Could thick cloud coal dust cause flashover on hi-voltage line

A cloud of dust with a high enough concentration of coal dust could cause a flashover. Do I know what the concentration would have to be? No. Smoke from brush fires in South Africa has been documented as a cause for flashovers on transmission lines there. Coal dust is probably more conductive than smoke particles.

Charles Perry P.E.

Dust clouds I think have been known to actually explode under the right conditions. Coal I suspect in a fine enough dispersion might be a possibility. Coal is pretty dirty carbon, so it's reasonable to assume it'd be somewhat conductive. If that's so, I can imagine a discharge propagating through it. After all, there are lots of sharp edges, and that leads to high field gradients. When one has a bit of a corona discharge it effectively reduces the air gap, and cascading could happen.

Or not.

Coal dust has been known to explode. I would suspect that if a spark occurs and the dust was dense enough, there would be explosive combustion. Flame stability is a major concern with coal fired steam generators using fine coal (as dust in an air suspension) - if the flame goes out and fuel is still being blown in there is a chance of a hot spot igniting the coal or sudden explosive ignition when the flame returns - bye bye boiler. It has also happened with flour as well due to static or other sparks. Any combustable material in a fine enough suspension can explode.

Didn't a range fire in New Mexico a few years ago flash over some 345kV spans?

?s falke

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blowing towards

over? Thanks

Carbon is a conductor... coal dust is largely carbon in one of the more combustible forms... the issues are voltage in the lines, distance between the lines, density of dust cloud, relative humidity at the time. There are charts on the density of coal dust necessary for a explosion... called the LEL... lower exposive limit. etc.

You can search that on google, but measurements would have to be taken regardless.. my guess..if the power lines are within

50' of the emmisions source and you have a dense emmision, the risks are higher than if the power lines are 100' or more away.

the higher the voltage, the denser the dust cloud and the higher the humidity within limits, the more likely the cloud will ignite. the force of the blast determined by size of the dust cloud, and if it is pyrotechnically linked to any relatively closed in spaces ...such as a building or large bin or hopper containing part of the cloud.

Uncontained...the blast will for the most part create a lot of noise... not destruction as with a bomb. Contained within a building it will take out the walls or maybe worse.

The odds of ignition?

fairly slim if its a 6,000 volt line probably not a major issue over 50'...but its still an issue. If its high voltage lines, well over the above number, into the tens of thousands of volts...its a much more serious issue..particularly if its old wiring with transformers in the cloud area and close to the source of coal dust.

What can the company do? they can put knock out water sprays at the source of the carbon dust emmission at the very least.. not a legal solution. but it would reduce the hazard IF applied well.. that would be a mist created from a 1500 lb pressurized water source (search misting equipment on google) and sufficient to cover the emmisssions opening with a 3' thick mist...that will take maybe 20 spray nozzels.. my wild guess a 5 gpm flow rate total or 0.2 gpm per nozzle... controls would be to turn the mister on when dust was detected (broken light beam across the stack)..and turn it off when the area cleared.

A scrubber is the legal solution. (contained water spray mist over ceramic fill, it washes the air so to speak)

The above is not a recommendation as I have not seen the site... just an opinion. get a qualifed local PE to review the situation... chemical or mechanical...take that persons advice.

Phil Scott Mech engr.

On Wed, 27 Oct 2004 19:15:40 GMT, RQ put forth the notion that...

I haven't tried it, but I'd certainly think so. It would be much more combustible and conductive than grain dust, and that's been known to explode.

that...

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dust blowing towards

over? Thanks

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Coal dust explosions are common.. even powdered metals not normally considered combustambe when disbursed in the air can burn.. even steel.

In fact you can buy a fine steel wool pad and set it on fire with a match..it will go up in flames (leaving a small pile of rust).

Phil Scott '

I took a chemistry course back in 1964, where the introduction to the class was the prof providing a little demonstration. This was done in an auditorium, where he had a 20-30 foot high ceiling. He had a 1 gallon paint can modified to have two items inside it. One was a candle holder on the bottom. Half way up the side was a small container about 1/2" diameter, maybe 5/8" high, with a small tube running from the bottom of the container out through the side of the can, connected by rubber tubing to a squeeze bag.

He put plain flour into the container, lit up a candle and set it on the bottom, and the put the lid securely on the paint can.

And squeezed the bag. Air rushes into the bottom of the container full of flour, which is blown in up in a cloud of dust inside the can, and explodes. The top of the can didn't reach the ceiling in the demonstration I saw, which clearly disappointed the professor! He said on good days it hit hard.

Given the right "coal dust", there could be one *big* explosion!

There's at least one case of someone having this happen in a shopping bag, where a steel wool cleaning pad came into contact with a small 9V battery's terminals.

| In fact you can buy a fine steel wool pad and set it on fire | with a match..it will go up in flames (leaving a small pile of | rust).

I saw a lightning arrestor made with steel wool stuffed into a PVC pipe along with the coax it was protecting ... after it took a lightning strike. The steel wool didn't just burn ... it exploded, shredding the PVC pipe. I suppose you could call it an "expulsion arrestor". But it had the desired effect of protecting the radio transmitter from the lightning apparently by giving the lightning a "more interesting" way to dissipate the energy.

Many times with deadly consequences. Elevator explosions are another thing that can be reduced with the use of modern sensors etc. I guess there is some sort of chemical that can be released just as an explosion starts that will lessen the explosive force.

Dean