Workplace safety Dust is our business But it’s our menace too. By Staffan Melin pellets are designed to burn – they’re made from highly combustible dust that is pressed into pel-let form and must be treated with care to prevent this fuel from lighting before it has been added to the boiler. The business of making dust is chal-lenging due to the risk of dust explosions. The dust is present in all stages of produc-tion and is also generated during transpor-tation and storage due to attrition. The fin-ished product is dropped dozens of times before reaching the destination and the drop height can be quite considerable, for example up to 25 metres during the initial stage of loading an ocean vessel. Dust collection, dust suppression, ex-plosion containment and explosion sup-pression are employed in facilities for manufacturing, handling and storage to minimize the risk of injuries and damage to equipment and facilities. The risk factors for initiating a fire or explosion includes hot particles or sparks landing on a dust layer setting the dust on fire, often with explosive force, which in turn may trigger a secondary massive explosion when dust in the rest of the facility becomes lofted. A rule of thumb to be used as a guideline for housekeeping is that a maximum thickness of 3.2 mm (1/8”) of dust should not be cov-ering more than five per cent of the floor area. Also, dust on surfaces with tempera-tures higher than 250ºC will catch fire. If the burning dust is dispersed at high speed in what is referred to as deflagration, it will light everything in its path and continue to burn until the fuel is exhausted. Surprisingly, dust lofted in the air catch-es fire if heated to +450ºC or higher. Small particles of dust may stay lofted in the air for a long period of time, particularly in tur-bulent air. In still air a dust particle with an equivalent diameter of 70 micron may stay in the air for several minutes and a one mi-W ood cron particle or smaller will stay constantly lofted in still air. If the concentration of loft-ed particles exceeds 70 gram/m3, a defla-gration may be initiated by, for example, an electro-static discharge. STATIC RISK Generation of tribo-electric charges due to fast moving equipment is a major concern. Wood dust has an extremely high electrical resistivity, which means that electrons are accumulated in the surface of dust particles and if materials in contact with the dust also have high resistivity, an electrostatic potential will build up and eventually dis-charge as a spark. All materials in contact with wood dust have to be electrically conducting to bleed off the charge from the dust. All equipment has to be properly earthed, grounded and bonded to deflect any electrostatic voltage build-up. There is static paint available to minimize the dust sticking to walls and floors and there are also static dissipative hard toe shoes (CSA – SD certified) avail-able to minimize discharge from workers using synthetic clothing. ENGINEERED FOR SAFETY Strict engineering design rules apply for mitigating the risk of fires and explosions in manufacturing, handling and storage Canadian BIOMASS 13
Canadian Biomass September October 2014: Page 13
