explosive situation?

Flour would be another prime example. I'm sure we're all familiar with the many reports of kitchen dust explosions when flour particles became airborne and explosively ignited from a lit burner on the stove.

todd

No, the situation is exactly the same. It's a question of max possible discharge energy vs. minimum energy needed to initiate a dust explosion. These don't change for industrial scale wood dust collectors - the volumes get bigger, the densities don't.

Well this nonsense comes up two or three times a year so didn't even bother to read most. A research done 3 - 4 years ago found that according to Gov. specs. you would have to reduce something like a 6" x 6" x 1 ft. pc. of Oak to dust in one minute to get enough concentration of dust to support an explosion. Anyone got one of these machines in their shop? Anyone actually have FIRST HAND evidence of a home wood shop explosion?

Wheat dust, bean dust, corn dust in grain elevators, etc, is not wood dust in home workshop.

Walt Conner

RE: Subject

What everybody is dancing around is "LEL" or "Lower Explosive Limit".

It is that point where the dust volume in the air WILL explode.

An entire industry exists to provide instrumentation to measure "LEL" for various products.

There are published tables of LEL values and are usually available from the instrumentation manufacturer.

Been there, done that, but that was a while ago.

Lew

No, not at all. Fill a reasonable router extractor hose with typical levels of fine larch dust, initiate it pyrotechnically and I have no doubt at all that you can have your very own wood dust explosion.

You'll not do it from static though. That's the demonstrable case where you can't initiate it.

No, far from it. It's the lower limit for where the mixture _can_ explode. Below this it's impossible to explode, at this limit it's barely possible. It isn't especially likely (depending on other conditions regarding ignition sources) and it certainly isn't any sort of spontaneous ignition as suggested by your emphatic "WILL explode" comment.

As written, not true.

It should have read: It is that point where the dust volume in the air WILL explode IF an ignition source is provided.

Blame it on a lazy typist, ME.

Lew

The RayV entity posted thusly:

Yes, I do believe in facts.

No idea of the concentrations or how hot a spark it would take to set it off.

I was, however, rudely awakened one Sunday morning in North Vancouver when a grain elevator about 5 miles from me was subject to a grain dust explosion. That was in the mid-70s or so, if you feel like looking it up.

I also participated in several experiments in which we ignited clouds of flour, cornstarch, and sawdust. No real explosion, because it was a small and unconfined cloud, but it was, nonetheless, spectacular.

My mother couldn't explain the seemingly rapid disappearance of her cache of corn starch when I, unbeknownst to her, would sneak baggies of it down to the creek bed to huff it out my mouth at a candle held before my face--much to my inner pyromaniac's delight.

er

Can you support that statement? At any rate, in an industrial setting the greater volume of air and dust being moved would have the potential to create a more powerful static discharge.

And all the comparison to grain silos; The dust that creates the danger of an explosion in them is MUCH finer than the sawdust created in the typical wood shop operation, except perhaps for sanding dust.

No disrepect intended, but I've read dozens if not hundreds of MSDS and had some training as well, and I have never seen a reference to LEL/UEL that was not for gas or vapor concentration in air. In fact, for dust any LEL would vary with the particle size.

That must have been fun, but how many times were you successful igniting it with a static discharge?

Agreed, but ... still don't throw a box of sanding dust into the wood stove if there's still a small fire in there. Took a LONG time to grow back eyebrows! Woof!

Pop

One of the things that people seem to keep missing here when citing grain silo explosions is the difference between spark sources. In all of the grain elevator explosions that I have read about, yes, a spark caused the explosion, but the spark was not a static spark, it was an electrical spark from a faulty connection or exposed electric motor. An electrical spark carries a much larger amount of energy than a static spark. The only static-caused explosions I have read about have been those which ignite gasoline fumes. Gasoline fumes are significantly more volatile than an air/dust mixture.

Even in the cites given, "The third major factor is the ignition source. Sparks from welding and cutting equipment, or cigarettes, can ignite dust. If a bolt, or some other piece of metal, located on moving equipment rubs or scrapes against another metal surface, sparks again could result, leading to an explosion. A choked bucket elevator, a conveyor belt slipping, an electrical malfunction, or lightning could also start a fire leading to an explosion." OK, lightning is static electricity. If your dust collector can generate a static spark with equivalent energy to a lightning bolt, some wimpy little wire wrapped around the pipe isn't going to help you anyway.

The OSU article, and a couple of the other sources, cite static as a possible ignition source, but provide no information to back up that assertion.

OK, dust can explode! But I'm not putting a candle in my DC anytime soon.

Any standard industrial handbook of safety electrostatics. I'd suggest Luttgens & Wilson's "Electrostatic Hazards" for readability, although it's expensive and light on some tables of hard figures that you need to do real work.

"Discharges" are categorised into different causes, of which "sparks" are only one.

The non-spark discharges occur around insulators and it's a relatively easy matter to calculate their maximum energy. Put simply, the fact that it's an insulator allows charge to build up, but it also limits the area that can contribute to the discharge when it happens. Maximum energy is a function of the materials, not the dust flow.

Sparks are discharges between conductors. These are rarer, as only an insulated conductor builds up charge. However the whole conductor can contribute, so there's no intrinsic limit on the discharge energy. However we just don't build dust collectors this way.

But that wimpy little wire will bleed off the charge before it can get as big as a lightning discharge.

While I doubt that an explosion in a home DC will occur - how about a fire? The only difference between a fire and an explosion is the rate of combustion.

Mike

From static electricity? Bwahahahahahaha!

I should have been more specific. My sentence above should have read, "I seriously doubt that static electricity has enough energy to ignite a wood dust explosion from the dust that would be generated in a typical home woodshop."

This might help: