Controlling Combustible Dust
A combustible dust explosion can be catastrophic and cause serious injuries, loss of limbs, property damage and even death. In many combustible dust incidents, employers and employees were either unaware of the potential threat of dust explosions or failed to recognize the hazard within their own organization.
Combustible dust is defined as a solid combustible material, composed of distinct pieces or particles, 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. It can be either an organic material or metal dust that is finely ground into very small particles, fibers, fines, chips, chunks, or flakes.
Five elements are required to initiate a dust explosion: a fuel source (combustible dust), oxygen, ignition source, dispersion of dust particles, and confinement of the dust cloud.
Foundries hold potential risks of catastrophic events associated with combustible dust. Knowing this, one company set out to evaluate its risks. Understanding its limitations, the company contracted a consultant to assist with the assessment.
The scope of the project included:
- Identifying processes, operations and locations where potentially combustible dusts may be generated or accumulated.
- Conducting sampling and analysis to characterize the combustible properties of the dust from different processes and operations.
- Developing and implementing a process that personnel at all facilities can use to assess the combustible dust situations at their own locations.
- Developing recommendations for corrective actions needed to mitigate any identified combustible dust hazards.
The company divided the project into five phases, beginning with a pilot study. Following the pilot study, the company developed a performance-based process and then implemented the program. In Phase 4, control measures were implemented, and the final phase resulted in a corporate combustible dust program.
Phase 1: Pilot Study
With assistance from the consultant, site surveys and dust explosive tests were completed at five sites. These sites represented the different business units within the company. The initial surveys provided the baseline information used to characterize the combustible dust risks for the rest of the other manufacturing locations within the company.
For the study, 90 dust samples were collected and then analyzed by a certified laboratory. Of the 90, 49 were considered explosible, identifying a potential risk that led to the development of the company’s performance-based management guidelines. The remaining 41 were not ignitable and therefore neither classified as combustible dusts nor deemed a risk.
Phase 2: Program Development
To effectively and efficiently evaluate the combustible dust risks throughout the entire company, a process was needed for existing personnel with limited expertise to identify combustible dust, complete the combustible dust risk assessment, determine proper corrective actions, and implement the appropriate controls in their own facilities. This led to the development of “The Performance Based Guidelines for Managing Combustible Dust Fire and Explosion Hazards.”
The guidelines included a training module for site personnel, a procedure for the sites to conduct dust surveys, resources to characterize the potential risks of the dust, and decision trees to help determine what mitigating controls were needed. The decision trees covered dust sample explosibility, dust collector explosion protection, explosion isolation, and dust collector explosion venting. An example is shown in Figure 1.
Phase 3: Program Implementation
After the plant personnel were trained, the company evaluated all other facilities that were not included in the original pilot study. The other facilities were surveyed, additional dust samples were collected and tested, and the dust collect systems were evaluated with the decision trees and dust loading calculations.
Once the data was collected, it was vetted and refined. It was also used to assure that an accurate comparison of the previously collected dust data was done.
During Phase 3, the company discovered most of the combustible dust concerns were already being addressed through existing engineering controls or current housekeeping practices. However, many of the existing housekeeping practices were informal and not on a regimented schedule. The housekeeping practices are now documented in the facilities’ combustible program.
Phase 4: Establish and Implement Controls
After completing the site surveys, risk assessments and data analysis using the decision trees, the individual sites developed specific action plans. The use of the decision trees and the dust loading calculations helped with developing the most efficient and cost-effective control strategies on a site-by-site basis.
Phase 5: Develop a Corporate Combustible Dust Program
To sustain the program and address any potential new combustible dust risks that may be introduced into the company, it started a corporate combustible dust program containing the following:
- Site-specific list of combustible dust hazards and current control measures.
- A powder spray paint compliance checklist and audit process per the requirements of NFPA 33.
- Requirements for each site to perform an annual physical review of potential combustible dust hazards using the processes outlined in the performance-based guidelines.
- A process to review any new material or process or modification of existing processes that may introduce or change the status of the existing combustible dust risks at a site.
- An audit process for the overall program and specific measures to audit existing combustible dust controls, including combustible dust housekeeping plans and engineering controls.
Findings and Corrective Actions
The pilot study and the other self-surveys identified a total of 309 dust concerns.
Of the 309 dust concerns, 201 were ruled out as not combustible dust hazards. The remaining 108 dusts were classified as combustible or possible combustible dusts. Of those 108, 17 were from powder coating paint operations, which falls under the requirements of NFPA 33. That left 91 possible combustible dusts. According to the NFPA 499 Explosivity Index Table, 75 of the 91 dusts were considered Weak, 12 were classified as Moderate, four were considered Strong and none were Severe (Table 1 on page 32).
Even though most of the dusts had a low propensity to cause a combustible dust explosion, appropriate hazard controls still had to be implemented.
At the sites, almost all combustible dust risks are controlled through engineering dust collection systems and/or housekeeping initiatives. The performances of the dust collection systems were verified by dust loading calculations and maintained with consistent preventive maintenance programs.
The fugitive combustible dust is being addressed with strong housekeeping programs.
The decision to develop and use the Performance-Based Process to complete the thorough evaluation of the combustible dusts at the remaining U.S. sites proved to be very effective and provided a significant cost savings compared to sampling and analyzing dusts at every single location. Even more significant was the use of the performance-based process to determine the effectiveness of the dust collection systems in controlling the combustible dust hazards.
The outcome of this study has also provided a valuable model to predict future potential combustible dust risks and controls at other locations outside the U.S.