– 8 – IEC 60079-10-2:2015 © IEC 2015 EXPLOSIVE ATMOSPHERES – Part 10-2: Classification of areas – Explosive dust atmospheres 1 Scope This part of IEC 60079 is concerned with the identi
General
This standard utilizes area classification, akin to the approach for flammable gases and vapors, to evaluate the probability of an explosive dust atmosphere developing.
Dust can create explosive atmospheres only when concentrations fall within a specific range Even if a dust cloud has a high concentration, it may not be explosive; however, a decrease in concentration could lead it to enter the explosive range Additionally, not all release sources will generate an explosive dust atmosphere, and dust clouds often have varying densities, necessitating careful consideration of concentration fluctuations within any given cloud.
Dust that is not eliminated through mechanical extraction or ventilation settles at varying rates based on its properties, including particle size, forming layers or accumulations It is important to consider that even a small, continuous source of dust release can lead to the formation of a potentially hazardous dust layer over time.
The hazards presented by dusts are as follows:
– the formation of a dust cloud from any source of release, including a layer or accumulation, to form an explosive dust atmosphere (see Clause 5);
Dust layers can pose significant fire hazards, as they may ignite from self-heating or contact with hot surfaces and thermal flux While these layers are unlikely to form a dust cloud, their ignition can lead to overheating of equipment and potentially serve as an ignition source for explosive atmospheres.
Since explosive dust clouds and dust layers may exist, any source of ignition should be avoided
To minimize the risk of ignition, it is essential to implement measures that reduce the presence of dust and ignition sources, ensuring that the chances of their occurrence together are extremely low, thereby making the overall risk negligible.
In situations where the risk of explosion cannot be entirely eliminated, implementing explosion protection measures is essential This may include strategies such as explosion venting, explosion suppression, or explosion isolation to enhance safety.
After completing the area classification, a risk assessment is 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.
This standard distinguishes between explosive dust atmospheres and dust layers, addressing area classification for explosive dust clouds while recognizing dust layers as potential sources of release Detailed considerations for dust layers are outlined in Clause 7.
Area classification procedure for explosive dust atmospheres
Area classification is based on a number of factors and may require informed input from a number of sources These factors include:
• Whether the dust is combustible or not Dust combustibility can be confirmed by laboratory tests to ISO/IEC 80079-20-2
To ensure accurate material characteristics for dusts, it is essential to gather data from various published sources, consult a process specialist, or conduct testing However, characteristics sourced from publications must be validated for specific applications, as significant variations in dust characteristic values can occur between different data sources.
• Nature of dust releases from particular process sources Specialist engineering knowledge may be required for this information
• Operational and maintenance procedures for the plant, including housekeeping
• Other equipment and safety information
Effective collaboration among safety and equipment specialists is essential While dust zone definitions primarily address cloud risks, it is important to also consider layers that may be disturbed to create dust clouds The identification procedure begins with assessing whether the material is combustible and evaluating its characteristics, including particle size, moisture content, minimum ignition temperature for clouds and layers, and electrical resistivity It is crucial to classify the material into the appropriate dust group: Group IIIA for combustible flyings, Group IIIB for non-conductive dust, or Group IIIC for conductive dust.
Information on dust characteristics is detailed in ISO/IEC 80079-20-2 The second step involves identifying equipment that may contain explosive dust mixtures or sources of dust release, as outlined in Clause 5, which may require reviewing process line diagrams and plant layout drawings This step should also consider the potential for dust layer formation, as specified in Clause 7 The third step is to assess the likelihood of dust being released from these sources, thereby evaluating the risk of explosive dust atmospheres in different areas of the installation, as indicated in Clause 5.3.
After completing the necessary steps, the identification and definition of zone boundaries can occur Documentation of the zone types, their extent, and the presence of dust layers is typically recorded on an area classification drawing These documents serve as the foundation for assessing potential ignition sources.
Documenting the rationale behind decisions in the area classification study is essential for future reviews Area classification reviews should occur after any process changes, alterations in materials, or increased dust emissions due to plant deterioration Additionally, a review is anticipated after the commissioning of a plant or process and should continue on a regular schedule thereafter.
