We previously addressed the basic concerns around combustible dusts, many of the standards that address dust hazard guidance, and the properties and testing for combustible dusts. This article will build on those topics to address potentially unrecognized dust hazards at your site and common potential ignition sources for both internal and external dust clouds or layers.
by Judith Lesslie, CFSE, CSP
Identifying Dust Hazards
If a process includes particulate or dust handling steps, then the process owner should determine if that dust is combustible and if so take the appropriate protective measures. That is accomplished by testing samples for the properties as described in part 2 of this series of blogs; then performing a Dust Hazard Analysis (DHA) and executing the necessary recommendations for process risk reduction. Safeguards and DHAs will be further discussed in later blogs in this series.
If you are new to a dust or particulate handling facility, then there are a variety of ways that could alert you to a potential combustible dust hazard other than site orientation and area training. For example, clues that the process dust is potentially combustible include safeguards such as a nitrogen inerting system, electrostatic discharge control measures, nitrogen suppression systems, or building or equipment blast panels in the field and on the facility P&IDs. Equipment clues include the presence of dust collectors, eductors, air locks, screeners, pneumatic conveying system, rotary locks, silos or several different types of dryers. From a documentation perspective, if the facility area classification drawings note that any areas are rated as Class II (NEC) or Group III (IEC), then the likelihood the facility is handling combustible dust or particulates is high. Housekeeping regimes that are focused on preventing process dust or particulates from accumulating internally or externally are another obvious indicator. Lastly, and most concerningly, if you are present in a facility with the routine presence of dust layers or dust clouds, that is a clear indication of a dust issue that has potentially not been adequately addressed.
Ignition sources
In a DHA for combustible dusts, there are three basic tasks once the presence of a combustible dust is established. The first task is to identify credible ignition sources for potential dust clouds and layers internal to equipment and external to equipment (compartment or room-based dust clouds or layers due to undesired system leakage). The second task is to establish the potential frequency of the ignition scenarios and the potential consequences; this is often called risk ranking, similar to a process hazard analysis. The third task is to identify recommended improvements if a dust explosion risk ranking justifies it per facility risk criteria. We will discuss the first task, identification of ignition sources in this blog.
For dust clouds, a catalog of ignition sources and their potential ignition energy should be compared to the relevant dust parameters, including minimum ignition energy (MIE) for an explosible concentration of the dust. What are ignition sources? Basically, any electrical, mechanical, or electrostatic energy source with enough energy to ignite a combustible dust. There are a variety of ignition sources for any given process, and they are not necessarily the same from process to process. Ignition sources are commonly listed with their typical ignition strength in millijoules, for ease of comparison to the minimum ignition energy (MIE) of the subject dust. A few ignition sources that are commonly encountered include:
Ignition Source | Typical Strength (mJ) | Example Sources |
Brush Discharge | 5-10 | - Plastic Surfaces - Emptying/Filling Plastic Containers |
Bulking Brush/Conical Pile Discharge | 10-200 | - Filling bulk storage or portable containers |
Propagating Brush Discharge | 1000-3000 | Double layer of opposing charges on opposite sides of a non-conducting material surface: - Non-conductive/lined piping exposed to high dust load or velocity - Filling large insulated containers |
Spark | Overall: <1000 Flange: 0.5 Drum: 20 Person: 15 | - Conductive material isolated from ground |
Mechanical Sparks | <10,000 | -Metal contaminants (“tramp metal”) impact equipment wall - Higher-speed (>1 m/s) rotating equipment malfunction |
Electrical Sparks | <10,000 | - Electric sparks and arcs that occur in switches, motors, instruments (during malfunctions) and in short-circuiting caused by damaged cables |
An additional consideration for dust clouds or layers internal or external to equipment is whether or not hot surfaces may potentially be present, for example due to process reactions, use of steam or fired heat, or equipment surface temperatures (including during malfunction conditions). These potential surface temperatures should be compared to the auto-ignition temperature (AIT) of the dust to determine if an ignition hazard may exist from simple contact.
Building on the concern for dust layer buildups both internal and external to equipment, a phenomenon called the maximum rate reaction is of concern for some combustible dusts. Some dusts are capable of self-heating in an exothermic reaction if allowed to persist in a layer above the maximum rate reaction onset temperature for long enough. You should be aware if your material is subject to this condition via the testing conducted as described in part 2 of this series of blogs. If you have no knowledge of whether or not your material is subject to the maximum rate reaction, this would be a good time to initiate testing!
Testing results for the time to maximum rate/exothermic reaction at a given temperature is typically provided as a set of temperatures and times of interest. If the maximum rate reaction does actually begin due to layer buildup and a sufficiently warm surface temperature, then it can produce smoldering and fire; and combustion products can be carried along in a moving particle stream and become a secondary source of ignition. It can also produce toxic or flammable gases, depending on the process and conveying gas. The information obtained through maximum rate reaction testing is highly relevant to protocols for cleaning of dust layer buildups; keeping process temperatures and equipment temperatures at a safe level; and housekeeping & maintenance regimes to minimize dusting around process equipment.
If you are handling a dust subject to the maximum rate reaction, then properly managing layer buildups via internal and external housekeeping and maintenance practices; keeping process temperatures below the maximum rate reaction onset temperature; and properly setting the equipment temperature classes to below the onset temperature for your electrical equipment are all of paramount importance.
The bottom line is that even dusts with weak explosivity and high ignition energy requirements can be dangerous under the right circumstances. Development of a set of credible ignition sources, then identification of a set of adequate safeguards is key to managing combustible dusts safely. Later articles in this blog series will address potential safeguards and commonly used dust hazard assessment (DHA) methods.
The Stakes
Do you handle potentially combustible dusts or particulates 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.
How do you know if you have credible ignition sources present? By conducting a DHA with sufficient technical expertise involved to identify credible ignition sources and compare them to the relevant dust parameters.
So What?
If you have not previously taken a deep dive into the properties of your particular dust(s) and ignition sources 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.
Next in the series: Dust Hazards Pt. 4 – Dust Handling Safeguards