Assessing Combustible Dust – New Codes

May 7th, 2015

Dust Fairy And CodesAccording to OSHA and the U.S. Chemical Safety Board1 over the last 35 years more than 450 accidents involving dust have killed nearly 130 workers and injured more than 800. Between the years of 2008 and 2012 alone, a total of 50 combustible dust accidents2 occurred resulting in 29 fatalities and 161 injuries. Given these grim statistics, a lot of attention and effort has been spent over recent years on highlighting the risks of combustible dust, educating the public, and developing non-disruptive, cost-effective solutions that ensure life safety in the workplace.

Much of these efforts led to further improvements in the understanding and better mitigation of the risks. This resulted in the creation of new building and fire code proposals and the creation of a new NFPA standard, both of which we would like to discuss in some detail.

Furthermore we would like to provide some background on the dangers and risks of combustible dust, and a basic strategy on how compliance can be achieved.

Combustible Dust And Its Dangers

The dangers of combustible dust exist, if the following criteria are met:

Existence of combustible dust (when in doubt send a sample to a testing laboratory)
Dispersion of dust and potential for suspension in air
Confinement of dust (e.g. room or building)
A form of ignition source (e.g. static electricity spark, flame, etc)
Oxidizing medium (e.g. air)

What Is Combustible Dust And What Makes It So Dangerous?

First, let’s address the question of what is considered dangerous combustible dust:

The California/International Fire Code defines it as ‘finely divided solid material, which is 420 microns or less in diameter and which, when dispersed in air in the proper proportions, could be ignited by a flame, spark or other sources of ignition. Combustible dust will pass through a U.S. No. 40 standard sieve‘. Therefore, the Fire Code will not classify anything greater than 420 microns as combustible dust. However, be aware that with most dry ingredients some portion of the material will be small enough (similar to the bottom of a cereal box) to be considered less than 420 microns!
NFPA 6544 defines combustible dust more general as “a combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.” Again the reasoning here is that material can break down during shipping, conveying, mixing, and use. It will then become an immediate hazard in its dust form, if a source of ignition is available.
Examples include dust from wood, coal, rubber, grain, sugar, flour, cellulose, combustible metals such as lithium or magnesium, and aluminum among others.

What makes combustible dust so dangerous? Danger arises when such fine, small-particle-sized combustible dust is suspended in air in the presence of an ignition source, such as a spark, electrostatic, an open flame or a hot surface. Depending on the makeup and concentration of the dust, a rapid combustion (deflagration) leading to an explosion can occur.

Imperial Sugar Explosion,  Port Wentworth, Georgia, February 7th, 2008

Imperial Sugar Explosion, Port Wentworth, Georgia, February 7th, 2008 (Click To Zoom, Image Credit3)

This cannot only lead to life-threatening situations in the affected immediate locations, but also often impacts everything and everyone else nearby and around that originating location due to the enormous power (heat and pressures) released by the dust explosion(s). Hence, it is not uncommon for dust explosions to have catastrophic results with devastating impact (e.g. ‘Imperial Sugar’ explosion).

Why is dry dust on surfaces critical since only suspended dust in air, as mentioned above, creates an explosion hazard?

Major dust explosions typically occur as a secondary incident. Usually there is a smaller incident caused by air turbulence creating a disturbance of the dust particles, suspending them into air. Once suspended in air, the secondary incident occurs when the dust mixed with air reaches an ignition source resulting in an explosion.

Industries at risk of dust explosions include wood product manufacturing, chemical processing and manufacturing, food and pharmaceutical production, grain storage and processing facilities, metal processing facilities, fabrication of rubber and plastic products plants, recycling operations and textile facilities.

New Standards And Codes

State and local building and fire codes are often based on the International Building and Fire Codes published by the International Code Council (ICC). Various NFPA standards are referenced within the combustible dust related chapters of these codes. These NFPA standards were developed to educate the stakeholders and to prescribe conditions and activities that enable a safe working environment. Yet each industry has different requirements based on operational business needs, and as a result separate standards have been created that address these specific needs and requirements.

