ATEX is widely understood to be an electrical engineering problem. It is not. The majority of ignition sources in potentially explosive atmospheres are mechanical — hot surfaces, friction sparks, mechanical impact, static electricity generated by process equipment, and adiabatic compression. The mechanical engineer who specifies a pump, a fan, a gearbox, or a coupling in a hazardous area is making decisions that directly affect whether equipment is correctly rated for the zone it will occupy — and whether people around it are safe if a flammable atmosphere forms unexpectedly.
This article explains the ATEX framework from the perspective of a mechanical engineer who needs to understand it without pretending to be an electrical engineer or an ATEX specialist. It covers zone classification, equipment categories, gas groups, temperature classes, and the practical decisions involved in specifying ATEX-compliant mechanical equipment.
The Legislative Framework — ATEX and DSEAR
Two pieces of legislation govern explosive atmospheres in the UK and EU:
- ATEX Directive 2014/34/EU (Equipment Directive, ATEX 114 in EU, retained in UK law post-Brexit as the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 2016) — governs the manufacture of equipment intended for use in explosive atmospheres. Equipment manufacturers must certify and mark their products in accordance with this directive before they can be placed on the market.
- DSEAR 2002 (Dangerous Substances and Explosive Atmospheres Regulations) — governs the use of equipment in potentially explosive atmospheres. Places duties on employers to carry out hazardous area classification, select appropriate equipment for each classified zone, and document the arrangements in an Explosion Protection Document (EPD).
The practical consequence: you cannot simply buy a pump and put it in a hazardous area. The zone must be classified, the equipment must be selected to match that zone, and the selection must be documented. ATEX-marked equipment has been certified as suitable for a defined range of hazardous atmospheres — but the responsibility for matching the right equipment to the right zone sits with the duty holder, not the equipment manufacturer.
Zone Classification — Defining the Hazard
The starting point is zone classification — a systematic assessment of where and for how long a flammable atmosphere might exist. Zone classification is performed by a competent person (typically a process engineer or specialist hazardous area consultant) using the guidance in IEC 60079-10 and the Energy Institute's widely-used Model Code of Safe Practice IP15.
For gases, vapours and mists, three zones are defined:
- Zone 0 — a place where a flammable gas cloud is present continuously, or for long periods, or frequently. Typically the interior of storage tanks, the vapour space above a liquid surface in a vessel, or within a vent stack. This is the most hazardous zone designation.
- Zone 1 — a place where a flammable gas cloud is likely to occur in normal operation occasionally. Typical Zone 1 locations: the area within 1–3 metres of a flanged joint on a flammable service line, around pump seals and valve glands, near sample connections, and around breathing vents on atmospheric storage tanks.
- Zone 2 — a place where a flammable gas cloud is not likely to occur in normal operation, and if it does, will exist only for a short time. Typically 1–5 metres beyond the Zone 1 boundary for most equipment items. The majority of outdoor process plant areas are classified Zone 2 where the process involves flammable fluids.
For dusts, equivalent zones are defined as Zone 20 (continuous presence of explosive dust cloud), Zone 21 (occasional in normal operation), and Zone 22 (infrequent and short duration). Dust explosions are if anything more energetic than gas explosions and are treated with equivalent rigour, but are less commonly encountered in the general process industries.
Equipment Categories — What Equipment Can Go Where
ATEX equipment is classified into categories that determine which zones it may be used in. Equipment Group II (surface industry — not mines) has three categories:
| Category | Zone permitted | Protection level |
|---|---|---|
| Category 1G (gas) / 1D (dust) | Zone 0 / Zone 20 | Very high — two independent means of protection; safe even if two faults occur simultaneously |
| Category 2G (gas) / 2D (dust) | Zone 1 / Zone 21 | High — safe even if one fault occurs |
| Category 3G (gas) / 3D (dust) | Zone 2 / Zone 22 | Normal — safe in normal operation |
A Category 2G pump can be used in Zone 1 or Zone 2. A Category 3G pump can only be used in Zone 2 — it must not be installed in Zone 1. Category 1G equipment for Zone 0 is uncommon and expensive; where Zone 0 exists inside a vessel, the standard approach is to keep all equipment outside the Zone 0 boundary rather than to specify Zone 0-rated instruments inside it.
