In industries like manufacturing, food processing, pharmaceuticals, wood, mining etc., combustible dust poses a significant hazard. To prevent catastrophic incidents like dust explosions flashfires and fires, comprehensive testing is crucial. These tests assess the explosibility and flammability of various dust types, identifying materials prone to explosion or fire. This knowledge helps implement safety protocols such as ventilation, dust control, explosion prevention and protection as well as ignition source management. Regulatory bodies like OSHA and NFPA provide guidelines for handling combustible dust, and testing ensures compliance. Testing results also influence equipment design and maintenance in dust-prone industries, minimizing ignition risks.
Powder dust fire and explosion properties vary significantly from material to material and from type to type of the same material (depending on the handling process) and are strongly dependent on powder dust moisture content, Particle size distribution, and particle shape. The finest and driest particles will normally exhibit greater ignition sensitivity and explosion violence.
NFPA 652 (Chapter 5) requires that a determination be made about the explosibility of powders/dust that could be handled/processed or otherwise generated within the plant. If the powders/dust are determined to be combustible, additional testing should be performed to acquire the data necessary to support the Dust Hazard Analysis (DHA). The Annex of NFPA 652 lists important tests that should be considered. These are;
- Go/No Go Explosibility Screening
- Burn Rate/Fire Train Test
- Minimum Ignition Energy (MIE)
- Minimum Autoignition Temperature-Cloud (MAIT – Cloud)
- Layer Ignition Temperature of Dust (LIT)
- Minimum Explosible Concentration (MEC)/Lower Explosible Limit (LEL)
- Dust Explosion Severity (Kst/Pmax/dP/dt)
- Limiting Oxygen Concentration (LOC) Test
Go/No Go Explosibility Screening
The Go/No-Go explosibility testing of combustible dust is a standardized screening procedure used to assess the propensity of a dust cloud to cause an explosion under specific conditions. This test aims to determine whether a particular dust possesses the potential for explosiveness or if it remains non-explosive under defined circumstances.
The test derives its name from the binary outcome it provides; either a “Go” indicating that the dust sample has the potential to cause an explosion, or a “No-Go” indicating that the dust sample is non-explosible within the test parameters. Â The Go/No-Go explosibility testing is conducted in accordance with the American Standard Testing Method (ASTM) E1226.
Data Interpretation and Applications
The material is deemed explosible if the pressure ratio generated in the 20-liter sphere is equal to or greater than 2.0. or when Pmax value is equal to or greater than 0.5 barG. The results of the screening test may prevent the need to subject the material to dust explosion dispersive testing and can still be used in the explosible hazard assessment of the material. Overall, the application of Go/No-Go explosibility testing spans various aspects of safety, material handling, process optimization, compliance, and risk management in industries where combustible dust is present. It serves as a fundamental tool in evaluating the potential for dust explosions and implementing measures to ensure workplace safety.
Burn Rate / Combustibility Screening Test
A material that is non-explosible does not necessarily mean that it is non-combustible. A material can burn vigorously to pose a hazardous situation to any processing plant. The Burn rate/fire train test is used to determine the burning or combustibility characteristics of a substance and to classify the substance into the appropriate packing group. Burn rate testing is used to measure the rate at which a substance, typically a solid propellant or combustible material, burns or undergoes combustion under controlled conditions. This testing process is crucial in various industries, particularly in aerospace, pyrotechnics, and ammunition manufacturing, where the performance and safety of materials are paramount. The test is conducted according to the UN/DOT, section 33 ‘’Classification Procedures, Test Methods and Criteria relating to class 4’’ and VDI 2263.
Data Interpretation and Applications
If the material does not ignite or smolder and propagates the flame to the 200 mm mark within the test period of 2 minutes or 5 minutes, the material will not be classified as Division 4.1. If the material ignites or smolders and propagates the flame to the 200 mm mark within the test period of 2 minutes or 5 minutes, the material will be classified as Division 4.1 and be subject to the full burn rate test to classify the material into the appropriate packing group. The result of the test is used in classifying the packing group of materials during transportation. Knowing the burning or combustibility characteristics of materials also helps to mitigate flash fire hazards arising from introduction of hot embers and heat generated from friction. The burn rate test for combustible materials also holds significant importance in various industries, particularly in aerospace, pyrotechnics, manufacturing, and other fields dealing with propellants, explosives, and energetic materials. Based on the reaction type, the material will also be classified into the appropriate combustibility class as indicated in the chart below
Dust Explosion Severity (Kst/Pmax/dP/dt)
The dust explosion severity test is performed to determine the degree of “explosibility’’ of a dust cloud characterized by the dust explosibility parameters, maximum explosion pressure, Pmax; maximum rate of pressure rise, (dP/dt)max; and explosibility index, Kst, under specified operating conditions. Kst is the rate of pressure increase during a dust explosion event. Pmax provides insight into the maximum achievable pressure generated during a dust explosion event.
