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Organizations in EU must follow ATEX directives to protect employees from explosion risk in areas with an explosive atmosphere. There are two ATEX directives, one for equipment manufacturers and one for end-users. ATEX gets its name from the French title of the equipment directive: Appareils destinés à être utilisés en ATmosphères Explosibles. Under ATEX, employers must classify areas where hazardous explosive atmospheres may occur into zones. The classification given to a particular zone, and its size and location, depends on the likelihood of an explosive atmosphere occurring and its persistence if it does. Areas classified into zones 0, 1, 2 for gas-vapor-mist and 20, 21, 22 for dust must be protected from effective sources of ignition. Equipment and protective systems used in zoned areas must meet the requirements of the directive. Zones 0 and 20 require Category 1 marked equipment, zones 1 and 21 require Category 2 marked equipment and zones 2 and 22 require Category 3 marked equipment. Zone 0 and 20 are the zones with the
highest risk of an explosive atmosphere being present.
All new installations after July 2003 must follow the directive, and since 1 of July 2006 all old installations must be updated to comply with the new rules.
The explosive power of a dust is denoted as “Kst”, the rate of pressure rise. Figure 1 compares the Kst values of a number of common dusts. For a much more comprehensive compendium, go to: http://
staubex.ifa.dguv.de/?lang=e. This web site contains a European database known as “GESTIS-DUST-EX” that lists the combustion and explosion characteristics of more than 4,000 dusts. The database provides a useful reference point, although it is not a substitute for the required dust testing. To determine whether your particular dust is combustible, it must undergo explosibility testing. Explosive dusts can be organic or metallic in nature and are present in a long list of manufacturing industries including agricultural, chemical, food, paper, pharmaceutical, textile
and woodworking. ST 1 dusts are below 200 Kst, ST 2 dusts range from 200 – 299 Kst, and ST 3 dusts are 300 and above. Under the new directives, any dust with a Kst value >0 is considered to be potentially explosive. This is significant because, to ensure compliance, many plants now have to install updated dust collection/explosion venting equipment that is manufactured in accordance with the latest EU standards.
3. WILL MY COLLECTOR FIX THE PROBLEM?
Though compliance is a major issue, it is not the only one. What else is the dust collector expected to do? Perhaps it must reclaim valuable product, maintain a higher level of cleanliness in manufacturing areas, accommodate changes or expansions in the plant, or solve a performance problem experienced with an older dust collection system. A good way to pinpoint objectives is by using a site survey form available from most equipment suppliers. This form typically calls for information on the process and the material to be collected,
Figure 1: Kst values of common dusts
operating hours and conditions, electrical
requirements, airflow and pressure ratings and other specifics of the application. The survey will also call for detailed information on the physical properties of the dust. Even if the dust is a common type such as wood dust, something in the process may cause it to behave differently. Therefore dust should always be tested, preferably using a sample collected from used filters. What are the median size and particle distribution of the dust? Is it in the shape of long fibers, uniform spheres or jagged crystals? Is it combustible? Is it sticky or hygroscopic? These are just some of the characteristics that can be determined through a series of bench tests available from independent laboratories and many equipment suppliers. A site survey coupled with lab testing is the best approach for determining the dust collector’s required filtration efficiency and
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pressure drop across the filter media and from this, what type of collector design and media will be most effective for the application.
4. WILL IT PERFORM RELIABLY?
Dust collection equipment can often be a maintenance headache, but this need not be the case. Reliability problems typically stem from neglecting or misunderstanding details about the unit’s performance during the initial selection process or when changes are made in the plant. By following the steps above, you can help to ensure more reliable performance from your dust collector. Although the site survey and lab analysis typically provide enough data, in some cases you may opt to commission full-scale dust collection testing. Full-scale testing typically requires a large (200 L) dust sample that is run through a full-size dust collector on a test rig in a simulation of real-life operating conditions. Dust particle size, pressure drop and other parameters can be precisely monitored and real-time emissions monitoring can be performed. Full-scale testing is usually limited to analysis of difficult or hard-to-handle dusts, or applications where emissions control requirements are particularly stringent. When selecting
equipment, it also helps to be aware of design and technological improvements that can enhance reliability and performance. Examples include:
Horizontal vs. vertical This dust collector
explosion vent is manufactured in accordance with the latest standards to protect against combustible dust hazards.
cartridge mounting: Some pleated filter cartridges are mounted on their sides. The biggest problem with horizontal mounting is that the dust does not get
cleaned off the top of the filter, causing the dust to blind at least one third of the filter. Also, because the incoming dust is dumped on top of the filters, there is no pre-separation of heavy or abrasive particles from the air stream. This situation can shorten filter life, or in spark-generating applications, pose a fire hazard because any spark entering the collector will come into direct contact with filter cartridges.
An antidote to this problem is a system using
vertically mounted cartridges. The best designs incorporate a high, side entry inlet with a
>>> Solids and Bulk Handling March 2014 9
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