Dust explosion testing: combustible dust testing jargon buster

Understanding the flammability characteristics and properties of combustible dusts is crucial for preventing fire and explosions in an industrial setting. To ensure the safety of workers and facilities, various tests – based upon national and international standards – have been developed.

The following list provides a brief overview of the various jargon, tests, and measurements used to assess combustible dusts. It should serve as a valuable resource for professionals involved in dust hazard management and fire and explosion mitigation.

A/B classification test

This determines whether a dust cloud in air can present an explosion hazard. It is a purely qualitative test and will derive a result of either combustible (Group A) or non-combustible (Group B).

Minimum ignition energy (MIE, dust cloud)

MIE is critical for assessing potential incendivities of electrostatic ignition sources. MIE by the purely capacitive method is used for electrostatic ignition hazard assessment, whereas the inclusion of a 1mH inductor in the earth circuit, allows for the assessment of ignition from mechanical sparks caused by grinding, impact, and friction.

Minimum ignition temperature (MIT, dust cloud)

MIT, in conjunction with layer ignition temperature, is used to select equipment with the correct surface temperature range for use in hazardous areas. MIT (BAM) data is used in conjunction with MIE (inductive) to assess whether frictional, grinding, and impact sparks could ignite combustible dust clouds.

Layer ignition temperature (LIT)

LIT tests determine the minimum temperature a surface reaches that will result in decomposition and/or ignition of, typically, a 5mm depth dust layer. LIT results are used in combination with MIT to determine the maximum surface temperature of ATEX compliant equipment within a hazardous area.

Explosion severity

Maximum overpressure (P_max) and dust constant (K_St) tests assess the explosion severity properties of explosions initiated at ambient pressure and temperature. These parameters are used to help design new and verify existing explosion protection systems. Explosion classes from K_St values are as follows:

Dust explosion class	K_St(bar m s^-1)	Explosion characteristics
St 1	> 0 to 200	Weak^[1] to moderately strong explosion
St 2	> 200 to < 300	Strong explosion
St 3	> 300	Very strong explosion

^[1] The term “Weak” is a misnomer. St 1 explosions are capable of destroying plants and buildings, and endangering people.

Limiting oxygen concentration (LOC)

LOC testing is needed if preventing dust ignition is to be achieved by inerting. At the LOC, there is insufficient oxygen to permit ignition or support combustion. LOC is used to determine the maximum permissible oxygen concentration (MPOC) within inerted vessels.

Minimum explosible concentration (MEC)

MEC tests are used to determine if there is sufficient powder to form flammable concentrations. However, as dust clouds are non-homogenous, care needs to be taken when relying on MEC as a Basis of Safety.

Burning behaviour (BZ)

The BZ assesses whether powder layers can propagate flame or smouldering. Powders are assigned from BZ 1 (no ignition) to BZ 6 (very rapid combustion). Test results provide a good insight into the risk of fire spread associated with layered dusts.

Powder volume resistivity and charge relaxation time

These tests examine the physical properties of bulk powders to determine whether they are electrically insulating, and their propensity to accumulate and retain hazardous electrostatic charge for prolonged periods. This data is important if the MIE of the dust is very low and there is a high charging mechanism, such as pneumatic conveying.

Thermal stability

Multiple laboratory scale tests have been developed to study exothermic decompositions of powdered solids. These tests vary according to scale, air availability, and geometry of the processed powder. Selecting the most appropriate test method is critical for obtaining meaningful results.

Bulk powder (diffusion) cell tests allow only limited natural diffusion of air through the powder — simulating conditions in a bulk powder dryer where the material slumps to the base in the form of a heap.
Aerated powder cell tests examine the thermal stability when pre-heated air is passed through the substance under test simulating fluidised powder conditions within a dryer.
Air over (powder) layer tests are specifically designed to examine the thermal stability of thin powder layers (up to 15mm depth) exposed to air. In this test, pre-heated air is passed over the powder simulating conditions such as those within a tray dryer.
Isothermal basket tests are a series of thermal stability tests conducted in steel aperture mesh. Tests can use cubic test cells, of various sizes, to provide thermal stability information that can be directly extrapolated to large-scale conditions, such as bulk silo storage facilities.
Differential scanning calorimetry (DSC) determines the onset temperature of energetic events and the total energy associated with those events. It uses a very small quantity of material and is useful in obtaining thermal stability data for high value substances.

For further information on dust testing and interpreting test data, reach out to your usual Marsh contact.

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Article

Jargon buster – combustible dust testing

A/B classification test

Minimum ignition energy (MIE, dust cloud)

Minimum ignition temperature (MIT, dust cloud)

Layer ignition temperature (LIT)

Explosion severity

Limiting oxygen concentration (LOC)

Minimum explosible concentration (MEC)

Burning behaviour (BZ)

Powder volume resistivity and charge relaxation time

Thermal stability

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Jargon buster – combustible dust testing

A/B classification test

Minimum ignition energy (MIE, dust cloud)

Minimum ignition temperature (MIT, dust cloud)

Layer ignition temperature (LIT)

Explosion severity

Limiting oxygen concentration (LOC)

Minimum explosible concentration (MEC)

Burning behaviour (BZ)

Powder volume resistivity and charge relaxation time

Thermal stability

Related insights

Jargon buster: Flammable gas and vapour testing

The importance of selecting credible over generic flammability test data

Letters of Credit: Challenges to insurer collateral requirements

Managing risk in a volatile business environment: The role of integrated risk quantification

Related insights

Jargon buster: Flammable gas and vapour testing

The importance of selecting credible over generic flammability test data

Letters of Credit: Challenges to insurer collateral requirements

Managing risk in a volatile business environment: The role of integrated risk quantification