Hazardous Area Classification (HAC): Constructing, Outcomes and Flammable Material Differences

Table of Contents

A Hazardous Area Classification (HAC) classifies an operational area based primarily on the likelihood of the presence of a flammable atmosphere (the combination of a fuel in oxidant (typically air)).  

Once the extent or size of a flammable atmosphere is determined, any equipment located within the flammable atmosphere must be appropriately designed to reduce the likelihood of that equipment item acting as an ignition source.

A HAC includes the identification of potential flammable materials, understanding ventilation characteristics (natural or mechanical/electrical equipment), and the assessment of other factors that can create an explosive atmosphere.

It will serve as a blueprint for safeguarding lives, property, and the environment from potential ignited events (fires or explosions).

Although sharing similar philosophic foundations and protection concepts, the United Kingdom/Europe and the United States have minute differences in approach when undertaking a hazardous area classification.

In the first of this two-part UK/USA hazardous area classification comparison blog series, we will delve into the differences in building classification systems, the implications/outcomes of hazardous area classifications and what criteria are needed for identifying flammable atmospheres related to dusts and gases/vapours, all while examining the regulatory frameworks.

Building a HAC system (UK/EU)

Building a suitable HAC system requires the identification of hazardous locations where fuel and air can result in an explosive atmosphere. Often, this will occur within equipment, but it equally applies to releases from equipment (either intentionally or during malfunction).

UK/EU Risk-Based Zonal Classification

UK and European regulations provide specific definitions of hazardous areas and are assessed and classified via a risk-based approach.

Think of it from the point of view of risk. The risk of an ignited event is a combination of the likelihood of a flammable atmosphere being present and the simultaneous likelihood of an ignition source being present. Sometimes, the flammable atmosphere will be present continuously (such as above a liquid surface at a temperature above its flashpoint).

In such cases, it makes sense to ensure that any potential ignition source present in the flammable atmosphere have an extremely low likelihood of acting as an effective ignition source.

The majority of the risk reduction is achieved by the probability of an ignition source being present.  On the other hand, if the flammable atmosphere only occurs very infrequently (for example, when a flanged connection leaks), risk reduction is generally achieved by not having a flammable atmosphere present. 

Hence any potential ignition source does not need to have such a low likelihood of becoming an effective ignition source. Remember, for the ignited event, which we are trying to avoid, both the flammable atmosphere and the effective ignition source must be present simultaneously.

UK/EU regulations highlight that equipment present within a flammable atmosphere must be special from the point of view of acting as an ignition source. Equipment can be constructed in different ways to achieve the likelihood of it becoming an ignition source.  

The equipment must undergo a special ATEX certification process which will decrease the probability of the equipment acting as an ignition source.

The ATEX certification process stops processing equipment contributing to ignition risks, that is, the completion of the fire triangle which comprises three components: fuel, oxygen (the flammable atmosphere), and an ignition source.

By ensuring that equipment does not act as an ignition source in hazardous areas, the risk of fire or explosion because of ignitable events reduce.

Building a HAC system (USA): NFPA 497 and NFPA 499

HAC, in the United States, shares the same fundamental goal as the UK—assessing and managing potentially explosive atmospheres.

But variations in terminology and language exist, reflecting the distinct regulatory landscape in the USA.

In the USA, there are two primary guiding documents that serve as the foundation for the hazardous area classification process: NFPA 497 and NFPA 499. NFPA 497 is the reference document for situations involving flammable liquids, gases, or vapours, while NFPA 499 pertains to materials with the potential to generate combustible dusts or clouds.

If a facility deals with materials falling under these NFPA categories, it is obligated to adhere to the NFPA codes. These documents provide essential guidance when dealing with dangerous substances that can lead to hazardous conditions.

HAC in the USA: A Multi-Step Process

As stated above, the regulatory framework for managing flammable and hazardous substances, and flammable atmospheres associated with flammable gas, vapours, powders, and dusts is governed by the National Fire Protection Association (NFPA).

This multi-step process includes:

  • NFPA Codes Application: If a facility handles materials with the potential for hazardous atmospheres, the NFPA codes are applied to determine whether the area should be classified as hazardous.
  • NFPA 70 (National Electric Code): If the area is classified as hazardous according to NFPA 497 or 499, the next step involves referring to NFPA 70, also known as the National Electric Code. This code specifies the requirements for electrical installations within the determined hazardous area to ensure safety.

The HAC process in the US involves a prescriptive approach that considers various factors beyond the “yes or no” determination, and includes:

  • Process Size: The scale and scope of the process are evaluated. This is also undertaken as part of a HAC in the United Kingdom.
  • Potential Weak Points: Identifying vulnerable areas in the process. This is also undertaken in the United Kingdom, but as part of the DSEAR risk assessment rather than a HAC.
  • Material Characteristics: Assessing the ignition energy requirements of the material involved. This is also undertaken in the United Kingdom, but as part of the DSEAR risk assessment rather than a HAC.
  • Classification Zone Determination: Based on the holistic assessment, a classification zone is determined, following NFPA recommendations. This is, in effect, the example approach used in the United Kingdom.
  • Electrical Code Compliance: Within the designated zone, the requirements of the National Electric Code (NFPA 70) are applied to ensure that all electrical installations meet safety standards. In the United Kingdom this is dependent on the zone as it helps specify, along with other information from the risk assessment to get the correct EPL (Equipment Protection Level) for equipment in a specified zone.

Hazardous Area Classification Outcomes

The outcome of a HAC system is generally a drawing. 

This serves as a visual representation of the hazardous zones within a given process or facility. Again, these zones are based on the probability of a flammable atmosphere’s presence and can vary depending on a range of factors.

