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mikem
09-24-2002, 09:24 PM
Several people have told me not to use plastic pipe to connect my sawdust collection system in my home shop because of static sparks that could ignite an explosion. I intend to run the main lines underground under my concrete floor. I think that metal pipe under ground would rust in time and I don't want to have to repair it in the future. What if I would only use plastic underground and metal above? Would the sand and concrete around the plastic pipes ground it well enough to avoid static build-up?

The plastic pipe is about half the cost compared to metal and would last forever. It goes together easier also. I am about ready to pour my floor so I have to quit pondering and do something. What should I do? Anybody have any first-hand experience? Thanks--Mike.

Gizmo
09-24-2002, 09:59 PM
This was pretty popular when the dust collectors first came out, using PVC trunk lines and gates to each machine. Static build-up was eliminated by running a bare ground wire through the pipe. I never built one, but remember it clearly. Standard wiring connections, to a ground.
Worked at a place that made IC's with MOSFET's in the mid 70's, very static sensitive. SOP was a ground strap to your wrist, and metal table tops that you plugged it into. So the grounding of the PVC pipe makes sense to me.

mikem
09-24-2002, 10:10 PM
I forgot to mention that I will probably use the Oneida Cyclone 1.5HP collector. Does anyone have any experience or opinion about Oneida? Are they as loud as a regular vacuum? I have sinus allergies to dust and want to make my shop as clean as possible. Will I need a air cleaner besides the cyclone collector? Thanks--Mike.

mbensema
09-24-2002, 10:18 PM
this has been discussed often on http://talk.woodmagazine.com/woodtalkindex.html
The research mentioned shows that there is not a danger of sparks causing an explosion in a home shop and using plastic pipe without the ground wire is fine. If you do a search on that website, you will find a link to some guy who did alot of research on the subject, can't remember his name. When I install a dust collection system this winter, it will be plastic piped.

Oso
09-24-2002, 10:35 PM
Well, I DON't have plastic collection pipe, in fact I don't have a system.

BUT, my shopvac shocks me nicely through the plastic hose if I pick up a lot of sawdust.

I understand that the use of plastic is verboten in industrial applications for the spark reason.
Areas with sawdust are classified "hazardous" under the electrical code, and require different wiring methods than regular areas. Essentially the same requirements apply as for a grain elevator, which as you may remember, used to explode regularly.

As far as the ground wire in plastic, I have seen that done, and on average it MAY help. It certainly will NOT "ground" the pipe, since the pipe remains non-conductive, and can still create isolated charges on the pipe or the dust. These can then discharge by arcing, or carry a substantial charge to another portion of the equipment by the same action that makes a VandeGraaf generator work.
I presume you have seen the sort of discharge those devices can create.

Nothing beats metal pipe, bonded together and grounded as per the industrial requirements.

And, your pipe in the floor might be a tad hard to clean out if there is a clog.............

mikem
09-25-2002, 12:12 AM
Thanks for all the input guys. I'll check out that website for more info. Does the static build up on the inner surface of the pipe only or can it be drained from the outside with metal shielding or drain wires? My shop has a partial crawl space and I will put in some clean-outs in the ends of the mainlines to be albel to clear clogs. Thanks-Mike.

Paul Gauthier
09-25-2002, 08:15 AM
As yet there has not been any instances of explosion reported due to static buildup in sawdust collection systems. Plastic pipe in a dust collector can create static, and it can be removed. Do not run the wire inside the pipe this will only cause problems such as clogging, wind it around the outside about 1 twist in 6 to 12 inches and ground it at the end.

------------------
Paul G.

Stepside
09-25-2002, 10:47 AM
Just some thoughts;
Unless you are going to be making a lot of sawdust, Why not use a large shop vacuum? In my experience they have a lot more suction and are a lot cheaper. I have one mounted to a 55 gallon drum and have made an adapter to use two filters. The two filters let you work longer before they clog up. We are cutting mostly particle board so it is a real fine dust.
We have mounted an air filter on the ceiling. One of those that are just a box with a fan and a set of filters. This is well worth your time as it cut our dust down to a minimum. We also have a large Jet dust collector---we disconnected it and use the shop vac instead.

mbensema
09-25-2002, 02:48 PM
A shop vac does not have the CFM to remove sawdust from tools like a jointer or planer. The minimum size you need is in the range of 500-600 cfm at the machine for those. A shop vac will pick up just about anything, but it can't handle the volume.

