a valuable tool for analyzing the interplay of various factors.
The assessment methods and tools must be validated for suitability and used with caution Assessors should be aware of the limitations and requirements of these tools, adjusting input conditions or results as necessary to draw accurate conclusions.
5.3 Use of industry codes and national standards
Industry codes and national standards may be used where they provide guidance or examples appropriate to the application and comply with the general principles of this standard
Annex K identifies some relevant industry codes and national standards that may provide further detail as well as examples
Where it is not practicable to make required assessments from individual sources of release, a simplified method may be used
Simplified methods will identify sources for zone types 0, 1, and 2, ensuring a conservative approach to potential release sources without requiring individual details This judgment should be guided by a set of criteria rooted in industry experience and tailored to the specific plant.
A comprehensive evaluation of every item in a plant is not required; instead, assessing a single item or condition can suffice to establish a conservative classification for all similar items or conditions within the facility.
Simplified methods often lead to larger zone areas due to the need for more conservative zonal classification in situations where hazards are uncertain This approach prioritizes safety by erring on the side of caution.
To achieve more precise estimates of the boundaries of the classified area, it is essential to refer to illustrative examples or conduct a more detailed assessment of point sources of release, as appropriate.
The use of different methods may be appropriate for classification of a plant at various stages of its development or for various parts of the plant
In the early conceptual phase of a plant, a simplified method is suitable for determining equipment separations, layout, and boundaries, especially when detailed data on release sources is lacking As the design progresses and more information becomes available regarding potential release sources, it is essential to enhance the classification by employing a more comprehensive assessment method.
In certain situations, a simplified assessment can be utilized for groups of similar equipment, such as piping sections with flanges, while a more thorough evaluation is reserved for critical potential release sources like relief valves, vents, gas compressors, and pumps.
In many cases the classification examples provided in relevant national or industry codes can, where appropriate, be used to classify some components of larger plants
The release rate of a flammable substance is the most important factor that affects the extent of a zone
Generally, the higher the release rate the larger the extent of the zone
Experience indicates that ammonia, with a lower flammable limit (LFL) of 15% by volume, typically disperses quickly in open air, resulting in a negligible risk of creating an explosive gas atmosphere.
An introduction to the nature of releases that should be considered when approaching classification of potentially explosive areas is provided in the 6.2 to 6.5
The basic elements for establishing the hazardous zone types are the identification of the source of release and the determination of the grade or grades of the release
To determine the presence of an explosive gas atmosphere, it is essential to assess whether flammable gases or vapours are present alongside air Typically, these flammable substances, including gases, vapours, and potentially flammable liquids or solids, are found within process equipment, which may not always be fully enclosed Identifying potential locations for flammable atmospheres within process equipment and recognizing areas where the release of flammable substances could lead to an external flammable atmosphere is crucial for safety.
Each item of process equipment (for example, tank, pump, pipeline, vessel, etc.) should be considered as a potential source of release of a flammable substance If the item cannot
– 18 – IEC 60079-10-1:2015 © IEC 2015 zone 2 classified areas Where experience or documented evidence indicates that a particular plant design and operations are sound this may be used to support the classification chosen
Furthermore, it is conceivable that an area could be reclassified based on industry experience or new evidence
5.2 Classification by sources of release method
Classification may be approached by calculation, considering appropriate statistical and numerical assessments for the factors concerned, for each source of release
Refer Annex F The source of release approach can be summarized as follows:
• Determine the release rate and grade of release for each source based on likely frequency and duration of release;
• Assess ventilation or dilution conditions and effectiveness;
• Determine zone type based on grade of release and ventilation or dilution effectiveness;
Formulae relevant to determining the release rates under specified conditions can be found in
Annex B These formulae are generally accepted as providing a good basis for calculating release rates for the conditions provided
Annex C offers guidance on evaluating ventilation and dispersion, while alternative assessment methods, such as computational fluid dynamics (CFD), can be effective in certain scenarios Additionally, computer modeling serves as a valuable tool for analyzing the interplay of various factors.
The assessment methods and tools must be validated for suitability and used with caution Assessors should be aware of the limitations and requirements of these tools, adjusting input conditions or results as necessary to draw accurate conclusions.
5.3 Use of industry codes and national standards
Industry codes and national standards may be used where they provide guidance or examples appropriate to the application and comply with the general principles of this standard
Annex K identifies some relevant industry codes and national standards that may provide further detail as well as examples
Where it is not practicable to make required assessments from individual sources of release, a simplified method may be used
Simplified methods will identify sources for zone types 0, 1, and 2, ensuring a conservative approach to potential release sources without requiring individual details This judgment should be guided by a set of criteria rooted in industry experience and tailored to the specific plant.
