by Judith Lesslie, CFSE, CSP
Following on from the first aeSolutions blog on the subject of the basics of combustible dust concerns, this blog provides a deeper dive into the properties of combustible dusts. It is necessary to first understand your specific dust testing and properties to assess the potential hazards of a given dust handling process.
A past blog provided an overview of the generalities of combustible dust hazards, including some serious past incidents, and provided some of the relevant guidance documents for those who are interested in more detailed information. Once the article’s technical basis is understood, future blogs will cover signals that there may be previously unrecognized dust hazards at your site, common ignition sources and safeguards, and potential dust hazard assessment (DHA) methods.
Combustible Dust Properties
Technical definitions of combustible dust vary only slightly across industries and agencies. OSHA, for example, defines it as "a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations". The NFPA defines it as “a finely divided combustible particulate solid that presents a flash-fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations” (NFPA 652, 2019). The CCPS defines it similarly to the NFPA (CCPS Guidelines for Combustible Dust Hazard Analysis, Wiley, 2017).
So what are the parameters of interest when it comes to deciding whether or not a dust presents a combustibility or explosion hazard? There are a number of relevant parameters that can be determined through testing per various ASTM, EN and IEC standards, including:
An initial explosivity screening test, which disperses dust samples in a chamber at various concentrations and exposes them to an ignition source, then determines the resulting pressure rise. If the standard pressure rise is by more than a factor of 2, then more detailed testing is indicated.
Deflagration index testing (Kst, given in units of bar-m/sec) measures how quickly pressure rises in a closed container when ignited. Kst is used to classify the dust hazard at St. 1, 2 or 3. Kst, of 0-200 is St. 1 (lowest hazard), 201-300 is St. 2 and greater than 300 is St. 3 (highest hazard). However, “low” is deceiving in this context. For example, the Kst of sugar (the dust involved in the serious dust explosion incident mentioned in a previous blog) is about 138. There have been numerous other serious explosion incidents from low Kst dusts as well. OSHA considers a Kst of 1.5 bar-m/sec as the minimum threshold for an explosion.
Maximum pressure testing (Pmax, given in units of bar) provides the peak explosion pressure developed by a dust explosion at its optimal concentration. Pmax is helpful in identifying potential worst-case equipment and compartment consequences due to a dust explosion; and is used in vent, explosion panel and explosion containment designs.
Minimum explosible concentration testing (MEC, given in g/m3) provides the minimum concentration of a dust that will deflagrate. The MEC is helpful in defining potential damage as well; and is also very useful if a site plans to use concentration control as a safety measure.
Minimum ignition energy testing (MIE, given in mJoules) provides the minimum electrical energy stored in a capacitor, which, when discharged, is sufficient to ignite the most ignitable mixture of dust. MIE is useful for evaluating what sources of ignition energy are credible to ignite a dust cloud. One item to be aware of for MIE is that it is typically determined at standard temperature and pressure conditions; and the MIE may be expected to be somewhat lower at higher actual process temperatures.
Limiting oxygen concentration testing (LOC, given in vol% O2) provides the minimum concentration of oxygen in a mixture of dust, air and an inert gas that will support combustion. This information is useful when considering the design of explosion prevention systems involving the use of inert gases.
Minimum auto-ignition temperature testing (MAIT, degrees C) provides the lowest surface temperature that will auto-ignite a given dust cloud. MAIT is useful for evaluating equipment and process temperatures to minimize the risk of auto-ignition.
The remaining parameters are conducted on dust layers or bulk samples rather than dispersed samples:
Volume (or bulk) resistivity testing (ohm-m) provides the electrical resistance of a solid material. It is useful when assessing the insulating and electrostatic properties of a material, and hence the potential for a material to generate and retain charge. Higher resistance dusts have the potential for charging during material handling (including loading activity) and releasing electrostatic charges. This is a concern if the energy of the electrostatic charge may exceed the minimum ignition energy for the dust under the operating conditions.
Layer (or hot surface) ignition temperature testing (LIT, given in degrees C) provides the lowest surface temperature capable of igniting various thicknesses of a dust layer, such as buildup that could occur in dyers, classifiers, or packaging equipment. LIT is used together with MAIT to evaluate equipment and process temperatures to minimize the risk of auto-ignition in dusty areas and equipment.
Time to maximum rate/exothermic reaction at a given temperature testing (usually provided as a set of temperatures and times of interest) is a less obvious concern. Some dusts are capable of self-heating in an exothermic reaction if allowed to persist in a layer above the maximum rate onset temperature for long enough. The maximum rate reaction can produce smoldering, fire and combustion products that can be carried along in a moving particle stream and become a source of ignition. If a dust has this property, it can also cause development of toxic or flammable gases, depending on the process and conveying gas. The information provided through maximum rate testing is highly relevant to cleaning of dust layer buildups, process temperatures, equipment temperatures and housekeeping regimes to minimize dusting around process equipment.
The bottom line is that even dusts with weak explosivity and high ignition energy requirements can be dangerous under the right circumstances. The right circumstances include:
· Presence of fuel, e.g., combustible dust
· Presence of oxygen
· An ignition source (including hot surfaces)
· Dispersion of the combustible dust
· Confinement of the combustible dust cloud
The dispersion and confinement factors above are specifically required for dust explosions. Without the presence of confinement, a dust cloud flash fire is possible.
There will be more on this topic in this continuing series. Stay tuned!
The Stakes
Do you handle potentially combustible dusts at your site? It is difficult to adequately control a hazard that is not well-understood, and no company wants to learn of dust explosion hazards the hard way.
How do you know if you have combustible dust hazards present? Screening and testing of a representative sample of the dust for the parameters noted above is the clear answer. This type of testing is available from many reputable labs and suppliers and can even be coordinated on your behalf by reputable process safety consultancies.
So What?
If you have not previously taken a deep dive into the properties of your particular dust(s) at your site, now would be a good time to do so. If you do not have the right technical expertise in your company to assess dust hazards, consider selecting a process safety consultancy with deep experience and expertise to assist you. Their range of experience enables assessors to recommend reputable testing labs and to share the general and specific methods proven to minimize dust explosion hazards across industry. This independence from the site and company has the best probability of a careful assessment with fresh eyes on the relevant critical systems and leads to more efficient compliance with the necessary standards.
Stay tuned for more. Future blogs in this series will cover signals that there may be previously unrecognized dust hazards at your site, common ignition sources and safeguards, and potential dust hazard assessment (DHA) methods. Part 3: Dust Ignition Sources