Electrostatic discharges are among the most important causes of fires and explosions in industry. Their energy can exceed 1000 mJ, so they are able to ignite not only gases and vapours, but also many dusts. The video above shows the electrostatic discharges that can occur during the operation of belt conveyors. When this happens in an explosive atmosphere, the conveyor can work like a piezoelectric spark generator in a lighter.
How does an electrostatic discharge occur?
The triboelectric effect, i.e. an increase in electrical charges due to friction, contact or induction, occurs when the electrical resistance of the path from a charged object to the ground prevents the discharge of excess charges. When two objects with different electrical potentials are near to each other, an electric field forms between them. In the case when its value exceeds the breakthrough strength of the atmosphere between the two objects, a spark discharge may occur.
How to calculate the potential electrostatic discharge energy?
The potential energy of an electrostatic discharge W [mJ] can be calculated when the capacity of the object on which charges are accumulated C [pF], and the object potential generated by the accumulation of charges V [kV] are known:
W = ½ CV2
Example 1 – human
Based on the formula above, we can calculate that as a result of the triboelectric effect, a human with a capacity of 200 pF can be electrically charged to the level of 30 kV [IchemE]. This can lead to an electrostatic discharge with the energy of 90 mJ.
Example 2 – road tanker
Based on the formula above, we can calculate that as a result of the triboelectric effect, a road tanker with a capacity of 5000 pF can be electrically charged to the level of 30 kV [IchemE]. This can lead to an electrostatic discharge with the energy of 2250 mJ.
Example 3 – bucket
Based on the formula above, we can calculate that as a result of the triboelectric effect, a bucket with a capacity of 20 pF can be electrically charged to the level of 30 kV [IchemE]. This can lead to an electrostatic discharge with the energy of 9 mJ.
Minimum ignition energy for dusts
Below are some examples of minimum ignition energy values for dusts. At this point, it should be emphasised that this value depends on several factors, such as the moisture content of the material or the diameter of its particles. For this reason, the best practice to determine the MIE for a specific dust is to test it.
|Dust cloud||Minimum ignition energy [mJ]|
|Certain pharmaceutical semi-finished products||1|
Minimum ignition energy for gases / vapours
|Explosive atmosphere||Minimum ignition energy [mJ]|