Low Mie Powder
The type of ESD (electrostatic discharge) that is possible in your system is a brush discharge. In large quantities and high bulk transfer rates, it could be possible to get a bulking brush discharge that might be sufficient to ignite a dust cloud with an MIE = 5 mJ, but it is very unlikely that this could occur in your situation. You are not dealing with a sufficient mass of materia in all likelihood.
A brush discharge is unlikely, if possible at all, to ignite any pure dust, even one with a low MIE such as you describe. If there were a flammable vapor or gas component in your product, however, then a brush discharge could provide adequate ESD for ignition.
If you are still concerned, you could have your dust tested to determine the charge density and charge retention characteristics of your particular dust. Let me know if you need recommendations for a lab that can do this testing for you.
Regards,
Bill ■
Low Mie Powder
Bill,
Thanks for your reply. Your response is very much in line with my 'gut' feeling. My problem is that so far I have 'very unlikely' as my basis of safety which is not as easy to stand over as say the next operation (transfer along a 2" fully earthed pipe to sub-surface in an aqueous liquid - for the transfer I can reference the British Standard that for the MIE there is little risk of ignition (but at least I know this would stand up in court). It owuld be nice to know that say for materials less than 1000kg (or saome weight) that the powder is not considered bulk (as per your answer) - do you know of any reference material?
Thanks again for such a quick response,
Fearghal ■
Low Mie
The best reference on the subject in English is "Avoiding Static Ignition Hazards in Chemical Operations," by Laurence G. Britton, AIChE, 1999.
Larry is the Chairman of NFPA 77, "Recommended Practice on Static Electricity", 2000 ed., which is also a good reference.
The other go-to-guy on electrostatic hazards is Martin Glor of Ciba Giegy in Switzerland who has written much and done a lot of experimental work over many years. I do no have a direct contact for him, but could give you a contact at Ciba here in the USA if you want it.
Also, you can cite my comments to you as a reference. I am a member of NFPA Technical Committees: "On Handling and Conveying of Dusts, Vapors, and Gases"; and "On Explosion Protection Systems."
Regards,
Bill ■
Low Mie
Another thing. For a dust to be susceptible to bulking brush or propagating brush discharge it would have to have a resistivity of at least ten to the tenth ohms. If you don't know that value, you should definitely have it tested. If the resistivity is very high, then having it tested for charge density and charge retention could help to establish the risk for bulking brush disharge and for the required mass of material that would be required.
In my experience, the minimum mass would be approximately twice the amount you are handling per FIBC. As you know, however, every material is different and it is very difficult to make broad generalizations that are not full of holes.
Regards,
Bill ■
Re: Low Mie Powder
Declan Barry
Atex Explosion Hazards Limited
If you wish to discuss explosion prevention or Protection I or my collegues will be ablwe to assist you
Dublin 00 353 1 4738 007
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info@explosionsafety.com ■
Low MIE Powder
I am transferring powders from bags (via an isolator - large glovebox) to an IBC (fully earthed conductor). I have full continuity assurance of the transfer system (isolator, chute, valves and IBC), however the MIE of my powder is 5mJ. The transfer is under gravity (maximum free fall distance about 3m). Should I still be concerned about the risk of a discharge from the bulked powder? (The IBC is just over 1m3 volume) and the maximum charge of powder would be 900 kg - though this would be in discrete lots (as we weigh in the isolator prior to charge via the chute to the IBC). The powder cannot take nitrogen so inerting is not a possibility. Anyone have any thoughts? ■