Due to the broad scope of this standard, it is not possible to specify exact measures for every situation Therefore, it is crucial that the recommended procedures are executed by personnel who are knowledgeable about area classification principles, the materials involved, and the specific operations of the plant.
Competence of personnel
Area classification must be conducted by qualified individuals who possess a deep understanding of dust characteristics and their significance It is essential that these professionals are well-versed in the relevant processes and equipment, as well as safety protocols, electrical systems, mechanical aspects, and other engineering qualifications.
NOTE These elements are covered in several personnel certification schemes, such as the IECEx Unit of Competence Ex002 according to IECEx OD 504
General
Explosive dust atmospheres arise from specific sources where dust can be released or elevated, creating conditions for potential explosions These sources include locations where dust accumulates, which can be disturbed to generate a dust cloud.
Not all release sources will create an explosive dust atmosphere; however, even a small or dilute continuous release over time can lead to the formation of a dust layer.
Identifying the conditions that lead to explosive dust atmospheres or dust layer formation in process equipment and steps is crucial It is essential to evaluate these conditions both inside and outside of dust containment systems.
Dust containment
In a dust containment system, dust is prevented from escaping into the external environment; however, continuous dust clouds may form within the containment during the process The duration and frequency of these clouds depend on the process cycle It is essential to analyze the equipment under normal, abnormal, startup, and shutdown conditions to identify the occurrence of dust clouds and layers The findings from this analysis should be documented in the verification dossier, particularly noting the formation of dust layers as outlined in Clause 7.
Identification and grading of sources of release
Several factors can affect area classification outside of dust containment When higher than atmospheric pressures, such as in positive pressure pneumatic transfer, are present, dust can escape from leaking equipment Conversely, negative pressure within the containment significantly reduces the chances of creating dusty areas outside the equipment Additionally, factors like dust particle size, moisture content, transport velocity, dust extraction rate, and fall height can impact the potential for dust release Once the potential for release is assessed, it is essential to identify each source of release and determine its corresponding grade or grades.
Grades of release are as follows:
Continuous grade of release refers to a type of release that occurs consistently over time, whether for extended durations or in frequent short bursts An example of this can be seen in the operation of a mixing vessel or a storage silo, which is regularly filled and emptied.
A primary grade of release refers to emissions that are anticipated to happen periodically or occasionally during standard operations An example of this can be seen in the area surrounding an open bag filling or emptying point.
A secondary grade of release refers to an event that is unlikely to happen during normal operations and, if it does occur, is expected to be infrequent and of short duration An example of this is a dust handling plant, where dust deposits may be present.
When assessing potential sources of release, it is important to note that catastrophic failures do not need to be considered Specifically, certain items should be excluded as sources of release during both normal and abnormal operations.
– pressure vessels, the main structure of the shell including closed nozzles and man-holes; – pipes, ducting and trunking without joints;
– valve glands and flanged joints, provided that in the design and construction, adequate consideration has been given to the prevention of the release of dust
General
Explosive dust atmospheres are categorized into specific zones based on how often and how long these conditions occur For further details, examples of these zones can be found in Annex A Additionally, any layers, deposits, or piles of dust are regarded as potential sources that can create an explosive dust atmosphere.
Extent of zones
General
The extent of a zone for explosive dust atmospheres is determined by measuring the distance from the edge of a dust release source to the point where the associated hazard is deemed to be absent.
Explosive dust atmospheres are typically considered absent when the dust concentration is significantly below the minimum threshold needed for such an environment However, it is important to recognize that fine dust can be transported by air movement within a building, potentially leading to hazardous conditions.
To ensure comprehensive coverage, classification should extend to encompass all areas, particularly where small unclassified regions exist between classified zones In outdoor areas like Zone 21 and Zone 22, which are exposed to the elements, weather conditions such as wind and rain can impact the zones Therefore, it is essential to adjust the boundaries of these zones to account for such environmental variations.
Natural ventilation can dilute hazardous concentrations below the explosive limit, potentially reducing the extent of the danger zone However, it may also disturb existing dust layers, which could increase the size of the zone.
