Charging mechanisms

Occurs at a shortage or excess of charge. Kind of charges:

  • Contact charging.
  • Induction / influence.
  • Thermal.
  • Transmission of charge.
  • Change of phase.
  • Liquid atomization.

Accumulation of charge

Occurs when charge cannot be discharged. The resistance to earth is in such cases generally 10^6Ω till 10^8 Ω and/or exists a specific volume resistance of 10^8Ω.m or a superficial resistance of >10^10Ω. For organic powders specific resistances in the range of 10^108Ω.m until 10^128Ω.m are possible.  Accumulation of charge can be caused by:

  • Bad conductors as well solids as liquids.
  • Bad conducting plates (plastics) of <8mm thickness.
  • Isolated conductors as well solids as liquids.
  • Gasses and/or vapors containing droplets or particles.

Electrostatic discharges

Accumulation of charge causes an electrical field. If a bad conductor loses its isolating properties than we are talking about “disruptive field strength”. For air and VOC/air mixtures, this is about ~3.10^6 Volt.m.
Kind of discharges:

  • Spark discharges, energy content: 100mJ until 10.000mJ.
  • Propagating brush discharge discharges, energy content: ~3000mJ.
  • Brush discharges, energy content: till ~5mJ.
  • Corona discharges, energy content: till ~5mJ.
  • Cone discharges, energy content: 10mJ until 25mJ.

For ignition sensitive materials and for gas and/or vapor-air mixtures practically all electrostatic discharges are dangerous, for dust air mixtures only the first 2 and the last one are.

Hazards of solids

This deals mainly about granulates and/or powders. However, plastic plates less thick than 8mm also form a risk. For dust explosions of this matter,  4 “ingredients” have to be present, namely: fuel, an ignition source, oxygen and turbulence. For powders of unknown composition is for safety reasons calculated with a concentration of 20gr/m³. De particle size distribution is of importance for the explosivity as where particle sizes <400µm may cause Explosivity.

Potential dangerous situations:

  • Dust explosions, depending of the minimum ignitable concentration (MOC) and energy (MIE).
  • Explosions of hybrid mixtures, these are powder type of materials in an inflammable gas / vapor mixture.
  • Reactions causing the rise of temperatures and/or concentrations of inflammable matter.
  • Static charging because of transportation and/or movements in the broadest sense.
  • Processing like blending processes, grinding processes and sieving processes.
  • Filters, fans etcetera.
  • Multi-phases processes.

Hazards of liquids

In liquids in fact, the vapor and/or the liquid-gas ratio (atomization) is important for the explosivity. In addition, sparks or other ignition sources that may occur during operations in conjunction with the material, and the oxygen from the air may combine to the required ingredients for an explosion. Pure liquids with a conductivity of ≤ 50pS/m may cause a dangerous charge. The explosivity of mixtures and/or contaminants is not proportional to the mixing ratio.

Dangerous processes in liquids can be:

  • Liquid transfer in hoses or ducts.
  • Filling, storage and discharge processes.
  • Filter processes.
  • Atomization and/ or other surface-volume ratio enhancing activities.
  • Stirring and blending.
  • Operations at which electrostatic charge can be exchanged.
  • Handling of regular and /or viscous fluids.

Hazards of gasses & vapors

Gasses can hardly be charged electrostatically. However, it is possible that particles in a gas can accumulate charge because of the difficult chargeability of the gas. If the charge is high enough the discharging may cause a spark due to disruption. If this happens in a flammable gas air mixture than an explosion may be possible.

These dangers may be:

  • Electrostatic charging.
  • Charging caused by atomizing.
  • Condensation because of expansion of gasses.
  • Shock waves in gasses.

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