By the time there is a visible flame the spark detector can see, the material would have already caught on fire
(PRWEB) August 24, 2005
Five elements are necessary for a dust explosion: ignitable dust; suspension of the dust into a cloud (in sufficient concentration); confinement; oxidant (usually air); and ignition. Take any one of these away and there can be no dust explosion. ThatÂs the concept behind the latest infrared heat detection equipment, which seeks to detect and remove hot bodies before they can contribute to a devastating event.
In every system there are high-risk zones where the possibility of dust explosions is relatively high. Ignition sources include sparks and embers, hot surfaces, and static electricity to name a few. Frequently, the sources of ignition occur within the production equipment, such as metal striking metal, heat generated in the process, friction, or even loss of ground.
High-risk equipment includes mills and grinders, ovens and dryers, rotating equipment bearings, dust collectors, storage bins and hoppers, or anywhere a high concentration of dust may accumulate.
In the past, many at-risk plants have utilized spark detectors to detect a visible flame or spark. However, these devices operate by detecting light Â not heat - which poses several significant drawbacks.
Most importantly, the ignition temperature for many dust clouds is 500ÂºC or lower Â the approximate temperature of a recently extinguished match. That's too low for spark detectors to pick up, since visible sparks and embers are not detectable at temperatures below 700ÂºC.
"By the time there is a visible flame the spark detector can see, the material would have already caught on fire," says David Cvetas, President of Cv Technology (http://www.cvtechnology.com) a prominent gas and dust explosion protection consulting and technology company.
Furthermore, spark detectors are very sensitive to any form of light in the facility, increasing the likelihood of false positives.
ÂIf you get a light leak in a process line, a spark detector might detect it and then activate the suppression system,Â notes Cvetas. ÂAll of a sudden an alarm sounds, water starts spraying, and the process shuts down.Â
As opposed to spark detectors that look for light, the latest breed of fire and dust explosion prevention systems seek heat sources. An examination of the Firefly product -distributed by Cv Technology - illustrates the mechanism.
The Firefly system reacts to temperatures as low as 250ÂºC - well below the almost 700Âº C threshold of traditional spark detectors. The detector measures both the number of glowing particles and the highest registered energy value.
The Firefly system can detect and neutralize ignition sources within 100-300 milliseconds. Water, the most common extinguishing agent, is sprayed under high pressure through full-cone nozzles throughout the extinguishing zone. Carbon dioxide and nitrogen extinguishing agents, when indicated, are regulated using high-speed actuators with closure times from 50-300 milliseconds. Mechanical diversion involves a switching valve that rapidly routes any abnormally hot material off the process conveyor into an isolated container. The process itself need not be stopped.
ÂThe system detects a hot body before it can ignite any particles and, if necessary, takes fast action to remove it from the process product,Â says Cvetas. ÂIt is a terrific dust explosion prevention tool.Â
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