Thread: Blast gates for 6" PVC ducting

Rev,
No.1 will slightly decrease the inside diameter and thus restrict flow a bit. No.3 will give the maximum diameter but there will be slight turbulence where the pipe fits into the bell. No.2 with a sleeve over the outside will give maximum diameter with minimum turbulence, however to be honest, I don't think you'll really notice the difference between any of these options when using a 6" pipe.

Mick

Rev,

My ducting has stopped about three feet (ok 900mm- but that is just too precise a measurement) short of having to make that decision! One more length of pvc and I'm with you.

I plan to terminate three rigid duct runs with multi-port outlets, to allow some flexibility in future, so the "in" duct at least will be permanently fixed.

I plan to use 18mm mdf, cut a press-fit hole and fix/glue it with silicone. I can't see the point in creating a flange if it isn't necessary.

I'm going to try the same with flexiduct, but will use a 6 mm piece agianst the gate, cut to the inside diameter of the duct to use as an end stop, and press fit as above, I may need to double the 18mm to get enough fixing.

I must admit that I have considered once again just slipping plastic gates on the 100mm tails....but that's not the point is it?

Since you are clearly ahead of me, I'd love to see the outcome!

P

Thanks for the replys.
Mick, I thought it would probably not make much difference but didn't want to overlook anything obvious.

b/midge, I think I will make some collars out of sheet metal left over from the cyclone to fit snugly inside of the PVC or flex, cut tabs in one end and bend outward to make the flange to attach to the mdf of the blast gate with shotr screws and silastic. I did think about assembling it all as I went - as you suggest. Thought this could be a pain if I ever wanted to modify the layout of the ducting later though. Unfortunately, I wont get to it for a couple of weeks now but I'll let you know how it goes.

One more question please...Is 3mm 3ply or 4mm mdf sufficiently robust for the shut off slide in the blast gate? It shouldn't have too much vacuum againt it because there should always be at least one duct open.

Good Question! I'm using 6mm MDF but can't see a problem with using thinner.

My reasoning: you need a certain amount of thickness to allow simple cleaning of the slides etc...too thin and it gets too hard. On the other hand, too thick and it may be too heavy to rely on the vacuum to keep sealed.

I am thinking of sealing the MDF with shellac...(my contribution to the cost of this forum! )

Over to the experts!

Cheers,

P

A thread on a UK forum woodwork site mentions that there is a potential hazard using plastic pipe as ducting. It appears static electric can build up and cause an explosion. The system states earthing is required.

Originally Posted by jow104

It appears static electric can build up and cause an explosion. The system states earthing is required.

Jow104,

I suggest you read this article by Dr. Rod Cole
http://mywebpages.comcast.net/rodec/.../DC_myths.html

This article has been published in several USA mags btw

Cheers,

jow104,

In certain applications, static discharge can certainly be a problem. Most dry, finely divided dusts are capable of generating static charges. If a dust handling system were installed in a commercial environment, static would be neutralised in accordance with our local standard (AS1020:1995).

Earthing of plastics is not as simple as attaching earthing and bonding wires. There is a whole bunch of novel ways to reduce static potential. With plastics, a common method is to incorporate antistatic agents into the material, which form a conducting layer on the surface allowing any static potential to dissipate quickly (ie Festool vac hoses etc).

An explosion is only going to occur if conditions are just right (particle diameter, moisture content, concentration of dust between the lower and upper explosive limits, and the spark etc). The reality is that the risk is probably low (but the consequences high!)

Having said that, there are some things that you can do to dissipate static charge in dust handling equipment:

Nozzles from which dusts are ejected (outlet nozzles) should be made wholly from conducting materials, and be bonded and earthed;

Objects should not be placed in the dust stream, but where unavoidable should be made wholly of conducting materials, earthed and bonded (ie partially closed blast gates, flow restrictors etc);

Bags and containers used for the collection of the dust should be conducting and earthed and bonded (cloth bags should be rinsed in an antistatic solution - from the manufacturer)

Would I do this myself - yes, I will be incorporating the above precautions in my dust handling system (when the shed is completed!)

Hope this helps

Originally Posted by Wayne Davy

Jow104,

I suggest you read this article by Dr. Rod Cole
http://mywebpages.comcast.net/rodec/.../DC_myths.html

This article has been published in several USA mags btw

Cheers,

Thanks Wayne - needn't have pounded out the reply.

