Editor’s note: This is part one of a two-part series focused on dust hazard analysis. In this article, we focus on the dust hazard analysis process. In our next issue, we will take a look at how to put the analysis into practice at your food plant.
Confused about the dust hazard analysis (DHA) process? You’re not alone. Many bakers and food processors have questions about DHA requirements. Here’s what you need to know and how to get it done.
Why Do You Need a Dust Hazard Analysis?
A DHA is required for facilities that handle combustible dust, which includes most food processing facilities. Dry food dust—including dust from sugar, flour, starches, cocoa powder, dry spices and flavorings, dehydrated milk products, and dust from processing grains and nuts—is combustible under the right conditions. These conditions include:
- Suspension of dust in the air in a cloud;
- Confinement of the dust cloud in an enclosed space, such as a storage silo, enclosed conveyor system or mixer, or dust collection system);
- Oxygen to fuel a combustion reaction (e.g., oxygen found in atmospheric air); and
- An ignition source, such as an open flame or high heat from ovens, sparks from friction in mechanical systems or conveyors, or static electricity.
Under the National Fire Protection Association (NFPA) standard 61, “Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities,” Chapter 188.8.131.52, a DHA is required for bakeries and food processing facilities every five years. A new analysis may also be required if the facility introduces new dust types, processes, or equipment that will substantially change the risk profile.
The DHA Process
NFPA 61 requires that “the DHA shall be led by a qualified person.” For most food processors and bakeries, that will mean getting outside help from an expert to complete the process. The organization does not mandate a specific format for the DHA but, in general, the process will include the following:
- Material characterization;
- Process characterization and hazard identification;
- Evaluation of existing safeguards;
- Mitigation recommendations; and
The first step in the DHA process is to determine the material characteristics of the dust. In some cases, it may be possible to use published industry values for your dust type; however, your dust must be substantially similar to the dust used for comparison in the published values. The explosibility of dust is dependent not only on its chemical composition but on factors such as particle size distribution, particle morphology, moisture content, and other variables. For this reason, it is usually advised that facilities collect a sample of their specific dust for analysis. The sample must be sent to an independent accredited laboratory (ISO17025 Accredited Lab or Calibration Round Robin Lab) and tested using NFPA-approved analytical processes covered by the accreditation. You can find an accredited laboratory at dustsafetyprofessionals.com.
Testing may include all or some of the following:
- Go/no-go testing: A simple screening test to determine whether dust will ignite in a pile and/or explode in a cloud.
- Explosion severity testing: These tests are conducted in a pressure vessel to determine the explosion indices, measures of how severe an explosion would be if one were to occur. These indices include KST (the speed of pressure rise) and Pmax (the maximum pressure rise in a closed vessel).
- Additional explosion testing: Other testing may include indices such as minimum explosive concentration (MEC), minimum ignition energy (MIE), and minimum ignition temperature (MIT). These values provide insight into the specific conditions under which an explosion is likely to occur.
Process Characterization and Hazard Identification
The DHA will also include measurement of pre-mitigation conditions, analysis of the processes in the facility, and identification of specific hazards. For example, it can pinpoint:
- Where dust clouds tend to form (e.g., places where dust is disturbed);
- Where dust accumulates on surfaces;
- Where dust clouds are under confinement (e.g., enclosed conveyor systems, silos, dust collectors); and
- What potential sources of dust ignition exist.
Evaluating Existing Safeguards
The DHA should note the safeguards that are already in place and their effectiveness in reducing a combustible dust explosion risk. These may include:
- Housekeeping practices (e.g., wet or dry sanitation type and frequency);
- Administrative controls (e.g., training programs, access limits, hazard communication);
- Engineering controls (e.g., dust collection); and
- Safety systems (e.g., deflagration systems, fire breaks, fire suppression/sprinkler systems).