Here is a site that has a review of most of the systems as well as some good tips for selecting and installing your unit. http://cnets.net/~eclectic/woodworking/cyclone/CycloneReviews.html If you get the 0.2 micron filters and install dust chutes on all your equipment, then you probably won't need a ceiling mounted air filter. The air filter also circulates all the dust in the shop to filter it, so if you are standing in the flow path then you are going to breathe in all the dust it is filtering. Several people I have talked to are removing the air filters and just using the dust collector.

Mike

NSDesign
09-25-2002, 05:09 PM
Mike,

I just thought I would throw in my 2 cents. The problem with static charges lie in their name - static. They do not usually like to move. When you build up a charge on a non- conductor, putting a drain wire near it or against it can only discharge the area of contact. Areas near the ground source will build to a certain point and then discharge (arc) to the ground source. Areas far from the ground will not discharge very well. That's the nature of an insulating material.
Folks who work around Electronics and chips will tell you that plastics used in these environments (flooring, shoes, countertops) always have additives to make them conductive so that charges can be bled off to ground.

Practically, the plastic pipe probably will work, and from the other posts it seems there is no documented cases of fires. However, static can be dangerous just from the shock. Imagine that you recieve a nasty jolt from you dust collector pipe while operating a power tool - NOT GOOD.

I was taught a simple rule when putting things under concrete. Use the best material, oversize it, and put in extra. I personnally use electrical conduit. Thinwall (EMT) strong, galvanized, and cheap (relative to other pipe). Nice smooth bend with large radii are available pre-bent, or mooched from your favorite sparky. These bends are just what you need for material handling systems.

Good luck.

Alan

chip's
09-25-2002, 09:58 PM
Mike,
I have as system as you describe but my pipe is over head. I have wire running through all the pvc to ground. The pipe in the earth is a great idea, wish I was able to. It would still need ground because plastic being nonconductive won't get rid of the static to ground.

I hope this will help you, Rick.

Thrud
09-26-2002, 12:45 AM
mikem:
Static build up in dust exraction equipment can be extreme. If you are worried about corrosion then use aluminum dryer venting in the concrete and make sure a copper ground wire runs through all plastic piping so the entire system is grounded machine to machine to cyclonic cleaner & dust bins. Do not take the chance - explosions can and do happen when high volumes of air are moved.

www.leevalley.com (http://www.leevalley.com) has a book on these systems as well as piping, flexible hose, waste gates, fittings, ground wires, HEPA Dust bags (you may be interested in these) for your Cyclonic cleaner, etc.

Oso
09-26-2002, 01:04 AM
True...
My previous comment on VandeGraff generators was addressing the fact that

the dust being transported rubs the plastic and develops a charge on the plastic, and an opposite charge on the dust.

When that dust reaches a conductive item such as the collector bin, or some part of the system, it can give up its charge to the collector, raising the voltage of the conductive item to arbitrarily high levels.

If the item is not grounded, the developed charge can arc to some other piece, which if there is flammable dust around, can cause an explosion the same as with flammable vapor.

The ground wire in pipe idea is that the dust bits on average will hit the wire and dump the charge before getting to any collector of charge. if you also ground all of the metal parts, then you are pretty safe.

I don't know what wrapping wire outside does, might actually have an effect, or not.

Metal pipe, grounded, avoids both the charge creation, and the chance of a spark.

[This message has been edited by Oso (edited 09-26-2002).]

Jack
09-26-2002, 02:02 AM
I was called in to "do something" about the electricity arcing from the plastic pipe to the top of the men's head when they walked under the dust collection system. We used metal mesh like the kind that used to be used on window screens. Wrapped the screen around the pipe - held it on with tye wraps. Attached a wire [the size of wire didn't make any difference]and took the wire to the nearest ground. It worked..
Jack

mikem
09-26-2002, 10:45 AM
The heat and air guys put metal ductwork under ground all the time so I am going to the HVAC shop this AM and ask them how they do it and whether they have trouble with rust and collapses etc. I thought about aluminum but the regular gauges are so thin that they would likely collapse under vacuum.