Some of the industry-specific standards on combustible dust are (NFPA offers free access to their standards) NFPA 61 Agriculture and Food Processing, NFPA 120 Coal Mines, NFPA 484 Combustible Metals, NFPA 655 Prevention of Sulfur Fires and Explosions, or NFPA 664 Wood Processing and Woodworking Facilities.

Often referenced is NFPA 6544, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. It has been developed to provide a more general framework in order to address industries and businesses not covered under the industry-specific standards. However, application of the provisions and requirements are neither always straightforward nor consistent with other NFPA standards.

Taking steps towards developing more consistency within the NFPA combustible dust standards, a new standard2 was developed, NFPA 6525: Standard on Combustible Dusts. It will provide (the basic principles and) general requirements for management and a framework for assessing combustible dust fire and explosion hazards. It will reference NFPA 654, as well as industry-specific and related NFPA standards as appropriate.

NFPA provisions usually apply to “new facilities unless modified.” This language was changed in NFPA 652 and provisions, such as the Dust Hazard Analysis (DHA), will apply to new as well as existing construction, with the latter being based on a max. 3-year phase-in process from the effective date of the standard. DHA2 is defined as “a systematic review to identify and evaluate the potential fire, flash fire, or explosion hazards associated with the presence of one or more combustible particulate solids in a process or facility.” NFPA is expected to release the new standard by the second half of 2015.

During ICC’s last International Fire Code (IFC) hearings, the committee suggested a rewrite of the respective International (Building and) Fire Code chapters on combustible dust to address the gaps and enable a more wide spread adoption. The major goals of the rewrite are:

Build on current IFC codes and NFPA standards, understandings and experiences
Maintain references to industry-specific NFPA standards (no change)
Provide simple-to-follow, effective guidelines for businesses and industries not covered in the industry-specific NFPA standards
Provide a straightforward methodology and practice, which local building and fire code inspectors can utilize in their inspection and enforcement activities
Maintain cross-references to other related building and fire code chapters (electrical and mechanical codes)

This effort is expected to complete by end of this year. At which time the new codes will be reviewed with the respective IBC and IFC code committees, as part of the code release process for the next editions of the International Building and Fire Codes.

How To Comply

If combustible dust hazards are present and risk mitigation deemed necessary (see below on how to conduct a risk assessment8), compliance is crucial in ensuring a safe workplace and successful business operations. Often relatively little effort is required – of course the actual effort expended will vary and depend on the type and size of the business operation at hand.

Nevertheless, it should be noted that compliance could never be considered a ‘one-off’ event, but rather a continuous process of assessing and reassessing all conditions that can lead to dust fire and explosion risks. Therefore it is best to institute and adopt a ‘combustible dust program’ into the day-to-day business operations, for maintenance of safe conditions.

Appointed staff, which executes or reviews dust control processes and procedures, should have at the very minimum a basic understanding of the dangers of combustible dust and how to mitigate these risks. Reviewing the applied combustible dust process and its procedures including the effectiveness of their results should be a periodic event. Only then can a safe workplace be maintained.

Conducting A Risk Assessment To Determine If A Combustible Dust Hazard Exists

Start with conducting a thorough risk assessment to determine if a combustible dust hazard exists in the workplace. This assessment should cover the following items:

Review all materials handled in the workplace (laboratory tests will help)
○     Are there any dust producing materials?
○     Is the dust comprised of a single material or a (hybrid) mixture?
○     Based on which particle size is the dust (mixture) considered combustible?
○     Is the dust (mixture) capable of spreading the flame?
Review all operations (PHA*) including all byproducts created
Assess potential dust accumulation on all surfaces including difficult to access spaces
Assess potential of creating dust clouds, e.g. through improper placed relief valves
Review all potential ignition sources (flame, heat, hot surface, electrostatics, spark)

* Process Hazard Analysis (PHA) is conducted to understand the hazards in the processes generating combustible dust and to identify appropriate means of effectively controlling these hazards. The gathered information is used as the basis for the applicability of NFPA design criteria (in NFPA 654 or industry-specific NFPA standards).