The DHA will include a set of recommendations specific to the facility. These recommendations will address the hazards identified in the DHA; they may include updates to existing safeguards as well as new recommendations. Examples could include:
- New housekeeping procedures;
- Upgrades to dust collection systems;
- Changes to process controls (e.g., changing the operating parameters of conveyance systems or mixers to reduce dust cloud formation);
- Removal of enclosures that create dangerous confinement of dust; and
- Removal of ignition sources.
The final step of the DHA should include measurements to determine whether the mitigations have been effective. Specifically, have levels of dust been reduced on surfaces and in the air and have hazards identified in the DHA been removed?
Collecting Your Dust Sample for a DHA
When preparing for laboratory testing of combustible dust, the dust must be collected in accordance with NFPA 652 Chapter 5.5. This document outlines procedures for safe collection of a representative dust sample. Some things to keep in mind:
- Contact your laboratory for specific requirements for collecting, storing, and shipping your dust sample. Sample size requirements may vary by laboratory and test type.
- Dust samples must be representative of the dust present in your facility. If you have different types and levels of dust in different places, you may want to collect multiple samples from different locations. Elevated surface samples will identify hazards of dust accumulation in the facility. Raw material and final product samples provide a baseline for understanding hazards related to material unloading and conveyance or packaging.
- Be careful not to introduce new hazards while collecting the sample. Samples should be collected without introducing an ignition source or dispersing dust into the air. Use non-sparking equipment such as plastic antistatic shovels and natural bristle brushes when collecting dust. When collecting samples from elevated locations, be sure to follow all safety guidelines for working at heights.
- Take care to preserve sample integrity when collecting and storing the dust sample to avoid the introduction of contaminants that could confound the investigation. Samples should be collected with clean equipment and placed in a clean plastic bag or non-conductive container.
- Consult the U.S. Department of Transportation’s hazardous materials regulations prior to shipping your sample and take all required shipping precautions.
These dust sample collection guidelines provide additional information for NFPA 652 compliance for sample collection, types of laboratory testing, and tips on finding an accredited laboratory for combustible dust testing.
Dust Hazard Analysis Outcomes
The DHA should be considered a living document. While completion of a DHA is required for compliance with Occupational Safety and Health Administration regulations and NFPA guidelines, it should be the first step in an ongoing combustible dust safety plan.
The DHA will help you determine risks associated with your specific dust, identify areas of your facility in which specific combustible dust hazards exist, and make effective recommendations for the mitigation of combustible dust risks.
Once you understand your current state, risk profile, and mitigation options, it is time to put the recommendations into place and conduct validation testing to determine whether goals have been met. Risk assessment and validation should be an ongoing process. While the DHA is only required to be reevaluated every five years, facilities should take steps to ensure continued compliance and improvement. This includes:
- Monitoring airborne and surface dust levels;
- Ensuring compliance with administrative controls and housekeeping procedures;
- Maintaining safe operating limits for equipment as outlined by the DHA;
- Evaluating any changes in equipment, procedures, or processes to ensure that new hazards are not introduced;
- Training all employees in the hazards of combustible food dust and ensuring that they have job-specific training for their areas of responsibility; and
- Updating the DHA when significant changes to materials or processes have been made.
Few bakers and food processors have the necessary skill sets on staff to conduct a formal DHA and make mitigation recommendations. It is usually advisable to work with a qualified engineering partner when conducting the DHA and implementing recommendations.
Yinger is director of engineering at RoboVent. Reach her at [email protected]. For more information about the DHA process, view the Visual Guide to Combustible Dust, available at robovent.com.
Image 1: EXPLOSION PENTAGON
CAPTION: All five elements of the Explosion Pentagon must be present for a combustible dust explosion to occur.
Credit: Courtesy of RoboVent.
Image 2: STATIC-FREE CONTAINER
CAPTION: Collect dust samples in a clean, static-free container and follow all laboratory guidelines for storing and shipping your dust.
Credit: Courtesy of RoboVent.
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