However, I do disagree on one aspect. I have seen the result of dust explosions resulting from a spark generated from PVC apparatus. From memory, the cause was equipment shutdown and start-up in a short interval. Dust settled at a restriction when shut-down. When re-started the conditions for a bang were just right - and it went bang. However, the true risk in a back-yard installation is very low.

The explosive power of dust can be phenomenal - check out pics of grain mill explosions.

Bunyip,

Thanks for that, I've read a bit (a lot?) on the subject, and like many others got myself totally confused and have decided to take the risk, which appears slight...however if I may impose with a few questions for you re the above:

a common method is to incorporate antistatic agents into the material, which form a conducting layer on the surface

There are a number of anti-static materials on the market, do you know of anything paint that could be painted on the outside of the PVC, and is it likely to work?

Nozzles from which dusts are ejected (outlet nozzles) should be made wholly from conducting materials, and be bonded and earthed

Do I assume that the fan on my DC is the "nozzle" for the purpose of this discussion, and is therefore earthed by way of its manufacture?

Objects should not be placed in the dust stream, but where unavoidable should be made wholly of conducting materials, earthed and bonded

So the use of non-conducting materials (MDF) is BAD? I'm wondering how to overcome this, I can only think of using Aluminium for the gate, and hard wiring it to the duct. I have read somewhere else that the pvc doesn't actually conduct electricity all that well, so I would have to run a wire all the way back to the DC if I were to go this way?

Bags and containers used for the collection of the dust should be conducting and earthed and bonded

Does the standard Plastic bag attached to the DC meet this criteria...how "conducting" is conducting.


Many thanks....I'm about half finished so can change some of the details if I have to....hope you can talk me out of it!

Cheers,

P

Originally Posted by bitingmidge

.... There are a number of anti-static materials on the market, do you know of anything paint that could be painted on the outside of the PVC, and is it likely to work?

G'day.

This is an interesting & complex subject. I have been reading up on it.
I'm also a computer tech with antistatic training.
(but not nessicelery an expert so if you want to disagree, go for it.)

If you are going to use some form of anti-static material on the PVC, it has to be on the inside 'cos that's where the dust & the explosion risk are.
PVC is a VERY good insulator so putting it on the outside is a waste of time as far as reducing the risk of a dust explosion is concerned. (Unless your workshop is very dusty.)
All it will do is reduce the risk of you getting an electro-static discharge on the outside of the ducting.
The inside of the ducting material is where the conductivity is needed to avoid the electro-static charge build up.
The good thing about static electricity is that it doesn't really need to be very well grounded to be reduced rapidly.
That statement will draw a bit of flack from a few quarters but, consider this.... an electro-static charge is usually at a very high potential (voltage) but relatively low energy (power), therefore low current. (unlike lightning) Therefore it is easily discharged via anything with any sort of conductance at all. (Ohms law, I=E/R techo st*t, see the attached table)
IE: shuffle your feet on the carpet & touch the metal door knob, it bites you. Now do it again & touch the painted surface of the door & then touch the door knob, no bite. That's 'cos the static charge is discharged into the mildly conductive layer of moisture on the surface of the door slowly enough not to cause a spark (bite) but fast enough to remove the electro-static charge from your body before you touch the door knob.
Now what does that all mean????

In theory, (& I'm still working on this one,) we should be able to reduce the risk of a spark inside the ducting by making the inside of the ducting mildly conductive.
That's the bit we have to solve.... how do we make it mildly conductive????
If we use a conductive paint, we have 2 problems...
1. It's bl00dy expensive.
2. It wears off with the abrasive action of the dust, shavings, chips, nails, screws, fingers, etc going up it.

The other answer is moisture.... dust + moisture = conductive muck.
This is a REAL problem for electronics in the Tropics.
We don't have a major problem with static most of the year up here 'cos the air is usually too damp & discharges the static charge, instead, we have a problem in the wet season with older equipment that has a layer of dust on it.
The dust becomes mildly conductive with the moisture in the air & causes tracking & the electronic equipment becomes VERY unreliable. (also causes rust on everything)

SO… an electro-static build up ‘should not’ be a problem in a humid environment.
(I say ‘should not’ ‘cos lightning works just great in a humid environment so there are exceptions to that rule)

My theory…
We get our flash new dust extractor & connect it up to our nice new PVC/plastic/non conductive ducting system. Then we remove the dust filter (so it doesn’t get clogged,) & saw up a nice wet, freshly felled mango tree (or something similar) that is just dripping with sap. The result is the whole inside of our dust extraction system ducting gets a real good coating of this sticky muck that is mildly conductive & once it dries out, becomes mildly conductive again with the slightest amount of moisture in the air.
Now all we have to do is fit the dust filter & as long as we maintain a certain amount of humidity in the dust/air we put through it & we don’t have a problem. (Shame about you blokes out near Alice Springs.)
There are a couple of ways to maintain a certain amount of humidity in the air if you don’t live in the tropics.
Buy one of those humidifiers from the chemists or get yourself a hot water urn & set it up at a low boil so that it gives off a bit of steam to keep some humidity in the air. The other way is to only cut wet wood but then you wouldn't have a dust problem antway.