Implementing Dust Control And Preventative Measures

If it is determined that there is a risk of combustible dust fires or explosions, it is best

(1) To identify all areas with combustible dust and
(2) To control dust accumulation, dust dispersion and the presence of potential ignition sources

In the following we will be discussing general guidelines that the user will need to carefully review against each business scenario based on the chosen protection designs, procedures, and processes in order to comply with all required codes and standards.

Dust Collectors/Ventilation/Ducts/Vents. In order to minimize dust accumulation and dispersion, dust collectors or ventilation systems should be applied. These devices and systems must meet specific requirements, e.g. they should be listed for the specific purpose and application, and comply with restrictions based on their location. If air-material separators, filtering or air recycling systems are used, they need to be installed and maintained per NFPA 654 or industry-specific NFPA standards.

All equipment and metal transfer pipe/ducting, which handles combustible dusts should be bonded and grounded. Pneumatic/vacuum duct systems, storage enclosures, silos, mixers/blenders, particle size reductions, dryers, and blowers/fans may need to be provided with deflagration and explosion protection, and measures to prevent deflagration propagation (NFPA 696). Explosion venting must be directed away from work areas with employees. If relieve valves are present, locate these valves away from dust deposits.

Access To Hard To Reach Areas. Ducts for dust collectors and ventilation systems must have unobstructed access doors to allow for periodic cleaning and removal of dust residues. Any larger obstructions such as beams, joists, equipment, hoods, ledges, suspended ceilings and mezzanines, or other hard to reach areas need to be made accessible for cleaning and removal of dust residues, as well as for the purpose of inspection. Use surfaces that minimize dust accumulation and facilitate effective cleaning. Additional information on access issues can be found in the building and fire codes and referenced NFPA standards.

Housekeeping Procedures. Preventing fugitive, combustible dust from building up on surfaces will reduce the risk of fires or explosions. If dust levels are kept below threshold values calculated from NFPA 654, Section 6.1, additional safeguards for workers may not be required. Note that the control of fugitive dust does not indicate non-existence of dust hazards directly associated with process equipment.

As a basic safety measure, implement periodic and unscheduled housekeeping procedures to ensure that combustible dust isn’t allowed to accumulate to hazardous levels. Always use vacuum cleaners approved for dust collection and never use pressurized cleaning equipment (though few exceptions exist). When evaluating fugitive dust explosion hazards per NFPA 654, you may use the threshold mass equations, which consider all of the dust mass collected throughout the building. Or you may use the ‘Layer Depth Criterion’ method for dust layers based on their size of the accumulation area and their layer depth, see example calculation below. When in doubt about permissible dust layer thicknesses and volumes, always check with the applicable Fire Code requirements and referenced NFPA standards.

NFPA 654 and industry-specific NFPA standards offer provisions to minimize damage from a fire or explosion by allowing hazard areas to be detached, segregated (using physical barriers and explosion venting to limit and control the explosion hazard), or separated from other occupancies. Separation is the concept of applying distance between areas which contain processes generating the combustible dust and other operations that are in the same room. In the case of a multi-floor building, which is effectively separated at the floor levels, explosion and flash fire hazards can be evaluated on a floor-by-floor basis. These three concepts of segregation, separation and detachment may be applied whenever combustible dust threshold values are exceeded as determined in Section 6.1 of NFPA 654.

Methods Used Toward Controlling Ignition Sources

Open flame. Identify places that can result in ignition, such as areas with open flames. No smoking in these areas is mandatory.

Electrostatics/Friction Generating Mechanics. Investigate any equipment for potential electrostatic discharge (arcing or sparking) and frictional heat, such as blenders, mixers, grinders, pneumatic conveyors, dryers, hoses, silos, hoppers, hoses, loading spouts and any other mechanical or structural equipment that may create friction. Keep the equipment in good repair. Provide grounding, bonding for these pieces of equipment.