Thrud
09-27-2002, 04:11 PM
mikem:
You will not have that problem with the aluminum dryer venting - to crush the aluminum piping would require a very high vacuum (10^-3 Torr or less). Make sure the ground wire goes through the plastic (internally) and not around it. You could also use the stainless steel chiminey liners (they expand like an accordian a 2' box holds 40') that heating contractors use to bring existing stacks up to code. They come is various diameters.

mikem
10-02-2002, 03:09 PM
Hey Thrud:
Are we talking obout the same flimsy aluminum pipe you buy at the hardware store? Can I bury that in sand and not have it collapse?

Stopped at the HVAC shop and they have just the perfect duct--PVC coated spiral wrapped galvanized steel pipe. The problem is that it is $3.94 per foot plus elbows and couplers. Would cost over $450 versus the plastic being about $160. What do you think?
Thanks--Mike.

Arcane
10-02-2002, 06:12 PM
Spiral wrap the plastic pipe with a small multistrand bare tin plated copper wire - one wrap per foot works well. Make sure it is tightly wrapped and makes contact with the pipe along the entire length of the wire. Neater of course and works best. Running it inside runs the risk of it causing a plug up.(And correct me if I am wrong, but doesn't static buildup just on the outside of a container?)Connect both ends to a proper earth ground. Static buildup will be eliminated except for a small amount halfway between the wraps of wire and this won't be enough to cause any problems.

Thrud
10-03-2002, 03:11 AM
Arcane:
The outside is unimportant - it does not have an explosive mixture of dust and air. It is important to insure the ducting and all stationary machines are connected to an earth ground and the dust collector.

Wastgaes placed along the pipe to connect things such as a floor sweeper, router, or portable sanders do not need a gound wire in the hose from the tool to the wastegate - they do not flow enough air to present an explosive hazard. Even though explosion are rare, it is still prefered to run the grounding wire THROUGH THE PIPING SYSTEM as a safety precaution. It may even be code in your area.

Arcane
10-03-2002, 11:25 PM
Thrud, you have not thought your answer thru. Think about it somemore.

docsteve66
10-04-2002, 01:21 AM
been following this thread with interest. So far as wire insidethe pipe or out side goes:
I think the wire or metal shield on out side will prevent sparks from the out side, done that. I also think the wire on outside may INCREASE the static charge build up inside the pipe and thus increase the chances of a stoppage (blockage). The bare wire inside the pipe will, I think, keep the charge small alright, but increase the blockage chances. I'd think hard about putting the wire inside, pull it tight on all straight runs and inside bends. On outside bends where the wire would tend to cross the pipe, drill two holes and run the wire on out side where it would tend to cross the tube.

Just put a vaccumm pump on some dryer duct, 10 foot length of expandable stuff. it collapsed at about 25 inches HG. The stuff had no kinks or bends. End pieces were wood and taped. Stayed nice and round until it suddenly collapsed. If think the shape (a cylinder) has max volume for thesurface area. Once tube developed a tiny sag it tried to go to minumim volume for the surface area (which was a flat sheet, ten foot long). Ifyou think about the pressures (say 12/14 PSI under a gair vaccumm) the tube was under a great deal more stress than it would ever see in a dust collection system ( few inches of water when blocked, less when in operation). My newest fire code is near 20 years old and I have not looked up how they suggest grounding plastic air handing systems.

BTW, be sure not to get too many bends between clean out openings (just as a plumber would in a sewage system.

Steve-

PS: remember "one experiement beats a thousand expert opinions" AND "every body trusts the results of an experiment- except the man who conducted the experiement"

yf
10-04-2002, 02:13 AM
Here is my two cents worth.

Putting dust collection lines underground may seem like a good idea, until you have to break up the floor to find a failed or clogged section. One client of mine that had a fabricating shop in Manhattan poured his floors with trenches that he framed out with angle iron, then layed Aluminum treadplate into the iron frames. Anytime there was a problem or if they needed to run elctric lines, they had all the access you could want. They had galvanized flue pipe (18 Ga)with formed bends in the ground for over 18 years. When we pulled it out, it was still in pretty good shape. But its also a dry shop.

Another thing to think about is putting your lines overhead. Overhead you will always have access to them and can change your layout very easily or even upgrade your pipe size.

If you look at the literature from the manufacturers of dust collection and central vaccuum systems, they recommend running one main line overhead pitched towards the collection unit, with all branch tees taken off the top of the main. This is to prevent
dust from falling down every branch it passes
on its way to the unit.