Hazardous areas are not static; their classification depends on various elements, including:

  • Zone 0/20 (UK/EU) / Division 1 (USA) – Normal operation (Explosive atmosphere for more than 1000h/yr): Consideration must be given to the routine operations within the facility that run for long periods. For instance, in an atmospheric storage tank storing a liquid fuel, flammable vapours may be present if the temperature of the liquid is above the material flashpoint.
  • Zone 1/21 (UK/EU) / Division 1 (USA) – Static Situation (Explosive atmosphere for more than 10, but less than 1000 h/yr): In scenarios like static loading or unloading from a tank, a different set of conditions may apply.
  • Zone 2/22 (UK/EU) / Division 2 (USA) – Abnormal Situation (Explosive atmosphere for less than 10h/yr, but still sufficiently likely as to require controls over ignition sources): In situations, such as tank overfills or spills. These must a separate classification within the HAC system.

Creating Zonal Classifications

Once the zonal classification is determined via a drawing, there will be a clear visualisation of the hazardous zones within your facility. The purpose of these drawings is to help specify the placement of equipment, while considering the likelihood of it becoming an ignition source and the likelihood of a flammable atmosphere’s presence.

Standards and Regulations

While regulations set high-level requirements for HAC, the details on engineering equipment and the methodology on how to give equipment specific categories are defined in the standards.

Standards such as BS EN 60079-10-1 and BS EN 60079-10-2 in the United Kingdom and Europe, provide guideline for classifying areas for vapours, gases, and dusts. These standards offer detailed information and procedures to ensure the safe management of hazardous areas.

Flammable Atmospheres – Gases & Vapours

Flammable atmospheres occur when the concentration of gas or vapour in such an atmosphere falls within a specific range known as the flammability limits. These limits consist of the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL).

If the mixture contains insufficient air, then it is too rich to burn – the process is operating above the UEL. An excess of air, due to efficient ventilation, will result in operation below the LEL.

To manage flammable explosive atmospheres regulations, it is important to grasp the properties of the substances involved in the explosive mixture. Key data points that contribute to this understanding include:

  • Flashpoint: The flashpoint of a substance is the temperature at which it can produce enough vapours to mix with the air to produce a flammable atmosphere, which if ignited will result in a flash of flame.
  • Flammability Limits: These limits define the vapour volume concentration ranges within which a substance can create a flammable atmosphere.
  • Autoignition Temperature: The autoignition temperature is the minimum temperature at which a substance can ignite without an external ignition source.
  • Vapour Density: When considering Hazardous Area Classification and creating related drawings, understanding the vapour density of a substance is crucial. Substances with lower vapour density tend to rise and create buoyant flammable atmospheres. An example of this is hydrogen, which, when released, ascends and mixes with the air. Substances with higher vapour density, such as industrial solvents, tend to sink to the ground, creating flammable atmospheres closer to the surface.

Temperature’s Influence on Flammability

It’s essential to note that the flammability limits and temperature class of substances can change as temperature increases. This means that substance properties, especially the flashpoint, can be dynamic, making it necessary to consider variations in operational conditions when assessing the potential for using flammable substances.

Flammable Atmospheres – Powders & Dusts

Flammable atmospheres are not limited to gases and vapours; they also encompass powders and dusts. These dusts will present a unique set of challenges and considerations. To form a flammable atmosphere with powders, several conditions must be met:

  • Fuel Capability: The powder or dust must have the capability to act as a fuel, which means it can react with oxygen. Substances that are already oxidized, such as chalk or sand, cannot form flammable or combustible clouds.
  • Explosible Limits: The concentration of the powder or dust must fall within what is known as the Minimum Explosible Concentration (MEC). This range is essential for the formation of a flammable atmosphere.
  • Particle Size Distribution: To maintain suspension in air, the particle size distribution of the powders or dusts must be less than 500 microns. The particle size has an impact on combustion dynamics and the ability to generate a dust cloud.
  • Ignition Energy: An energy source above the minimum ignition energy of the powder is required for ignition.

Due to the intricate nature of combustible dust, it is challenging, if not impossible to calculate this data. This is why testing of your specific powders is required. Because particle size distribution etc determines whether a given powder or dust will ignite, it can differ significantly from supplier or source. Similarly, many powders, from different sources exhibit widely different minimum ignition energies. This is much less of an issue with gases and vapours where the necessary data can be obtained through the available literature.

Powder/dust testing is often conducted to assess the likelihood of forming a flammable atmosphere and how easily it can be ignited. These tests will provide crucial insights into the substance’s behaviour under different conditions.

Hazardous Area Classification (HAC): Your Practical Blueprint

Hazardous Area Classification (HAC) is not just a theoretical exercise but a practical blueprint for safety at your facility.

In the UK and Europe, the HAC methodology is formulated via a risk-based approach, where equipment’s potential to act as an ignition source for a hazardous substance or gas is meticulously examined and reduced. The multi-step process evaluates process size, potential weak points, material characteristics, determination of classification zones, fire and explosion risks and electrical code compliance, all under the guidance of the NFPA codes.

The outcomes of hazardous area classification in both regions manifest in drawings that visually represent hazardous zones. These zones are dynamic and subject to change based on various factors, including the nature of operations, materials involved, and the potential for flammable atmospheres.

Understanding potentially explosive atmospheres for flammable gases, vapours, powders, and dusts is a critical aspect of HAC. The UK and USA require an in-depth comprehension of dangerous substances and explosive gases’ properties, such as flashpoints, flammability limits, autoignition temperatures, and vapour densities, to create accurate classifications.

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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