Is every one still with me??? Anyone??? Hello???

Pay attent... I say pay attention boy, I’m a cut’n but you aitn’t a bleed’n…

Okay, I'll shut up now. Some fellas have to keep their tongues flappin' but not me. I was brought up right. My pa used to tell me to shut up and I'd shut up. I wouldn't say nothin'. One time darn near starved to death. WOULDN'T TELL HIM I WAS HUNGRY!!

I have never grounded mine which seems to generate a fair amount of static in a 90mm dust extraction system put up a few years back.
I therefore often get a boot from the wire enforced 100mm dust extraction hose connected to the dust extractor but in saying that its only a mild boot but still enough for you to keep your wits about you whilst working there.
cheers

Originally Posted by bitingmidge

There are a number of anti-static materials on the market, do you know of anything paint that could be painted on the outside of the PVC, and is it likely to work?

There are antistatic paints available, but they are expensive. We use them in the aerospace industry to facilitate conduction over a composite surface. Same plastics have conducting additives added to the plastic. A suggestion was made to run a wire through the ductwork (when you get to a blast gate, tap in some screws and make a loop over it). I can't see that this will do any harm; I will be doing this with mine. I suggest that this may be a simple solution. In industry, there are all sorts of nifty devices to remove static embodies in the particles, but they are not really relevant here.

Do I assume that the fan on my DC is the "nozzle" for the purpose of this discussion, and is therefore earthed by way of its manufacture?

Your fan would be considered significant source of static build-up. For those making a cyclone, a nozzle would also to be sectional change into the cyclone itself. Experience has taught me to assume very little. Easy to test the earthing using a multimeter, or even a battery and globe. All you are testing for is a closed circuit.

So the use of non-conducting materials (MDF) is BAD? I'm wondering how to overcome this, I can only think of using Aluminium for the gate, and hard wiring it to the duct. I have read somewhere else that the pvc doesn't actually conduct electricity all that well, so I would have to run a wire all the way back to the DC if I were to go this way?

Hmmm- difficult to answer this. Consider a half closed blast gate. At this point, the velocity of the gas stream is rapidly increased, and particle interaction and separation also increased and leads to a even greater static potential. Static charge concentrates at the extremity of a point. Therefore, charge will concentrate at the knife edge of the gate. The material of the gate knife will determine the potential generated. For some plastics this would be a concern. Not sure about MDF. Gut feel is that it would not be a problem, as I believe that it is difficult to get charge separation on a slab of MDF. Having said that, I am making my gate knives out of aluminium or tin plate, and joining them into the earth line.

Does the standard Plastic bag attached to the DC meet this criteria...how "conducting" is conducting.

It is quite possible to measure the magnitude of static charge, and therefore the effectiveness of any treated or earthed materials. Given the surface area of the plastic bag and that the frame should be earthed, I would not worry about it. In industry, cloth bags are sprayed with an antistatic, or metal fibres woven into them.

Remember, that for a fire or explosion to occur, the following conditions must be present:

an effective means of static generation
separated charges must be of sufficient potential
A static discharge must occur between the separated charges to be of sufficient energy to ignite the material
The discharge must occur in or on an ignitable material.

Interestingly, to give you a feel for the ignition energies:
bark dust is 40 mJ
very fine wood dust is 20 mJ
acetylene is 0.019 mJ
methane is 0.25 mJ
aluminium powder is 10 mJ
phenolic resin is 10 mJ
smokeless powder is 12 mJ
wheat flour is 50 mJ and
nitrocellulose is 62 mJ

Be aware of the issue of static discharge, but do not be alarmed. As has been stated on many other posts, the risk of ignition is real but very low.

Originally Posted by John Saxton

... I therefore often get a boot from the wire enforced 100mm dust extraction hose connected to the dust extractor but in saying that its only a mild boot but still enough for you to keep your wits about you whilst working there.
cheers

btw - the wire 'reinforcement' is not just there to keep the hose ridgid, it should be earthed at the point of connection to the system.