What is Hazardous Area Classification (HAC)? | Practical Guide
Hazardous area classification (HAC) is the process of identifying and classifying areas where explosive atmospheres may occur due to the presence of flammable gases, vapours, mists, or combustible dusts. It defines the extent, likelihood and duration of an explosive atmosphere (known as a hazardous zone), allowing appropriate equipment and control measures to be selected to mitigate the risk of explosion in hazardous areas. Hazardous area classification is a legal requirement which forms a key part of DSEAR compliance, and is often carried out alongside a DSEAR risk assessment.
What is a Hazardous Zone?
A hazardous zone - sometimes called an ATEX zone - is an area with a defined extent which defines how frequently an explosive atmosphere is likely to be present for. Ignition sources must be removed from or controlled within hazardous zones - common examples of ignition control measures include suitable ATEX rated equipment, electrostatic discharge (ESD) controls, and lightning protection.
Gases and vapours can be classified into the following zones:
Zone 0 - is a place where an explosive gas/vapour atmosphere is present continuously or for long periods of time
Zone 1 - is a place where an explosive gas/vapour atmosphere is present periodically or occasionally in normal operation
Zone 2 - is a place where an explosive gas/vapour atmosphere is not likely to occur during normal operation, and would only exist for a short time if it did occur
Zone 0/1/2 Negligible Extent (NE) - is a place where an explosive atmosphere could exist, but is so small that the consequence of it being ignited would be negligible and is therefore considered to be non-hazardous
Combustible dusts are classified in a similar way:
Zone 20 - is a place where an explosive dust atmosphere is present continuously or for long periods of time
Zone 21 - is a place where an explosive dust atmosphere is present periodically or occasionally in normal operation
Zone 22 - is a place where an explosive dust atmosphere is not likely to occur during normal operation, and would only exist for a short time if it did occur
How is a Hazardous Zone Determined?
A hazardous zone is the likelihood and duration of an explosive atmosphere being present. The extent of the hazardous zone is a measure of how far the explosive atmosphere could extend from the release source. The presence of a flammable material does not necessarily mean a hazardous zone will need to be classified - the following factors influence the likelihood, duration, and size of hazardous zones:
Material Properties - Lower and Upper Explosive Limits (LEL, UEL) for gases/vapours or Minimum Explosible Concentration (MEC) for dusts
Source of Release - the locations where a dangerous substance could be released impact the likelihood, duration, and size of the hazardous zone
Grade of Release - this is typically Continuous, Primary, or Secondary
Material Quantity - the amount of substance present can impact the duration and size of the hazardous zone
Process Conditions - process temperature and pressure impacts the likelihood, duration and size of the hazardous zone
Release Hole Size and Geometry - the size and shape of the release orifice impacts the size of the hazardous zone for gaseous and liquid releases
Process Containment - physical containment (e.g. effective flange guards) impacts the size of the hazardous zone
Ventilation and Inerting - the ventilation arrangement and inerting systems can impact the likelihood of the hazardous zone
All of these factors are considered when conducting a hazardous area classification assessment. An accurate hazardous area classification is vital so that ignition sources can be effectively controlled. Blanket zoning - where a whole room or area is zoned - should be avoided wherever possible. This often creates unmanageable zones, and significant effort and expense to install suitable ATEX equipment.
A good hazardous area classification should state the zone classification (0/20, 1/21, 2/22) and clearly define the extent of the zone, with an accompanying drawing showing the zone in plan and elevation view.
How is Hazardous Area Classification Carried Out?
Step 1: Gather Data
The first step in HAC is to gather information on your site, processes and substances used. This includes:
Material properties (dusts, gases, and vapours)
Process descriptions and operating conditions
Drawings (site layout, PFDs, and/or P&IDs)
Ventilation details
This will involve a site visit for existing processes, and a data request for design stage desktop-based assessments.