Zone 20
The extent of zone 20 includes the inside of ducts, producing and handling equipment in which explosive dust atmospheres are present continuously, for long periods, or frequently
If an explosive dust atmosphere outside dust containment is continuously present, a Zone 20 classification is required
Zone 21
In most circumstances, the extent of Zone 21 can be defined by evaluating sources of release in relation to the environment causing explosive dust atmospheres
The extent of Zone 21 is as follows:
– the inside of some dust handling equipment in which an explosive dust atmosphere is likely to occur periodically, for example starting and stopping of filling equipment;
Zone 21 is established outside equipment based on a primary grade of release, influenced by various dust parameters including dust quantity, flow rate, particle size, and moisture content It is essential to consider the release source and the conditions that led to the release to accurately define the extent of the zone.
– where the spread of dust is limited by mechanical structures (walls, etc.), their surfaces can be taken as the boundary of the zone
A non-confined Zone 21 (not limited by mechanical structures, e.g a vessel with an open man-hole) located inside, will usually be surrounded by a Zone 22
NOTE 1 If dust layers are found to have accumulated outside the original Zone 21, then the classification of the zone 21 area might be required to be extended (it could become a Zone 22) taking into account the extent of the layer and any disturbance of the layer that produces a cloud
NOTE 2 If the boundary between Zone 21 and Zone 22 is difficult to determine, it might be practical to classify the entire area or room as Zone 21.
Zone 22
In most circumstances, the extent of Zone 22 can be defined by evaluating secondary grade sources of release in relation to the environment causing the explosive dust atmospheres
The extent of Zone 22 is as follows:
The extent of an area affected by a secondary grade dust release is influenced by various parameters, including dust amounts, flow rate, particle size, and moisture content It is essential to consider the conditions surrounding the release source to accurately define the zone's boundaries Additionally, when dust dispersion is restricted by mechanical structures such as walls, these surfaces can serve as the zone's limits.
If dust layers accumulate beyond the original Zone 22, it may be necessary to extend the classification of this area This extension should consider the extent of the dust layer and any disturbances that could create a cloud.
Based on the likelihood of the formation of explosive dust atmospheres, the areas can be designated according to Table 1
Table 1 – Designation of zones depending on presence of dust
Presence of dust Resulting zone classification of area of dust clouds
Inside containment, where dusts are handled or processed, layers of dust of uncontrolled thickness often cannot be prevented because they are an integral part of the process
Effective housekeeping is crucial for managing dust layer thickness outside containment, and the level of housekeeping must be clearly defined for proper classification It is important to coordinate housekeeping arrangements with plant management Additional risks related to dust layers and housekeeping practices are detailed in Annex B.
Conditions that may cause dust layers to be raised to form a dust cloud, such as ventilation, wind or other conditions, must be taken into consideration during the area classification
General
Area classification, and the various steps taken which lead to the area classification, shall be documented
All pertinent information must be referenced, including recommendations from applicable codes and standards, evaluations of dust dispersion from all emission sources, process parameters and dust control strategies that affect the creation of explosive dust environments and layers, operational and maintenance factors, housekeeping initiatives, and designated EPLs.
The results of the area classification study and any subsequent alteration to it shall be included in the verification dossier
The properties, or basis for properties, used for the area classification concerning all process materials used on the plant shall be listed The information should include items such as:
– ignition temperatures of a dust clouds,
– ignition temperatures of dust layers,
– minimum ignition energy of a dust cloud,
While reference materials exist that can offer some dust parameters for explosive atmosphere safety, the variability among different dust types necessitates testing to accurately determine all relevant parameters.
Drawings, data sheets and tables
Content of documents
Area classification documents, whether in hard copy or electronic format, must contain plans, elevations, or three-dimensional models that illustrate the types and extents of zones, the allowable thickness of dust layers, and the minimum ignition temperatures for both the dust cloud and the dust layer Additionally, these documents should provide other pertinent information related to the classification.
The IEC 60079-10-2:2015 standard emphasizes the importance of identifying and locating sources of release in large and complex plants, suggesting that numbering these sources can aid in cross-referencing with area classification data sheets and drawings It also highlights the need for information on housekeeping and preventative measures that support the classification, as well as methods for maintaining and regularly reviewing the classification in response to changes in process materials, methods, and equipment Additionally, a distribution list of the classification should be maintained, along with documented reasons for decisions regarding the establishment of zones and the extent of dust layers.
