Combustible Dust & Static Electricity Q&A
Generally speaking, the use of non-conductive materials (plastics) should be avoided in locations where flammable gas/vapor atmospheres could be present.
Electrical grounding/earthing of plastic objects is not usually an effective method of controlling electrostatic discharge/ignition hazards from plastic objects.
There may not be a need for grounding of a steel drum using a grounding strap, provided that the electrical resistance between the drum, placed on the concrete floor, and your plant reference electrical ground point is less than 1MΩ (1×106 ohm). A concrete floor that is free from any insulating deposits, coating, or paint may have a resistance-to-ground of less than 1MΩ.
There are static dissipative coatings that may be considered. The suitability and effectiveness of the static dissipative coating (once applied) should be verified by a subject matter expert using an appropriate ohm meter and floor electrodes.
It is possible that there may be additional potential static discharge hazards (e.g., those arising from ungrounded metal objects such as tools, sampling devices etc., ungrounded personnel, insulating liquids, transfer hoses etc.) in the area. It is advised to seek an expert’s evaluation regarding the specific hazards and measures to control/abate such hazards.
One could determine the minimum explosible concentration (MEC) of the dust being conveyed to the vessel and based on some assumptions calculations can be done to compare the dust cloud concentration in the vessel’s headspace with the dust’s MEC. It should be noted, however, that it is often very difficult to maintain the dust cloud concentration in the headspace of a receiving vessel below the minimum explosible concentration (MEC) at all times, even if there is an exhaust ventilation installed. In addition to taking steps to reduce the dust cloud concentration by the use of effective exhaust ventilation, one should avoid all potential ignition sources (including but not limited to electrostatic discharges from flexible duct, isolated metal components, bulk bag and powder material etc.). This would require testing the dust cloud for minimum ignition energy (MIE) and also minimum ignition temperature (MIT), if mechanical/friction ignition sources could be present. If the MIE is below 25 mJ, you should consider determining the volume resisitivity and chargeability of the powder.
It is suggested to perform a hazard assessment on this process to determine the level of risk associated with this operation and suitability of preventive and/or protective safeguards to control/abate the hazard. For more specific guidance, expert advice should be sought.
Electrically insulating coatings with a breakdown voltage greater than about 4 KV, if adequately charged, could give rise to propagating brush discharges with energies as high as about 2,000 mJ. Such discharges could readily ignite most explosible dust cloud atmospheres.It is therefore suggested that a metal plate with dimensions of about 0.5 m x 0.5 m is coated with the said epoxy according to the same exact specifications as that used for the coating of the silo and subject that sample to the breakdown voltage testing according to ASTM D3755 standard. The suitability of the coating would then be based on the result of this test.
It may be appropriate to conduct a non-electrical equipment ignition risk assessment using perhaps the methodology specified in BS EN 15198 standard. This methodology is a logical extension of hazardous area classification assessments done for electrical equipment to define hazardous areas where special precautions are required to control ignition hazards from electrical equipment and devices. The purpose of the non-electrical equipment ignition risk assessment is to ensure, as far as reasonably practicable, that the design and operation of the non-electrical equipment that are located in areas of the facility where flammable materials are used/present, is undertaken in such a way that the health and safety of the operators (from a fire or explosion) is not compromised. In the non-electrical equipment ignition risk assessment report, individual sections are presented, which include the following:
1. The hazardous area classification inside and outside the equipment is reviewed, and the relevant equivalent classification category – Division 1, or Division 2 – for the equipment specified.
2. The maximum allowable surface temperature for parts of the equipment located within the hazardous area is determined based on those flammable atmospheres that could occur.
3. Relevant engineering details of the equipment such as materials of construction and safety features and the actual risk assessment are detailed with respect to each identified potential ignition source (a standard source list is used).
4. A general discussion of the main issues identified and any other general points is then carried out, including, where deemed necessary, the need for a basis of safety for the equipment other than ignition source control.
