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From: (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: solvent vapor, LEL
Date: Fri, 25 Feb 2000 18:42:16 GMT wrote:

>I'm working on safety issues for my company's product. My company
>manufactures commercial ink jet printers. The ink is solvent based.
>Solvent evaporates when ink flows out of a nozzle. It creates a
>concentration (exceed LEL - lower explosion level for that solvent
>vapor)in a small confined area. In that confined area, there might be
>possibility of arcing when certain conditions are met. We tried to
>generate arcs but have never observe any explosions. So we think that
>the arc has to carry enough energy (temperature or some other energy
>form) to make the explosion to happen. Does anyone in this forum know
>something like this or can steer me to right sources. Thank you.

Almost all major reference books on fire and explosion safety will
have sections on the relationship between minimum ignition energy
and limits of flammability for various fuel/air mixtures needed
for ignition. The relationship varies according to temperature,
pressure and spark gap. You may get more information from the group, but just to give you an idea.

For n-butane in air at 25C and 1 atm, the limits of ignitability are

Energy        Fuel/air ratio
0.25               0.10
0.5             0.06-0.13
1               0.055-0.15
10              0.04-0.17
100             0.04-0.18

The minimum spark ignition energy in air and oxygen at 25C and 1 atm
for some common chemicals is below. I've included oxygen, even
though it's not relevant to you, because many don't realise how
easy it is to ignite fuel/oxygen mixtures, which also have
wider flammability limits as the upper limit expands significantly
eg methane ( air 5-15%, oxygen 5-60% ), diethyl ether ( air 1.9-36%,
oxygen 2-82% ), acetone ( air 2.6-18%, oxygen 2.6-60% ), methanol
air 6.7-36% oxygen 6.7-93% ) .

Chemical           Minimum ignition energy
                          ( mJ )
                     Air       Oxygen
Methane             0.30       0.003
Propane             0.26       0.002
n-Hexane            0.29       0.006
Acetylene           0.017      0.0002
Ethylene            0.07       0.001
Benzene             0.22         -
Acetone             1.15       0.0024
Ammonia         >1000            -
Diethyl ether       0.20       0.0013
Methanol            0.14         -
Hydrogen            0.017      0.0012

The above information comes from a US Dept of the Interior
Bureau of Mines Bulletin 680. " Investigation of Fire and
explosion Accidents in the Chemical, Mining, and Fuel-related
Industries - A Manual. by Joseph M. Kuchta.

If you don't understand the complexities of hazardous
properies of fuel-air mixtures, perhaps the cheapest solution
would be to hire a safety consultant that specialises in
calculating and mitigating such risks. You wouldn't like
to tell us the name of your employer would you?. Then we can
consider the experience of the staff conducting safety
assessments when purchasing printers.

          Bruce Hamilton

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