A symbol key shall always be provided on each drawing
The preferred area classification symbols shown in Figure 1 may be varied,, e.g for drawing clarity or to show differing dust characteristics.
Preferred Symbol key for area classification zones
Figure 1 – Identification of zones on drawings
Examples of zones
General
The examples provided should be adapted to fit specific process equipment and circumstances, as they are not meant to be strictly followed Additionally, it is important to acknowledge that certain equipment may demonstrate multiple grades of release.
Zone 20
Examples of locations that may give rise to Zone 20:
– locations inside the dust containment;
– hoppers, silos, cyclones and filters, etc;
– dust transport systems, except some parts of belt and chain conveyors, etc;
– blenders, mills, dryers, bagging equipment, etc.
Zone 21
Examples of locations that may give rise to Zone 21:
– areas adjacent to dust containment and in the immediate vicinity of access doors subject to frequent removal or opening for operation purposes when internal explosive dust atmospheres are present;
Areas near filling and emptying points, feed belts, sampling points, truck dump stations, and belt dump over points are often left without measures to prevent the creation of explosive dust atmospheres.
Dust accumulation can occur in areas outside of containment, where process operations may disturb the dust layer and create explosive atmospheres Additionally, inside dust containment, explosive dust clouds may form, although not continuously or for extended periods Examples include the filling of silos with low-dust bulk materials and the dirty side of filters, particularly when large self-cleaning intervals are present.
In many cases, a distance of about 1 meter around the release source is adequate for defining a Zone 21, extending vertically down to the ground or to a solid floor level.
Zone 22
Examples of locations that may give rise to Zone 22:
– outlets from bag filter vents which, in the event of a malfunction, can emit explosive dust atmospheres;
Locations near equipment that open infrequently or are prone to leaks, such as pneumatic equipment and flexible connections, can easily accumulate dust and debris due to potential damage.
– storage of bags containing dusty products Failure of bags can occur during handling, causing dust emission;
Areas typically designated as Zone 21 may be reclassified to Zone 22 when appropriate measures, such as exhaust ventilation, are implemented to mitigate the risk of explosive dust atmospheres These safety measures should be applied near critical locations, including bag filling and emptying points, feed belts, sampling points, truck dump stations, and belt dump over points.
Controllable dust layers can create explosive atmospheres if disturbed, making it essential to remove these layers through proper cleaning before hazardous conditions arise This proactive approach is crucial for designating areas as non-hazardous, highlighting the importance of effective housekeeping practices.
In many cases, a distance of about 3 meters from the release source is adequate for defining a Zone 22, extending vertically down to the ground or to a solid floor level.
Bag emptying station within a building and without exhaust ventilation
In this example, illustrated in Figure A.1, bags are regularly emptied into a hopper, which then pneumatically conveys the contents to another area of the plant without the need for exhaust ventilation Typically, a portion of the hopper remains filled with product.
Zone 20 Inside the hopper because an explosive dust atmosphere is present frequently or even continuously
Zone 21 is established around an open man-hole that lacks exhaust ventilation, making it a significant source of release This zone extends a certain distance from the man-hole's edge and reaches down to the floor.
Zone 22 may develop next to Zone 21 due to the buildup of dust layers or the presence of very fine particles that can occasionally escape the typical boundaries of Zone 21 during abnormal operating conditions.
If dust layers build up, additional classification may be necessary based on the thickness of the layer and any disturbances that create a dust cloud, along with housekeeping standards (refer to Annex B) During bag discharges, air movements can sometimes extend the dust cloud beyond Zone 21 under abnormal conditions, potentially necessitating a Zone 22 classification as outlined in section 6.2.4.
6 to process via a rotary valve
NOTE 1 The relative dimensions are for illustration only In practice other distances may be required
NOTE 2 Additional measures such as explosion venting or explosion isolation, etc may be necessary but are outside the scope of this standard and are not therefore given
Figure A.1 – Bag emptying station within a building and without exhaust ventilation
Bag emptying station with exhaust ventilation
The system illustrated in Figure A.2 features extract ventilation, similar to the example in Clause A.2, which effectively minimizes dust escape by containing it within the system.