Step 2: Identify Sources of Release and Select Applicable Standards / Guidance
We examine the process and identify where the potential sources of release are. Common sources of release are flanges and connections in pipework, vents, compressor and pump seals, powder tipping points, dust collector exhausts, and inside process equipment.
An appropriate standard, guidance document, or modelling method should then be selected to conduct the hazardous area classification. It is important that there is a sound understanding of the assumptions and limitations of the selected methodology.
Step 3: Develop the HAC Document and Drawings
A detailed assessment of the process to classify the hazardous zones, taking into account material properties, quantity of substances used, process conditions, ventilation, and other considerations should be conducted. The findings of the hazardous area classification should then be recorded in a clear and concise document. Typically, the format given in BS EN 60079-10-1:2021 is used. This includes:
List of release sources
Grade of release
Process conditions
Ventilation assessment
Zone classification and extent
Minimum ATEX equipment specification required for each zone
Reference to any standards, guidance, and calculations used
HAC drawings should then be produced to show the hazardous zones in both plan and elevation view, as required by the DSEAR ACOP. It is usually useful to display these drawings at the entrance to areas where hazardous areas are present around site to allow employees and contractors to effectively control ignition sources in these areas.
Step 4: Managing Hazardous Zones
The hazardous area classification and drawings will allow you to determine where ignition sources should be controlled and select appropriate ATEX equipment. Examples of other ignition control measures which may be required include ESD footwear and conductive flooring, or lightning protection - the DSEAR risk assessment should advise on what is required.
An asset register of ATEX equipment should be developed so that it can be inspected and maintained in line with the manufacturer’s instructions and the requirements of BS EN 60079-17:2024. ATEX equipment should be installed and inspected by a competent person (e.g. CompEx certified).
Signage should also be erected at the entrance to areas where hazardous zones have been classified - guidance on Ex signage can be found here.
What Standards and Guidance Should I Use for Hazardous Area Classification?
There are two main standards for conducting hazardous area classification - EN 60079-10-1 for gases, liquids, and vapours, and EN 60079-10-2 for combustible dusts. There are also numerous industrial guidance documents which are commonly used for hazardous area classification in specific industries. These are typically derived from the methodology in the main hazardous area classification standards, empirical data, or dispersion modelling. All standards and guidance documents should be applied by competent personnel using sound engineering judgement, with a sound understanding of the assumptions and limitations of the methodology.
A list of commonly used hazardous area classification standards and guidance documents is provided below:
BS EN 60079-10-1:2021 - This is the main standard for gases, liquids, and vapours
BS EN 60079-10-2:2015 - This is the main standard for combustible dusts
EI 15 (5th Edition) - This guidance covers flammable liquids and gases, and is focused mainly for the petroleum and petrochemical industries. It contains useful direct examples (such as LPG storage and tanker offloading, and has a both point source and risk-based approach for HAC.
IGEM/SR/25 (Edition 2) - This guidance covers natural gas installations, and also has a supplement which considers hydrogen and mixtures of natural gas and hydrogen.
IGEM/UP/16 (Edition 2) - This guidance covers natural gas installations up to 2 barg, ands is typically used for energy centres at the design stage. It asks a series of questions with a Yes/No answer which either allows a Zone 2 NE to be classified, or directs you to assess the installation using IGEM/SR/25.
REA Landfill Gas: Area Classification for Landfill Gas Extraction - This guidance focuses on hazardous area classification for landfill gas (LFG) and leachate. It contains useful direct zoning examples for different types of gas wells.
NFPA 660 - This standard is produced by the National Fire Protection Association (NFPA), U.S.-based organisation, and consolidates multiple existing NFPA dust standards (including NFPA 652 and NFPA 654, among others). It lays out the requirements for analysing and managing dust hazards, and the process of Dust Hazard Analysis (DHA)
How Does Ventilation Reduce Hazardous Zone Size?
Ventilation plays a key part in hazardous area classification of gases, vapours, and combustible dusts. Effective ventilation can reduce or even remove hazardous zones and their extent, if correctly designed and implemented.
