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Combustible Dust & Static Electricity Q&A

  •   Can a type A or type B super sack be effectively grounded by inserting a grounding rod into the material, i.e. the rod is attached to a wire that is attached to a ground?

    Type A and Type B FIBCs are made from fabrics that are insulating by nature and as such placement of a grounding rod in these FIBCs during powder filling and particularly during powder emptying operations will not prevent the generation and subsequent accumulation of hazards levels of electrostatic charge on these FIBCs. As I am sure you are aware, electrostatic discharges from Type A and Type B FIBCs can potentially ignite flammable atmospheres with Minimum Ignition Energies of up to about 1,000mJ and 3mJ respectively. The insertion of a grounding rod into the bulk powder might perhaps be effective in dissipating electrostatic charges from the powder itself (and not the Type A or Type B FIBC walls) only if the powder is conductive or static dissipative. It is therefore recommended that an appropriate FIBC be selected based on the Minimum Ignition Energy of the flammable atmosphere present as well as the electrostatic chargeability and volume resistivity characteristics of the powder.

  •   Is expanded polystyrene dust explosive?

    Expandable polystyrene (EPS) dust is explosible and under the right conditions could cause flash fires and/or explosions.  It is therefore suggested that a Dust Hazard Assessment (DHA) be conducted and measures for ensuing safety of people and facilities against the consequences of explosions are based on data derived from the ease of ignition (Minimum Ignition Energy. Minimum Ignition Temperature of a Dust Cloud and a Dust Layer, and the Minimum Explosible Concentration) and explosion severity (Maximum Explosion Pressure and Kst) tests that are conducted on representative samples.

  •   Inside a large tank there are wooden forms used to pour concrete and during the removal process of these forms dust is created. Is this concrete dust explosive?

    Concrete dust is reported in public literature as being a non-explosible. However, if any combustible dust in sufficient quantity is mixed with concrete dust, the mixture could become explosible, in which case explosion prevention and protection measures may be required. If you have any doubt about the combustibility of the dust in question, then you may consider conducting an Explosibility Screening (Go/No Go) test according to the ASTM E1226.

  •   Can humidification ever be used as a way to eliminate a dust explosion hazard? Is there any relationship between static buildup or discharging vs relative humidity?

    Depending on the nature of a dust, if water vapor (moisture) is present in air, it absorbs onto the dust surface. Humidification of the atmosphere in which a dust cloud is present can therefore make the dust cloud more difficult to ignite (a decrease in the ease of ignition) and the severity of the dust cloud explosion is also expected to decrease. The level of humidification necessary to eliminate a dust cloud explosion can be determined by laboratory testing.

  •   I read a piece on cement dust and was interested in the possibility of quantifying the level of dust contamination that would make a cement tanker explode. I was shown a tank that had catastrophically failed and it was claimed to be the result of a pressure of 5 psi which to me is not rational compared with the possibility of it being a cement and dust explosion, possibly due to static.

    Cement dust is reported in public literature as being non-explosible and as such it is unlikely that the explosion you are referring to could have been caused by a cement dust cloud explosion. However, if a combustible material is mixed with cement, depending on the concentration of the combustible material, the mixture could become explosible. Explosibility of a cement dust cloud containing various concentrations of a combustible dust can be established by conducting Explosibility Screening (Go/No Go) test according to the ASTM E1226 on the dust cloud mixture.

  •   Are there US or EU standards that apply to packaging of powdered products that other than being combustible dusts are non-hazardous? NFPA 654 covers FIBC but are there guidelines for bags and cardboard boxes, specifically whether standard polyethlyene is okay from a static perspective? As a supplier of the powder we cannot control how our customers use the product or in what plant conditions, i.e. cannot preclude they won’t dump into equipment with flammable gas although we can warn about SDS.

    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.

  •   Does talcum powder become explosive if flooded with high percentage oxygen as in nitrox diving (40% oxygen , 80% nitrogen)?

    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.

  •   My company builds stone walls (mostly fieldstone or bluestone). During construction we cut and grind the stones. If we are working in an enclosed plastic shelter (in winter) the dust can be so thick in the air it takes 15 minutes to settle and you can only see probably 20 ft. If we were to have an open flame such as a propane heater would this be a recipe for a deadly explosion? Or is dust from grinding stone/mortar inert?

    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.

  •   Is plastic of any sort allowed in explosive atmospheres? Where we use alcohol / ethanol should the plastic be earthed in anyway? Are steel drums allowed in the same area, without earthing straps attached, and moved around a concrete floor? Should a certain type of floor covering be down, and should there be a certificate of some sort to prove it is a safe floor?

    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.

  •   We are discharging a combustible dust from a conveying system through 6 in. flexible duct to a weigh tank. Particle size is a broad range from fines to 1700 micron. What guidance is available for quantifying and evaluating the risk of a combustible dust atmosphere (i.e., dispersion) in the head space of the tank? Does it matter if the head space has general exhaust ventilation?

    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.