Explosion-proof ventilation for petrochemical areas uses spark-proof impellers, ATEX or IECEx certified motors in flameproof enclosures, and full static earthing so no ignition source can form. Air velocity is held above 0.5 m/s to stop combustible dust settling, and the fan is selected for the zone, typically Zone 1 or Zone 2, with a 6 to 12 m/s duct velocity.
The legal framework for hazardous areas is strict because the consequence of a single spark is a loss of plant and life. In the European Union, the ATEX 2014/34/EU directive places a duty on the employer to classify zones, select certified equipment, and document the risk assessment, while IECEx gives the equivalent international certificate accepted across most markets. In the United Kingdom the DSEAR regulations mirror ATEX, and in the United States the NFPA 70 Article 500 area-classification system serves the same role. A fan placed in Zone 1 without an Ex d or Ex e certificate is not a near-miss, it is a prosecutable breach, and after any incident the certificate file is the first thing an investigator opens. The economics are also clear: a certified flameproof motor costs perhaps 30 to 60% more than a standard unit, but a single ignition in a petrochemical area can destroy tens of millions in equipment and shut a unit for months. Earmarking the small premium at spec time is trivial against that exposure, and the certification also protects the supplier in any later liability dispute. Zone classification is therefore the first line of the whole design, set by a competent person before any fan is chosen.
What "Explosion-Proof" Actually Requires
In hazardous areas the fan itself can become the ignition source if mishandled. A stray spark from a standard motor, a aluminium blade striking steel, or a static discharge in a duct can light a gas or dust cloud. Explosion-proof design removes each path: motors carry ATEX or IECEx certificates, impellers are non-sparking alloys or stainless, and the whole path is bonded to earth below 10 ohm. The equipment is matched to the zone classification, never guessed, because under-certifying in Zone 1 is a prosecutable safety failure.
Zone Classification and Equipment
The table contrasts the two common gas zones and the equipment each demands.
| Zone | Gas present | Motor certificate | Impeller material |
|---|---|---|---|
| Zone 0 | Continuous | Ex ia intrinsic safety | Non-spark, earthed |
| Zone 1 | Likely in normal use | Ex d flameproof / Ex e | Stainless, aluminium-free |
| Zone 2 | Only in fault | Ex n / Ex e | Coated steel acceptable |
| Zone 21 dust | Likely dust cloud | Ex tD enclosure | Anti-static, earthed |
Dust zones 20, 21, 22 mirror the gas scheme but focus on combustible powder layers, requiring IP6X enclosures and temperature class below the dust ignition point, often 200°C or less.
Selection Checklist for Hazardous Areas
- Choose Ex d flameproof motors in Zone 1 where gas is expected during normal running, never a standard TEFC motor.
- Choose Ex n equipment in Zone 2 to cut cost where explosive atmosphere appears only on fault.
- Choose non-sparking stainless impellers where a blade strike could ignite, avoiding aluminium in Zone 1.
- Choose duct velocity 6 to 12 m/s to keep dust airborne and pipes clear without excess static build-up.
- Choose full bonding with earth resistance below 10 ohm and verify it before the first energisation.
Velocity, Static, and Temperature
Velocity is a safety variable, not just a flow one. Below 0.5 m/s, combustible dust settles on blade and wall and waits for a spark; above 12 m/s, static charge and erosion rise. Keep 6 to 12 m/s in ducts and confirm the surface temperature class stays below the auto-ignition point of the specific vapour, commonly 200 to 300°C for hydrocarbons. Flexible connectors must be conductive and earthed, and the whole system is continuity-tested below 10 ohm to ground. Routine inspection checks for coating damage that could break the bond and for dust build-up that could breach the velocity floor.
Worked Example: A Zone 1 Solvent Tank Farm
A tank farm vents hydrocarbon vapour and is classified Zone 1, because release is conceivable during normal filling. The extract must move 8,000 m³/h at 6 to 12 m/s, so a 500 mm duct at 11 m/s is chosen. The fan uses an Ex d flameproof motor and a stainless non-sparking impeller, with no aluminium that could strike and ignite. Bonding is verified below 10 ohm to a common earth bar, and the flexible connector is conductive. Surface temperature class is T3, capped at 200°C, safely under the 250°C auto-ignition of the solvent. At 8,000 m³/h and 400 Pa the unit draws 1.5 kW; had velocity dropped to 0.4 m/s, vapour would stratify and dust would settle, so the setpoint is held by a flow switch that alarms on droop. Annual inspection checks the coating for damage that could break the 10 ohm bond and measures dust depth against the 0.5 m/s floor, keeping the ignition path closed for the plant's life.
Inspection frequency should track the duty, not the calendar alone. A continuous tank farm warrants a bonding and velocity check every 3 to 6 months because coating damage from cleaning or impact can break the 10 ohm path between visits, while a standby extract used a few hundred hours a year can be reviewed annually. Each check records the earth resistance, the duct velocity at the flow switch, and the dust depth on blades, so a trend reveals slow degradation before it becomes a hazard. Keeping the last three records on file also demonstrates due diligence to an ATEX or DSEAR auditor, turning maintenance from a cost into a compliance asset that protects the operator after any incident.
Common Mistakes to Avoid
- Fitting a standard TEFC motor in Zone 1, where a spark during start-up can ignite the vapour.
- Using aluminium impellers where a blade strike could throw a spark, when stainless is required.
- Skipping earth bonding, because a 10 ohm limit only works if it is measured, not assumed.
- Letting duct velocity fall below 0.5 m/s, so combustible dust settles and waits for ignition.
- Choosing the wrong temperature class, exceeding the vapour auto-ignition point by even 20°C.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What makes ventilation explosion-proof for petrochemical areas?
Explosion-proof ventilation uses spark-proof impellers, ATEX or IECEx certified motors in duct or flameproof enclosures, and static-earthing throughout so no ignition source forms. Air velocity stays above 0.5 m/s to prevent settled combustible dust, and the fan is sized for the zone classification.
What is the difference between Zone 1 and Zone 2 equipment?
Zone 1 means explosive gas is likely during normal operation, requiring Ex d flameproof or Ex e increased-safety motors, while Zone 2 means it occurs only in fault, allowing lower-cost Ex n equipment. The zone sets the motor certificate and the impeller material, such as aluminium-free stainless to avoid sparking.
Why must fan velocity stay above a minimum in hazardous areas?
Velocity above 0.5 m/s keeps combustible dust from settling on blades and duct walls where a later spark could ignite it, and it prevents gas stratification. Too low a speed lets a flammable layer build; too high a speed raises static and erosion, so 6 to 12 m/s in ducts is the usual band.
How is static electricity controlled in explosive atmospheres?
All metal parts are bonded and earthed with a resistance below 10 ohm to ground so charge cannot accumulate to a spark. Flexible connectors use conductive material, and non-metal ducts get a static-dissipative liner. This is verified by a continuity test before the fan is energised in the hazardous zone.