Hazardous Area Classification (HAC): Undertaking, Procedures and Zones

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Hazardous Area Classification (HAC) is a method used to evaluate the likelihood of a flammable atmosphere forming and how long it might persist. This methodology helps determine whether electrical, mechanical, or other equipment should have specific protective features to prevent the risk of fire or explosion.

HAC is a specific requirement of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) in the UK, which mandates a comprehensive assessment of fire and explosion hazards in industry. Alternatively, in the United States, there is the Dust Hazard Analysis (DHA), which is a methodical approach employed to enhance plant safety by pinpointing any combustible dust hazards linked to a given process.

Although sharing similar philosophic foundations and protection concepts, the United Kingdom/Europe and the United States have unique distinctions when undertaking a Hazardous Area Classification, as both systems entail comprehensive evaluations and considerations concerning the risks of fire and explosion.

In the second instalment of this two-part UK/USA hazardous area classification comparison blog series, we will discuss the divergences between how the UK and USA undertake a Hazardous Area Classification. This blog will focus on the steps involved in the methodology, the procedures for determining a grade of release, and how hazardous zones are identified. We’ll also consider the details of classification systems, terminology, and factors that influence the extent of a zone.

Undertaking a HAC

Determining the Grade of Release

HAC zones are based on the probability of a flammable atmosphere’s presence and can vary depending on a range of factors.

Once potential sources of release are identified, the next step is to determine the grade of release. The grade of release categorises the nature and persistence of a flammable atmosphere. This will help to define the hazardous zones:

  • Continuous Release: Continuous release implies the presence of a flammable atmosphere continuously or for long periods of time. This can occur when flammable liquids are present at temperatures above its flashpoint, and the containment system is continuous. Ultimately, a flammable atmosphere will persist.
  • Normal Operation: In certain situations, a flammable atmosphere may be generated only during specific operations. For example, a vent will release a flammable atmosphere when in operation, such as during tank filling.
  • Infrequent Release: Some scenarios involve occasional spillages and leakages, leading to infrequent flammable atmospheres.

Zonal Classification (UK)

With the grade of release established, the next step is to determine the hazardous zones. The classification system is as follows:

  • Zone 0 (Gas/Vapour) or Zone 20 (Powder): These are areas with continuous release of flammable atmospheres.
  • Zone 1 (Gas/Vapour) or Zone 21 (Dust): These are zones with normal operation-related release.
  • Zone 2 (Gas/Vapour) or Zone 22 (Dust): These are secondary zones where infrequent release from spillages and leakages may occur.

Class and Division System (USA)

While the UK and Europe use a zone system, the US has employed a Class and Division system.

  • Class 1 designates an explosive gas atmosphere.
  • Class 2 pertains to environments where combustible dust is present.
  • Division 1 signifies the presence during normal operation.
  • Division 2 is for abnormal conditions.

The distinction between normal and abnormal conditions aligns more with UK/EU zone system. In the future, the US may shift towards adopting the zone system, but for now, the Class and Division framework remains a critical aspect of area classification in the country.

Protection Against Ignition

Once hazardous areas have been defined, the next step is to safeguard against ignition sources. Any mechanical or electrical equipment operating within these zones must be protected from becoming an ignition source.

Various methods are employed to ensure that these devices do not serve as triggers for flammable atmospheres. The two most common methods are the use of “flameproof” and “intrinsically safe” equipment (although there are other protection concepts available).

Applicability to Specific Zones

Different zones come with different ignition risks and need unique approaches to safety.

Some equipment can only be deployed in specific zones due to their specialised protective measures, while others are versatile and can be utilised in all explosive atmosphere zones. Flameproof equipment, for example, can only be used for gas and vapour atmospheres and cannot be used as a protection concept with combustible dust atmospheres.

The allocation of equipment to specific zones must be considered as it is a fundamental aspect of maintaining safety within hazardous areas.

Area Classification Procedure – USA

The process of determining hazardous areas and categorising them into specific zones, while similar in principle to the UK and Europe, has distinct characteristics and terminologies in the USA.

