Dust explosions from combustible dust can occur in any industrial process where materials are processed. This combustible dust could exist in the form of a cloud or by build-up, which when circulated can cause an explosive atmosphere. This occasion is rare in normal operation and often temporary. Zone 22 specifically relates to the presence of an explosive dust atmosphere that occurs only by accident, but not in normal duty, category 3D and equipment level Dc is required at a minimum.
In ATEX coding, G represents gas and D for dust, while the second letter refers to the protection level required. Dc equipment protection levels offer enhanced protection; the equipment remains safe in normal operation and may have extra protection to minimise ignition risk in fault situations. A single fault may cause the equipment to shut down on Gc or Dc labelled equipment.
ATEX fans certified for zone 22 use are designed to exacting standards and are suitable for dust group IIIB non-conductive dusts such as milk powder, sugar, or flour, and IIIC conductive dusts such as graphite powder and toner. Some of these materials are not normally combustible, but they can burn or explode if the particles are the right size or concentration.
Motors characterised as “Increased Safety” Ex ec or ex eb, are used on industrial zone 22 explosion proof fans. Ex ec is the lowest and most normal level of Ex protection, known as reduced risk protection. The design of ex ec motors prevents an internal malfunction from creating sparks in normal operation and therefore is not capable of causing an explosive atmosphere to ignite. The construction ensures reliable prevention of unacceptably high temperatures and sparks or electrical arcs that could act as ignition sources in hazardous areas. It also aims to prevent excessive heating and offers a safer design against the risks of sparks during starting.
If a dust cloud enters in contact with a hot surface, it may auto ignite and explode. The MIT, or minimum ignition temperature is the minimum temperature for which a surface will ignite a dust cloud. MIT values are generally between 150-700 degrees Celsius depending on the material. During design phase, a dust explosion risk analysis is carried out on test material to determine the ignition point. This is then used to ensure the system is designed so that the ignition temperature cannot be reached by any component in the process.
Unlike gas zones, where maximum surface temperatures fit into a class that is dependent on the carried gas ignition properties, for dust, the maximum surface temperature should be used as an absolute figure. Industrial fans suitable will advise the absolute figure on their ATEX label; for example, 120° and 135°. Maximum admissible temperatures should be equal or lower than the lowest value of the temperature limit. This is given as 2/3 of the maximum ignition temperature of the dust.
Axair are an award winning team of industrial fan integration experts but cannot by law assess an area to determine its hazardous area classification. We encourage anyone working with hazardous areas to contact an independent body for a zone assessment before requesting information about ATEX certified components for gas or dust extraction. For industrial fan assistance contact our team who will be happy to discuss key ATEX concepts and the suitability of explosion proof or material handling components for dust rich environments.
Axair Fans UK Limited