requirements of the National Fire Protection Association (NFPA) 484: Standard for Combustible Metals. Grinding or polishing operations can create fi ne dust and must be ventilated, with the collector located outside the building, unless it is a wet collector. Dust must be removed from collectors daily and fi nes kept in sealed conductive containers. In addition to metal, any other com-
bustible material may present a hazard if fi nely divided to smaller than 420 microns (small enough to pass through a No. 40 standard sieve). A hazard also may exist if fi ne dust is separated from larger particles. This can occur if a leak develops in a ventilation system that allows only fi ne particles to escape. Separation most often oc- curs through gravity, as small fugitive particles fl oat and settle on overhead surfaces while larger, heavier ones fall to the fl oor. An explosion hazard may exist ac-
cording to NFPA 654: Standard for the Prevention of Fire and Dust Explosions from Manufacturing, Processing, and Handling of Combustible Particulate Solids if: • the dust is combustible; • accumulations are more than 1/32- in. deep (if you can write your name in the dust or if it obscures the color of the surface underneath it, it is too deep);
• the area of surfaces covered by combustible dust exceeds 5% of the fl oor area. In the metalcasting industry, areas to
review include wood and plastic dust from pattern shops, seacoal and other solid organic mold additives, powdered core resins or organic binders like dextrin, and melt alloying materials. For metalcasters melting aluminum or magnesium, fi ne metal particles from shot blasting and metal fi nes from grinding, polishing and machining
A dust explosion has two phases: a primary explosion and a secondary explosion that occurs when dust is lifted into the air and ignited by the primary explosion.
operations must be handled with care.
Evaluating Combustible Dust Evaluating combustible dust consists
of fi nding the answers to two questions: 1. How easily does the dust ignite (igni- tion sensitivity)?
2. How powerful is the explosion if the dust does ignite (explosion severity)? The questions are usually answered
one after the other, fi rst by using the A/B Classifi cation test, performed ac- cording to the ASTM E1226 Standard Test Method for Explosibility of Dust Clouds.. In this test, a sample of the dust is suspended in a test chamber (usually a 20-L sphere) with an ignition source. If the result is negative, the dust is not combustible. If positive, additional test- ing may be appropriate, but the result of the initial test can be used for explosion venting criteria. What happens when the dust ignites?
Table 1. The hazard denoted by the KST value is usually ranked as shown in this table.
Three measurements are required: 1. Maximum Pressure (PMAX)—The maximum pressure created when the dust ignites.
2. Maximum Rate of Pressure Rise ((dP/ dT) MAX)—How fast does the pres- sure build up when the dust ignites?
3. Defl agration Index (KST)—How fast does the pressure wave travel? The hazard denoted by the KST
value usually is ranked as shown in Table 1. While the table indicates KST values less than 200 are less of a concern, explosions have occurred with materials in the 1 KST category. The U.S. Occupational Safety and Health Administration (OSHA) cur- rently considers any KST above 0 to be a problem. OSHA’s Compliance Program Language document for the Combustible Dust National Emphasis Program (CPL 03-00-008 - Combustible Dust National Emphasis Program, Reis-
32
MODERN CASTING / February 2011
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