The process of safety testing can be include such as; Dust and Powder Flammability Testing, Dust Combustibility (Group A/B), Minimum Ignition Energy (MIE), Minimum Ignition Temperature (MIT),Layer Ignition Temperature (LIT), Explosion Severity (Pmax, Kst), Minimum Explosive Concentration (MEC), Limited Oxygen Concentration (LOC), AND Burning Behaviour. https://sigma-hse.com/testing/dust-powder-testing/

1. Sigma-HSE (UK) Ltd
Unit 2 Moorside Point, Moorside Road,
Winchester, Hants, SO23 7RX
Tel: 01962 840570
Email: info@sigma-hse.com
Web: www.sigma-hse.com
REGISTERED IN ENGLAND & WALES NO. 06837763
REGISTERED ADDRESS: AS ABOVE.
Frequently Asked Questions from our 2017
Dust Explosion Seminars
Q1
What is the significance of MIE (with inductance) and MIE (without inductance). Which
one of them is lower?
A1
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.

2. Page 2 of 4
Q2
How do you establish the maximum
allowable temperature for equipment using
powder properties?
A2
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.
Q3
What is the significance of particle size for dust explosions. What is the smallest particle
size where a dust explosion could not occur?
A3
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.

3. Page 3 of 4
Q4
What is the difference between St1, 2, 3 and St0 H, 1H, 2H?
A4
Explosion severity is quantified in terms of the Maximum Explosion Pressure and the Max. Rate of
Pressure Rise (which in-turn relates to the Kst using Cubic Law, i.e. (dP/dt)max * V1/3
= Dust Constant or
Kst). As can be seen Kst is a measurement that is used as it is a constant and does not change with vessel
volume. The St Class is based on Kst value, 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
1m3
vessel 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.

4. Page 4 of 4
Q5
What explosion protection measures should be selected, such as Explosion Pressure Relief
Venting, Explosion Suppression and Explosion Containment?
A5
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.