That's how I did central vac systems and never had a callback.

One more thing to worry about is flammability of plastic pipe and erosion by dust and chips passing through at 100 MPH.
Another reason to put it overhead.

I think that's more than two cents worth http://bbs.homeshopmachinist.net//biggrin.gif

Thrud
10-05-2002, 01:03 AM
Arcane:
Fine. Do what ever you want. Just remember you will be liabel for your own negligence if the legal requirements and an inspection is not carried out for your locale.

I will not beat my head into the wall...
I will not beat my head into the wall...
...
ad nauseum

Oso
10-05-2002, 01:20 AM
For Arcane, yeah, what Thrud said..........

For mikem: Thought of another issue with floor pipes;
if you ever get water into them it will be trapped, and hard to get out except by drying out with the blower. Meanwhile it will be cementing sawdust into blobs. No fun.


The overhead pipe is what we have for our big woodshop (50 x 200 feet) where we have a couple CNC routers and saws, plus pin routers, tenoners, shapers, drills, etc.

We didn't plumb over the top, its big pipe about 24 inches or so, and going in the side is OK to avoid falling crud.
Its all spiral metal tube, except for machine connections.

We did have a fire in it once, not sure why, we think it was a piece of something that was sucked up and made a spark when it hit metal in the pipe, because the area is no smoking. Also could have been a spark from debris in the particleboard.
If it had been plastic pipe, would have been a much bigger problem.
They just shut down while shooting a big extinguisher into it, as I recall (was not present at the time).

mikem
10-05-2002, 12:24 PM
docsteve--what a helpful friend. If you ever get to Nebraska, I will buy you a McDonald's lunch! (I are such a generous fellow, aren't I?)

I think that I will make channels around the perimeter in the concrete and put the pipe in there and use 3/4" plywood for a lid. That way I can use cheaper pipe and still fix problems when they occur. It will also give the flexibility to change setups later if I ADD or subtract tools. Thanks--Mike.

docsteve66
10-05-2002, 06:23 PM
Mike, You blew it bad! I'll get the burger one day. Wife graduated Chadron (North west corner), her family was from Wayne (left Wayne long ago)(north east corner), best mueseum (naw but one of the best) is The Americas (southern side), always spend sometime at autohenge north of alliance. Neb is good state. Wife is from Hot Springs South Dakota, not too sold on Neb herself, but I run the family!

OSO has good advice about using the over head. On air lines- avoid pvc pipe as every one knows (and many including my self use), and use "the tap on top" (as OSO suggests for vac lines) for air lines. Friend had a high quality 25,000 rpm abrasive disk explode and would have suffered severe belly damage had his cell phone not slowed the shrapnel down before he was hit. A slug of non compressible water entered his tool and over reved it instantly. Now he drains tanks more frequently and all air line taps now come from top of the line.

When we laid out this house (a square 80' by 30' I had 16" duct put under floor for A/C return. Vents at both ends, tee'd at middle to HVAC room. Long Nylon cord from all outlets. Its sure been nice being able to fish wire and cable from HVAC room to other places. All to do over, I would have made more runs for future use.

The (what i would call 'cable trenches') covered with ply wood lids are sure a good idea. I'd say make a side run so you can bring electrical/air up in the middle of the floor at later dates. Be sure to consider air velocities in your vac system. Large cross section areas will have lower velocity, and may not have the ability to lift the trash back into your fan/storage areas. Its easy to figure velocity- cubic feet per minute divided by area in square feet gives velocity in feet per minute. Course the cfm varies with vac. need the mfr tables there.

Do a lot of thinking about OSO's fire hazards. under the floor lines with metal shavings, oil, saw dust etc might bemore susceptable to spontanous combustion. Think about how you will extinguish the fire before it starts. CO2 ready to blast while the fan is still sucking? a "davies" type screen just before the holding tank? Maybe the under floor duct work is safer because the fumes will extinguish the fires? I have no experience in this area, but think it all out.
Steve
P.S.Gotta sendthis BEFORE wife sees my comment about me running the joint.

mikem
10-06-2002, 02:09 AM
Hi Doc and all:
Have been to Chadron once (took family to a talent contest--didn't win)and never went back. It's not quite the end of the world but you can see it from there.

Think I will use metal duct within the channels. I have had enough advice here to convince me so. With the channels, the dirt won't be in contact with the duct to rust it out. If it doesn't work out under the floor, I can always put it overhead later. Thanks-Mike.

