Traditionally, combustible dusts posing potential deflagration or explosion hazards have been defined as dusts with particle diameters below a specified size (420 or 500 microns). However, this definition has recently disappeared from engineering standards, as it tends to oversimplify the issue. For example, some dusts with particle diameters up to 1000 microns have been found to be explosible through standardized testing methods; other factors such as moisture content can also dramatically affect a dust’s explosivity characteristics. The question is: how should one go about determining if a combustible dust hazard exists?
The first step in determining whether process materials are combustible dusts is performing dust combustibility hazard characterization testing. Where combustible dusts are determined to be present within process equipment or facilities, it does not necessarily mean that all process equipment should be equipped with the highest level of protection available. The reality is that proper protection depends on the quantity and location of the combustible dust, the type of equipment in question, the specific operating conditions at a particular facility, housekeeping, and other process and administrative parameters.
Planning for a dust hazard analysis
The best way to assess the hazards and ensure adequate safeguards are in place is to perform a dust hazard analysis (DHA). Various resources are available on the methodologies available for the performance of DHAs; the industry benchmark is described in NFPA 652, “Standard on the Fundamentals of Combustible Dust.”
The purpose of a DHA is to identify hazards in the process and document how those hazards are being managed. Each part of the combustible dust handling process is considered in a DHA and the specific hazards addressed are fire, flash-fire (i.e., deflagration) and explosion hazards of combustible dusts. A DHA consists of three main parts:
- Material hazard evaluation.
- Equipment hazard evaluation.
- Building hazard evaluation.
This article focuses on the first and most important part of any DHA, the material hazard evaluation. This step identifies the extent to which combustible dusts are present in a given facility and quantifies any important explosibility parameters that can be used in the design of explosion protection features. The characterization of a particular material is accomplished through a series of tests following standardized methods prescribed by the American Society of Testing and Materials (ASTM). The minimum recommended set of tests to characterize dusts are the following:
- ASTM E1226 - Explosibility Screening (Go/No-Go): This common screening test determines whether a material qualifies as an explosible dust. If the result is positive (“go”), additional testing is required to quantify the combustibility hazard.
- ASTM E1226 - Explosion Severity (Pmax and KSt): These parameters describe the maximum overpressure and volume-normalized rate of pressure rise that can be experienced during a deflagration or explosion involving a specific material. These values are used in the design of explosion protection systems.
- ASTM E2019 - Minimum Ignition Energy (MIE): Identifies the minimum amount of energy required to ignite a suspended dust cloud. This test is useful in determining and eliminating credible ignition sources within and proximal to dust-handling equipment.
- ASTM E1515 - Minimum Explosible Concentration (MEC): This test determines the minimum dust concentration required to sustain combustion in a suspended cloud. This value can be used to determine whether sufficient materials are present in a process to result in deflagration hazards under normal and abnormal operating conditions.
- ASTM E1491 - Minimum Ignition Temperature (MIT): This parameter will determine the minimum surface temperature that could ignite a suspended dust cloud. This value is useful in managing equipment temperatures when combustible dust clouds are present.
- ASTM E2021 - Layer Ignition Temperature (LIT): The LIT test determines the minimum ignition temperature of an accumulated layer of dust. This parameter allows the facility to manage dust accumulations on equipment to prevent the equipment itself from serving as an ignition source.
- ASTM E2931 - Limiting Oxygen Concentration (LOC): This parameter is usually only required when designing explosion control systems that maintain the oxygen concentration low enough to avoid combustion.
A sampling plan should be developed that identifies specific points of interest within a process where dust samples should be taken. The sampling plan should also specify which tests should be performed in order to obtain the information required to fully characterize the material hazards at a facility. Only once the material hazards of a process have been identified can hazard zones be defined and hazard mitigation strategies be planned.
Performing a dust hazard analysis
The three most widely recognized and acceptable methods for performing dust hazard analyses are briefly outlined below, and each uses a different level of rigor when applying material hazard properties:
- Prescriptive: A prescriptive DHA uses established engineering codes and standards to perform gap assessment between good engineering practice and existing conditions. This method is most appropriate for simple processes where the dust hazards are well understood.
- Performance-based design (PBD): Performance based design uses performance goals and objectives, engineering analysis and quantitative assessment to develop a performance-driven solution. This method can be appropriate where prescriptive protection measures do not directly align with operational goals of the facility.
- Risk-based: Risk-based analysis is used to estimate the risk associated with a process and determine if the associated risk is acceptable, in accordance with industry and societal accepted levels of exposure. This method is best suited for companies that have many of the same types of equipment operating in similar conditions, since the rigor and engineering effort to develop a risk-based DHA is significantly higher than a prescriptive or performance-based approach.
Performance-based solutions can use material characteristics to demonstrate that although the materials handled are explosible, there is no risk of deflagration or explosion under particular operating conditions and recommend safeguards accordingly. A risk-based methodology estimates the risk present within a piece of equipment using material characteristics and specific operating conditions and compares it to what is considered a tolerable risk level before making recommendations on appropriate safeguards. These methodologies tend to be beneficial for facilities that handle combustible dusts that are harder to ignite and do not pose as great of a deflagration or explosion risk.
In the pulp and paper industry, the materials handled (e.g. paper & cardboard dusts, fly ash, starch, sugar) tend to be less sensitive to ignition and generally have lower explosion severity parameters when compared to other industries such as metals, foods and pharmaceuticals. Thus, the use of prescriptive methods can result in conservative and sometimes impracticable designs. A comprehensive sampling plan and a careful selection of the hazard analysis methodology can result in significant capital and operational cost savings in terms of safeguards and mitigating features against dust explosion hazards, while ensuring that the facility is adequately protected from the hazards that are present.
Luc Cormier, M.Eng., P.Eng. is a project engineer at Jensen Hughes, an engineering firm that specializes in evaluating risks and safety solutions.