Fire/Arson Investigations : Fire/Arson Investigations
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Re: Minimum Ignition Energy (mJ) of gases
Posted by: dcarpenter (IP Logged)
Date: January 02, 2018 01:58PM
It is not just "plus or minus," it is also order(s) of magnitude. The most cited data is for methane. The ignition energy changes by at least one order of magnitude (0.2 to 2 mJ), possibly two if the data was extrapolated at both ends of the flammability range.
Before saying that plus or minus 20% is significant, one needs to look at the other side of the equation. That is, what are the energies associated with common ignition sources of flammable gases.
The minimum ignition energies for most hydrocarbon gases and vapours range from 0.1 - 1 mJ (milliJoule). The level for methane is 0.29 mJ for example. A person walking across a carpeted floor can develop a potential difference large enough for a 40 mJ discharge – more energy than it takes to ignite methane.
Humans generally only sense discharges of 0.6 mJ or more, which means that discharges we can’t detect may carry enough energy to ignite a flammable mixture.
"Sparks" generated from electrode gaps, such as light switches have orders of magnitude higher ignition energy that a static spark. If a switch exists in a flammable vapor concentration of methane, do we really need to take into account the uncertainty of the minimum ignition energy of a gas if the ignition source is orders of magnitude greater than the required energy?
I think there is an inherent assumption of this assessment in the material presented in NFPA 921, but you can be the judge of that when your assessing the value proposition of your proposal.
Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com
Before saying that plus or minus 20% is significant, one needs to look at the other side of the equation. That is, what are the energies associated with common ignition sources of flammable gases.
The minimum ignition energies for most hydrocarbon gases and vapours range from 0.1 - 1 mJ (milliJoule). The level for methane is 0.29 mJ for example. A person walking across a carpeted floor can develop a potential difference large enough for a 40 mJ discharge – more energy than it takes to ignite methane.
Humans generally only sense discharges of 0.6 mJ or more, which means that discharges we can’t detect may carry enough energy to ignite a flammable mixture.
"Sparks" generated from electrode gaps, such as light switches have orders of magnitude higher ignition energy that a static spark. If a switch exists in a flammable vapor concentration of methane, do we really need to take into account the uncertainty of the minimum ignition energy of a gas if the ignition source is orders of magnitude greater than the required energy?
I think there is an inherent assumption of this assessment in the material presented in NFPA 921, but you can be the judge of that when your assessing the value proposition of your proposal.
Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com
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