Arcane
10-06-2002, 03:12 PM
Thrud, Oso...STATIC CHARGE IS ONLY ON THE OUTSIDE OF A CONTAINER!! GET IT!? DRAIN THE CHARGE AND NO SPARK! NO SPARK AND NO EXPLOSION! POSITIVE CHARGE ON THE INSIDE, NEGATIVE ON THE OUTSIDE. POSITIVE CHARGE ISNT THE PROBLEM, IT`S THE NEGATIVE CHARGE ON THE OUTSIDE THAT WILL CAUSE THE SPARK AND IT IS THE DUST IN THE AIR THAT FLOWS THRU THE FILTERS THAT CREATES THE EXPLOSION HAZARD. THE EXPLOSION HAZARD ISN`T INSIDE THE DUCTS, IT`S THE WHOLE DUST FILLED ROOM.

Oso
10-06-2002, 10:07 PM
AND, no plastic, no charge, no charge no spark no problem.

Use metal.

Mr Arcane, I will say this one more time, in slightly different words.

If you doubt the ability of a non-conductor to develop sparks across it, rub a synthetic blanket in a dark room. Or pet the cat in a dark room.

Those flashes you see across the non-conducting blanket are arcs equalizing the charge distribution.

Ditto for inside the pipe. Or outside, for that matter.

Use grounded metal pipe and no further problem exists.

mikem
10-06-2002, 10:12 PM
Dear friend Arcane:
There is no reason to shout. The best idea will prevail regardless of how loudly presented.

I think you are using Gauss' law to describe the charge on the pipe and I think if my Physics class comes back to me, that Gauss' law only applies to a closed area. When you put a differently charged object(a ground wire) into the area, it is no longer a closed area and discharge is possible. Even without a ground wire, the dust collector itself will present a break in the closed area and could cause a spark(or explosion). Not likely maybe, but possible. I will use metal and put it into concrete channels with a plywood lid so I can change things if I need to. Thanks for your input--Mike.

docsteve66
10-07-2002, 01:50 PM
Arcane: You are complelty correct. well, almost- the charge might be a deficiency of electrons (which might act like a positive charge). Almost- GOT IT?.

One page one of the "junior scientist's text book for boys" they speak of Faraday and some of those other old codgers, pith balls, and then go to CONDUCTING containers. The charge on the CONDUCTING container must reside on the outter surface, distributed along equipotentiel lines, affected by the distance from other charges of like sign. PIth balls and non conductors can have charges localized.

Now if you progress to para one , page two, they discuss things such as conductors (sheets usualy) around a non conductor- The Lyden jar is an example. the charge there is contained in the non conductor. remove the conducting sheets and the charge stays in the glass (or plastic if you prefer). This is the princple of the what us old codgers call a condenser now a capacitor. The charge there stays in the insulator (forgive my use of "insulator"). remove the conductors, trash can them, replace them with new foil and the charge reappears on the conductors. Wonderful stuff on pages one and two. some day I am going to read on up to page 1001 myself. I am sure I will be humbled by my lack of knowledge.

The problem we have been discussing is more that of a long lyden jar with charges desposited by moving particals (such as has been said before a static electricity generator (a special case of which is the Van DeGraff). As in the case of pith balls (see page one again) the charges may be localized in the extreme. Placing a conductor (such as wrapping the pipe with wire, may actualy increase charges inside the pipe while draining the charge from the out side of the pipe. Then again it might not http://bbs.homeshopmachinist.net//smile.gif.

The empirical evidence submitted so far has been the results of a bunch of what i believe to be actual observations and theories developed from those actual observations.
In case YOU missed thepoint, we are discusssing in a gentlemanly fashion, those observations and applicable theories and possible applications to the practical problems encountered or anticipated.

I think you may have applied your theory with out considering the problem being addressed. Hopefully the stuff on page two is basic stuff to you and you just got tired of us fumbling and wanted to help us. I suspect there are persons following this thread that could embarass both of us with their knowledge, but have restrained them sleves because they don't have a sure fire solution to present.

As i say, I suspect you missedthe problem- if I am wrong please explain it again to us. If you did miss the problem, you are forgiven- we all do that.

