Dust and fine powders, whether from catalysts, active ingredients or inert fillers, are an unavoidable presence in chemical processing plants. Unless you're breathing them in because of a faulty hood or collection unit, it's all too easy to forget about these nearly invisible particles.
However, you ignore them at your peril. All that's needed to ignite fine particles is a small flame or electric spark, and air. In many cases, problems with filters, baghouses or dust-collection equipment are to blame. But even friction or mechanical failure in mills, grinders, filters, conveyors, cyclones and other equipment can start a conflagration.
This year, two fatal accidents, occurring within a month of each other, underscored the importance of controlling dusts, and the need for standards to prevent dust explosions in the process industries.
Together, explosions at the CTA Acoustics plant in Corbin, Ky., and the West Pharmaceutical Services facility in Kinston, N.C., killed 13 and injured more than 50 people. The tragic lessons left in their wake, and analyses by the U.S. Chemical Safety Board (CSB), may help other chemical processing companies ensure the safety of their processes, operations and facility designs. They may also lead to safety standards and better guidance for manufacturers.
Cleaning and maintenance procedures, false ceilings blamed
The January 29th explosion at West Pharmaceutical took place in an area where a nonstick coating of moistened polyethylene powder was added to rubber to manufacture sheeting. Numerous dust-collection systems were placed throughout the plant, but they were not linked to the accident. Instead, the potential for explosion was overlooked during an earlier plant expansion. Apparently, when the company enlarged the Kinston facility in the 1980s, it used suspended ceiling tiles to contain process areas and facilitate their cleaning. According to CSB, as the rubber was dried, fans blew some of the polyethylene dust into the air.
Most of it fell on equipment surfaces, walls and floors within the processing area, and was cleaned during routine maintenance. However, some of the dust entered air conditioning intakes above the suspended ceiling, and accumulated above it, out of eyeshot of engineers at the facility. Selecting a different type of ceiling might have prevented the accident, the Board says.
Why do such accidents continue to occur in the process industries? One problem is significant knowledge gaps, CSB member Gerald Poje, lead investigator for the Corbin explosion, wrote in July 16th's Louisville Courier Journal. The National Fire Protection Association (NFPA), based in Quincy, Mass., has established best practices governing dust in manufacturing facilities, but "industry awareness remains low, and compliance, voluntary," Poje wrote. Although OSHA has set specific standards for coal, grain and sawmill dust, there are no standards for dust in chemical processing plants.
In contrast, Europe has set very strict standards for dust in its ATEX directive, which took effect throughout the European Union last month, augmented, in the U.K., by the Dangerous Substances and Explosives Atmosphere Regulation (for more information see www.mvo.nl/milieu/download/non-binding-guide-of-good-practice.pdf). The regulation divides process areas into zones where dust clouds are continually, occasionally, or potentially present, and establishes strict guidelines for equipment, process and workplace safety within each zone.
More study needed
Meanwhile, consultants recommend that companies monitor closely any areas where fine particles are present. The first place to check for potential problems is in baghouses, filters and other dust-collection units. Malfunctioning collectors are one of the leading causes of dust explosions across all industries, experts say. Potential ignition sources should be eliminated, and containment, venting, suppression and inert gas blanketing systems should be evaluated. (For a brief, comprehensive article, see www.powderbulksolids.com/docs/safetywatch2.doc).
Process and powder-handling equipment also should be checked. (For a review of friction-related dust explosions, see www.safetynet.de/Seiten/articles/HSL.pdf. )
Codes and resources
The American Institute of Chemical Engineers' (AIChE) Center for Chemical Process Safety (CCPS) also offers guidance on dust www.aiche.org/pubcat/seadtl.asp?ACT=S&Keyword=ON&Title=ON&ISBN=ON&Pubnum=ON&srchText=dust+explosions).
The issue also is treated in the CCPS handbook "Essential Practices for Managing Chemical Reactivity Hazards," published this year. OSHA plans to make the book available for free download via its Web site.
By Agnes Shanley, managing editorThe NFPA is revising standards for chemical processing plants, and a new standard is due in 2005; However, its current standard 654 is available (For a catalogue and more information, see www.normas. com/NFPA/index3.html and www.nfpa.org/Codes/NFPA_Codes_and_Standards/List_of_NFPA_documents/NFPA_654.CSB has recommended that the issue of dusts in chemical processing plants be studied in more depth, and will continue to examine the two explosions to determine the adequacy of current regulations and good manufacturing practices, said CSB spokesman Daniel Horowitz.At the CTA plant, which made automobile insulation, the fire occurred on February 20 at a production line that was being cleaned. According to CSB, which released preliminary findings last month, the temperature control devices on ovens used to dry the insulation had been malfunctioning, and one or two oven doors had been left open to allow them to cool. Flames escaped through the open doors and ignited phenolic resin dust that had been disturbed during cleaning. The initial explosion stirred up more dust, causing more secondary explosions in a runaway reaction typical of these accidents.