This is a case study which explores the reasons why a combustible dust cloud caught fire during a manual powder processing operation. The operator had the task of tipping over about 18 kg or 40 lbs of powder contained in a polyethylene drum into a metal vessel in which the process was to be carried out.
The minimum ignition energy of the powder was 12 milli-joules. The drum had a metal chime around the rim to protect it from the daily impacts that would arise during its routine use. The operator rested the drum against the edge of the metal process vessel and tipped the powder over into it, removing it after all the powder had been transferred. At this point the dust cloud that had formed just over the process vessel during the transfer ignited.
Calculating the Static Buildup
The investigators suggested that static electricity built up on the metal chime and discharged as a spark when the drum was being removed, bringing the chime very near the metal vessel. The vessel itself had a fixed grounding connection. This was verified by an experiment in which the same amount of identical powder was tipped over from the same type of drum into a specialized equipment called a Faraday cage which allows electrostatic charge to be measured.
In this case it was 3.6 microcoulombs. The friction between the powder and the sides of the drum created static electricity, which read 500 KV/m on a field meter used at one isolated area of the drum (the highest possible reading on the instrument). Such a high buildup is sufficient to induce a charge on the metal chime that is limited only by its surface area (0.0641 m2 or 99 in for both faces).
Considering the 3.6 microcoulombs of static charge in the powder created by friction, if the whole of it was induced on the chime, it would have crossed the barrier for charge density on a surface in air which is about 27 microcoulombs per square meter for a surface in air. In this case, we can calculate the total charge density of the chime to be 56 microcoulombs per square meter.
(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
This maximum charge density was achieved by simply tipping the powder in the drum into the vessel quickly. The capacitance of the metal chime was 71 pico-farads.
In this case the potential energy of the spark (electrostatic discharge) is calculated as follows:
(i) Q = σA
The maximum charge on the chime => 27 x 10-6 x 0.0641 = 1.7 x 10-6 C
Therefore, the total charge on the chime is approximately 1.7 micro-coulombs.
Therefore, the voltage of the chime would be roughly about 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
Knowing that air has an average breakdown voltage of 3000 volts per mm, this voltage would be able to discharge as a spark of static electricity over at least 8 mm or 0.3 in to the process vessel which was grounded.
To calculate the chime’s potential energy:
Potential energy (W), = Q2 /2C
Where Q = charge on chime and C = capacitance of chime
Therefore W = (1.7 x 10-6)2 / (71 x 10-12). (2)
= (2.89 x 10-12) / (142 x 10-12)
= 20 milli-joules.
This crosses the threshold of minimum ignition of the powder in air (12 milli-joules), which leads to the conclusion that the electrostatic charge accumulation on the equipment led to the dust cloud catching fire, in the absence of any other source of ignition.
What Actions Could have been Taken to Prevent this Explosion?
It is easy to believe that the same type of loading might have happened many times without any visible ignition following a spark discharge. This is explained by the absence of a flammable dust cloud in the spark gap at the moment of discharge. Many similar operations share this feature, as a matter of fact. But the real question is, why static was allowed to build up on the chime in the first place.
In the situation described above, the chime was not properly connected to a true earth ground, allowing the charge to accumulate instead of finding its way to earth. To comply with industry standards such as NFPA 77 and IEC 60079-32-1, the metal chime, as a isolated metal part, should have been grounded by, for instance, connecting it to the process metal vessel, by a pathway with 10 ohms or less of resistance. Both these standards state, “Temporary connections can be made using bolts, pressure-type earth (ground) clamps, or other special clamps. Pressure-type clamps should have sufficient pressure to penetrate any protective coating, rust, or spilled material to ensure contact with the base metal with an interface resistance of less than 10 Ω.”
Many devices are available for use in connecting the drum to the grounded process vessel. At least a grounding clamp should be provided with FM/ATEX certification which is capable of penetrating any barriers such as paint or deposits on the surface of the equipment, to make full contact with the underlying metal, such as the VESX45 dual clamp assembly.
The Bond-Rite* EZ clamp is a more advanced device with an LED indicator on the clamp to show when a successful grounding connection has been made between a metal drum and the process vessel, with 10 ohms or less of resistance. The LED pulses green to tell the operator whether the powder tipping is GO or NO GO.
Another issue in this situation is the presence of the plastic powder loading drum within the EX/HAZLOC area. Materials such as polyethylene have low conductivity and the static charge generated on the drum by friction during the powder transfer is almost impossible to discharge properly even if a grounding connection is provided.
It is also impractical or even impossible to totally avoid charge buildup on a resistive powder because it involves specifically changing the powder formula. Thus, the use of plastic containers or other objects in this area involves a significant risk of inducing static charge accumulation, which can induce charge on either other equipment or operators who are positioned close to or in contact with the charged plastic item.
To avoid this, only metal drums should be used in connection with the grounded metal process vessel.
Finally, footwear which is designed to dissipate static should be mandatory for all process operators, so that static buildup due to their body movements during the process can be discharged safely to the ground, rather than discharging as a spark onto a grounded object.
The industry standards IEC 60079-32-1 and NFPA 77 describe the type of preventive measures that companies should adopt to minimize fire and explosion risks due to static accumulation and sparking. Most are centered on the installation and use of grounding devices before the event occurs, and this is made easier by the availability of a range of devices from basic to advanced interlocking ground status indicators, to suit any of an array of processes.
VESX45 – dual clamp assembly for connecting portable objects.
Bond-Rite® EZ with clamp mounted LED indicator that pulses green when a connection resistance of 10 ohms or less is made between conductive portable equipment.
This information has been sourced, reviewed and adapted from materials provided by Newson Gale.
For more information on this source, please visit Newson Gale.