Combustible Dust & Static Electricity Q&A
NFPA (National Fire Protection Association) 77 – Recommended Practice on Static Electricity – should be a good starting reference as it addresses the electrostatic ignition hazards associated with the use of non-conductive containers for liquids and powders. You may also visit http://www.chilworth.com/library/ to access a series of Focus articles on the assessment and control of electrostatic hazards associated with powder and liquid handling/processing operations, including containers for powders. As far as standard (insulating) polyethylene bags/liners/containers are concerned, they can become electrostatically charged when they are being filled with powder or emptied. Depending on the “electrostatic chargeability” of the powder, electrostatic “brush” or “propagating brush” discharges could be expected from the bag/liner/container. It is well documented that “brush” discharges can ignite flammable gas and vapor atmospheres with minimum ignition energies (MIE) below about 4mJ and “propagating brush” discharges can ignite gas/vapor and dust cloud atmospheres with MIE less than about 1,000 to 2,000mJ. Therefore, unless it can be reliably verified that electrostatic charging levels are not adequately high enough to give rise to “brush” discharges, plastic bags/liners/containers must not be used in areas where flammable gas/vapor atmospheres could be present. The ignition sensitivity of potential flammable atmospheres, both inside the containers and in the surrounding environment (around the containers) is generally considered when selecting a particular container type (with certain electrostatic characteristics) that would be suitable for use in that atmosphere. It is generally a good engineering practice to seek expert advice to evaluate such situations.
According to various public references (Merck Index and NIOSH Pocket Guide to Chemical Hazards), talc is hydrous magnesium silicate with the formula Mg3Si4O10(OH)2 or H2Mg3(SiO3)4. Thus, talc is a fully-oxidized mineral, consisting [essentially] of 3 MgO [magnesium oxide], 4 SiO2 [silicon dioxide], and H2O, requiring 12 oxygens, as are present in the molecule. Thus, a dispersion of talc dust in air [or in 100% oxygen] would not be explosible or combustible. According to various public references (Merck Index and NIOSH Pocket Guide to Chemical Hazards), talc is hydrous magnesium silicate with the formula Mg3Si4O10(OH)2 or H2Mg3(SiO3)4. Thus, talc is a fully-oxidized mineral, consisting [essentially] of 3 MgO [magnesium oxide], 4 SiO2 [silicon dioxide], and H2O, requiring 12 oxygens, as are present in the molecule. Thus, a dispersion of talc dust in air [or in 100% oxygen] would not be explosible or combustible.
Fieldstone and bluestone are inorganic minerals that are composed of oxides of magnesium, aluminum, silicon, and aluminum. Thus, these materials could not be further oxidized, and they and their dusts are non-combustible. Further, any suspension of the dust in air could not explode. However, dense clouds of stone dusts would be hazardous, because the concentration would far exceed – by several orders of magnitude – the recommended limit for “particles not otherwise specified”, which is 10 milligrams per cubic meter. Thus, persons within or near such clouds should be protected from inhaling the dust by wearing appropriate PPE.
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.