Assessing Dangers of Dust Explosions
Dust explosions pose a severe hazard in the workplace, but there many ways to detect danger before a problem occurs.
- By Dr. Ashok Ghose Dastidar
- Feb 01, 2021
Dust explosions are a serious hazard in the process industries. They have led to the destruction of property and damage to facilities and equipment. In the worst circumstances, they may also lead to injury of plant personnel and even fatalities. In 2017 alone, there were 68 globally reported cases of dust explosions with 163 injuries and 13 fatalities. They are not as common as flammable vapor or gas explosions, but they do occur often.
With the potential for such a dangerous workplace hazard, the reader may assume that there are federal and state laws in place protecting worker safety. However, there are currently no laws in place at the state or federal level protecting workers against dust explosion hazards.
Nevertheless, there are consensus standards developed by the National Fire Protection Association (NFPA) that provide detailed guidance for preventing and mitigating dust fires and explosions. These guidelines are considered to be widely effective. Fortunately, the standards are voluntary unless they have been adopted by a municipality or state as local code. The NFPA dust codes are mentioned in the International Fire Code as well as the Uniform Fire Code, which are globally acknowledged model codes that various governments base their own local, state and national fire codes upon.
Due to this, the NFPA dust standards are considered recognized and generally accepted good engineering practices (RAGAGEP). Worker safety organizations and insurance companies then use these standards as safety guidelines for plants and facilities. OSHA used these standards to establish their own Combustible Dust National Emphasis Program (CPL 03-00-008) back in 2008 and have successfully enforced them through the use of the General Duty Clause OSHA Act of 1970 section 5(a)(1). The clause states that “each employer shall furnish to each of his employees' employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.”
Anticipating Dust Incidents
Five factors are required for a dust explosion to occur. Often, these factors are arranged pictorially in a pentagon to facilitate hazard analysis and investigation. Figure 1 depicts an Explosion Pentagon and the five factors that are required for a dust explosion.
Fuel. The material at your facility—either a resource, a product or byproduct—that when present in sufficient quantities can sustain combustion.
Oxygen. This can come from either the air or another chemical oxidizer in unique circumstances.
A credible ignition source. This can be an open flame, electrical/electrostatic spark, hot surface or mechanical spark, etc.
These first three components taken together constitute a Fire Triangle and can be used to look for fire hazards at the facility. It is the addition of the last two factors that elevate the event to an explosion.
A credible mechanism for the dispersion of the dust into a cloud. This facilitates rapid combustion, which increases heat and pressure development from the oxidation/combustion reaction.
Confinement. The enclosure for the suspended dust cloud where pressure, due to rapid combustion, can increase until a violent failure occurs. This final violent failure can be in the form of an exploding dust collector, storage silo, dryer, mixer or even a building.
Industry Specific Standards
There are five industry specific dust standards:
- NFPA 61 is for agricultural and food industries.
- NFPA 664 deals with woodworking facilities.
- NFPA 484 covers metal dust fires and explosions.
- NFPA 655 encompasses sulfur handling industries.
- NFPA 654 is for chemicals, polymers, resins and pharmaceuticals.
Governing these five industry specific standards is NFPA 652, which is a core fundamentals rule that applies to all industries if one of the specific industry standards does not apply to your facility.
There are three objectives for NFPA 652. The first is “Life Safety” to ensure that all persons are protected against the adverse effects of a dust fire or explosion. The second objective is “Mission Continuity,” where continued safe operations of the plant or facility are ensured if a dust fire or explosion were to occur. The third and final objective is “Mitigation of Fire Spread and Explosions,” which not only safeguards the facility against these affects, but prevents them from spreading to adjacent facilities and properties.
As mentioned above, NFPA 652 applies to all industries and to all facilities and operations manufacturing, processing, blending, conveying, repackaging, generating or handling combustible dust. It does not apply to commercial retail or commercially packaged warehousing facilities. So, the facility that makes breakfast cereal is covered, but the supermarket selling the cereal is not. It is the owner/operator of that breakfast cereal facility with the combustible dust who has to determine the combustible and explosible nature of the material handled at the facility.
Most materials that are not pure minerals or salts are combustible or explosible. They are responsible for identifying and assessing any fire, flashfire or explosion hazards and managing those risks. They then should communicate these risks to both employees and contractors. The deadline to comply with these NFPA standards was September 7, 2020, with the exception of agricultural and food facilities, which had until January of 2022.
