Plastic or metal ducting?

[malky boi]

In a previous life I had a small dust extaction system using drainage pipes. Then 1 day there was an almighty fire at 2 in the morning, fire engines all over the place. The cause, according to fire service, was started in the dust extractor. Most likely a spark from a hidden nail in some oak boards I was ripping went through the ducting, into the extractor, simmered for 5/6 hours, plastic sack melted, let in the oxygen and, whoosh. (wasn't insured, ouch)

I moved into a new workshop and had a full metal dust extaction system fitted. Both extractors were 3 phase, 1 for the spindle and planer, and 1 for the saws and panel sanders. Worked wonderfully well.

I sold up when I got to that age, and I am wanting to re-develope my small (d/garage) workshop. The use of plastic piping is scary for me, but 1000s of others seem to use this system. What safety precautions should be employed? I'm thinking along the lines of static.
I would like the metal but it seems expensive for a small shop.
I know the plastic ducting wasn't at fault, but I associate it with my first workshop.

I would appreciate any advice.

 

[adidat]

Very scary!! Plastic ducting is easy and cheap to install and provides excellent air tight ducting as most fittings have a gasket on.

Adidat

 

[beech1948]

I have plastic soil pipe 150 mm. Its been up for about 15 years and in daily use.

I have had no problems with static at all. I think its a bit of a myth and much referred to by the nit pickers.

I did for a while use an internal earthing rod connected to blast gates and earthed via an external line but it was more trouble than it was worth.

Like you I'm a bit paranoid about fire and so last thing in the day I frequently empty the D/C bag. After all it was not the piping material which caused your fire but the hot nail in shavings.

I also religiously check any material for nails and metal even though I seldom use reclaimed timber.

My D/C is now also outside my garage in a small hut of its own alongside my compressor.

 

[SVB]

I have plastic ducting but have a camvac dust extractor that sucks into a metal waste container so even if there is a small fire, my hope is that it should be contained. (note mine extract system is really dust only, not chippings / shavings from p/t etc which I accept is more voluminous and may require the plastic sack approach.)

 

[Jelly]

I have had no problems with static at all. I think its a bit of a myth and much referred to by the nit pickers.

I did for a while use an internal earthing rod connected to blast gates and earthed via an external line but it was more trouble than it was worth.

Static electricity buildup during fluid/powder flow most certainly is not a myth...

But, it all depends on the rate of flow, and amount of material in contact with the surface. A big mill which has a very high flow rate and enough material to run along the sides of the ducting non-stop, is going to have to pay careful attention to it, whereas a small joiners shop where the volume of air moved massively outstrips the volume of dust, and where the flow rate is lower, would really struggle to create a risky situation.

 

[beech1948]

Jelly,

I was going to give a robust reply but the second half of your message helps put it in perspective. The issue is about Fire and not static electricity. My shop has an air flow which does a great job for me but would be woeful in a joinery factory. To create any fire or explosion problems the density of particles in my air flow would need to be 40x more than my current maximum.

This was tested by a friend who works for AWEA who was completing his PhD on the use of fluid dynamics in recovery of atomic particles from atomic waste.

The maximum volume was established by attaching and using a P/T, table saw, two bosch sanders and bandsaw simultaneously. I had no more points to access the D/C left to use. A somewhat arbitrary to call this the maximum but true of this system and the available kit.

The chances of creating the conditions for fire or explosion were infinitesimally small but not quite zero.

The chances of hitting a small bit of metal and getting this red hot into the dust bags is quite a bit higher. So the prevention activity should be prioritised towards avoiding metal and red hot particles.

I also have searched for examples of workshops where the cause of a fire was hot metal particles in the D/C and have found nothing. Search was of Google, UK, EU and US and was checked by reference to an insurance claims database for the UK and a FoI Act request to the appropriate insurance associate/representative body.

As you see I do not take this problem lightly mainly because a friend was badly disfigured due to a grain silo explosion.

But for small workshops and hobbyists its a myth.

 

[malky boi]

Hello Beech, I wasn't insured at the time so not sure it would be in any database. The fire service and scene of crimes people were both pretty sure a spark in the d/c smouldered for a few hours, and when the heat reached the plastic sack it melted and bang.

Am going to go with every one else and and do it in drain pipe.

Thanks all