Newson Gale’s latest series of articles that contain case studies of fires and explosions caused by static electricity draw attention to the wide range of processes that are susceptible to electrostatic charge generation and accumulation on portable and fixed plant equipment.
This case study investigates the factors behind the ignition of a combustible dust cloud during a manual powder processing operation. In this example a process operator was tasked with manually tipping approximately 18 kg (40 lbs) of powder from a plastic drum, constructed from polyethylene, into a metal process vessel. The plastic drum contained a combustible powder that had a minimum ignition energy of 12 milli-joules. A metal chime was positioned around the circumference of the top of the plastic drum to provide it with impact protection from daily usage in the plant.
The operator tipped the powder into the process vessel, resting the drum on the edge of the vessel. As he removed the drum from the vessel when the powder was fully deposited there was an ignition of the dust cloud that had formed at the top of the vessel.
It was postulated that the accumulation of electrostatic charge on the chime resulted in a static spark discharge from the chime as it came into close proximity with the vessel when the drum was removed. The vessel was grounded through its own fixed connection to the plant.
In order to verify this theory an experiment was conducted to determine how much electrostatic charge could have been generated by the movement of the powder. 18 kg (40 lbs) of the same powder was tipped from a similar drum into a Faraday cage from which electrostatic charge measurements were taken.
A charge of 3.6 micro-coulombs was measured on the Faraday cage which received the powder. In this case the powder was charged due to the friction caused between the powder and the plastic drum as the powder slid down the inside surface of the drum. A field meter reading of 500 KV/m (the maximum voltage the meter was capable of measuring) was recorded on an isolated area of the plastic drum which would have had the effect of charging the metal chime by induction.
Given the high rate of charge generation caused by frictional charging, the amount of electrostatic charge that could have been induced on the chime would have been limited by the surface area of the chime. In this case the surface area of the chime was approximated to 0.0641 m2 (99 in2).
If the total quantity of electrostatic charge (3.6 micro-coulombs) created by the movement of the powder was induced on the chime this would have exceeded the maximum charge density any surface can hold in air. The maximum charge density of a surface in air is equivalent to 27 micro-coulombs per square metre. The total charge density of the chime in this case, theoretically, would have been 56 micro-coulombs per square metre.
(I): Charge density (σ) = Total charge (Q) / surface area (A)
Charge density (σ) = 3.6 x 10-6 / 0.0641
Charge density (σ) = 56 x 10-6 C/m2
It can be assumed that the maximum charge density, i.e. the total possible amount of charge that could be held on the chime, was achieved through the simple and rapid act of tipping the powder from the drum into the vessel. In this study the capacitance of the chime was estimated to be 71 pico-farads. Knowing these values it is possible to estimate what the potential energy of the spark discharge was.
Taking the above formula (i), Q = σA, the maximum charge on the chime can be calculated:
=> 27 x 10-6 x 0.0641 = 1.7 x 10-6 C
Therefore, the total charge on the chime would have been close to 1.7 micro-coulombs. Hence the voltage of the chime would have been in the region of 24,000.
(II): voltage = total quantity of charge / capacitance of charged object
V = 1.7 x 10-6 / 71 x 10-12
V= 24 KV
The average breakdown voltage of air is 3000 volts per milli-metre, therefore the voltage of the chime would have been capable of discharging an electrostatic spark from a distance of at least 8 mm (0.3”) to the grounded process vessel.
The potential energy of the chime can be calculated from:
Potential energy (W), = Q2/2C
• Q = charge on chime
• C = capacitance of chime
Therefore the potential energy of the chime:
= (1.7 x 10-6)2 / (71 x 10-12).(2)
= (2.89 x 10-12) / (142 x 10-12)
= 20 milli-joules.
This exceeds the minimum ignition of the powder which was 12 milli-joules.
Given that the minimum ignition energy of the powder dispersed in air was 12 milli-joules and that the circumstances of the process proved there would have been significant electrostatic charging of the equipment, and other sources of ignition being eliminated, a static spark caused the ignition of the dust cloud that formed around the grounded process vessel.