Process Safety Consultancy | Sigma HSE

1. Health, Safety and Environment Frequently Asked Questions from our Dust Explosion Seminars

2. 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.

3. How do you establish the maximum allowable temperature for equipment using powder properties? • The maximum surface temperature of enclosure for equipment located in hazardous zones is based on Minimum Ignition Temperature (MIT of a dust cloud) and Layer Ignition Temperature (LIT of powder layer). After establishing these values, then the lowest value of the two is selected to give the maximum surface temperature after applying a factor of safety as follows: • For MIT data, Max. Allowable Surface Temperature = 2/3 * (MIT) • For LIT data, Max. Allowable Surface Temperature = LIT – 75o • Methods of protection along with Ingress Protection are equally important along with the maximum allowable temperature of equipment.

4. 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 Pmaxof 8 barg and a Kstvalue of 150 bar.m.s-1 whereas at < 10µm the Pmaxrises to 17 barg and the Kstto > 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.

5. What is the difference between St1, 2, 3 and St0 H, 1H, 2H? Explosion severity is quantified in terms of the Maximum Explosion Pressure and the Max. Rate of Pressure Rise (which in-turn relates to the Kstusing Cubic Law, i.e. (dP/dt)max* V1/3= Dust Constant or Kst). As can be seen Kstis a measurement that is used as it is a constant and does not change with vessel volume. The St Class is based on Kstvalue, i.e. St 1 (Kst<200 bar.m.s-1), St 2 (200bar.m.s-1< Kst< 300 bar.m.s-1) and St 3 (Kst> 300 bar.m.s-1). The complete explosion severity test is normally carried out in a 20 Litre Sphere, although the 1m3vessel can also be used. Sometimes, for screening purposes (St 0H, 1H, 2H determination), you can use the Modified Hartmann tube apparatus (1.2 L), which can indicate Explosibility and dust explosion class. The MIE tube is fitted with a hinged lid and a constant arc ignition source. When an ignition occurs, the lid opens to various angles depending upon the severity of the explosion. Therefore, the results are not accurate but do provide an indication of explosion violence. • St 0H – No explosion – St 0 • St 1H – indicates an explosion which indicates a St 1 powder. • St 2H – indicates an explosion which indicates a St 2 powder. • Note: there is no ST 3 indication and design of Explosion Protection Measures cannot be defined based on Modified Hartmann tube apparatus results. This can only be achieved with full 20L Sphere testing.

6. 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.

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