Zone 20 Inside the hopper because an explosive dust atmosphere is present frequently or even continuously
Zone 22 is designated around the open man-hole, which is classified as a source with a secondary grade of release Under normal conditions, the dust extraction system prevents any dust escape, effectively sucking in any released particles The boundaries of Zone 22 extend a certain distance from the man-hole's edge and down to the floor, with the specific area needing to be determined based on the dust characteristics and the associated process.
5 to process via a rotary valve
NOTE 1 The relative dimensions are for illustration only In practice other distances may be required
NOTE 2 Additional measures, such as explosion venting or explosion isolation etc may be necessary but are outside the scope of this standard and are not therefore given
Figure A.2 – Bag emptying station with exhaust ventilation
Cyclone and filter with clean outlet outside building
In a suction extraction system, as illustrated in Figure A.3, the cyclone and filter work together to minimize dust accumulation The extracted product is directed through a rotary valve into a closed bin, resulting in minimal dust in the filter and extended self-cleaning intervals Consequently, the interior experiences only occasional dust clouds during normal operation, while the extraction fan efficiently expels the extracted air outside.
Zone 20 Inside the cyclone because an explosive dust atmosphere is present frequently or even continuously
Zone 21 There is a Zone 21 on the dirty side of the filter only if small quantities of dust are not collected by the cyclone in normal operation
Zone 22 refers to the area where a dust cloud may form on the clean side of a filter if the filter element fails This zone encompasses the interior of the filter, the downstream extract ducting, and the area surrounding the discharge of the extract duct It extends a considerable distance around the duct outlet and reaches down to the ground The specific boundaries of Zone 22 must be established based on the dust characteristics and the associated process.
If dust layers build up on the exterior of plant equipment, additional classification may be necessary This should consider the thickness of the layer, environmental conditions, and any disturbances that could create a dust cloud.
2 Zone 20, see 6.2.2 8 to fines bin
5 to product silo 11 Zone 21, see 6.2.3
NOTE 1 The relative dimensions are for illustration only In practice, other distances may be required
NOTE 23 Additional measures, such as explosion venting or explosion isolation etc may be necessary but are outside the scope of this standard and are not therefore given
Figure A.3 – Cyclone and filter with clean outlet outside building
Drum tipper within a building without exhaust ventilation
In the process illustrated in Figure A.4, powder is transferred from drums into a hopper for transportation via a screw conveyor to a nearby room A full drum is placed on the platform, and its lid is removed Hydraulic cylinders secure the drum to the closed diaphragm valve The hopper lid is then opened, and the drum carrier is rotated to align the diaphragm valve with the hopper Once the diaphragm valve is opened, the powder is gradually conveyed by the screw conveyor until the drum is completely emptied.
When a new drum is needed, the diaphragm valve is closed, and the drum carrier is rotated back to its original position The hopper lid is then closed, and the hydraulic cylinders release the drum Finally, the lid is replaced before the drum is removed.
Zone 20 The interior of the drum, hopper and screw conveyor will contain dust clouds frequently and for long periods and are therefore classified Zone 20
Zone 21 is designated for areas where dust can be released as a cloud, particularly when the drum lid, hopper lid, or diaphragm valve is manipulated The boundaries of Zone 21 are defined by the distance surrounding the drum, hopper, and diaphragm valve To accurately establish the extent of Zone 21, it is essential to consider the specific characteristics of the dust and the associated processes.
Zone 22 The remainder of the room is Zone 22 due to the possibility of accidental spillage forming dust layers and disturbance of large quantities of dust
NOTE 1 The relative dimensions are for illustration only In practice, other distances may be required
NOTE 2 Additional measures such as explosion venting or explosion isolation etc may be necessary but are outside the scope of this standard and are not therefore given
Figure A.4 – Drum tipper within a building without exhaust ventilation
Introductory remarks
Area classification in this standard is based on definitions for zones Any hazards presented by dust layers should be considered separately from dust clouds
Three risks are presented by dust layers:
A primary explosion in a building can create dust clouds that may lead to secondary explosions, which can be more destructive than the initial blast To mitigate this risk, it is essential to manage and control dust layers effectively.
2) Dust layers may be ignited by the heat flux from equipment on which the layer rests, which may be a slow process
3) A dust layer, even a thin layer, may be raised into a cloud, ignite and cause an explosion
The risks associated with dust are influenced by its properties and the thickness of the layers, which are affected by housekeeping practices Proper equipment selection and effective housekeeping can significantly reduce the likelihood of a dust layer igniting a fire.