Hot Surfaces/Overheating/Chemical Reactions. Maintain hot surface temperatures below 80 percent (in degrees Celsius) of the lower of the dust surface ignition temperature or the dust-cloud ignition temperature per NFPA 654. Look for blocked heating vents of electric motors and similar to avoid any type of overheating. Check for wear on mechanical and electrical equipment (e.g. worn out bearings can overheat). In general keep the equipment in good repair. Ensure that oxidizing chemicals or metals are stored safely and cannot create heat, flames, or sparks during storage and use.

Classified Electrical. When using classified electrical, sparking of electrical wiring and equipment can be avoided. Classifications depend on the properties of the combustible dusts and the likelihood that a flammable or combustible concentration or quantity is present. Areas where hazards of combustible dusts are present are classified as Class II locations. Class II locations are further classified into Division 1 and Division 2 locations depending upon the likelihood of the dust being suspended or dispersed in air. Then combustible dust locations are even further classified into Groups E, F, and G according to the type of combustible dust and its properties. See NFPA 707, National Electrical Code, for more detail.

Maintenance/Repair/Irregular Operations. Maintenance or repair of equipment may add (often unknowingly) a potential ignition source, which is usually not occurring under normal business operations. Therefore it is crucial to assess and prepare these workplaces beforehand and educate any personnel performing unusual or infrequent operations (i.e. maintenance and repairs) about the dangers of combustible dust (NFPA 654).

Applicable Codes And Standards

Achieving compliance requires you to ensure that all applicable codes and standards (Building and Fire Codes and all referenced NFPA standards) for your business are reviewed and properly implemented. The new code provisions (to be published and adopted) will make this process much more straightforward and consistent. Nevertheless, this should not stop anyone from starting this process today. After all a safe workplace is in the interest of everyone.

You may document and provide the gathered information on risk assessments and improvements to mitigate the risks including modified designs, process and procedural changes to the building inspector, Fire Department, or AHJ (Authority Having Jurisdiction) as part of proof for compliance.


If your business is in an industry that is vulnerable to combustible dust fires and explosions, you need to ensure to take the necessary steps in preventing combustible dust from becoming a safety hazard. Dust explosion protection can be economical while ensuring life safety and protect business operations.

We discussed this subject in very general terms while attempting to be as accurate as possible in describing the issues and solutions. However, there needs to be an understanding that we cannot cover all or any of the very specific business situations prevalent. Therefore, if any questions or disagreements on compliance issues arise, readers will consult with qualified experts and not solely rely on this document (disclaimer).

Example Calculations For Combustible Dust (NFPA 654)

  Layer Depth Criterion Method
We would like to investigate the potential of a dust explosion hazard in a distillery. The room dimensions under consideration are 100 ft by 100 ft. The room contains a dry grind process used for creating mash in the distillation process. The grind process uses dried grains with a bulk density of 30 lb/ft3. There is a 1/32 inch thick layer of dust (equivalent thickness of a standard paper clip) covering an approximate floor area of 300 ft2 around the grinder. We will be using the Layer Depth Criterion Method in Section 6.1 in NFPA 654 in order to determine if a dust explosion hazard and dust flash fire hazard exist.

First we will determine the allowable dust layer thickness applying the Layer Depth Criterion Method using Equation in NFPA 654:

(1) Layer Depth Criterion (in) = (1⁄32 in) x (75 lb⁄ ft3 ) divided by 30 (lb⁄ft3 )
     = 0.078 in (~ 1⁄16 in)

(Note: NFPA 654 applies 75 lb⁄ ft3 for the reference bulk density in all dust related calculations).

This means that the allowable combustible dust layer thickness, 1/16 in, based on a grain bulk density of 30 lb⁄ft3, is larger than the measured existing dust layer thickness, 1/32 in. In other words, the actual dust layer thickness is below the threshold value and considered safe, but only IF the area of dust accumulation does not exceed the threshold value associated with the room footprint area.

