1. Electrostatic Effects

2. 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

3. 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

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

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

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

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

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

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

10. 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

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

12. 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.

13. 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

14. 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.

16. Electrostatics Effect on Flow Patterns in Pneumatic Conveying