ATEX certified dryer 1

[Technopriest]

We have a little problem on the ATEX protection on a horizontal dryer.
The dryer is little more of a stainless steel cylinder with an agitator in the middle. There is a jacket for heating, usually at about 60°C.
The dryer will have to dry a powder that is nonconductive and potentially explosive. The customer requires a Zone 0 protection (so class 1).

The cylinder itself is safe; the agitator is slow-moving, and with the same electric potential of the cylinder's walls. There is also a temperature control device on the wall that prevents a dangerous overheating of the walls.

So the machine itself do not present any active ignition source. However the powder (the product to be dryed) can have electric discharges on it's own, due to it's nonconductive nature.

My questions are:
1) do the static discharges that are due to the product inside the machine (the powder) need to be contemplated in our ATEX protection? They are not an ignition cause due to the machine..
2) If the powder has to be protected from it's own static discharges, are there any other ways than inert atmosphere/vacuum?

3) During discharge and load (when it is not possible to have vacuum or inert atmosphere inside the machine), is there any need of ATEX protection? If yes, do you have any suggestions?

Any help will be greatly appreciated. Any reference to directives or such will be, too.

Che Dio ce la mandi buona, o almeno ce la mandi- Massimiliano Eusebi
Check eng-tips rules: faq731-376

 

[PeterCharles]

A couple of points -
"The customer requires a Zone 0..." This raises the question of whether the customer should be defining the the Zoneing of the INTERNALS of a machine. Is he actually competent to do this, does he fully understand what is going on inside the machine? This should surely be down to the machinery manufacturer whos actually understands his machine.
"..the agitator is slow-moving .." Does it actually create an explosive dust/air atmosphere? ATEX is about potentially exposive atmospheres NOT an atmosphere that contains dust. The atmosphere in my house contains dust but it is not an explosive atmosphere!

"..the static discharges that are due to the product.." How much energy does a discharge spark contain, is it sufficient to ignite a dust/air atmosphere at the explosive concentration? Some tests on the material may be worthwhile to show the extent of any spark causing an explosion.

ATEX is all about risk assessment. If the dust can result in a potentially explosive atmosphere and there can be a source of ignition within the machine then you do a risk assessment and put measures in place to to deal with the risk i.e. that the dust will be in an explosive concentration and the spark will be large enough for ignition resulting in an explosion. Maybe your machine can be strong enough to contain the explosion. If not maybe you can fit explosion vents or maybe you can fit an explosion suppression system.

When considering your problem knowledge is the key. Too many people (including those who should know better) see dust, say ATEX, say Zone 20!! In normal operation the dust may be minimal, and the possible source of ignition is minimal, so a realistic approach is Zone 22. But as a manufacturer you know your machines. How long have you been making them, how may have exploded? This information can be used in your risk assessment.

The following links may be of assistance :-

http://europa.eu.int/comm/enterprise/atex/direct/newapproach.htm

http://europa.eu.int/comm/enterprise/atex/guide/index.htm

http://www.dti.gov.uk/strd/exatps02.pdf

 

[Technopriest]

The dryer is a pressure vessel; inside the vessel the process takes place (in this case, the drying of the product).

We believe that if the customer ask for a zone 0, the product that will be processed inside the dryer will generate a potentially explosive atmosphere (and it's surely possible).
In this case we will have a potentially explosive atmosphere inside the vessel.

The agitator do not generate an explosive atmosphere by itself; it's far too slow to create a mixture of air and product. Anyway the dust (that will be present inside to a thickness of about 400+mm, as this 'dust' is the product to be processed) IS explosive in nature if ignited. The sparks that can be generated by the product inside the product are strong enough to ignite it (customer's data).

We've built many similar machines, for a wide array of different (and potentially extremely dangerous) products and none of them encountered safety problems. Anyway, it's not really about safety, but it's about laws and codes. We can make sure that nothing happens anyway (our customer did produce this product for about 15 years, without inertization or ATEX certified machines, and it never ignite), but we would like to be compliant with what laws require.

I'll check your links and thank you very much.


Che Dio ce la mandi buona, o almeno ce la mandi- Massimiliano Eusebi
Check eng-tips rules: faq731-376

 

[PeterCharles]

Remember :
Zone 0 is for gases and vapours, explosive atmosphere present continuously
Zone 20 is for dust air mixtures, explosive atmosphere present continuously
Just because an explosive atmosphere CAN be formed does not mean it's continuously present so does not mean Zone 20.
You say yourself that the agitation is too slow to create a dust/air explosive atmosphere so I would not classify the internal conditions as greater than Zone 22.

You also say the dryer is a pressure vessel. If it is truely a pressure vessel the you must also check the Pressure Vessel Directive

http://europa.eu.int/comm/enterprise/newapproach/standardization/harmstds/reflist/equippre.html

 

[Jalink]

Technopriest,
The replies given are, according to my view, all quite right, but let me give my view on your questions, assuming the static discharges by the product are continously (zone 20) capable to cause an explosion in the vessel:
1) Yes, if static discharges from the product are capable to cause an explosion, this should be considered, as the equipment is designed/sold for operation with that product.
2) If the vessel, inclusive seal of agitator, is not capable to protect operators against harm, due to a design pressure above the pressure caused by the explosion, other protecting means, as inert atmosphere or vacuum should be considered.
3) The simple goal of the ATEX and other safety directives is nothing more than to have a safe piece of equipment. If the explosion will be limited within the container due to design pressure exceeding the pressure caused by the explosion, or when the remaining gas can be vented to a safe location, or when the equipment is located on a safe location, e.g. Mars, you would have a safe design.
Kind regards,
Jalink