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Combustible Dust & Static Electricity Q&A

  •   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.

  •   We are planning to line a silo with a new coating of epoxy. The supplier was asked to provide the breakdown voltage of the coating so that we could be sure that we did not create conditions suitable for generating a propagating brush discharge. (Vb <= 4 kV)The silo supplier, and his coating suppliers, are unable to provide this information and are unwilling to conduct a test. The required coating dft is 6 mils. Do we have a problem and do we need to pursue obtaining the breakdown voltage?

    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.

  •   We are providing conveyors – either chain or belt driven live rollers-into filling areas that are rated Class 1/Division 1 or Class 2/Division 1. Besides supplying explosion proof motors and control devices, is there anything else we should be doing to the conveyors in these environments?

    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

  •   Throughout my childhood, I’ve seen cartoons with explosions, sometimes portrayed as being so huge that sound disappears from the surrounding area. I didn’t think much of it, until I saw it in movies, such as the movie ‘2012’ where Yellowstone erupts and all sound disappears for a couple of seconds. Could you tell me what this effect is called?

    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.

  •   We have come across two situations this year whereas multi-national companies are requesting a cross comparison / reference from ATEX to US standards. Do you know of any such cross reference or standard being developed? We offer ATEX but don’t want to insist customers pay for the certification when it’s not required.

    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.