A comprehensive evaluation of every item in a plant is not required; instead, assessing a single item or condition can suffice to establish a conservative classification for all similar items or conditions within the facility.
Forms of release
General
The characteristic of any release depends upon the physical state of the flammable substance, its temperature and pressure The physical states include:
• a gas, which may be at an elevated temperature or pressure;
• a gas liquefied by the application of pressure, e.g LPG;
• a gas which can only be liquefied by refrigeration, e.g methane;
• a liquid with an associated release of flammable vapour
Releases from plant components like pipe connections, pumps, compressor seals, and valve packings typically begin with a low flow rate If these releases are not promptly addressed, erosion at the source can significantly escalate the release rate, thereby increasing the associated hazards.
When a flammable substance is released above its flashpoint, it generates a flammable vapor or gas cloud that can be either denser or less dense than the surrounding air, or it may remain neutrally buoyant The different release forms and their behavior under various conditions are illustrated in a flow chart in Figure B.1.
All types of releases ultimately result in gaseous or vapor emissions, which can be classified as buoyant, neutrally buoyant, or heavy These characteristics significantly influence the size of the zone created by each specific release type.
As relative density increases, the horizontal extent of the ground-level zone typically expands, while a decrease in relative density leads to a greater vertical extent above the source.
Gaseous release
A gas release generates a gas jet or plume at the source, influenced by the pressure at the release point, such as a pump seal, pipe connection, or evaporative pool The subsequent movement of the gas cloud is affected by factors including the gas's relative density, the extent of turbulent mixing, and the prevailing air movement.
In calm conditions, low-velocity releases of gases that are much less dense than air, such as hydrogen and methane, will rise In contrast, denser gases like butane and propane will settle at ground level or in depressions Eventually, atmospheric turbulence will mix the released gas with air, leading to a state of neutral buoyancy Gases or vapors with a density similar to that of air are considered neutrally buoyant.
Higher pressure releases will initially produce jets of released gas which will mix turbulently with the surrounding air and entrain air in the jet
At high pressures, gas expansion leads to a thermodynamic effect where the escaping gas cools down and may initially act as heavier than air However, this cooling, influenced by the Joule-Thomson effect, is countered by heat from the surrounding air Eventually, the gas cloud reaches a state of neutral buoyancy, transitioning from being heavier than air This shift can occur at any point, depending on the release characteristics, and may happen after the gas cloud has diluted below the Lower Flammable Limit (LFL).
NOTE Hydrogen demonstrates a reverse Joule-Thomson effect, heating up as it expands and so will never exhibit a heavier than air effect.
Liquefied under pressure
Some gases can be liquefied by the application of pressure alone, e.g propane and butane, and are usually stored and transported in this form
When pressurized liquefied gas leaks from its containment, it typically escapes as a gas from vapor spaces or gas lines This rapid evaporation leads to significant cooling at the release point, which can result in icing from the condensation of atmospheric water vapor.
A liquid leak undergoes flash evaporation at the release point, where the evaporating liquid absorbs energy from itself and the surrounding air, resulting in a cooling effect This cooling prevents complete evaporation, leading to the formation of an aerosol In cases of significant leaks, cold pools of liquid may form on the ground, which will gradually evaporate, contributing to the overall gas release.
The cold aerosol cloud behaves similarly to a dense gas, and when a pressurized liquid is released, the cooling effect from evaporation condenses surrounding humidity, resulting in a visible cloud formation.
According to IEC 60079-10-1:2015, an area will not be classified as hazardous if it foreseeably contains a flammable substance that cannot be released into the atmosphere For instance, a fully welded pipeline is not deemed a source of release and therefore does not create a hazardous area.
To assess the potential release of flammable substances into the atmosphere, it is essential to identify the grade of release based on its frequency and duration Additionally, activities such as filter changes or batch filling in enclosed process systems must be considered as potential sources of release during area classification Each release is categorized as either 'continuous', 'primary', or 'secondary' to ensure proper safety measures are implemented.
NOTE 1 Releases may form part of process, e.g taking samples, or may occur as part of a routine maintenance procedure These forms of release are generally classified as continuous or primary grades of release Accidental releases are generally classified as secondary grades of release
NOTE 2 One item may give rise to more than one grade of release For example, there may be a small primary grade release, but a larger release could occur under abnormal operation; thus giving rise to a secondary grade release In this situation, both release conditions (both grades of release) need full consideration as described in this standard
After determining the grade or grades of the release, it is essential to assess the release rate and other factors that could impact the characteristics and scope of the zone.
