Frequently Asked Questions
What is the significance of MIE (with inductance) and MIE (without inductance). Which one of them is lower?
MIE (without inductance) simulates a purely capacitive electrostatic discharge such as from isolated conductors in an industrial situation. MIE (with inductance) simulates longer duration discharges as the introduction of the inductor into the circuit, delays the spark discharge to earth, hence this corresponds to MECHANICAL SPARK SENSITIVITY. As the spark duration is shorter for "pure" static electric discharges so the probability of ignition is less than for the MIE (with inductance) and therefore, in many cases the MIE (with inductance) produces a lower value of MIE. A review of your facility will determine whether electrostatic spark discharges or mechanical sparks need to be assessed and the corresponding MIE test performed.
What is the significance of particle size for dust explosions. What is the smallest particle size where a dust explosion could not occur?
As the particle size distribution decreases so the explosion properties of the powder changes, sometimes dramatically. Explosion severity will increase, for example aluminium with a particle size of 150µm will have a Pmax of 8 barg and a Kst value of 150 bar.m.s-1 whereas at < 10µm the Pmax rises to 17 barg and the Kst to > 650 bar.m.s-1. The same applies to explosion sensitivity where the material becomes more sensitive to ignition especially with MIE. Using the same example of aluminium then the MIE value drops from 100 mJ to <10 mJ as particle size distribution decreases. Usually it is accepted that when the particle size distribution is > 500µm then ignition of a dust cloud cannot occur. However, even where a powder has a large particle size distribution, attrition can occur which can result in fines being present. Due to their nature these fines will remain in suspension in air for a considerable period of time and should be taken into consideration when performing a risk assessment.
What explosion protection measures should be selected, such as Explosion Pressure Relief Venting, Explosion Suppression and Explosion Containment?
All Explosion Protection Measures serve the same purpose and can be used depending on feasibility with the process. Explosion Venting is the most common example that is being used in industry. The important aspect is that all three measures must be designed using Explosion Severity data of the material being handled. Another important aspect is that explosion isolation needs to be provided along with explosion protection measures to prevent explosion propagation to unprotected systems. Also, it is necessary to assess the specific requirements for any operation. Explosion venting needs to be designed and installed correctly and it may not be suitable for materials of a toxic nature, explosion suppression is expensive to install and maintain but can be used in places where venting is not an option. Explosion containment needs to be maintained over the longevity of the process and maintenance engineers must understand the complexities of the system. As stated previously, all three methods of protection will require explosion severity data (20 litre sphere) to ensure safe design.