Hazardous Area Classification

In practice, hazardous area classification is often seen too quickly as a drawing with coloured areas or hatched zones. A proper classification, however, is not a drawing exercise. It is the result of a technical assessment of substances, release sources, process conditions, ventilation, operating practice and ignition hazards. The drawing is only the visual representation of that assessment. If the technical justification is weak, the classification becomes vulnerable during inspections, audits, insurance reviews, regulatory visits and, above all, when modifications are made to the installation.

A good hazardous area classification therefore always starts with understanding the process. It must be clear which flammable or combustible substances are present, in what form they occur, under which temperatures and pressures they are used, where they may be released and how often this can realistically happen. For gases, vapours and mists, relevant properties include LEL, relative vapour density, ignition temperature, gas group, volatility and operating conditions. For combustible dusts, the assessment must consider particle size distribution, moisture content, dust cloud behaviour, dust layer formation, minimum ignition energy, maximum surface temperature, Kst value and the way in which the product is handled, conveyed, mixed, stored or processed. A classification can never be better than the quality of the substance and process data on which it is based.

The methodology must be aligned with the principles of IEC 60079-10-1 for explosive gas atmospheres and IEC 60079-10-2 for explosive dust atmospheres. In the UK, the same technical logic is applied within the DSEAR framework; HSE describes hazardous area classification as a technique for assessing the probability of formation of a flammable atmosphere and its likely duration. The first step is always to determine whether flammable gases, vapours, mists or combustible dusts can form an explosive atmosphere under normal or reasonably foreseeable operating conditions. The next step is to identify the release sources. These are not only obvious points such as flanges, pumps, sampling points, filling openings, vents, filters or transfer points, but also less visible sources such as drains, seals, breather lines, cleaning openings, pressure relief devices, product transitions, damaged flexible connections or temporary connections.

The nature of the release must then be assessed. A continuous, primary or secondary release leads to a different risk profile. In practice, this is where many classification errors occur. A source is easily described as secondary because the design intent assumes that nothing is released during normal operation, while the actual operating practice shows otherwise. A sampling point that is opened several times a day, a big-bag station that releases dust during every changeover, or a pump seal that regularly shows minor leakage is not the same as a theoretically rare fault. Good hazardous area classification therefore requires an assessment of real operating practice, not only the original design intention.

The next step is to assess how the explosive atmosphere can disperse. For gases and vapours, ventilation, density relative to air, temperature, pressure, release velocity, room geometry and obstructions are all important. A light gas such as hydrogen rises quickly and requires a different approach from a heavy vapour that remains close to the floor or can collect in pits, trenches and channels. Outdoor installation does not automatically mean that no hazardous area exists. Natural ventilation may be favourable, but it can be significantly restricted by buildings, wind-sheltered corners, canopies or recessed areas. Indoor spaces always require a critical assessment of ventilation rate, air distribution, ventilation availability and behaviour in the event of ventilation failure.

For dusts, dispersion must be assessed differently. The issue is not only the dust cloud at the moment of release, but also dust layers that may later be re-entrained into the air. An installation may generate little visible dust cloud during normal operation, yet still build up a relevant risk through leakage, insufficient cleaning or structural parts on which dust can settle. In dusty environments, the assessment must therefore consider not only the release source, but also horizontal surfaces, cable trays, motors, luminaires, pipework, platforms, roof structures and difficult-to-reach areas. A dust layer is not merely a housekeeping issue. It can provide thermal insulation, become re-suspended and contribute to secondary explosions.

The outcome of the assessment is the zone classification. For gases, vapours and mists, areas are classified as Zone 0, Zone 1 or Zone 2. For combustible dusts, the corresponding zones are Zone 20, Zone 21 or Zone 22. The zone does not describe the severity of an explosion; it describes the probability and duration of the presence of an explosive atmosphere. This distinction is essential. A small hazardous area can still represent a serious risk if an effective ignition source is present or if an explosion can propagate into equipment, pipework or connected systems. A good classification must therefore always be read together with the ignition source assessment, equipment selection, inspection regime, maintenance strategy and organisational controls.

