Pressure Rise and Explosion Limits
Combustible/explosive substances mixed with air are only explosive between their lower and upper explosion limits (minimum and maximum concentration of dust in a given volume), within their explosion range. When determining the proper safety of workplaces, the lower explosion limit is one of the most important measurable parameters. The presence and amount of oxidizing medium (mostly oxygen) is also a concentration-dependent parameter since the chemical reaction uses it. Below this minimum amount, the explosion is also prevented.
Figure 3.: Characteristic pressure rise and its significant values
The intensity of the reaction, the maximum explosion overpressure (pmax), and maximum rate of pressure rise (dp/dt)max) give information during the explosion process, as shown in Figure 3. These characteristic values vary as a function of the concentration of the explosive substance within the lower and upper explosion limits, in most cases with a well-defined maximum value. Naturally, the aim of designing the appropriate protection is to take into account the worst-case scenarios, so the maximum values of these curves must be taken into account in such cases. The explosion constant (Kst for powders, KG for gases) can be obtained from the maximum value of the pressure rise rate. It is calculated based on the so-called cubic law (Equation (1)). This value is equal to the maximum rate of pressure rise that would be measured for a given substance in a 1 m3 cubic vessel under specified test conditions. With the help of the explosion constant, the explosion intensity of each powder becomes comparable.
Considering KSt values, the powders are classified into three classes according to their explosion hazard: St-1, St-2, St-3. (Table 1.) If the explosion constant for a given powder is 0, the powder is not explosive. The values in the table were determined for a closed tank with a volume of 1 m3 and an ignition source energy of 10 kJ.
Hazard classes and their characteristics
With the above knowledge, it is easy to see that the determination of the minimum ignition temperature, minimum ignition energy, explosion limits, and explosion characteristics of powders is the cornerstone of the development of an adequate level of protection.