Thursday, September 27, 2012

Ask The Experts - Spark Detection vs black body ember detection

August @ 2011 @ Ask The Experts

Near Infrared Spark Detection vs. black body ember detection. 

Definition of spark detection as defined by the experts.

Q. We have differing recommendations on spark detection systems for dry wood dust collection. One recommends only detecting and extinguishing black body radiation for specific ignition temperature. Another specifies to detect and extinguish any and all sparks prior to the collector. Given limited information available, which method is preferable?

NFPA 664, Paragraphs A. states: “Provide a spark detection and extinguishing system on the main airflow duct between the dryer drum and cyclone. The spark extinguishing system should activate every time a single spark is detected [emphasis added]. It will reset after a few seconds (if no additional sparks have been detected), and the dryer can continue to operate. The spark counting features available in some approved spark extinguishing systems can be used to shut down dryers when an excessive number of sparks are encountered, but they should never be used as a measure of when to actuate the extinguishing spray.”

There is no discussion concerning “black-body radiation” in NFPA 664, and the only discussion of “black body” is in NFPA 1971 and concerns flammability of clothing. However, there is good discussion of detection methods – including a mention of “Planck’s Law” [regarding black-body radiation] – in NFPA 72, in Sections 3.3, 5.8, A.5.8, and B.5.1.4.

If small, low-energy sparks occur so frequently that spark extinguishment interferes with production AND a prolonged historical record indicates no significant explosion hazard from such sparks, then a “cut-off” based on thermal [black-body] radiation might be appropriate. However, for infrequent sparking, a large low-temperature spark or firebrand might have a temperature below an established black-body temperature criterion but might have more energy – to ignite a dust cloud – than a small particle having a temperature far above the black-body temperature criterion.

Thus, the “bottom-line” response to this question would be a suggestion to consider the above-quoted guidance from NFPA 664, and detect and extinguish every single spark.

Based on the results of a recent explosion investigation by Chilworth, it also would be prudent to count all sparks and document the counts, rather than document the number of extinguishment actions. An increasing number or frequency of sparks could indicate serious problems with upstream equipment.

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