Combustible dust testing lab

Before starting the trial tests, it is necessary to prepare the sample according to requirements of particular norms. The preparation includes drying, milling, separation etc. Our laboratory is equipped with different kinds of mills and drying devices.

Photo: Rotor speed mill, company FRITSCH

Sieve analysis is used for determining of mean grain size. This parameter is required by trial norms when assessing other fire technical parameters.

Primary chemical analysis includes determining content of water, volatile combustible matter, ash and fixed carbon.

Photo: Testing device MAC-500 for assessing of primary chemical analysis

Minimum ignition temperature of dust in a layer is defined as minimum temperature of hot surface. At that temperature the dust layer of determined thickness (usually 5 mm) ignites when placed on the hot surface. The method is suitable especially for industrial devices where a thin layer of dust is present, e.g. electric motors, light fixtures. The parameter determines maximum allowable surface temperature, which prevents fires of dust on hot surface. The testing proceeds in accordance with European norm, EN ISO/IEC 80079-20-2. Dust layers are formed by filling a cavity, made of metal ring of corresponding height, placed on heated surface. The trial is repeated with more dust layers as long as the minimum ignition temperature is determined. The lowest measured ignition temperature is finally taken into account.

Photo: Testing device in accordance with ČSN EN 50 821-2-1 for determination of minimum ignition temperature of dust layer.

Minimum ignition temperature of agitated dust is the lowest temperature of hot inner furnace wall. At the temperature the agitated dust ignites in the air inside the furnace. The trial is proceeded in accordance with EN ISO/IEC 80079-20-2 (Methods of determination of minimum ignition temperatures). Heated tube of the furnace is vertical and opened in its lower part to the atmosphere. The upper part is connected to a dust dispenser with help of a glass adapter. The dust is dispersed into the furnace by opening a solenoid vent, which releases the pressed air from a tank. The furnace is installed in its stand so that it is easy to observe the lower part of the furnace tube. Under the tube a mirror is placed which enables us to observe the inner part of the tube. Determination of the minimum ignition temperature is important for dust explosion prevention.

Lower explosive limit is defined as the lowest concentration of the mixture of flammable dust and air which can support the explosion. The value is very important for assessing of environment according to valid norms and to protect the device from the danger of explosion by keeping the concentration of flammable dust in the air under the dangerous value. Lower explosive limit is determined on autoclave VA-20L, from a Swiss company Kühner AG. It is fully in compliance with a European norm ČSN EN 14034-3+A1.

Graph: Dependence of explosive pressure on concentration of tested sample

Maximum explosion parameters include maximum explosion pressure – pmax, maximum speed of explosion pressure growth - (dp/dt)max of agitated dust and explosion constant – Kst. Explosion testing with defined mixture of dust and air are carried out in compliance with European norms ČSN EN 14034-1 + A1 and ČSN EN 14034-2+A1. Explosion pressure –pex and speed of explosion pressure growth - (dp/dt)max are determined for every concentration. Both values are marked in a graph in dependence on concentration. It is repeated until the maximum value of explosion pressure – pmax and maximum speed of explosion pressure growth - (dp/dt)max are found. The values of explosion indicators are necessary for qualified calculation and proposal of anti-explosion protection elements (membranes, vents etc.).

Photo: Autoclave VA-20L, Graph: Dependence of explosion pressure growth speed on oxygen concentration

Limit oxygen concentration is the highest concentration of oxygen, at this concentration the dust-air mixture is not capable of explosive reaction. We carry out the test on the autoclave VA-20L, made by Kühner AG company, according to the testing norm ČSN EN 14034-4+A1. By gradual, step-by-step increasing of ratio of inert gas (nitrogen) to air and by change of dust concentration, the oxygen concentration is decreased to the level at which the explosion does not come up at any dust concentration. This value is the limit oxygen concentration (LOC). The parameter is necessary when proposing the inertization technologies.

Photo: Autoclave VA-20L

Minimum initial energy of agitated dust is the energy of spark that is necessary for ignition of air-dust mixture. As the explosion container a modified glass Hartman tube of 1.2l is used. The system of dispersing the dust in the base of the tube is the “mushroom type” and the sample of dust is freely dispersed around it. For dispersion of the dust into a glass cylinder where the dust is ignited by a sparkle between two electrodes, we use a thrust of air with 7 bars pressure. The value of minimum initial energy is used for excluding of potential initiation source. In practical usage the data is needed mostly to protect the devices against electrostatic sparks. The test is carried out in compliance with the European norm ČSN EN 13821, on MIKE 3 device – it was bought from a well-known Swiss company Kühner AG.

Photo: Testing device MIKE 3

Calorific value is the characteristics of fuel that defines how much energy is released by complete combustion of a unit (usually 1kg). Unlike combustion of heat, in the value there is not included the specific state of steam heat, contained in the combustion products.

Bulk density is a basic value for calculation of weight when doing stock taking of separated kinds on low-capacity waste disposals, to calculate the volume of dispensers etc. Our laboratory provides the test in both, small and large container.

The test is suitable for fuels and other solid substances in powder, granular or fibrous form. It enables us to determinate volume weight of material. It is poured into a container and later condensed under definite conditions. This parameter simulates the conditions of long-term stored material.

The test of speed of fire spread on the sedimented dust layer is carried out according to our approved proceedure. The aim is to determine the speed of burning of the sample in 1mm and 5 mm layers. To determine the parameter we use the testing device whose main part is made of special glass tube with oxygen intake.

The test gives evidence on the tendency of powder materials to self-ignite during storage. The puprose of the test is to asses induction period of self-ignition of powder mass in certain conditions. Hence the materials are classified for the purposes of transport according to RID/ADR.

Photo: Testing device for determination of tendency to self-ignition

Determination of self-ignition tendency by Olpinski method, determination of lower explosive limit in gases, assessment of gas explosion indicators.

Other activities: research in the field of material explosiveness, expert judgements in the field of explosiveness, carrying out the tests for risk analysis, carrying out tests for “Explosion protection documents”.