For gases and vapours, ventilation should be designed to keep the release below its Lower Flammable Limit (LFL) and provide adequate air changes to prevent a release from accumulating to hazardous concentrations. The methodology for hazardous area classification in BS EN 60079-10-1:2021 provides a methodology and calculations to determine the required ventilation velocity local to the release, and the volumetric flowrate / air changes per hour to ensure the background concentration is not hazardous. This is typically a key input into the ventilation design for hazardous areas.
For combustible dusts, ventilation is usually referred to as Local Exhaust Ventilation (LEV). For LEV to effectively capture dusts, it must be properly positioned and designed - it is often undersized and placed too far away from the source of release to be effective. The capture velocity of LEV systems drops massively the further way from the release source it is placed, significantly reducing its effectiveness. LEV systems for combustible dusts will typically be serviced by a suitable dust collector. A further consideration for LEV systems is the velocity achieved in the ducts - this should be sufficient to avoid dust accumulating in ductwork and causing process issues (i.e. blockages) or explosion propagation and fire hazards.
We frequently support clients with zone reduction exercises and can specify the flowrates required to achieve different zoning outcomes in your process. This can provide significant capital and operating cost savings if the need for ATEX equipment is reduced or eliminated.
Do I Need Hazardous Area Classification Drawings?
Hazardous area classification (HAC) drawings are not a legal requirement of DSEAR, but the DSEAR Approved Code of Practice (ACOP) document states that drawings should be produced to show the zones in both plan and elevation to give a 3D representation of the hazardous zone.
It is useful to display these drawings at the entrance to areas where hazardous areas are present around site to allow employees and contractors to effectively control ignition sources in these areas.
We produce hazardous area classification drawings as standard as part of our hazardous area classification report.
Common Mistakes in Hazardous Area Classification
Here are some of the common mistakes we have encountered when reviewing hazardous area classification:
When to Conduct Hazardous Area Classification
Hazardous area classification should ideally be carried out at the design stage of a project to ensure that zones are eliminated or reduced as far as possible, and suitable ATEX equipment is installed in the residual hazardous areas. Hazardous area classification should be reviewed periodically along with the DSEAR risk assessment to capture any creeping changes - we recommend every 3-5 years.
A common issue is DSEAR not being considered until the commissioning stage where equipment has already been procured and installed. At this point, any required changes are likely to incur significant cost and delays to the project. It is vital to engage a DSEAR expert throughout the project lifecycle to avoid this, and also to ensure compliance with all aspects of DSEAR, including DSEAR risk assessment, verification, emergency response plans, and training.
Blanket Zoning
Blanket zoning is where a whole room or area is zoned, and should be avoided wherever possible. It often creates unmanageable zones, and results in significant effort and expense (both upfront and ongoing) installing suitable ATEX equipment.
The sources of release should be identified and assessed individually, with discrete zones classified. A zone reduction exercise should then be carried out, which may involve improving the ventilation to reduce the zone extents, or installing measures such as effective flange guards to contain releases at the source.
Cherry Picking Standards and Guidance
Mixing and matching standards and guidance should generally be avoided unless you are confident you understand the underlying assumptions and limitations.
A common example is guidance given in EI 15, which suggests that a release should be regarded as continuous if it is present for more than 1000 hours in a year, primary if present between 10 and 1000 hours in a year, and secondary if present for less than 10 hours in a year. This assumption is fine for a continuously operating petrochemical plant under certain conditions, but it is often cited for other processes. If we apply this to a process which operates relatively infrequently, there is a danger that the likelihood of an explosive atmosphere being present will be underestimated.
Lack of Understanding
Hazardous area classification can be a complex process, and requires an in-depth knowledge of the relevant legislation, standards, and guidance to ensure that all of the sources of release are correctly identified and assessed. This information then needs to be conveyed in a clear and coherent way in order for the site to include into their DSEAR management system.
Expert Support for DSEAR
Our team supports clients across the UK with all aspects of DSEAR compliance, including:
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