Electrostatic Effects

Electrostatic Effects Jones and King – Lewis Publishers (1991) Dust explosions are in high in polymer industries – PE/bisphenol – A/ABC etc For an explosion one needs Fuel Oxidizing Agent Ignition Source

Breakdown strength of air is Eb=30kV/cm Charge relaxation τ=κε0γ where γ is the bulk resistivity κ is the dielectric constant γ~107 ohm m giving τ=10-3 sec γ~1013 ohm m giving τ=102 sec Polystyrene and polyethylene τ=104 sec

Capacitance Energy – Gibson states that for safety Ue<100V Vmax350 V – minimum sparking potential

Discharge Types Capacitance – 90% of dust and vapor ignitions Brush discharge Lightning discharge Maurer discharge

Plastic Liners can be a source of capacitance discharge Conducting object ++++++++++++++++++++++++++++++++ liner

Level of Hazard Level Specific Relaxation Resistivity Hazard Charge Time (µC/kg) (sec) ohm-m Low <10-3 <10-3 <108 Moderate 10-3 – 10-1 10-3 -102 108-1013 High >10-1 >102 >1013

Minimum Ignition Energy Values Recommended MIE < 5 mJ In the presence of HC vapors <0.25 mJ For Plastic MIE < 10 mJ

Pneumatic Conveying Charging Largest method of Charging Plastic ball to Eliminate Maurer Effect Polyethylene granules Sharp Point Promotes Corona Discharge

Important Notes on Charging MIE for polymer dust >> flammable vapors Bulk Storage is most dangerous Relaxation times>100 sec are dangerous 1 mm plastic particles can become charged

Materials having an Explosivity Index >10 Wheat starch 17.7 Nylon Sulfur Aspirin MMA Corn dust Alpha cellulose

Summary of Electrostatic Effects Electrostatic forces dominate the minimum pressure drop region Inertial effects dominate the high velocity ranges Clustering accentuated by electrostatic forces Additives can help reduce electrostatics but care needs to be taken Electrostatic forces can be used to your advantage.

Do’s and Don’ts with Electrostatics Use Conductive pipes, hoses and couplings Have good grounding procedures Use metal level gauges Regularly inspect grounds Avoid rough surfaces in storage vessels Use metal scoops and shovels

Don’ts Never use non-conductive hoses and tubing. Don’t use insulating linings on inside surface of vessels. Don’t cover the floor with insulating mats. Don’t use metal foil labels on containers with plastic liners. Don’t use metallic threads woven into fabric of filters. Avoid manual loading of powders. Watch for flammable vapors.

Electrostatics Effect on Flow Patterns in Pneumatic Conveying