Static buildup in DC systems ?

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On Mon, 15 Nov 2004 13:41:22 GMT, "Keith Carlson"

yup. he bought the whole load of BS too. it's a shame, 'cause he's a pretty good teacher.

let's make a clear distinction between production shops and home shops here.
in production shops there are large machines, multiples of them, running continuously, each making lots of dust. this is an environment where plastic piping is a hazard and AFAIK illegal.
in home shops this is not the case.
there are plenty of non-static discharge hazards associated with dust collectors in home shops. big bags of sawdust are a ready source of fuel just waiting for a source of ignition. vacuum up a cigarette butt or a nail that sparks off of some metal part inside the DC and you could have a smouldering fire inside the DC that erupts into full ignition hours after you've shut off the lights and gone to bed.
I fail to see how a pilot light could do this short of sucking big piles of dust through the piloted device, but someone somewhere might be able to pull it off ; ^ ) more likely is that the DC would blow out the pilot...
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On Mon, 15 Nov 2004 12:57:49 -0700, snipped-for-privacy@thanks.com wrote:

Why ? The physics is the same.
If plastic piping is a no-no for commercial workshops in your jurisdiction, then that's to reduce the _consequences_ of a fire (and they do happen, for many reasons). Plastic piping, and unearthed too, is in regular use for the flexible sections of DC hookups.
If I had a roof mounted central DC system, I'd want steel pipe too.
--
Smert' spamionam

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On Tue, 16 Nov 2004 03:42:35 +0000, Andy Dingley

home shops never involve anywhere near the volume of dust needed to generate a hot static discharge into a dust cloud dense enough to sustain it. that takes a dust collector in the 40 HP range pulling from multiple high volume dust producing machines at once. while in theory a home shop could create the right conditions, in practice it's never happened and never will.

not just to reduce the conscequences- also to reduce the sources.

that's ABS flex, not long straight sections of PVC

steel pipe is the preferred installation. for home shops, PVC is very convenient.
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On Mon, 15 Nov 2004 21:36:38 -0700, snipped-for-privacy@thanks.com wrote:

Nor does a commercial DC. It's not the volume that's the issue, it's the energy per discharge.

How are machines like panel saws connected to a DC in the USA ? Here we commonly see ten-twenty foot drops of clear flexible PVC.
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Andy Dingley wrote:

You're right that it's not the volume, it's the concentration. To get an explosion you need x quantity of suspended particles per unit volume and you need an ignition source. If the concentration is high enough for an explosion to occur, then it is not a good idea to rely on the hope that any discharge will be of too little energy to cause ignition. In a home shop it's not likely that you'll get that kind of concentration. In a commercial shop it might be a different story.

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On Tue, 16 Nov 2004 09:53:19 -0500, "J. Clarke"

It's the concentration and the energy. If there's inadequate energy in the discharge, it won't ignite. If the concentration is inadequate, it won't propagate.
However, even low concentration is not a guarantee of safety for wood dust handling. There's a problem with smouldering and a burning particle _may_ settle out in a dead-air zone where there's likely to be a build up of other dust. In forensic examinations of many dust collector fires, they began not in the filter or the collecting bin, but at stagnant corners and sudden pipe expansions.
For a real treatment of this problem, read Luttgens & Wilson's "Electrostatic Hazards" or a similar industry-standard handbook. I still haven't found one that has evidence of static discharge caused fires in wood dust.
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On Tue, 16 Nov 2004 11:44:28 +0000, Andy Dingley

dust volume *and* the speed that dust is travelling do have a bearing on the static charges built up in plastic pipe. high volume + high speed = large static energy being generated. in systems handling tons per day like grain silos and millwork factories the risk of static discharge becomes real. in home shops it's pure unadulterated urban mythology...

and that one machine is unlikely to make enough dust to be a problem. it's the main trunk lines that do.
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On Tue, 16 Nov 2004 10:03:07 -0700, snipped-for-privacy@thanks.com wrote:

In the grain elevator explosions with which I am familiar, static was not the cause of the explosion, it is usually either a faulty electric motor that generates a spark (much longer duration and thus more energy supplied to start the ignition), or an open flame.
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On Mon, 15 Nov 2004 21:36:38 -0700, snipped-for-privacy@thanks.com wrote:

