Dust resistivity

Physical meaning and use in risk assessment of powder resistivity

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Section summary
Definition of resistivity
Determination of powder resistivity
Typical range
Use in risk assessment
Resistivity of common dusts

Bulk Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust

1. Definition of powder resistivity

As every materials, powders will conduct electricity to a certain degree. During processing, the movement of the particles during for example pneumatic transport or gravity chute will create electrostatic charges on the particles. How well the dust will be able to release those charges will depend on its resistivity.

The resistivity is defined in Ω.m as the ratio of an electric potential across a dust layer to the induced current density [Mastropietro]. The higher the potential to get a certain current, the higher the resistivity will be.

2. Experimental determination of powder resistivity

The experimental measure of dust resistivity consists in applying a difference of potential through a layer of powder of known thickness and measuring the current obtained. The higher the current, the lower is the resistivity and thus the higher is the capability of the dust to conduct electrical charges.

ρ = (V/I) * (A/L) [Mastropietro]

With :

ρ = resistivity in Ω.m
V = applied voltage in Volts
I = measured current in A
L = powder thickness in m
A = current measuring electrode face area in m2

It should be noted that powder resistivity varies a lot with powder porosity, humidity or temperature.

Values of resistivity should therefore be considered according to the conditions in which they have been obtained.

3. Powder resistivity typical range

[Glor] report that plastics like polyethylene have a high resistivity and will not conduct electrical charges well, however the paper mentions as well that most of the pigments, dyestuffs, additives, drugs or foodstuffs have resitivities higher than 1010 Ω.m which makes them sensitive to risks like cone discharge in silos.

Each processor must carry out a dust explosion risk analysis in order to assess the risk related to a specific material in a specific process and take necessary precaution and mitigation measures.

4. Use in risk assessment

Powder resistivity will have to be considered especially for risks related to cone discharge in a hopper or container that is being filled. [Glor] is mentioning that such a risk is to be considered as soon as the powder resistivity is higher than 1010 Ω.m, which is, as seen above, very common.

All decision in the risk analysis should however be supported by documented assessment of the concentrations observed in the process.

5. Resistivity of common dusts

Please find below some resistivity data given in the literature. WARNING : these are general values given without guarantee, a risk assessment and design must ALWAYS refer to the MSDS of the ACTUAL product used for tests carried out specifically on the ACTUAL material by a reputable institute.

As mentioned above, the determination of the value depends on many parameters including the temperature, when no tests conditions are defined, new experience relevant for the application should be foreseen.

Table 1 : Resistivity of some powders

Material Resistivity in Ω.cm
Teflon
1017 [Dahn]
Nylon
1014 [Dahn]
Nitrocellulose
1014 [Dahn]



Sources

# [Glor] Ignition hazard due to static electricity in particulate processes, Martin Glor, Powder Technology, 135-136 2003
# [Dahn] Bulk powder electrostatics hazard evaluation, Dahn and Reyes, Safety Consulting Engineers
# [Mastropietro] Dust resistivity measurement for industrial processes, Robert Mastropietro, International Conference Electrostatic Precipitation, 2013




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