Combustible dust analysis

A dust explosion may occur when an ignitable concentration of fine dust particles dispersed in air comes into contact with an ignition source, which can be a hot surface, static electricity, self-heating material or a naked flame. Determination of explosive characteristics for dusts is vital for many industries to comply with the user directive 1999/92/EC, linked to ATEX. The parameters achieved through testing can be used to dimension vent sizing, to specify maximum surface temperature that may be achieved on equipment to be installed and much more.

RISE offers testing of nine different explosive parameters which can be used to determine the explosive characteristics of dust, RISE also provides sample preparation and chemical analysis of both organic and inorganic materials.

Screening test: ISO/IEC 80079-20-2

A yes/no method to determine if the dust can form an explosive atmosphere or not. If explosion occurs further testing is recommended.

Minimum Ignition Energy (MIE): ISO/IEC 80079-20-2, EN 13821

The minimum energy required to ignite an explosive atmosphere. Testing can be performed without increased inductance to simulate common electrostatic discharges or with inductance corresponding to e.g. mechanical sparks or sparks generated by short circuits.

Minimum Ignition Temperature of dust cloud (MIT dc): ISO/IEC 80079-20-2, EN 50281-2-1

The lowest temperature of a hot surface which will ignite an explosive atmosphere. The test measures at which temperature a dust cloud in contact with, for example, a hot pipe, will ignite.

Minimum Ignition Temperature of dust layer (MIT dl): ISO/IEC 80079-20-2, EN 50281-2-1

The lowest temperature where a dust layer resting on a hot surface will ignite. The test simulates dust gathered on equipment, for example, and at which surface temperature the dust will ignite.

Pmax – Maximum explosion pressure: EN 14034–1

The maximum explosion pressure attained in a 20 L sphere. Together with the Kst value these are essential data to be able to properly dimension vent sizing etc.

dP/dt och KSt – Maximum pressure rise and dust explosion constant (severity): EN 14034–2

An indication of how fast the pressure rises in the 20 L sphere and the corresponding pressure rise in a 1 m3 vessel. This value is the basis for the St-classes

LEL/MEC – Lower Explosion Limit/Minimum Explosible Concentration: EN 14034-3

Lower explosion limit/minimum explosive concentration of dust which is ignitable in atmospheric conditions.

LOC – Limiting Oxygen Concentration: EN 14034–4

Determination of the lowest oxygen concentration required to sustain an explosion

Test of resistivity: ISO/IEC 80079-20-2

An indication of the electrical conductivity of the dust. High resistivity equals low conductivity. The risk of accumulating electrostatic charges increases with increasing resistivity.  

Sample preparation and characterization

The particle distribution and the moisture content are of eminent importance for the dusts’ explosive character. Generally smaller particles of dust generate a stronger reaction, since finer particles have a larger specific area (i. e. area per mass) and combustion is more easily spread between smaller particles.

According to EU regulations testing is performed on particles with diameter <500 μm. Moisture content shall be declared but there is no recommended range specified. The dust can be milled or dried upon request to simulate a worst case scenario, otherwise testing is performed as received to correspond to dust naturally occurring in the process (although the particles need to be less than 500 μm).

 

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Johanna de Grahl

Phone: +46 10 516 62 63

RISE Research Institutes of Sweden, Phone 010-516 50 00, E-mail info@ri.se

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