Experience shows that the best results are achieved when hazardous area classification is not prepared solely from behind a desk, but in combination with a site survey and discussions with operations, maintenance and engineering. Operators often know where dust actually accumulates, where vapour is noticeable, where doors remain open, where spillages occur during unloading and which activities deviate from the procedure. Maintenance personnel know where seals weep, where cable glands corrode, which ventilation systems frequently fail and which items of equipment are regularly replaced. Engineering knows the design, but the shop floor knows the reality. A good classification is created when these three sources of information are brought together.

In the chemical industry, hazardous area classification often focuses on gas and vapour releases. Pumps, flanged connections, seal systems, sampling points, reactors, vents, tank breathing, loading and unloading points, solvent systems and CIP or cleaning agents determine the risk profile. A common weakness is that old classification drawings remain in use even though the medium, temperature, pressure or operating practice has changed. A higher process temperature may significantly increase evaporation. A different solvent may have a lower LEL or a different gas group. A new pump seal system may change the likelihood of release. A good classification in chemical plants must therefore be closely linked to Management of Change. Every change in substance, process or ventilation can affect the classification.

In the food, feed and powder industries, classification is mainly driven by dust behaviour. The question is not only where dust is released, but also where dust remains. Transfer points, mixers, sieves, filters, cyclones, elevators, screw conveyors, big-bag stations, silos and extraction systems are decisive. In this sector, the external areas around equipment are often classified, while the internal explosive atmosphere inside filters, silos, elevators and mixers receives too little attention. Inside equipment, an explosive dust cloud may be present much more frequently than outside the installation. A proper classification must therefore distinguish between internal and external zones. The outside of an installation may be Zone 22, while the inside may be Zone 20 or Zone 21. This has direct consequences for mechanical equipment, ignition source assessment, constructive explosion protection, explosion isolation and maintenance.

In pharmaceuticals and fine chemicals, classification is often complex because products, solvents, powders, batches and temporary set-ups change frequently. A room may be used today with a highly volatile solvent and tomorrow with a powder with a low minimum ignition energy. Pilot plants, laboratories and multipurpose installations therefore require a classification methodology that considers not only the fixed installation, but also use scenarios. It must be critically assessed which substances are representative, which worst-case scenarios are realistic and how temporary equipment is approved. In this sector, the main weakness is often that the documentation is impressive, but does not keep pace with operational reality.

In wastewater treatment plants, biogas installations and waste processing facilities, the focus is often on methane, hydrogen sulphide, moisture, corrosion, pits, pumping stations, digesters, gas trains, compressors and flare systems. Classification can be difficult because installations are often old, have been extended in phases and drawings do not always match reality. A pit or technical room may be sufficiently ventilated on paper, but due to contaminated ventilation grilles, failed fans or changed operating modes, the actual risk profile may be different. Biogas also requires consideration of variable gas composition, condensate formation, leakage points and outdoor installation conditions. A good classification in this sector mainly restores clarity: where the zone boundaries are, which equipment is located within them, which ventilation assumptions apply and what must be checked periodically.

In energy systems, utilities and battery charging environments, attention shifts towards hydrogen generation, ventilation and ignition sources. Battery charging stations, standby power systems, battery rooms and certain energy storage systems are still too often treated as ordinary technical rooms. With lead-acid batteries, hydrogen can be released during charging. The key questions are whether the ventilation is sufficient, where hydrogen may accumulate, which electrical equipment is present and whether charging behaviour, maintenance and room use match the assumptions of the assessment. Not every battery charging room automatically has to become a hazardous area, but every battery charging room must be demonstrably assessed as safe.

