The explosiveness of a  powder can be demonstrated by testing in a closed vessel. A standard vessel designed for this purpose has a nominal volume of 1 m3, but the test may be carried out in a significantly smaller 20 litre vessel.

Testing equipment:

The test chamber is a stainless steel double-walled sphere with an internal volume of usually 20 litres or 1 m3, depending on the design. The double wall allows liquid to flow around the chamber. This may serve the purpose of cooling, that is, to dissipate the heat generated by explosions and, on the other hand, it can also allow heating to a predetermined temperature.

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Test:

Before starting the test, the powder sample to be tested is carefully measured and then filled into the sample holder. Using compressed air controlled by a solenoid valve, the measured powder is delivered to the interior of the container, where it forms a homogeneous mixture with the air. The explosion is triggered by a chemical ignition charge placed in the center of the vessel, which, after the delay time, releases 10kJ energy in the 1 m3 vessel and 2 kJ in the 20 litre vessel. The pressure transmitters built into the interior of the vessel transmit the data they record to the controlling PLC for recording. They are evaluated by a graphical evaluation software. The tested powder is not explosive if the pressure rise measured during the test does not exceed 0.3 bar (Dp ≤ 0.3 bar). In this case, there is no need to carry out further explosion safety technical tests. If the dust explodes during the test, the lower explosion limit, maximum pressure rise rate and maximum overpressure of the dust must be determined to prepare the explosion protection documentation and risk assessment.

Lower explosion limit:

The lowest concentration (typically unit of measurement: gpowder/m3 air) at which an explosion occurs is called the lower explosion limit value. During this determination, the mass of the powder flowing into the interior of the vessel is reduced until the mixture can no longer be ignited.

The lower explosion limit is a material characteristic that cannot be used directly, because the concentration of dust present in process systems is difficult to judge. Its value mainly helps to estimate the risk of developing an explosion hazard.

Maximum explosive overpressure (pmax):

Dust explosions can be interpreted in a closed or semi-closed system. During the explosion, the small dust particles in the air ignite, and the heat released as a result also ignites the particles floating around the dust particles, thereby starting a chain reaction. The self-excitation process — being a closed system — lasts as long as there is enough oxygen and combustible material to sustain combustion. As a result of the large amount of energy released during rapid combustion, the initially atmospheric pressure in the closed vessel begins to intensively increase. If the vessel is robust enough to hold the resulting overpressure, the process stops due to the consumption of combustion products or oxygen, the pressure in the vessel does not increase any further, it has reached its maximum.

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It is important to know that the maximum pressure that can be measured during a dust explosion can be considered independent of the size of the closed vessel, so its value in a silo of several thousand cubic meters is the same as in a much smaller cyclonic precipitator. The maximum overpressure is essentially the material characteristic of the powder used, which is a function of the powder concentration.

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Determining the maximum explosion overpressure may be necessary when sizing pressure-resistant casings, casings, and structures. However, this is quite rare, as large-scale technological systems will not withstand the typically 6-10 bar explosion overpressure, so they are sized for reduced pressure (pred) by providing them with splitting, splitting-opening surfaces.

Maximum pressure increase rate (dp/dt) max:

After the onset of the explosion, the pressure in the closed vessel continues to increase in time. The rate of growth can be defined as the first derivative of the above pressure-time curve with respect to time. In terms of the ideal process, the pressure rise rate curve is initially progressive, then constant, and finally degressive. At the zero point of the first derivative of the curve, the maximum explosion overpressure can be interpreted. The maximum pressure increase rate depends on the powder concentration, therefore, to determine it, the tests must be carried out within a wide concentration range.

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Pressure rise constant (Kst)

While the maximum explosion overpressure can be considered quasi-independent of vessel size, the rate of pressure rise tends to decrease with increasing vessel volume. Of course, this does not mean that in a larger vessel the effects of the explosion are more harmless, since the total energy released increases in proportion to the increase in volume. The maximum pressure rise rate is therefore normalized in all cases for a vessel with a volume of 1 m3. This can be done using the following formula:

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Thus, both the maximum explosive overpressure and the pressure rise constant are independent of the volume of the vessel used, their value is only affected by the properties of the given powder.

Danger classes (St-0, St-1, St-2, St-3)

According to the determined Kst and Pmax values, the dusts are classified into different classes according to their danger:

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