Introduction
Coal dust, some organic additives and other combustible powders can create a deflagration hazard in a cement or bulk-solids plant. Do not infer combustibility from the plant name or assume every biomass ash is combustible: identify the actual material and representative conditions. An initial explosion may disperse deposited dust and cause a secondary event, so the review must include surrounding structures and connected equipment.
A cement screw conveyor intended for a hazardous area needs a defined project specification. ATEX includes a product framework and a workplace framework; UK DSEAR addresses workplace risks. These are not interchangeable labels, and a certified motor does not establish conformity of the complete conveyor. VOGO’s public catalog does not claim that every standard machine is ATEX certified. This guide explains zoning, ignition control, mitigation, testing and procurement responsibilities.
Area classification and equipment selection
Classify locations from the frequency and duration of an explosive atmosphere. For combustible dust clouds, Zone 20 concerns continuous, frequent or long-duration presence; Zone 21 concerns occasional presence in normal operation; Zone 22 concerns presence unlikely in normal operation and brief if it occurs. These definitions do not assign a screw’s interior or exterior automatically. Depending on the evidence, its interior may need a different classification from Zone 21, and an external location may be classified or unclassified.
Assess inlet and outlet releases, shaft seals, covers, ventilation, dust deposits and cleaning. A damaged seal can change the release conditions and require reassessment; it does not mechanically convert every Zone 22 location into Zone 21. Record boundaries and assumptions on a responsible party’s approved drawing. Clouds and layers need related but distinct attention, as described in the public scope of IEC 60079-10-2:2026.
For the usual non-mining dust selection framework, Category 1D is used for Zone 20, Category 1D or 2D for Zone 21, and Category 1D, 2D or 3D for Zone 22, subject to the complete suitability assessment. Category is not the only criterion: dust characteristics, maximum surface temperature, ambient range, protection concept and conditions of use also matter. Do not confuse ATEX equipment Group II with the dust subdivisions used in relevant equipment standards.
Review the assembly, internal and external ignition hazards and its conformity route. Individual flights are not necessarily separately certified products, and collecting certificates for components does not assess their combination. A site’s decision to standardize higher-category spares can be a maintenance policy, but it is not a universal rule that every plant uses Category 2 everywhere.
Ignition control and mechanical design
Consider friction, hot surfaces, electrical faults, electrostatic discharges, hot work and incoming hot or foreign material. Conductive parts may need bonding and earthing, including continuity across joints and flexible connections. Non-conductive materials need their own electrostatic assessment. An “anti-static flight” label does not prove that all ignition mechanisms are controlled.
Specify a verification method and acceptance value for each bonding or electrostatic function using the applicable design and standards. A blanket resistance limit of 10^9 ohms is not a suitable acceptance criterion for every metallic protective connection. Nor is routine insulation-resistance testing automatically the correct continuity test. Select instruments and a safe test procedure for the particular circuit and location.
Maintain designed clearances while accounting for runout, shaft deflection, thermal growth, liner thickness and deposits. Both interference and inappropriate geometry can create problems; there is no universal 3–6 mm clearance for all conveyors. Prevent or detect foreign-object entry through measures selected for the material and process. Seal and bearing selection must address contamination and movement rather than relying on a labyrinth name alone.
Bearing-temperature sensors, including PT100 devices where appropriate, can support ignition control if their locations, response, limits and fault handling are engineered. A sensor does not guarantee it will detect every hot spot in time. Set alarms and protective actions with the complete thermal assessment, including dust layers and the equipment’s permitted surface temperature, not simply at the measured dust ignition temperature. See the bearing-temperature guide for comparable baselines and investigation of trends.
Venting, suppression, containment and isolation
A standard screw casing is not automatically resistant to an internal explosion. The protection concept must consider the strength of the entire enclosure, including covers, joints and connections, and the possible propagation into silos, filters and receivers. Ordinary process ventilation and a silo pressure-relief valve are not explosion venting systems.
Purpose-designed explosion vents release pressure to a suitable location. Their design depends on more than Kst and Pmax: enclosure geometry, operating conditions, opening characteristics, ducting and the permitted reduced explosion pressure also matter. Do not place generic panels at arbitrary intervals. Compare the design pressure effects with the verified resistance of the complete assembly, not only the yield strength of a sheet material.
Indoor arrangements need assessment of flame, pressure, heat, dust discharge and personnel exposure. Flameless venting, suppression, pressure-resistant containment or another justified concept may be considered within its certified application limits. None is automatically suitable solely because an external vent route is inconvenient. Suppression needs detection, agent delivery, controls and periodic verification matched to the installation.
Isolation limits propagation through connections. Passive mechanical devices and active detection-triggered systems operate differently; a passive flap does not contain an electronic detector merely because it responds to pressure. Select the isolation method for flow direction, location, material, connected volumes, operating states and validated limits. An ordinary process flap, slide gate or rotating screw is not verified isolation. Define trip logic, interface responsibility and fault response without assuming every arrangement requires the same safety instrumented system.
Dust characterization and test conditions
Use representative laboratory data when reliable information does not cover the actual powder. Kst characterizes explosion pressure-rise behavior under the specified test method; Pmax is the measured maximum explosion pressure under those conditions. They support consequence and protection design, but do not alone determine ease of ignition, equipment temperature or every isolation requirement.
Select additional tests for the decision, which may include minimum ignition energy, cloud and layer ignition temperatures, or electrical properties. Link results to composition, particle distribution, moisture, sample preparation and method. Deposited or collected fines may differ from the incoming bulk sample. The public scope of ISO/IEC 80079-20-2 describes identification methods for combustible dust and layers; it is not a source of one universal design value for every powder.
Dust layers can insulate hot surfaces and have different ignition behavior from clouds. Apply the relevant selection method and margins for layer thickness and the complete duty. Subtracting 75 K from one layer test result is not a complete temperature specification for all conditions. Likewise, there is no universal transition from “safe at 5% moisture” to “explosive at 1%”; testing must cover credible operating changes. Record the limits beyond which a material or process change requires reassessment.
Writing a procurement specification
Separate the operator’s workplace obligations from the manufacturer’s product obligations. For EU supply, define the applicability of Directive 2014/34/EU and the intended use, while the workplace assessment follows its own framework. Specify internal and external classifications, required equipment category, relevant dust designation, maximum surface temperature and ambient range. Dust temperature selection should be explicit; do not substitute an unexplained gas T-class for the dust requirements.
Request the EU Declaration of Conformity where applicable, identification and marking, instructions, drawings, required conformity-assessment documents and any certificates relevant to the chosen route. For components with a different documentation route, identify that route explicitly. The review must cover the complete assembly and protective systems, not only the motor. Confirm who designs and supplies detection, isolation, vent paths, controls, installation verification and commissioning tests.
Supply the actual dust data, layout, pressure and temperature envelope, start and stop states, feed duty and connected volumes. Agree maintenance access, inspection intervals, spare-part restrictions, proof tests, bypass controls and response to faults. Evaluate initial cost together with testing, air or energy use, maintenance and downtime. Do not promise that certification automatically lowers insurance premiums or guarantees a particular lifetime cost saving.
Conclusion
A defensible specification connects material evidence and area classification to ignition controls, a designed protection concept, assembly conformity and verifiable maintenance. Neither enclosure nor a certified component proves the whole conveyor suitable. Use the explosion-risk assessment guide to assemble the project evidence before requesting VOGO’s proposed configuration and documentation scope.