In storage, transfer and logistics, classification is strongly determined by activities. Loading, unloading, pumping, filling, flushing, venting, sampling, IBC handling, tank bunds and temporary storage determine where releases can occur. Actual practice often differs from the written procedure. Hoses are connected differently, drums are opened in places where this was not foreseen, earthing clamps are not used, or ventilation conditions change because doors and shutters are left open. A good classification must therefore not be based only on the ideal procedure, but also on reasonably foreseeable use. Especially for flammable liquids and vapours, the differences between indoor and outdoor locations, high-level and low-level release, natural and mechanical ventilation, and open or enclosed spaces are decisive.

In manufacturing industries, hazardous areas are often localised. Spray booths, cleaning baths, degreasing stations, bonding processes, printing processes, woodworking, plastic dust, metal dust and additive manufacturing can create explosive atmospheres locally. Because such companies do not always see themselves as “ATEX companies” or “DSEAR sites”, hazardous areas are sometimes assessed too late or too narrowly. A good classification starts by identifying local release sources. Where does solvent evaporate? Where can a dust cloud form? Where can dust settle? Is the local extraction sufficient and available? Which equipment is located in the immediate vicinity? Are temporary activities included? In this sector, a practical and understandable classification is particularly important because the users are often not explosion protection specialists.

A good hazardous area classification only creates value when it is translated into control. The classification determines which equipment may be used, which equipment category or EPL is required, which type of protection is suitable, which inspection requirements apply and which organisational measures are necessary. A zone drawing alone is insufficient. There must be a clear technical justification covering release sources, substance data, ventilation assessment, dust assessment, assumptions, limitations, abnormal operating conditions and operating conditions. If, for example, mechanical ventilation is essential for limiting a zone, it must also be defined what happens in the event of ventilation failure. If cleaning is essential for preventing hazardous dust layers, the cleaning frequency and verification of cleaning must form part of the explosion protection concept. If a zone exists only during loading or unloading, the procedure must secure that condition clearly.

The best result is achieved when the classification is built as a technically defensible chain. First the substances and process conditions are assessed, then the release sources, then ventilation, dispersion or dust deposition, followed by zone type and zone extent, and finally the consequences for equipment selection, inspection, maintenance and operating instructions. If one link in this chain is missing, the classification becomes weak. A well-presented drawing without justification is not defensible. A good justification without a clear drawing is difficult to use. A correct zone without a connection to the equipment register and inspection programme does not provide sufficient control.

From experience, the best hazardous area classifications are neither the most conservative nor the smallest, but the best justified. Over-classification may appear safe, but can result in unnecessary cost, wrong priorities and loss of credibility. Under-classification reduces cost on paper, but increases risk and makes the installation vulnerable during audit, regulatory inspection or after an incident. A good classification is realistic, technically justified and practically manageable. It shows where explosive atmospheres may arise, why the zone has been selected, which assumptions apply and what the operator must do to keep those assumptions valid in operation.

Hazardous area classification is therefore not an end product, but a foundation for design, maintenance, inspection, the Explosion Protection Document or DSEAR assessment, training and Management of Change. As soon as a substance changes, ventilation is modified, an extraction point is moved, a production line is extended, a cleaning regime changes or a room is used differently, the classification must be reviewed. This is where weaknesses often arise in practice. The zone drawing remains on the wall while the plant changes. A good system ensures that hazardous area classification becomes part of Management of Change and is not only reviewed when an auditor or regulator asks for it.

The value of a good classification ultimately lies in reducing uncertainty. Management understands where the significant risks are. Engineering knows which equipment and installation methods are required. Maintenance knows where inspection and repair are critical. HSE has a defensible basis for the Explosion Protection Document or DSEAR assessment. Production understands which activities influence explosive atmospheres. When hazardous area classification is carried out in this way, it does not result in a paper drawing, but in a practical control instrument. That is the result companies need: a classification that reflects reality, is technically defensible and remains usable in everyday operation.