Determination of Dust Explosion Characteristics of Dusts/Powders
There are several tests that can be performed in order to determine the explosion characteristics of powders and dusts:
The result of the test provides information about whether the dust sample is combustible or not. If the result shows that the sample is combustible, it is recommended to perform other tests to be able to take precautions against dust explosions and fires in the process. Gexcon performs explosibility screening tests using the 20-litre explosion apparatus. Powder or dust samples of various sizes are dispersed inside the sphere and are exposed to a 2 kJ ignition source (chemical igniters).
The explosibility test is performed according to the Kühner AG 2009: 'Operating Instructions 20-l-Apparatus', Ver. 7.0.
The Minimum Ignition Energy (MIE) test determines the lowest spark energy capable of igniting a sample when dispersed in the form of a dust cloud. The test is used primarily to assess the potential vulnerability of powders and dusts to electrostatic discharges, but is also relevant to frictional sparks.
Gexcon performs MIE testing in accordance with EN 13821: 'Potentially explosive atmospheres – Explosion prevention and protection – Determination of minimum ignition energy of dust/air mixtures', using the Modified Hartmann Tube apparatus. Powder or dust samples of various sizes are dispersed in a 1.2-liter vertical tube and attempts are made to ignite the resultant dust cloud with discrete capacitive sparks of known energy.
When performing the MITdc (dust clouds) the parameter is used to assess the hazards of hot surfaces for a dust cloud. The parameter can be used to limit surface temperatures to safe levels. The residence time of the dust cloud in the hot furnace is limited. Therefore this determination is relevant to situations where a dust cloud can come in brief contact with a hot surface. Examples of such situations can be found where steam pipes or electrical equipment is present. In order to assess the hazards of mechanical sparks the MITdc as well as the MIE must be known.
The minimum ignition temperature (MIT) test determines the lowest surface temperature capable of igniting a powder or dust dispersed in the form of a dust cloud. The MIT is an important factor in evaluating the ignition sensitivity of powders and dusts and is relevant to define the maximum operating temperature for electrical and mechanical equipment used in dusty environments. MIT testing is performed using the Godbert-Greenwald Furnace in accordance with EN 50281-2-1: Part 2-1: Test methods: 'Methods for determining the minimum ignition temperatures of dust'. Powder or dust samples of various sizes are dispersed into the furnace and the minimum wall temperature capable of igniting the dust cloud is determined.
When determining the dust layers, i.e. (MIT dl). This parameter describes the minimum ignition temperature when a dust layer of a given thickness is resting on a hot surface. The MIT of the dust layer is the lowest temperature of a heated free surface at which a dust layer of 5 mm deposited on it will ignite (starts smouldering). The MIT of a dust layer is used together with the MIT of a dust cloud to define the maximum operating temperature of electrical and mechanical equipment used in dusty environments. The test involves heating a circular layer sample 5 mm thick and 150 mm in diameter on a hot plate at constant temperature. The temperature of the sample layer and the hot plate are monitored and the minimum surface temperature capable of igniting the powder or dust layer is determined. In practice the following hot surfaces exist: surfaces of hot equipment, heaters, dryers, steam pipes and electrical equipment.
This test is performed according to 'EN 50281-2-1: Part 2-1: Test methods: "Methods for determining the minimum ignition temperatures of dust”'.
The maximum explosion pressure forms the basis for explosion protection in design and construction of equipment, protective systems and components to reduce the effects of an explosion. The maximum explosion pressure test can be used when designing explosion proof equipment. The maximum explosion pressure together with the Kst-value test are used in dust explosion vent sizing. The result provides information on the necessary size of vent openings (to see whether the existing ones are sufficiently large). The explosion pressure development test is performed in a 20-liter sphere apparatus. The sample size is varied to determine the optimal dust cloud concentration. The maximum pressure and rate of pressure rise are measured and used to determine the Kst-value and St hazard class of the material. These data can be used in designing dust explosion protection measures and equipment.
The investigation is performed in the 20-litre spherical vessel according to prEN 14034-1: 'Determination of explosion characteristics of dust clouds - Part 1: Determination of the maximum explosion pressure pmax of dust clouds' and EN 14034-2: 'Determination of explosion characteristics of dust clouds' – Part 2: 'Determination of the maximum rate explosion pressure rise (dP/dt)max of dust clouds'.
The Lower Explosion Limit (LEL) test determines the smallest concentration of material in the air that may cause flame propagation upon ignition when in the form of a dust cloud. The test involves dispersing powder or dust samples in a vessel and attempting to ignite the resulting dust cloud with an energetic ignition source. Trials are repeated for decreasing sample sizes until the LEL is determined. The parameter is used for assessing dust explosion hazards inside dust handling equipment. The test is performed according to EN 14034-3: 'Determination of explosion characteristics of dust clouds - Part 3: Determination of the lower explosion limit LEL of dust clouds'.
The Limiting Oxygen Concentration test is used when applying inerting. LOC testing is performed using a 20-liter sphere apparatus. Powder or dust samples of various sizes are dispersed in the vessel and attempts are made to ignite the resulting dust cloud with an energetic ignition source (2kJ). Trials are repeated at decreasing oxygen concentrations until the LOC is determined. It should be emphasised that the LOC test is inert gas type dependent because of the different capacities of these inert gases. Gexcon usually uses nitrogen as the inert gas. If you want us to use another inert gas, please inform us. Nitrogen has a higher heat capacity than e.g. argon and the result of the LOC value found for nitrogen may be therefore higher than in an atmosphere where a part of the oxygen is replaced by argon.
Determination of the limiting oxygen concentration for inerting of dust clouds is performed in a 20-litre spherical vessel according to EN 14034-4: 'Determination of explosion characteristics of dust clouds - Part 4: Determination of the limiting oxygen concentration LOC of dust clouds'.
The self-ignition temperature is used to assess ignition hazards on hot surfaces arising from mechanical sparks. Self-heating is most commonly caused by oxidation processes. Oxidation processes generate some heat locally which, due to the insulation of the powder, is not easily transferred to the environment. Locally, the surrounding dust particles are heated slightly, resulting in faster oxidation. This leads to even more local heating. If the heat transfer to the surroundings is not big, this process could lead to high temperatures in dust heaps and even in ignition of the dust. Self-heating properties of dusts and powders are investigated according to EN 15188: 'Determination of the spontaneous ignition behaviour of dust accumulations'.
This test is performed in order to see whether a dust layer may propagate a smoldering combustion. This is done according to VDI-GL 2263 Part 1 1990: 'Test methods for the determination of the safety characteristics of dust, Burning behaviour'.
Similar to the MIE test mentioned above, however, this test is performed in an atmosphere with reduced oxygen content.
This test is performed according to IEC 61241-2-2: 'Method for determining the electrical resistivity of dust in layers'.