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Combustible Dust Basics, Part 3: What is Overdriving?

Written by AnnMarie Fauske | Apr 9, 2014 8:13:00 PM

This article is the final in a three part series following, "Combustible Dust Basics, Part 2: What Testing Do I Need?"

The “Explosion Severity Test” is a standard dust test used to quantity the maximum pressure of a dust cloud explosion (Pmax) and the speed of the pressure rise (KSt). The test is generally conducted in a 20L sphere (pictured at top) because it is directly scalable to the 1m3 sphere (pictured at bottom), which is the original instrument used to test combustible dust. The 20L sphere is a practical substitute for the 1m3 chamber, as it requires significantly less sample to conduct the test. On the other hand, the 1m3 sphere  is considered to be the “gold standard” for dust testing, and it is useful for providing data whenever the results from the 20L tests raise questions about the explosive characteristics of a dust.

Per Ashok Dastidar, PhD, MBA, Vice President, Dust & Flammability Testing and Consulting Services at Fauske & Associates, LLC (FAI): "The 20L chamber has become the modern workhorse of dust cloud explosibility testing. Explosibility testing in these chambers are performed per ASTM E1226, E1515 and E2931 as well as the EN 14034 methods. Units worldwide provide valuable data to help create dust explosion hazard mitigation strategies in various process industries ranging from agriculture, wood working, pharmaceutical, plastics, fine chemical as well as metal working.

The relatively small size of the 20L sphere results in two limitations," continues Dastidar. "The first is 'overdriving'. Overdriving occurs when the powerful ignition source used to conduct experiments in the 20L chamber preheats the test material and burns the dust cloud under study without really generating a propagating flame. The second limitation is 'underdriving' – where the walls of the 20L chamber abstract heat from the dust cloud explosion and thereby partially quenching the intensity of the deflagration. Both of these phenomena degrade the test data and thus impede the establishment of adequate explosion hazard mitigation. Though the vast majority of dusts and powders are not affected by these phenomena – there are still a few that are."

The solution to both these issues is to perform the relevant tests in a large vessel such as the 1m3. Because of the size and geometry of the 1m3 vessel, it is not as susceptible to overdriving or underdriving. The following blog explores the history of the overdriving issue as well as the methodological response designed to solve overdriving concerns.

Studies going back to 1992 conclude that false positives can occur due to overdriving. False positives may result because materials that will not self-propagate a deflagration will burn due to the overdriving of the large pyrotechnic igniters in the small vessel. It was found that testing in a 20L vessel indicated that a dust was explosible even though the dust was not explosible in a larger 1m3 vessel or in full scale mine tests.  However, strong ignitors may produce overly conservative data in a 20L chamber, and that further tests in a 1m3 chamber would be necessary for a "more realistic hazard determination,” and that “the final determination should be made in a larger system, such as a 1m3chamber”.

A more recent 2007 paper by Proust compared testing results in a modern 20L vessel (consistent with the ASTM E1226) and a 1m3 vessel and again found that overdriving caused false positives in the 20L vessel. The paper found that some dusts with KSt values as high as 65 bar-m/s in the 20L vessel were not actually explosible when tested in a 1m3 vessel and concluded that “a significant proportion of dusts (5 over 21) explode, although weakly, in the 20L sphere and not at all in the 1m3 vessel. From the present analysis, a dust with KSt of 45 bar m/s in the 20L sphere may not explode in the 1m3 chamber.” As a general rule, dusts with a Kst under 50 bar m/s are candidate for clarification testing in the 1m3 chamber.

The ASTM E1226-10 Standard Test Method for Explosibility of Dust Clouds also describes that overdriving can occur in the 20L vessel and recommends testing in a larger chamber such as a 1m3 chamber to determine if a dust is actually explosible. NFPA 68 – 2007 Standard on Explosion Protection by Deflagration Venting also describes that overdriving can occur in the 20L vessel and states, “It can be impossible to unequivocally determine whether a dust is noncombustible in the case of small vessels [e.g., the 20 L (0.02 ft3) vessel]. The ideal solution is to use large (10 kJ) igniters in larger [1m3 (35 ft3)] vessels." Given that the same standard (ASTM E1226-10) governs the performance of dust testing in the 1m3 and 20L chambers, there is no reason not to perform a 1m3 test to ensure that the data is correct.

For more information on your dust testing questions or needs, contact Jeff Griffin at griffin@fauske.com or 630-887-5278. www.fauske.com