sued, 5(a)(1)) states: “citations may be issued for deflagration and explo- sion hazards if [OSHA’s Salt Lake Technical Center (SLTC)] finds KST values of the submitted dust sample to be greater than zero. Citations may also be issued for other fire hazards if SLTC determines that the dust is combustible.” Although simple, two problems
can arise when using this approach: 1. The 20-L chamber used for most KST tests is prone to false posi- tive results (studies indicate KST results below 30 are suspect), so definitive KST tests require a 1-cu. m chamber.
2. The KST-only approach to hazard evaluation ignores ignition sensi- tivity information. The question of how easily the dust will ignite is answered by four tests:
Testing Laboratories. Only a handful of testing laboratories are
able to conduct combustible dust testing: • Chilworth Technology Inc., Plainsboro, N.J.,
www.chilworth.com
• Ciba Expert Services, McIntosh, Ala.,
Dean.Hamel@ciba.com
• Fauske & Associates LLC, Burr Ridge, Ill.,
www.fauske.com
• Kidde Fenwal Inc., Holliston, Mass., www.
kidde-crc.com
• Fike Explosion Protection Services, Fike Corp., Blue Springs, Mo.,
www.fike.com
• EMSL Analytical Inc., Centennial, Colo.,
www.emsl.com
1. Minimum Ignition Energy— Cloud. How much energy is needed to ignite a cloud of dust? The greatest concern is with
Table 2. The explosivity index is evaluated by multiplying ignition sensitivity and explosion severity.
dusts that can be ignited by the energy in a static spark.
2. Minimum Ignition Tempera- ture—Cloud. What temperature is needed to ignite a cloud of the dust? Dust clouds that can be ignited by a hot surface are a concern.
3. Minimum Ignition Tempera- ture—Layer. What temperature is needed to ignite a layer of the dust? Layers of dust can act as an insulator and cause equipment to overheat. Dust layers that can be ignited by a hot surface are a concern.
4. Thermal Stability—Exothermic Reactions. Is there a temperature at which the dust will begin to react and cause a rapid rise in temperature?
Finding the Explosivity Index One way of evaluating the overall
hazard of dust accumulation is to combine the information obtained in the testing above to develop an explosivity index according to a for- mula developed by the U.S. Bureau of Mines and incorporated into NFPA 499 Recommended Practice for the Clas- sification of Combustible Dusts and of Hazardous Locations for Electrical Installations in Chemical Process Ar- eas. Ignition sensitivity and explosion severity are determined according to the following formulas:
Ignition Sensitivity =
(T(c) x E x M(c))(1) (T(c) x E x M(c))(2)
Explosion Severity =
(P(max) x P)(2) (P(max) x P)(1)
Where T(c) = Minimum Ignition Temperature (oC), E = Minimum Ignition Energy (mJ), M(c) = Minimum Explosive Concentra- tion (g/m3), P(max) = Maximum Explosive Pressure (barg), and P = Maximum Rate Pressure Rise (KST) (bar-m/s)
Subscript 1 refers to the appropri-
An explosion hazard may exist if accumulations are more than 1/32-in. deep. If you can write your name in the dust, it is too deep.
MODERN CASTING / February 2011
ate values for Pittsburgh seam coal, the standard dust used by the U.S. Bureau of Mines. Subscript 2 refers to the values for the specific dust in question. The explosivity index is evaluated by
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