Modifications in the dust layer's state, such as its moisture absorbency, can diminish or completely remove the potential to transform a dust layer into a dust cloud Consequently, this may lead to a reduced risk of secondary explosions, while the fire risk could either remain unchanged or also decrease.
Levels of housekeeping
Cleaning frequency alone does not adequately assess the risk posed by dust layers, as highlighted in B.1 The dust deposition rate significantly influences this risk; for instance, a secondary grade of release with a high deposition rate can form a hazardous layer more rapidly than a primary grade with a lower rate Therefore, both the frequency and effectiveness of cleaning are crucial factors to consider.
Thus, the presence and duration of a dust layer depends on:
– the grade of release from the source of the dust,
– the rate at which dust is deposited, and
– the effectiveness of housekeeping (cleaning)
Three levels of housekeeping can be described
Dust layers are minimized or eliminated regardless of the release grade, significantly reducing the risk of explosive dust clouds and fire hazards associated with dust accumulation.
Fair: Dust layers are not negligible but are short-lived (typically less than one shift) The dust is removed before any fire can start
Poor: Dust layers are not negligible and persist for a long period of time (typically more than one shift) The fire risk and secondary explosion risk may be significant
To ensure safety, it is essential to prevent poor housekeeping and conditions that may generate a dust cloud from existing layers Any activities, such as individuals entering the room, that could lead to the formation of a dust cloud must be taken into account during hazardous area classification.
NOTE When a planned level of housekeeping is not maintained, additional fire and explosion risks are created
General
A hybrid mixture consists of a flammable gas or vapor combined with combustible dust or flyings, which can exhibit unique behaviors distinct from the individual components Due to the diverse scenarios encountered in industrial settings, it is impractical to offer specific guidance Nevertheless, this Annex outlines key considerations to address when dealing with hybrid mixtures.
Ventilation
Ventilation is an important control measure that can mitigate gas and vapor hazards; however, it may inadvertently increase dust hazards or affect other components of the mixture in unpredictable ways.
Explosive limits
A hybrid mixture can create an explosive atmosphere even when the gas or vapor concentrations are below their individual explosive limits It is advisable to classify a hybrid mixture as explosive if the gas or vapor concentration surpasses 25% of its Lower Explosive Limit (LEL).
Chemical reactions
It is important to take into account the potential chemical reactions that can happen within the materials or the trapped gas in dust, as these reactions may lead to the release of gas during the process.
Minimum ignition parameters
In hybrid mixtures, the minimum ignition parameters, including minimum ignition energy and auto ignition temperature for gas/vapor mixtures, as well as the minimum ignition temperature of dust clouds, may differ from those of individual components In the absence of specific data, it is advisable to use the worst-case parameters of any component within the mixture.
Final classification
In environments where a hybrid mixture of gas and dust is present, it is essential to assign both gas and dust zones based on the most severe potential hazards This approach ensures that the worst-case scenario is taken into account during any Equipment Protection Level (EPL) assessment.
IEC 60050-426, International Electrotechnical Vocabulary – Part 426: Equipment for explosive atmospheres
IEC 60079-2, Explosive atmospheres – Part 2: Equipment protection by pressurized enclosures "p"
IEC 60079-11, Explosive atmospheres – Part 11: Equipment protection by intrinsic safety "i"
IEC 60079-14, Explosive atmospheres – Part 14: Electrical installations design, selection and erection
IEC 60079-18, Explosive atmospheres – Part 18: Equipment protection by encapsulation "m"
IEC 60079-28, Explosive atmospheres – Part 28: Protection of equipment and transmission systems using optical radiation
IEC 60079-31, Explosive atmospheres – Part 31: Equipment dust ignition protection by enclosure “t”
IEC 60079-32-2, Explosive atmospheres – Part 32-2: Electrostatics hazards – Tests 2
IEC 61241-2-1, Electrical apparatus for use in the presence of combustible dust – Part 2: Test methods – Section 1: Methods for determining the minimum ignition temperatures of dust
IECEx OD 504, Specification for Units of Competency Assessment Outcomes
ISO 4225, Air quality – General aspects – Vocabulary