We will test this requirement by calculating the allowable dust accumulation area (our room is less than 20,000 ft2 and therefore the ‘5%-footprint rule’ applies):

(2) Allowable Area (ft2) = 100 ft × 100 ft × 0.05 = 500 ft2

For final determination of dust deflagration hazards, we will use the ratio test between actual to allowable dust layer properties:

(3) Ratio = (300 ft2 x 1/32 inch) / (500 ft2 x 0.078 inch) = 0.24 ≤ 1

From (3) it is shown that the ratio is less than one, which means that a dust deflagration hazard does not exist in the room per NFPA 654. Ratio (3) can also be applied to a single room or building with areas of multiple dust accumulations. Just sum up the individual actual dust layer properties (area x depth) and divide the sum by the allowable dust layer property (see Annex D, NFPA 654).

It should be mentioned that at times the argument is made that the dust layer under consideration can accumulate in more than 5% of the room footprint area and still not present a dust deflagration hazard. The assumption is that the bulk density of the dust is less than the 75 lb/ft3 used as the basis for defining the threshold value for allowable area in the Layer Depth Criterion method. This argument is confirmed in Annex D of NFPA 654: “A dust with a bulk density less than the basis 75 lb/ft3 can accumulate to 1/32 inch layer depth in more than 5% of the room footprint area and still not present a dust deflagration hazard.” This is also easily demonstrated, if we substitute a higher value for the actual dust accumulation area, say 900 ft2 (instead of the 300 ft2). In this case, Ratio (3) is still less than one; no deflagration hazard exists.

Occasionally it is necessary to relate the existing dust to the permissible total dust volume. This is usually required when significant dust accumulations are found in evenly distributed dust layers, as well as in additional dust piles.

It needs to be demonstrated that the actual dust volume is below its allowable threshold value (NFPA 654, Section Therefore we will calculate the actual volume of the dust dispersed on the floor:

(4) Actual Volume of Dust (ft3) = (1⁄32 in) × (1 ft)/(12 in) × 300 ft2 = 0.78 ft3

If additional dust piles on the floor exist, add their volumes to the result in (4). Then determine the volume of allowable dust per NFPA 654:

Our room is less than 20,000 ft2 and again the ‘cannot exceed 5%-footprint rule’ for non-separated/non-segregated dust accumulation areas applies. We calculate the allowable volume of dust by multiplying the layer depth criterion (1) by 5% of the footprint area (2).

(5) Allowable Volume of Dust (ft3) = (0.078 in) x (100 ft × 100 ft × 0.05) ×
     (1 ft)/(12 in) = 3.25 ft3 (~ 97 lbs)

Finally compare actual dust volumes with allowable dust volumes:

The actual volume of dust (4) is considerably less than the allowable volume of dust (5). As a result, a dust explosion hazard and dust flash fire hazard does not exist per NFPA 654.

Note that housekeeping efforts are still required to maintain dust layer thickness and volume below threshold values. Since this method only addresses fugitive dust hazards, additional dust control and prevention methods need to be applied to avoid deflagration hazards associated with the dry grinding process itself, e.g. generation of dust clouds of hazardous concentration.


[1] Unchecked dust explosions kill, injure hundreds of workers, The Center For Public Integrity,, Washington DC, 2012
[2] Credible Risk, NFPA Journal,, March 2015
[3] A view of the damage caused to the Imperial Sugar refinery at Port Wentworth in Georgia, United States, U.S. Chemical Safety and Hazard Investigation Board, February 2008
[4] NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, National Fire Protection Association, Quincy, MA, 2013
[5] NFPA 652, Standard on Combustible Dusts, Proposed Standard, National Fire Protection Association, Quincy, MA
[6] NFPA 69, Standard on Explosion Prevention Systems, National Fire Protection Association, Quincy, MA, 2014
[7] NFPA 70, National Electrical Code, National Fire Protection Association, Quincy, MA, 2014
[8] NFPA Guide to Combustible Dusts, Walter L. Frank and Samuel A. Rodgers, National Fire Protection Association, Quincy, MA, 2012

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