Respectfully, Steve
PS: I have seen dust collection systems with bulges the would indicate the explosion took place within the duct work not the dust filled room.
*******************************************
posted 10-06-2002 02:12 PM
--------------------------------------------------------------------------------
Thrud, Oso...STATIC CHARGE IS ONLY ON THE OUTSIDE OF A CONTAINER!! GET IT!? DRAIN THE CHARGE AND NO SPARK! NO SPARK AND NO EXPLOSION! POSITIVE CHARGE ON THE INSIDE, NEGATIVE ON THE OUTSIDE. POSITIVE CHARGE ISNT THE PROBLEM, IT`S THE NEGATIVE CHARGE ON THE OUTSIDE THAT WILL CAUSE THE SPARK AND IT IS THE DUST IN THE AIR THAT FLOWS THRU THE FILTERS THAT CREATES THE EXPLOSION HAZARD. THE EXPLOSION HAZARD ISN`T INSIDE THE DUCTS, IT`S THE WHOLE DUST FILLED ROOM.

Thrud
10-07-2002, 02:49 PM
Arcane:
What you have suggested has been tried before - they always fail. The explosion hazard is in the dust collection system itself. Look at it this way - fill a room full of flour dust and swirl the air around at 100-300 MPH. the Static build up will eventually discharge to the path of least resistance. In an empty room with no metallic grounding surfaces to remove the charge eventually "lightning" occurs and the explosive mixture is ignited and the room demolished. You can put a faraday cage around the room - which will protect the area surrounding the room itself from discharge, but you cannot prevent discharge in the room itself without introducing metallic surfaces that are earthed.

Because the dust extraction system removes most of the particulates (when properly installed and sized) produced at their source of production, the chance of explosion in the shop area is zero. The dust extraction system pipes can still have an explosion within the system. The ground wire run through the piping has proven to be an effective way to drain the dangerous static build up off in a safe manner. It has been shown that the wires statistically will completely drain the charges before the cyclonic separator (because the cone is metallic, it finishes the job) and even the fine sawdust falls into the bin. Commercial applications use large diameter metallic plenums and branches with flexible joints at the machines.

If you do not believe me, I invite you to check you national and local building codes, as well as UL and CSA regulations. Or better yet, call your local university and talk to the mechanical engineering department and ask them. I am only telling you what works and what does not.

Arcane
10-13-2002, 09:55 PM
Go learn about static electricity.

docsteve66
10-14-2002, 06:51 PM
Arcane: is that static electricity the stuff they talked about on page one of the book I mentioned? The stuff about pith balls, amber, cat fur? Lyden Jars later on? Some old codger named Faraday? The same static electricity faraday who says charges must reside on the out side of conductors, and contained in the dielelctric of non conductors? The same stuff the greeks played with? The single piece of folded metal that spreads itself (is that called an electroscope?) becasue the charge is distributed UNEVENLY in the middle and theend repeal themselves becasue they are both of the same charge? I seem to be missing a part of my staics (electrical not mechanical in the is case).
Would you PLEASE quote some archaic law that referes to what ever point you make that i am missing?
Peace
Steve

I admit to being a self made man. you can tell because of the big gaps and holes i left.

mbensema
10-14-2002, 09:53 PM
There are a lot of misconceptions about static in home shop dust collectors, this website explains what's happening http://www.gis.net/~dheaton/woodworking/articles/DC_myths.shtml

the conslusions are at the end if you don't wnat to read the entire article.

Mike

Oso
10-14-2002, 11:46 PM
I like the article, but not all the conclusions.
Many things are perfectly correct, including the correct location of the charge (on the INSIDE, for Arcane's benefit).

One thing appeared, as I skimmed thru, to be overlooked, and that is what I alluded to in a prior post.

The dust picks up a charge from the duct, and carries an opposite charge with it. That charge can be given up to a conductor.

It WILL be given up to a ground wire inside, IF it is contacted. All to the good, no doubt, but unlikley to do much.

BUT, if another item is in the system, such as a metal gate or other object, and that object is not grounded, it can be pumped up to a significant voltage in the same manner a vandegraaf generator is charged up.

THEN, that item can cause an energetic spark. I really could not care less what someone's website says, I have SEEN (felt) that effect, and constructed a number of static generators of all sorts myself. They work, and can knock you on your butt.

The most likely suspect as a collector is something metal at or around the dust bags, since that is where the dust may be blasted at a surface regardless of charge. And that is where even the author suggests dust is present, well....

