Combustible Dust Series: Part 2 – What Is a Dust Hazard Analysis?

As we learned in Part 1 of this series, catastrophic flash fires and explosions can occur from the ignition of airborne dust composed of material that may not be flammable except when in the form of powder. Recognizing and understanding the potential combustibility of dust present in a facility is the first step to implementing combustible dust control measures. Once the dust has been evaluated, and a Kst value above zero has been identified – indicating a flash fire or explosion hazard exists – a Dust Hazard Analysis (DHA) is warranted to identify, assess, and control the hazards.

As stated in our series intro, The National Fire Protection Association (NFPA) is developing a new Combustible Dust Document Consolidation Plan. Until “the dust settles” and it is published, the 2019 edition of NFPA 652, Standard on the Fundamentals of Combustible Dust, provides basic principles and requirements regarding the identification and management of fire and explosion hazards of combustible dust. NFPA 652 requires owners to conduct a Dust Hazard Analysis (DHA) where combustible dusts are present to identify hazards and implement the necessary safeguards. Other NFPA standards, such as NFPA 654, NFPA 61, NFPA 484, NFPA 664, and NFPA 655, contain additional requirements applicable to specific industries such as manufacturing, agriculture, food processing, combustible metals, wood processing, and others. These industry-specific standards also include DHA requirements. Before sifting through the features of a Dust Hazard Analysis, it’s best to begin by looking at the elements that cause a dust explosion to occur.

What Factors Cause Combustible Dust Explosions?

OSHA’s Hazard Alert: Combustible Dust Explosions Fact Sheet lists the five factors, known as the “Dust Explosion Pentagon,” which are required for an explosion to occur: oxygen, heat, fuel (in this case, the fuel is dust), dispersion, and confinement. Oxygen, heat, and fuel are common to any fire, but the dispersion of a sufficient concentration of dust particles (fuel) into the air can cause rapid combustion known as deflagration. Deflagration becomes an explosion when confined (the fifth factor) to a piece of equipment, a vessel, room, or building. A thorough Dust Hazard Analysis must examine measures to control the dust, eliminate or control ignition sources, and contain combustible dust in properly designed areas.

What Does a Dust Hazard Analysis Entail?

Chapter 7 of NFPA 652 provides requirements, criteria, and methodology for a Dust Hazard Analysis, specifying the completion of a DHA for all new processes and facility compartments and a review and update of the DHA at least every five years. A qualified person shall perform the DHA, who generally works with a team of individuals knowledgeable about the processes and facility. A list of equipment and areas to be evaluated is developed using process flow diagrams and by touring the plant. Dust collection devices and any potential ignition sources must also be examined. The fire, deflagration, and explosion hazards are identified, and recommendations to minimize or mitigate the hazards are provided. Lastly, a timeline to implement mitigation measures and new management systems is developed.

Process systems evaluation is covered in section 7.3.3 and specifies, “Each part of the process system where combustible dust is present or where combustible particulate solids could cause combustible dust to be present shall be evaluated.” An in-depth assessment of the process system should consider:

• Potential transport of combustible dust between parts of the process system.
• Combustible dust emissions into a building or area.
• Potential for an initial explosion to propagate a chain reaction of subsequent airborne dust explosions in other areas.

Any part of the process containing a layer of dust in an environment containing an ignition source and sufficient oxygen for the dust to burn – an “oxidizing atmosphere” – must be documented as a fire hazard. If this same oxidizing atmosphere containing an ignition source has dust suspended in the air in the form of a dust cloud, a dual flash fire and explosion hazard exists and must be documented. Airborne dust may be a result of ordinary processes or the result of an unintended accident. Both scenarios must be considered.

Building and building compartment evaluation is covered in Section 7.3.4 and is similar to the process system evaluation described above, with additional information noted below:

• In the case of multiple buildings and areas with essentially the same hazard, a single evaluation of a representative building or area is acceptable.
• Dust accumulation levels are also evaluated and compared to threshold dust accumulation levels that indicate a potential for flash fire or explosion hazards.
• Chapter 7 of NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, specifies methods to determine the maximum allowable accumulation of dust, as do additional NFPA standards for specific industries.

Once materials, equipment, processes, and areas containing combustible dust have all been assessed, it’s essential to review the safeguards and preventative measures that are in place, and examine maintenance procedures and confirm the reliability of existing safety systems. Once this has been completed, the results of the DHA can be used to develop a recommendation plan which prioritizes the most critical hazards. Part 3 of this series will examine mitigation methods used to reduce the potential for a combustible dust incident. These management systems are covered in Chapter 8 of NFPA 652. Section 8.6 discusses the use of flame resistant clothing as a final measure of protection to safeguard workers from serious injury due to combustible dust flash fires and explosions.