And MUCH less expensive. I just bought a bunch of the PVC for installing a cyclone currently on order from Bill Pentz.
Comparitive pricing between a local plumbing supply house for 6" ASTM-D2729 S&D PVC and the web prices from a well-known woodworking supply house:
Pipe:    PVC $8.50 for 10 ft length, Steel $28.00 (+ shipping) for 2 x 5 ft lengths Wye:    PVC $12.00 each, Steel $60.00 (+ shipping)
Well, you get the idea. Comparative costs for other fittings are similar. But, there is a wider variety of fittings available in the steel.
Tom Veatch Wichita, KS USA
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Andy Dingley wrote:

But the volume of dust handled by the collector may be different. CWG has a planer that is bigger than my truck. Somehow I think that when it's turning a piece of 8/4 lapacho into 5/4 it produces a little more in the way of shavings and dust and whatnot than my 13" Delta.
Whether the volume is high enough to constitute an explosion hazard in the event of a static discharge in the system, I have no idea.

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Keith Carlson wrote:

I could be that Sandor Nagyszalanczy is referring to a commercial wood shop and avoiding a law suit.. The book is not just written for the hobbyist.
-- Jack Novak Buffalo, NY - USA (Remove "SPAM" from email address to reply)
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Keith Carlson wrote:

As I said,
To date none of the experts have come up with a single itty-bitty bit of evidence that PVC piping will lead to a dust collector explosion.
And this includes Sandor.
UA100, who is mentioned by name on the acknowledgements page of Sandor's book Power Tools...
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On Mon, 15 Nov 2004 13:41:22 GMT, "Keith Carlson"

Very unlikely. The research I've seen (Sorry, I don't have a cite immediately available) indicates that in order for a dust cloud to support a flame front, the visibility in the cloud would be on the order of a meter. That's a pretty thick dust cloud. Based on that, the rule of thumb I use is - If I can still see the far wall of the shop through the dust, an explosion is the least of my worries.
Tom Veatch Wichita, KS USA
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Static Canon?
The myth is that a construction worker finds a really nice piece of large diameter, about 8-10 inch, plastic pipe and wants to use it but it's all dirty so he sandblasts it at the site and builds up such a huge static charge on it that when he walks around its end, it shoots a lightning bolt out of it and blows him across the compound.
They tried everything. Couldn't even make the thing spark. They had a voltmeter on it, and it kept building up a little charge and then dissipating.
As someone else has said, the stuff is an insulator. Grounding the outside grounds only the outside. Don't need a resistor, you're only sending a few volts to the ground. But any charge inside the pipe is still there.
I've also seen articles on attempts to duplicate a shop explosion. They found that the dust particles have to be a certain size, and be dispersed in the air in such a way that each ignited particle ignites the particle next to it, and in such a manner that it creates an explosion-like combustion. They found that when they finally got the conditions correct for a dust explosion, the air was so full of fine-particle dust they could barely see. And any air currents in the area screwed it up. The dust particles weren't close enough together. Their conclusion was, if you have the correct conditions to create a dust explosion, you should be more worried about breathing than an explosion.
Want me to hunt around for the article? I think it might have been posted to this very newsgroup.
Dan
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So the general word is that there's no safety hazard involved - saw dust is obviously different to flour, for example.
That's what I was looking to be sure about! My new shop will be in a building that doesn't belong to me and as the lawyers here in europe are starting to get almost as trigger happy as in the USA I wanted to be sure I wasn't not making myself negligent through ignorance. Thanks people ....
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True, but MDF dust is not much different than flour.
Lou
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nigel did say:

Sorry to hear that.
--
New project = new tool. Hard and fast rule.


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On Sat, 13 Nov 2004 13:57:13 -0700, nigel wrote

I read somewhere on-line (the dust collection info site?) about a factory using PVC and how everyone would get nasty static shocks from it. They tried the usual tricks of wire, etc but none really worked. What finally did work was a strip of the metal (aluminum ?) duct tape in a strip along the inside of the PVC and another strip along the outside. The two strips were then connected together with several holes drilled through the pipe wall and fastened with a bolt and washers. This was then earth grounded. Something about the greater surface area of the conductor in the pipe doing a better job of collecting those pesky extra electrons.... -Bruce
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On Sun, 14 Nov 2004 11:22:12 -0700, Bruce wrote:

Musta been fun getting that tape on the inside of the pipe...
-Doug
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