Categories are always accompanied by the G or D suffix — G for gas/vapour/mist, D for dust. Equipment marked Category 2G cannot be assumed safe in dust atmospheres; it must be marked 2D (or 2GD for dual certification) for dust service.
Gas Groups — Matching Equipment to the Substance
Flammable gases and vapours vary significantly in their ignitability — some ignite readily with small spark energies, others require far more energy to initiate combustion. ATEX uses gas groups to categorise substances by their worst-case ignition characteristics, so that equipment can be designed with appropriate ignition protection for the most demanding substance it might encounter:
| Gas Group | Representative substance | Characteristic |
|---|---|---|
| IIA | Propane, methane, acetone, most petroleum vapours | Least easily ignited — widest safe gap, highest minimum ignition energy |
| IIB | Ethylene, town gas, hydrogen sulphide | Intermediate — narrower safe gap, lower ignition energy than IIA |
| IIC | Hydrogen, acetylene, carbon disulphide | Most easily ignited — very narrow safe gap, extremely low ignition energy |
Equipment marked for Group IIB may be used in IIA and IIB atmospheres but not in IIC. Equipment marked for Group IIC is suitable for all Group II gas atmospheres. The practical implication: an LPG plant (Group IIA) can use less stringently designed equipment than a hydrogen facility (Group IIC). Specifying IIC equipment throughout a propane plant is technically acceptable but commercially over-engineered. Specifying IIA equipment in a hydrogen plant is dangerous and non-compliant.
For common process fluids: natural gas and most petroleum products are IIA. Ethylene cracker and chemical plant is commonly IIB. Hydrogen service, chlor-alkali, and hydrofluoric acid plants are IIC. When in doubt about a substance's gas group, the IEC 60079-20 database and NFPA 497 classify several hundred substances.
Temperature Classes — Surface Temperature Limits
A flammable atmosphere can be ignited not only by an electrical spark but by a hot surface. ATEX temperature classes define the maximum permissible surface temperature of equipment, which must be below the auto-ignition temperature (AIT) of the flammable substance in the atmosphere:
| Temperature Class | Max surface temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
The maximum surface temperature of the equipment under all operating conditions — including fault conditions for Category 1 and 2 equipment — must not exceed the temperature class limit. The temperature class must be matched against the AIT of the substance. Common auto-ignition temperatures: methane 580°C (T1 adequate), petrol/gasoline 280°C (T3 required), diethyl ether 160°C (T4 required), carbon disulphide 90°C (T6 required).
The relevance to mechanical engineers: friction, bearing overload, and brake application can generate significant surface temperatures on rotating equipment. A pump bearing running above its rated load, a brake applied to a rotating shaft, or a seized coupling can reach temperatures well above the normal operating surface temperature. The ATEX certification of the equipment must account for these fault-condition temperatures, not just normal running temperatures.
Reading an ATEX Marking
Every piece of ATEX-certified equipment carries a marking that encodes all the relevant information. A typical marking for a pump might read:
⟨Ex⟩ II 2 G Ex d IIB T4 Gb
Decoded:
- ⟨Ex⟩ — ATEX certified (the hexagonal symbol)
- II — Equipment Group II (surface industries, not mines)
- 2 — Category 2 (suitable for Zone 1 and Zone 2)
- G — Gas/vapour/mist atmosphere (not dust)
- Ex d — Type of protection: flameproof enclosure (d = druckfest, German for pressure-tight). Other common types: Ex e (increased safety), Ex ia (intrinsic safety), Ex nA (non-sparking), Ex p (pressurised)
- IIB — Gas group: suitable for IIA and IIB atmospheres but not IIC
- T4 — Temperature class: maximum surface temperature 135°C
- Gb — Equipment protection level: Gb = high protection for gas (Zone 1)
For mechanical equipment, the type of protection is often Ex h (formerly Ex c) — constructional safety — which covers protection by design features rather than enclosure or intrinsic safety. Mechanical pumps and compressors are commonly certified under Ex h, meaning the protection is achieved through features like restricted surface temperatures, controlled bearing clearances, non-sparking materials for impellers, and mechanical seals designed to prevent the pumped fluid from reaching a potential ignition source.
Mechanical Equipment Specific Considerations
Pumps and Compressors
For pumps handling flammable fluids in hazardous areas, the principal mechanical ignition risks are: bearing overheat from overload or lubrication failure, impeller contact with the casing (producing friction sparks), and mechanical seal failure allowing flammable fluid to escape and contact a hot surface. ATEX pump certification addresses all three through maximum bearing temperature specification, running clearance requirements, and seal face material selection. For pumps handling flammable fluids — regardless of ATEX zone classification — a mechanical seal with a barrier fluid system (dual seal, API Plan 52 or 53) is often specified to prevent process fluid reaching the atmosphere even on seal failure.
Fans and Ventilation Equipment
Fans handling flammable vapour-air mixtures (roof ventilation fans on solvent stores, exhaust fans from spray booths) must be ATEX-certified. Non-sparking fan construction — aluminium impellers with brass or bronze ring on the inlet, or GRP impellers — is required to prevent spark generation from impeller-casing contact. Motors must be appropriately rated for the zone in which they are installed (motor outside the hazardous zone with shaft penetration is the simplest arrangement; motor in the zone requires ATEX-rated motor).
Gearboxes and Couplings
Gearboxes in hazardous areas must be assessed for maximum surface temperature under all load conditions, including stall. Coupling guards must be non-sparking material. Flexible couplings must not generate static electricity — dissipative materials or grounding provisions are required. The mechanical engineer specifying a drive train in a Zone 1 or Zone 2 area must ensure each element carries an appropriate ATEX marking, not just the motor and the driven equipment.
Valves and Actuators
Hand-operated valves with no mechanical power source generate very low ignition risk and are generally exempt from ATEX equipment requirements (though their seals and packing must be considered in the zone classification around them). Actuated valves — pneumatic, hydraulic, or electric — require consideration of the actuator's ATEX status in the zone where it is installed.
The Explosion Protection Document
DSEAR requires the duty holder to prepare and maintain an Explosion Protection Document (EPD) for every workplace where explosive atmospheres may occur. The EPD must contain: the hazardous area classification (zone drawings), the measures taken to prevent and protect against explosions, the equipment installed in each zone and its ATEX certification, and the arrangements for maintenance, inspection, and competence of personnel.
The EPD is a living document — it must be updated when the process changes, when equipment is replaced, or when zone boundaries are revised. In a compliance inspection or following an incident, the EPD is the primary document the regulator will ask to see. An absent or inadequate EPD is a significant compliance failure regardless of whether the installed equipment is correctly specified.
Common Errors in ATEX Equipment Specification
- Installing Zone 2-only equipment (Category 3G) in Zone 1: A Category 3G pump is only certified for Zone 2. Installing it in Zone 1 is non-compliant regardless of the equipment's general quality. Check the zone drawing before specifying equipment.
- Specifying the wrong gas group: Equipment certified for IIA only is not compliant in an ethylene (IIB) or hydrogen (IIC) plant. The substance being handled, not its general category, determines the gas group requirement.
- Ignoring temperature class against fluid AIT: A T3 (200°C max surface) pump in a diethyl ether atmosphere (AIT 160°C) is a potential ignition source. Verify the temperature class against the actual AIT of the substance at its maximum expected concentration.
- Assuming ATEX compliance on replacement parts: Replacing an ATEX-certified pump impeller with a non-certified spare voids the ATEX certification of the assembly. Spare parts for ATEX equipment must be sourced from the OEM or certified equivalents.
- No zone drawing: ATEX equipment cannot be correctly specified without a zone classification drawing. If one does not exist, the EPD is incomplete and no equipment specification can be correct.
Summary
ATEX is a structured framework — zone, category, gas group, temperature class — that links the hazard in the atmosphere to the protection level built into the equipment. For a mechanical engineer, the key decisions are: understand the zone classification from the area drawing, specify equipment of the correct category for that zone, verify the gas group against the substance being handled, check the temperature class against the auto-ignition temperature, and ensure every component of a mechanical drive train or pump system carries appropriate ATEX certification — not just the motor.
Compliance with ATEX starts with the Explosion Protection Document and is maintained by keeping it current when anything in the plant changes. The EPD is not a one-off exercise — it is a live record of the basis on which every piece of equipment in a hazardous area has been selected and is operated.
Forgepoint has experience designing mechanical systems for ATEX hazardous areas across chemical, pharmaceutical and oil and gas applications. Get in touch to discuss your project requirements.
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