Exponent’s Combustible-Dust Testing Laboratory (CDTL) is located at our office in Natick, Massachusetts. A variety of methods are used to characterize the combustibility and explosibility of dusts. Testing performed at the CDTL includes standard ASTM test methods and customized test methods developed by Exponent. Our engineers are members of both the ASTM E27 Hazard Potential of Chemicals committee, which is responsible for many of the ASTM standards for testing combustible dust, and NFPA committees related to the assessment and mitigation of combustible dust hazards. Exponent can aid you in analyzing and mitigating potential combustible dust hazards in your facility.
Brief descriptions of the most common test methods are listed below. Exponent can design testing programs using additional methods in our laboratories or at other testing facilities.
Explosibility and Combustibility Screening Tests
Combustibility Screening Test: A torch is held at one end of a triangular cross-section trough or train of powder for a period of time, to determine whether the sample ignites, and the rate of propagation along the length of the sample. The test is one of two screening tests required by NFPA 484 Standard for Combustible Metals. Testing can be performed based on criteria in NFPA 484 or UN 4.1 test methods.
Explosibility (Go/No-Go) Screening Test: Dust samples of at least two different concentrations are dispersed in a 20-L spherical combustion chamber and exposed to a 5- or 10-kJ ignition source to determine whether the material is explosible (Go), or not explosible (No-Go). If a threshold overpressure is exceeded at any single concentration, the dust is considered to be explosible. Full explosion severity (Pmax/KSt) testing is often performed on dust samples found to be explosible during the Go/No-Go testing. Testing is performed in accordance with ASTM E1226 Standard Test Method for Explosibility of Dust Clouds.
Explosibility Characterization Tests
Explosion Severity (Pmax/KSt) Testing: Dust samples are dispersed in a 20-L spherical combustion chamber and exposed to a 10-kJ ignition source. Multiple tests are performed at increasing concentrations to find the maximum pressure and rate of pressure rise. The Pmax and KSt are used in the design of explosion protection systems such as explosion vents, and in hazard analysis calculations. Testing is performed in accordance with ASTM E1226 Standard Test Method for Explosibility of Dust Clouds.
Minimum Explosible Concentration (MEC) Testing: Dust samples are dispersed and exposed to a 2.5- or 5-kJ ignition source in a 20-L spherical combustion chamber, beginning with a very small dust concentration. The concentration of dust within the chamber is varied incrementally until a threshold explosion overpressure is measured. The MEC is used to evaluate whether dust concentrations in an enclosure are high enough to be ignitable. Testing is performed in accordance with ASTM E1515 Standard Test Method for Minimum Explosible Concentration of Combustible Dusts.
Limiting Oxygen Concentration (LOC) Testing: Dust samples are dispersed and exposed to a 5-kJ ignition source in a 20-L spherical combustion chamber in mixtures of diluent and oxygen. The limiting oxygen concentration occurs at the lowest concentration of oxygen at which a threshold explosion overpressure is exceeded. The LOC is used to determine the required level of inerting in some explosion protection systems. Testing is performed in accordance with the working draft of ASTM 2931 Standard Test Method for Limiting Oxygen (Oxidant) Concentration of Combustible Dust Clouds.
Flash Fire Characterization Tests
Flash Fire Screening Testing: Exponent engineers have developed novel test methods for assessing the potential for unconfined combustible dust deflagrations (flash fires). Current standards rely on confined explosibility testing to determine whether or not a flash fire is a potential hazard. The flash fire screening test is performed by dispersing dust with compressed air into localized dust clouds and subjecting it to various ignition sources including spark igniters, chemical igniters, or flames.
Ignitability and Self Heating Tests
Hot-Surface Ignition Temperature: A layer of dust of specific thickness is placed on a hot surface, and the surface temperature is varied in multiple tests to determine the minimum surface temperature at which the dust layer ignites or rises above a threshold temperature. Tests can be performed at multiple thicknesses as the ignition temperature generally decreases with increasing dust-layer thickness. The data can be used to determine maximum allowable surface temperatures of equipment where dust layers may be present. Testing is performed in accordance with ASTM 2021 Standard Test Method for Hot-Surface Ignition Temperature of Dust Layers.
Dust-Cloud Minimum Autoignition Temperature (MAIT) Test: Dust clouds are dispersed in a cylindrical furnace to determine the minimum temperature at which the dust cloud will self-ignite without an ignition source present. The MAIT is used to determine safe operating temperatures where dust clouds may be present in heated equipment such as ovens and dryers. Testing is performed in accordance with ASTM E1491 Standard Test Method for Minimum Autoignition Temperature of Dust Clouds.
Differential Scanning Calorimetry and Thermogravimetric Analysis (DSC/TGA): DSC/TGA is performed on small dust samples to determine the approximate onset ignition temperature and heat of reaction of materials. The weight loss or gain of material can also be used to determine the fraction of the sample that is reactive (or combustible).
Accelerating-Rate Calorimetery (ARC): ARC testing is used to determine the onset (ignition) temperature for self-heating of materials. The apparatus approximates an adiabatic condition that is more representative of self-heating at industrial scales than the DSC/TGA testing. The onset temperatures can be used to determine temperatures at which self-heating and ignition may occur in storage or heated process equipment like ovens or dryers.
Oven Tests: Exponent performs a variety of oven tests using different sized samples and controlled air- flow conditions to determine onset (ignition) temperatures of self-heating in various processes or equipment.
Particle Size and Morphology Analysis Sieve Analysis:
A series of sieve screens with different-size openings is used on a sieve shaker to analyze the particle size distribution of the material. The sieves can also be used to screen out specific size fractions of material for testing.
Size Reduction: Ball mills, grinders, and other equipment are used to reduce the size of particles when testing of a smaller particle size fraction is desired.
Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM/EDS): SEM/EDS can be used to examine individual particles for determination of their size, shape, morphology, and elemental composition. Exponent has also used SEM/EDS as a tool to examine post-explosion residues to determine whether the material served as a fuel in the explosion.
For more information about Exponent’s Combustible-Dust Testing capabilities, or to inquire about testing your material, please contact us at