In situations where a flammable substance is present in small quantities, such as in laboratory settings, the standard area classification procedure may not be suitable, even if there is a potential for explosion It is essential to consider the specific factors at play in these cases.
The classification of areas for process equipment that involves the combustion of flammable substances, such as fired heaters, furnaces, boilers, and gas turbines, must consider factors like purge cycles, as well as start-up and shut-down conditions.
Closed systems that adhere to specific construction codes can significantly reduce the risk of flammable substance leaks To ensure safety, a thorough assessment of hazardous area classification is essential, confirming that installations meet all relevant construction and operational standards This compliance verification must encompass design, installation, operation, maintenance, and monitoring processes.
Mists which form through leaks of pressurized liquid can be flammable even though the liquid temperature is below the flash point (see Annex G)
The characteristic of any release depends upon the physical state of the flammable substance, its temperature and pressure The physical states include:
• a gas, which may be at an elevated temperature or pressure;
• a gas liquefied by the application of pressure, e.g LPG;
• a gas which can only be liquefied by refrigeration, e.g methane;
• a liquid with an associated release of flammable vapour
Releases from plant components like pipe connections, pumps, compressor seals, and valve packings typically begin with a low flow rate If these releases are not promptly addressed, erosion at the source can significantly escalate the release rate, thereby increasing the associated hazards.
When a flammable substance is released above its flashpoint, it generates a flammable vapor or gas cloud that can be either denser or less dense than the surrounding air, or it may remain neutrally buoyant The different release forms and their behavior under various conditions are illustrated in a flow chart in Figure B.1.
All types of releases ultimately result in gaseous or vapor emissions, which can be classified as buoyant, neutrally buoyant, or heavy These characteristics significantly influence the size of the zone created by each specific type of release.
As relative density increases, the horizontal extent of the ground-level zone typically expands, while a decrease in relative density leads to a greater vertical extent above the source.
Liquefied by refrigeration
Permanent gases, such as methane and hydrogen, can only be liquefied through refrigeration Small leaks of these refrigerated gases evaporate rapidly, preventing the formation of liquid pools by absorbing heat from their surroundings However, in the case of a large leak, a cold pool of liquid may develop.
As the cold liquid absorbs energy from the ground and surrounding atmosphere, it boils, creating a dense cold gas cloud To manage leakages, dikes or bund walls can be employed to direct or contain the flow.
NOTE 1 Care needs to be taken when classifying areas containing cryogenic flammable gases such as liquefied natural gas Vapours emitted will generally be heavier than air at low temperatures but will become neutrally buoyant on approaching ambient temperature
NOTE 2 Permanent gases have a critical temperature lower than –50 °C.
Aerosols
An aerosol consists of tiny liquid droplets suspended in air, rather than being a gas These droplets form from vapors or gases under specific thermodynamic conditions or through the flash evaporation of pressurized liquids The scattering of light within an aerosol cloud often makes it visible to the naked eye The behavior of aerosol dispersion can resemble that of a dense gas or a neutrally buoyant gas Additionally, aerosol droplets can coalesce and fall out of the cloud, while aerosols from flammable liquids may absorb heat from their surroundings, leading to evaporation and contributing to the gas/vapor cloud.
Vapours
Liquids in equilibrium with their surroundings produce a vapor layer above their surface The pressure exerted by this vapor in a closed system is referred to as vapor pressure, which increases non-linearly with temperature.
Evaporation is an energy-intensive process that draws energy from the liquid itself or its environment, potentially lowering the liquid's temperature and mitigating temperature increases However, the temperature changes resulting from increased evaporation under normal conditions are typically too minor to influence hazardous area classifications Additionally, predicting the concentration of generated vapor is complex, as it depends on factors such as the evaporation rate, the liquid's temperature, and the surrounding airflow.
Liquid releases
The release of flammable liquids typically creates a pool on the ground, accompanied by a vapor cloud at the liquid's surface, unless the surface is absorbent The dimensions of the vapor cloud are influenced by the substance's properties and its vapor pressure at the surrounding temperature.
Vapour pressure indicates the evaporation rate of a liquid, with substances that have high vapour pressure at normal temperatures being classified as volatile Generally, a liquid's vapour pressure increases at ambient temperatures as its boiling point decreases Additionally, as the temperature rises, the vapour pressure also increases.
Releases of flammable liquids can happen on water surfaces, as many of these liquids are less dense and not miscible with water This results in the formation of a thin film on the water, whether on the ground, in drainage systems, or in open bodies of water like seas, lakes, or rivers The increased surface area from this film enhances the evaporation rate, making the calculations in Annex B inapplicable in such scenarios.