Dust clouds in the explosive region (above the minimum explosible concentration) are categorised into 3 zones, based upon the grade of release (as per EN60079-10-2:2009):
Continuous release inside a dust containment enclosure gives rise to Zone 20 - a place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is present continuously, or for long periods or frequently for short periods. For example, a mill or pneumatic conveying system.
Primary grade of release gives rise to Zone 21 - a place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is likely to occur occasionally in normal operation. For example, bagging points and inspection ports that are frequently opened.
Secondary grade of release gives rise to Zone 22 - a place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is not likely to occur in normal operation but, if it does occur, will persist for a short period only. For example, leaks from incorrectly fitted lids or spillages.
In many plants designed for the handling and processing of flammable materials, dust may escape from equipment and settle on the floor and on fixtures and fittings in the workroom. In the settled state the dust is not explosive but any unusual shock may displace some of it and disperse it in the form of a dust cloud. This shock is often caused by a relatively mild primary explosion inside the plant, the flame from which serves as a means of ignition for the dust cloud formed in the workroom. This secondary explosion disperses more of the dust accumulated in the workroom and ignites these clouds until the whole building is swept by the explosions. Accumulation of dust in workrooms is the cause of major explosions and its elimination is vital. The following precautions should be observed:(1)The dust must be prevented from escaping from the plant, which should be designed and constructed with this end in view. All joints should be flanged and packed. All inspection doors, slide valves etc., should be dust tight.(2)Where practicable, the plant should be operated under a slight negative pressure to reduce the egress of dust.(3)At charging and bagging-off points complete enclosure of the plant is not always practicable. To prevent dust from entering the workroom these points should be enclosed as far as possible by means of a hood or cabinet. The access openings should be reduced to a minimum. Local exhaust ventilation should be provided to control the dust cloud within the cabinet.(4)The handling of sacks containing dusty material is a common cause of dust in workrooms. Sack storage should be avoided in rooms in which milling and similar processes are carried out. Such storage should be in a separate warehouse from which all plant in which a primary explosion could occur is excluded.(5)In spite of all precautions some dust will always escape from the plant. It must be prevented from accumulating in workrooms by the regular and frequent cleaning of all parts of the room and plant on which it may settle. Careful design and construction of workrooms facilitates this cleaning. All horizontal surfaces on which dust can collect should be avoided. Window sills, door frames and projecting fittings should be made flush with the wall. Other horizontal surfaces like girders, beams and ledges should be bevelled to prevent settlement. Walls should be made smooth and corners should be rounded. Vacuum cleaning is recommended for dust removal. In small factories portable vacuum cleaners may be used provided the electrical equipment is safe for use when exposed to flammable dusts. A static vacuum pump fitted to a ring main is more suitable for large factories; the ring main should be provided at intervals with facilities for the connection of flexible cleaning hoses. Soft brooms may be used provided care is taken to avoid the generation of dust clouds. The use of compressed air for cleaning is not recommended.
Hazardous Area Zoning
Hazardous area zoning of dust handling plant is required to comply with article 7 of the ATEX workplace directive. The first step is to ascertain whether the dust is explosible (Group A) or non-explosible (Group B). This is done either by reference to explosion data or through dust explosion testing. Once flammability is established, zoning is determined following the grade of release: continuous grade of dust release is zone 20; primary grade of dust release is zone 21; secondary grade of dust release is zone 22. In certain circumstances, the zoning may be modified to accommodate dust cloud concentrations known to exist inside plant equipment. For example, screw conveyors do not promote the formation of dust clouds internally and this fact may be reflected in a reduction of the zone designation from zone 20 to zone 22. Note that screw conveyors have the ability to transfer an explosion through the mechanism of disturbing internally settled dust - hence zone 22, not non-hazardous. Such modification to hazardous area zoning should only be undertaken by a competent person.