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Combustible Dust & Static Electricity Q&A

  •   How do we select the area and pressure rating for explosion vents? We are a manufacturer of high powered transformers and electrical equipment in Connecticut. Our equipment is typically installed in rectangular steel enclosures that are not pressure vessels, i.e. low-strength enclosures. We have been using 1.5 psi rated vents. We need to confirm this rating and to calculate the area of the vent and how many vents are needed based on the volume of cabinet.

    .  In order to select the most appropriate and practical means of ensuring safety for the situation that you are describing some more information is required.  This includes the explosibility of the flammable atmosphere (Pmax, Kg, or Kst), the pressure rating/strength of the enclosure, and the type of location (e.g. occupied/open, proximity to other equipment and personnel, etc.) where the enclosure is normally used.  If you would like to discuss this matter in more detail please feel free to contact us at [email protected]

  •   How do you explain to your clients the difference between NFPA 652 and NFPA 654, and how should each be used?

    On September 7, 2015, NFPA 652, “Standard on the Fundamentals of Combustible Dust”, was issued by the Technical Committee on Fundamentals of Combustible Dusts. This standard provides the general requirements for management of combustible dust fire and explosion hazards and directs the user to NFPA’s industry or commodity – specific standard, as appropriate.

    The genesis of this document was the realization that there are inconsistencies with regard to the requirements outlined in the commodity-specific standards and that a more consistent approach needed to be developed to assist users of these commodity specific standards in the management of the fire and explosion risk associated with combustible dusts. This standard introduces a new term, dust hazards analysis (DHA), which is similar in nature to the Process Hazard Assessment (PHA) which is required by all of the commodity-specific standards.

    The NFPA 652 standard will be the governing document, moving forward, where industrial facilities manufacture, process, blend, convey, generate, or handle combustible dusts or combustible particulate solids.

    If your commodity specific standard is NFPA 654 then you do have the option of deferring to that standard. If  a requirement in NFPA 654 differs from a requirement specified in NFPA 652 then the NFPA 654 requirement is permitted to be used. If an NFPA 654 requirement specifically prohibits a requirement specified in NFPA 652, then NFPA 654 shall be applied. Finally where NFPA 654 neither prohibits nor provides a requirement, then the requirements of NFPA 652 should be used.

    I would recommend that you use NFPA 652 to conduct the required DHA and to evaluate hazards and risk associated with your operations.  The requirements in this document very closely parallel those of NFPA 654 and using it in a competent fashion should ensure that you are effectively managing your combustible dust fire and explosion risk.

  •   Where can I find a comprehensive list for Kst Pmax values for food ingredient powders?

    NFPA 61 (Standard for the prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities) lists the explosion properties of some agricultural products so do some other NFPA standards/guidelines such as NFPA 68 (Guide for Venting of Deflagrations). However, please note that published data on the combustibility/explosibility of dusts should not be used for the actual assessment and design of explosion prevention and protection measures and systems. This is because factors such as particle size, moisture content, age, exposure to contaminants, test method, etc. can all affect these properties. It is recommended that a qualified laboratory determine the relevant properties experimentally, using representative samples.

  •   We have very large dust collectors located at our plant. They are located outside the building – if the rotary air lock is also outside of the dust collector, is this classified as a hazardous area? When work is performed on this airlock, dust is present but the motor is not running.

    The area surrounding the airlock/dust collector will most likely be “non-classified” only if the rotary valve discharges into a closed and well maintained system [piping, or another closed container]. Often, the rotary valve under a dust collector discharges into a drum, and there might be considerable “dustiness” around the drum [and drive motor]. In that case, the area surrounding the drum would be classified as hazardous.

  •   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 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.