It is important to note that some of the requirements set forth in NFPA 652 and the other dust standards are not monitory, and parts of facilities or operations may be “grandfathered.” However, much of these standards do apply to existing operations and have to be adhered to for all new operations. A short list of the core parts of the standards that are not “grandfathered” are ignition source control requirements, facility housekeeping to remove hazardous dust level, personnel training, management systems to document hazards, process changes and conducting a Dust Hazards Analysis (DHA) to identify combustible and explosible dust hazards and recommend risk mitigation and engineering protocols.
Dust Hazards Analysis
The mandatory DHA requirement is a tool used to understand your specific materials and the hazards they pose, systematically evaluate your processes for risks, and then develop a roadmap to manage those risks by either administrative controls or engineering controls. This whole process has to be documented, and those documents are retained for the life of your process or facility—whether it be 10 years or 100 years.
The DHA should be conducted in such a way as to classify a given facility location, process step or unit operation as a hazard, potential hazard or not a hazard at all. Then the DHA should recommend process, operation, facility or material changes to mitigate the identified hazard. Given the complex nature of this task and the nuanced nature of the identification/classification of the hazard, the DHA team should be led by a person qualified in the field of combustible and explosible dust hazards, with support from engineering staff, maintenance staff and plant floor operators.
NFPA 652 does not give specifics about the methodology to be used for a DHA, and thus they can be varied. If the facility falls under OSHA’s Process Safety Management requirements of conducting a Process Hazards Analysis (PHA), it may be already using techniques such as a Hazard and Operability Study (HAZOP), a Failure Mode and Effects Analysis (FMEA), Fault Tree Analysis (FTA) or a multitude of other methodologies. It may be appropriate for the facility to incorporate the DHA into the existing PSM program. The rigors of a formal PSM program are a little excessive for a DHA.
NFPA 654 had originally recommended that a PHA be performed on operations handling combustible dusts, but since that lead to confusion with the complex OSHA PSM program requirements, the NFPA technical committee decided to re-term the hazards analysis to DHA and make it mandatory.
Methods of Analysis
Since a very elaborate methodology is not required by the NFPA standard, the approach selected for ease of implementation and adherence can be a simple Checklist Analysis combined with a “What If” investigation. In this approach, the requirements of the NFPA 652 standard are converted into a checklist against which the facility is audited for compliance. The gaps are then noted and plans can be made to add engineered or administrative solutions to fill the variances.
However, checklists and prescriptive mitigation schemes from the NFPA standards cannot deal with all possibilities. This is where supplementing the Checklist Analysis with a What If Analysis, carried out by an experienced team, can fill in the gaps. In short, the What If Analysis postulates various failure scenarios that can occur in the process under normal and abnormal conditions, then the DHA team can formulate corrective measures.
The best way to approach a What If Analysis for a DHA is to use the Explosion Pentagon depicted in Figure 1. For each unit operation, process line, room and building the team creates scenarios where the five components for an explosion, or three components for a fire, can come together to cause an accident. The team can then develop hazard prevention and protection plans.
For this What If Analysis and subsequent hazard prevention and protection plan to be effective, the combustion and explosion potential of the material must be ascertained. The schema present in Figure 2 is a method to follow to effectively characterize the combustion hazard of the material. Combustion characteristics of dusts, unlike gases and vapors, are specific to the sample.
Varying factors like particle size distribution, particle morphology, surface characteristics and moisture content can greatly affect the physical combustion parameters. This prevents the usage of historical data from previous studies of the material unless the sample of the material in question is exactly the same as the sample used in the historical study. Do not trust data from Safety Data Sheets unless there is information provided that the data applies to the specific sample in hand!
Once it has been decided which materials to consider for the DHA, unique samples can be taken for study from various locations in the plant. For example, the sample from the rafters and overhead lights can be different than the material on the floor, dust collector or cyclone. As a result, multiple samples of the materials at the facility may need to be studied.
Once the samples have been taken and uniquely identified, the analysis process moves to Stage 3 where the explosion potential of the sample is qualitatively determined in Stage 3a. The combustion potential of the sample as a pile of layers is qualitatively determined in Stage 3b. For all the samples in Stage 3a and 3b that demonstrate reactivity, further quantitative combustion and explosion qualities are determined in Stage 4.
From Stage 4a, the Explosion Severity, MEC and MIE are the three characteristics that are used in a DHA with the other Stage 4a tests used for unique process situations. In Stage 4b; the Burn Rate Test, LIT, Hot Storage Screening and Exothermic Decomposition Screening tests are more often used with the other tests being performed when identified as warranted in the DHA.
Ultimately, conducting quantitative testing to determine combustion and explosion characteristics of the dusts present at a facility in concert with performing the required DHA from NFPA 652 will be crucial to the reader in assessing the dangers of a dust explosion at a facility.
This article originally appeared in the January/February 2021 issue of Occupational Health & Safety.