The Kst/Pmax test is conducted by the American Standard Testing Method (ASTM) E1226 and BS EN 14034 parts 1 & 2.
Data Interpretation and Applications
The material is classified as St-1 (Kst between 1-200 bar.m/s), St-2 (Kst between 200-300 bar.m/s) and St-3 (Kst equal to or greater than 300 bar.m/s). St-1 rated materials have weak explosion characteristics while St-2 and St-3 rated materials have strong and very strong explosion characteristics respectively. The significance or application of the Combustible Dust Cloud Explosion Severity Test lies in its crucial role in assessing and understanding the potential risks associated with combustible dust clouds. This test provides valuable information about the severity and explosibility of dust clouds, offering critical insights for industries dealing with combustible dust. The results of this test can be used to design deflagration containment, venting and suppression systems. These values are applicable to the design of protective measures, such as deflagration venting per NFPA 68, VDI Method 3673 or ISO Method 6184.
Minimum Explosible Concentration (MEC)/Lower Explosible Limit (LEL)
The minimum explosible concentration (MEC) is conducted to determine the minimum concentration of dust-air mixture that will propagate a deflagration in a closed system such as 20L Sphere. It is the lowest concentration of combustible dust in the air that is capable of producing a dust explosion if ignited. It represents the lower limit at which a dust-air mixture becomes capable of sustaining combustion or explosion. The MEC is a critical threshold concentration below which the dust-air mixture is too lean (below the lower explosive limit – LEL) to support sustained combustion or explosion. However, once the concentration surpasses the MEC, it becomes capable of sustaining a combustion reaction if ignited.
The MEC test is conducted in accordance with the American Standard Testing Method (ASTM) E1515. ‘’Standard Test Method for Minimum Explosible Concentration of Combustible Dusts’ ‘and BS EN 14034 part 3.
Data Interpretation and Applications
The MEC of the material is determined from the pressure ratio generated in the 20L Sphere chamber. When the pressure ratio in the sphere at any dust-air mixture is 2.0 or greater, the dust is deemed to have ignited. This concentration must be repeatable as well as the non-ignition dust-air mixture concentration. The MEC is established at a concentration below which ignition of the dust is not possible. The determination of the Minimum Explosible Concentration (MEC) of combustible dust is recommended in various scenarios to assess the potential hazards associated with dust explosibility. The MEC is particularly important during Risk Assessment and Hazard Analysis, Safety Protocols and Preventive Measures, Process Optimization and Equipment Design, Regulatory Compliance Training and Awareness, and Incident Investigation and Prevention.
Limiting Oxygen Concentration (LOC) Test
The Limiting Oxygen Concentration (LOC) for combustible dust refers to the minimum oxygen level in the air at which a dust explosion or combustion involving the dust cannot be sustained, even in the presence of an ignition source or sufficient fuel. It represents the lower boundary below which the dust-air mixture is unable to support combustion or explosion.
The LOC test is conducted in accordance with the American Standard Testing Method (ASTM) E1291. ‘’Standard Test Method for Limiting Oxygen (Oxidant) Concentration of Combustible Dust Clouds’’ and BS EN 14034 part 4.
Data interpretation and Applications
The LOC of the material is determined from the pressure ratio generated in the Sphere. When the pressure ratio in the sphere at any dust-air mixture is 2.0 or greater or the Pmax is greater than 0.5 barG, the dust is deemed to have ignited. The lowest concentration of oxygen below which ignition of the dust is not possible is deemed the limiting oxygen concentration. The result of this test is used in the design of nitrogen inerting systems to ensure that concentration of oxygen during the process is kept below the threshold that can allow deflagration to occur. The Limiting Oxygen Concentration (LOC) of combustible dust holds significant importance in assessing and managing the risks associated with dust-related fire and explosion hazards in various industries.
Determining the Limiting Oxygen Concentration (LOC) is essential in various scenarios, especially in environments where flammable materials, including combustible dust, are present. When inerting is used as a measure to prevent dust explosions, it is recommended to establish the limiting oxygen concentration to understand the highest oxygen concentration that can be allowed in the process. Performing Limiting Oxygen Concentration (LOC) assessments at appropriate intervals, especially during new material introductions, regulatory compliance checks, and incident reviews, helps in ensuring workplace safety and preventing potential fire or explosion hazards associated with combustible materials.