However, I am pretty sure most home machines do not present the very fine dust hazard of the high speed commercial machinery. Some may. Probably that is why the incidence of explosions is low.

I have seen the old fire protection demonstrations where a box of flour is ignited by a spark plug after shaking it. So the "dust won't ignite" argument is not over-impressive to me, depends on the dust...

I wouldn't do it in my house, you can do as you please. I would not want a fire in plastic duct, no matter what the origin........

P.S.
I looked back and noticed that I also mentioned the best reason for metal pipes, which is they won't create a charge in the first place, just as the guy says.


[This message has been edited by Oso (edited 10-14-2002).]

docsteve66
10-16-2002, 11:27 AM
mbensema: Thats a good web site. thanks!.
The author has done a good job of presenting facts and especially theory.


Mikem's original question was pretty complex question but he wanted a simple answer- Plastic or metal? In my opinion there is no one correct answer. Neither system is apt to be dangerous. The hazards of either system are much less than the hazards of having no collection system at all (I include broom and dust pan as a collection system http://bbs.homeshopmachinist.net//smile.gif ). Personaly I am impresssedby the actualy observations we havemade. I disagree with some of the theories about what caused the observed events.
everyone should remember that just because you can reproduce an event, you have in no way proved that your explaination of why the event occurs is correct. Thewebsite author mentions grain elevators and dust explosions. I have seen the remains of several exploded grain elevators- and worked in several (none of which explodedwhile i was around). The solution there is pretty well established (at least in the old tin ones), first NEVER let dust accumlulate, second don't have any thing that would ignite the dust WHEN it DOES accumulate, third build the structure so that WHEN dust accumulates, AND WHEN the ignition occurs, the walls blow off and no damage is done http://bbs.homeshopmachinist.net//smile.gif.

Years ago We were converting a cotton gin from a lineshaft (steam or diesel powered) to electric motors. (thats an easy job compared to trying to convert a bunch of electric motors to a a line shaft- which is uspect would be the more energy efficient). he boss (there were only the 2 of us) mutters something about they should have cleaned this place before the end ofthe season, whipped out his zippo, and ignited some cotton lint stuck to the walls. Prettiest thing you ever saw (maybe not THE prettiest). A blue ring of light like maybe a tire of violet light, expanded in a circle, ran over the walls in just a few seconds . the whole cotton gin "burned" it's lint. Little piles of lint in the crevices just laid their. Kind of a "whoomp" followed. The whoomp may have been my heart!, I think I saw a cellolouse explosion, in a large confined tin building. The ignition could just as easy have been a spark, static electricity from a passing cloud. Theory be damned- the danger was that the conditions were becomming right for an exlosion, and somethingwould have triggered it sooner or later. I am sure the gin workers "burned the lint" themselves. What I really cantfigure out was why anyone would pass up a chance to see what we saw.

THe web site author missed explaining or considering another phenomena. If you have a pool of gas on the floor is is safe- the mixture is too rich to burn or explode. Some where above the pool the mixture is safe because its too lean to burn. Somewhere just by pure logic, there is a place where the mixture is ideal for rapid burning. Avoid the place where bad things can happen and you are safe- scared yes , but safe.

Some ofthe "facts" presented here have been theory misapplied to explain actual events. Despite the non agreement regaring how things happen, dust collection can be safe or dangerous with the same materials. Its one thing to demonstrate a man to be in error, but entirely diffferent problem to provide the a completely correct explaination of the errorouns conclusion.
Nuff philosophy- wish i have been more polite in my disagreements with some of you.

Don't jump on me any one- Its a long time observation that when you pour hot water on a pig it squeals !!!! http://bbs.homeshopmachinist.net//smile.gif
Steve



posted 09-24-2002 08:24 PM
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Several people have told me not to use plastic pipe to connect my sawdust collection system in my home shop because of static sparks that could ignite an explosion. I intend to run the main lines underground under my concrete floor. I think that metal pipe under ground would rust in time and I don't want to have to repair it in the future. What if I would only use plastic underground and metal above? Would the sand and concrete around the plastic pipes ground it well enough to avoid static build-up?
The plastic pipe is about half the cost compared to metal and would last forever. It goes together easier also. I am about ready to pour my floor so I have to quit pondering and do something. What should I do? Anybody have any first-hand experience? Thanks--Mike.