  • Assessing the Potential for Combustible Material Release: The procedure in the USA commences with an assessment of the industrial process. The aim is to identify areas where there is potential for the release of combustible materials i.e., gases, vapours, powders, or dusts. – In the United Kingdom, this is usually a starting point for our risk assessments. It is highly likely that the industry has done a HAZID of its processes which also gives this information. The reason the United States system actually specifically asks for this is because the risk-based approach of a HAZID is not commonplace (especially outside of oil & gas and chemical process industry).
  • Gathering Facility History and Process Information: The historical documentation is scrutinised to uncover patterns, i.e., a tendency for leaks during maintenance activities or a high rate of spillage. This historical perspective helps in understanding the likelihood of releases. – The assessment in the United Kingdom also specifically covers normal operation, anticipated malfunction, and maintenance.
  • Analysing Process Parameters: The assessment then shifts to process information, which involves examining various parameters of the operation. This includes an assessment of the volumes of flammable substances being handled, process pressures, and flow rates. This is also considered in the United Kingdom, but the DSEAR regulations also does this irrespective of process size or quantity of dangerous substance.
  • Classification Diagrams and Documentation: The heart of the HAC procedure lies in the utilisation of classification diagrams, provided by the NFPA. These diagrams are compared to the specifics of the process, ensuring alignment between the source of release and process details. This is, in effect, the example approach used in the United Kingdom.
  • Matching Ignitable Mixture Likelihood and Material Transport: The effectiveness of the procedure hinges on matching the classification diagrams to the likelihood of an ignitable mixture being present. Considerations are made for the potential for combustible materials to be transported within the facility, such as through trenches, pipes, conduits, or ducts. Ensuring that the selected classification diagram represents these aspects is vital. This is also undertaken in the United Kingdom, but as part of the DSEAR risk assessment rather than a HAC.
  • Documentation and Terminology: Once the appropriate classification is determined, it is documented using a combination of Class Division and zone terminology. This documentation includes the processing material’s name, respective material group, and either the auto-ignition temperature or an appropriate design T code. This documentation is consistent for combustible dust, vapours, liquids, and gases. This is also undertaken as part of a HAC in the United Kingdom.

Process Equipment Information (USA)

Quantification and Volume Assessment

The US takes a distinct approach when it comes to quantifying the volumes of materials and determining hazardous areas. Instead of focusing on the grade of release, emphasis is placed on factors such as size, pressure, and flow within a particular industrial process.

In the UK Hazardous Area Classification (HAC) process, similar considerations are made, but size, pressure, flow, etc., are used to determine the extent of the zone not the classification of the zone.

Process, Equipment, and Piping Details

The intricate details of the industrial process, the equipment used, and the piping configuration play a pivotal role in determining the appropriate classification diagrams. It is not a one-size-fits-all approach, and the specific nature of the operation influences the chosen classification method. This is also undertaken as part of a HAC in the United Kingdom.

Customised Classification Diagrams

To further ensure safety alongside the most accurate and tailored area classification, process safety professionals in the US compare process-specific factors with available classification diagrams.

The goal is to match the unique attributes of the operation to the diagrams that best represent the potential for hazardous conditions.

By allowing these factors, including size, pressure, and flow, to dictate the classification approach, HAC in the US ensures that the area classification procedure is custom-fit to each industrial setting. This precision minimises the chances of overlooking potential hazards and enhances safety measures within the facility.

Determining the Extent of Zones (UK)

One critical aspect for hazardous area classifications in the UK is the determination of the extent of the hazardous zones. There are various methods and approaches to do this, each tailored to specific situations and requirements.

The Example Approach

The simplest and often most accessible approach to determining the extent of hazardous zones is through the use of examples. This method involves referencing existing standards and guidance that pertain to a specific scenario.

For instance, if you have an atmospheric storage tank containing a low flashpoint material within a bund or dike, you can consult established standards and guidance documents to learn about the potential extent of your hazardous area.

If your situation mirrors a documented example, this approach can save valuable time and resources.

There are many established hazardous area classifications for typical pieces of equipment such as pumps, tanks, and reactors. These classifications have already been determined, and their parameters are well-documented.

When using the example approach, it’s essential to ensure that your situation aligns with the given example. Any significant deviations or unique factors may need more complex calculation methods to determine hazardous area extents.

Calculation Methods

While the example approach is convenient, there are scenarios where specific calculations are necessary to determine the extent of hazardous zones. These calculations involve assessing release characteristics, which can be determined using consequence modelling software or via calculation of relevant chemical engineering data.

By simulating potential releases and their consequences, you can calculate the spread of a flammable cloud. This method provides a more precise and tailored hazardous area classification, particularly when your situation deviates from established examples.

Determining the Extent of Zones (USA)

In the United States, the process of determining hazardous areas relies on diagrams and a profound understanding of potential releases. Key factors include the nature of the material, its vapor density (whether it’s lighter or heavier than air), and the Lower Flammable Limit (LFL).

The size of the processing or storage unit, relief sizes, and ventilation are also crucial considerations. By evaluating these parameters, operators can apply diagram methods tailored to their specific facility requirements.

Review and Regulation Compliance

Once a hazardous area classification (HAC) has been established, it’s not the end, but rather a starting point for ongoing safety.

Legislative requirements dictate that regular reviews of the HAC documentation should be conducted. These reviews are crucial to ensure that the classification remains accurate and up to date, particularly in the face of any changes within the facility.

Whether it’s the introduction of new processes, large product changes, or shifts in the physical layout of the plant, these developments warrant a revision of the hazardous area classification document.

The Hazardous Area classification involves the examination and categorisation of environments where explosive gas atmospheres might emerge. In the process industry your primary objective is to streamline the appropriate selection and installation of equipment for safe use in your processing environment while considering the characteristics of your materials present.

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