The minimum ignition energy is the lowest voltage spark that is capable of igniting a dust cloud at its most easily ignitable concentration in the air. A capacitive-generated spark is used in place of processing plant sparks such as propagating brush discharges, brushes, and mechanical and electrical sparks.
The MIE test is conducted in accordance with the American Standard Testing Method (ASTM) E2019. ‘’Standard Test Method for Minimum Ignition Energy of a Dust Cloud in Air’’. This link presents a video a MIE test at Prime Process Safety Center Lab – https://shorturl.at/disEG
Data Interpretation and Applications
The MIE is determined at the lowest possible voltage spark capable of igniting the material at its most ignitable concentration. Materials with MIE < 1000mJ are sensitive to ignition by electrostatic discharges, materials with MIE < 30mJ are very sensitive to ignition by electrostatic discharges and materials with MIE < 3mJ are extremely sensitive to ignition by electrostatic discharges. Materials with MIE > 1000mJ are considered to be not sensitive to ignition by electrostatic discharges.
MIE is performed when the assessment of the likelihood of ignition during powder handling is required and has specific application for the assessment and control of electrostatic hazards. Whenever the avoidance of an Ignition Source is a basis of safety for the operation and handling of materials in a processing plant, the minimum ignition energy must be determined. The result is used to determine how sensitive the materials are to ignition sources such as electrostatic discharge and mechanical sparks.
Minimum Autoignition Temperature-Cloud (MAIT – Cloud)
The Minimum ignition temperature (MAIT) of a dust cloud stands as a pivotal parameter in assessing the propensity for spontaneous combustion within environments where airborne combustible dust particles are prevalent. This temperature represents the lowest threshold at which a dust-air mixture, suspended in the atmosphere, can ignite spontaneously without the need for an external ignition source. The test is used to assess the maximum operating temperature for electrical and non-electrical equipment used in areas where test material is present. This link presents a video an MIT dust cloud test at Prime Process Safety Center Lab:Â https://shorturl.at/gvMU3
The MAIT test is conducted by the American Standard Testing Method (ASTM) E1491. ‘Standard Test Method for Minimum Autoignition Temperature of Dust Clouds’’
Data Interpretation and Applications
Materials with MAIT values between 400ᣞC and 500ᣞC are common for most organic solids and some metal powders. Other organic solids such as sulfur powder may be very sensitive to hot surface temperature and can have MAIT values of less than 250ᣞC. Factors such as the composition, particle size, moisture content, and specific characteristics of the dust significantly influence this critical ignition temperature. Industries handling combustible dust rely on the knowledge of the MAIT values to design operational controls, select appropriate equipment, and establish safe temperature thresholds to minimize the risk of spontaneous ignition and prevent fire hazards associated with combustible dust clouds.
Layer Ignition Temperature of Dust (LIT)
The Layer of Ignition Temperature or the Minimum Ignition Temperature of a dust layer is the lowest temperature at which the dust layer is capable of autoigniting on a hot surface. The test is used to assess the maximum operating temperature for electrical and non-electrical equipment used in areas where test material is present.
The LIT test is conducted in accordance with the American Standard Testing Method (ASTM) E2021. ‘’Standard Test Method for Hot-Surface Ignition Temperature of Dust Layers’’
Data Interpretation and Applications
Materials with LIT values between 300ᣞC and 400ᣞC are common for most organic solids and some metal powders. Other organic solids such as Lycopodium powder may be very sensitive to hot surface temperatures and can have LIT values less than 250ᣞC. The results obtained from the Minimum Temperature of a dust layer test are used to assess the susceptibility of a dust layer to ignition by heated environments, such as hot surfaces. It is also used in the selection of suitable electrical equipment for Class II locations.
LIT is performed when the assessment of the likelihood of ignition during powder handling is required and has a specific application for the assessment and control of hot surface ignition of a dust layer. LIT is an important parameter used in assessing the fire risks associated with combustible dust layers and is essential in establishing safe handling practices and storage conditions to prevent potential dust-related fires
Conclusion
Testing is a fundamental pillar of Prime-PSC’s commitment to safety excellence. Our extensive testing services encompass a wide array of critical areas, ensuring that organizations across diverse industries can effectively manage and mitigate potential risks. Dust Explosion/Fire Testing is a specialized service focusing on assessing and addressing the risks associated with combustible dust. Prime-PSC conducts rigorous tests to analyze the flammability and explosion characteristics of dust particles, crucial for maintaining a safe and compliant workplace.