For conveyers in Class I and 2 areas, obvious areas of concern include:
1. Friction spark potential from metal to metal contact
2. Friction heating of moving parts (continuous monitoring of bearing temperatures is possible to warn of overheating
3. Electrostatic discharges from non-conductive surfaces as well as those conductive components that could become electrically isolated from ground
4. Specific location and exposure potentials
When a detonation or deflagrating blast occurs, the resulting blast creates a temporary vacuum in the immediate area of the event and behind the propagating blast wave. Sound requires a medium, such as air, to propagate. Therefore it seems reasonable to assume that in close proximity to a detonation especially, sound transmission would be reduced or eliminated until the air rushes back in. The in-rush of air after a blast is readily seen in videos of such events. Typically the most readily observed in-rush of air is close to the ground, as it is hampered above the ground by the continuing outward expansion of any fireball produced by the event. In a large explosion, shock wave propagates noticeably for a much greater distance. The wave also elongates with distance from the blast. Therefore, the passing of the shock wave and subsequent partial vacuum at some distance from the event would, of course last longer.
There is no direct comparison between ATEX requirements and the US standards. One major difference is that ATEX requires compliance with the essential health and safety requirements for non-electrical as well as electrical equipment, while the US standard only considers electrical equipment. European manufacturing standards for electrical equipment are increasingly aligned with the IEC (international) standards, but the US standards applicable to the Class-Division system of area classification are different. Although the Class-Zone system of area classification is based on the IEC standards, that does not imply that an ATEX certificate can be exchanged for a US one. More specific information on this topic can be provided if the nature of the equipment in question are known.
The Minimum Explosible Concentration or MEC (sometimes commonly referred to as Lower Explosive Limit, or LEL), is the lowest concentration of dust cloud in air that can give rise to flame propagation upon ignition by a sufficiently energetic ignition source. Literature values for the MEC of Polyethylene dust vary from about 30-35 g/cu m to as low as about 10-20 g/cu m. The MEC test is usually performed according to ASTM E1515 (Standard Test Method for Minimum Explosible Concentration of Combustible Dusts) with the recommended particle size of 95% < 75 µm. Dust particle size and shape can obviously affect the MEC results and while literature values can provide a general guide, it will be very prudent to take a representative sample of the subject dust and perform an MEC test on that sample.
In the absence of any flammable vapor or gas atmosphere, a plastic scoop would be fine for transfer of non-conductive (insulating) powders. However, aluminum is a conductive material and any smear of aluminum fine dust forming on the plastic scoop could give rise to an ungrounded conductive patch on the scoop. This ungrounded conductive patch could become electrostatically charged through repeated use of the scoop, resulting in electrostatic sparks with perhaps sufficient energy to ignite a low Minimum Ignition Energy (MIE) aluminum dust cloud (MIE < ≈10mJ) in the vicinity. It is therefore suggested that an electrically grounded conductive scoop (stainless steel) be used. As a secondary precaution, the operators carrying out this operation should avoid any action that could cause energetic frictional impacts between the scoop and any other hard surface such as concrete flooring.
Phosphoric acid – H3PO4 – is “fully oxidized” and can be considered as the combination of 1/2 P2O5 and 3/2 H2O. Therefore, phosphoric acid powder or dust would not be a combustible material, with no fire or explosion hazard.
Similarly, calcium phosphate – Ca3(PO4)2 – is “fully oxidized” and can be considered as the combination of 3 CaO and P2O5. Therefore, calcium phosphate and other phosphates [such as sodium, magnesium, and potassium] would not be combustible materials, with no fire or explosion hazards.
Whereas measures such as maintaining the ambient relative humidity above about 65% and reducing the speed of separation/movement could reduce the magnitude of the electrostatic charge on the rollstock, perhaps a more reliable method of controlling the charge on rollstock would be through the use of electrostatic charge neutralizers. Correctly positioned passive and or active neutralizers could be quite effective in reducing the charge on such surfaces to an acceptable level. Selection of the neutralizer type and its positioning should be made after quantifying the magnitude and polarity of the charge that builds up on the surface through on-site electrostatic measurements.
As far as you second question is concerned, the electrostatic measures, if any, would very much depend on the type of flammable atmosphere that could be present and also on any activity in the storage facility that might be generating static charge.