Dust Explosion Risk Parameters

Dust explosions are an industrial hazard demanding consideration of key explosion risk parameters. Some of these parameters are deciding indicators for zone classification.

The remaining parameters play a supporting role in safety measures, such as selecting appropriate equipment or addressing electrostatic risks.

This blog provides an overview of these parameters and their practical applications.

Note: When dealing with gas explosion hazards, different risk parameters apply (see Gas and Vapor Explosion Risk Parameters: A Brief Guide).

Parameters Used for Zone Classification

These parameters define the likelihood of explosive atmospheres forming and, in turn, serve as the foundation for determining safety zones and implementing appropriate protective measures.

For example, the i.safe Mobile IS530.2 is a device that is certified to be employed in zone 22 (dust) environments.

Group Classification (A/B)

The evaluation of dust explosion risks begins with determining whether a dust is combustible. Dusts are categorized into Group A or Group B based on how they behave under ignition conditions.

• Group A dusts can ignite and propagate flames at normal temperatures. There fore, Group A dusts are are combustible.
• Group B dusts, in contrast, require elevated temperatures, typically above 110°C, to burn or become explosive.

When a dust is identified as combustible (Group A), the sensitivity to ignition is to be assessed (1.2 + 1.3).

Lower Explosive Limit (LEL)

The LEL defines the minimum concentration of dust in air required to sustain ignition. For most dusts, this limit lies between 20 and 60 g/m³, although some materials require significantly higher concentrations.

Unlike gases, the Upper Explosive Limit (UEL) is not practically significant for dust due to the difficulty of forming homogeneous, high-concentration clouds. In other words, no concentration of dust in air, which is higher than the LEL, would be enough to form a mixture that is not explosive. Especially since dust clouds are not solid and can form rapidly.

While temperature and pressure can influence flammability limits for dusts, these effects are generally minor. Changes in LEL values under varying conditions tend to have negligible impact on overall explosion risks.

Parameters Not Used for Zone Classification

Some parameters, while not used to classify hazardous zones, are used for evaluating ignition risks, explosion intensity, and equipment compatibility. These parameters support broader safety measures and mitigation strategies.

Minimum Ignition Energy (MIE)

MIE measures the smallest amount of energy needed to ignite a dust cloud. It tells us how easily a dust can catch a spark, which is useful in understanding whether static or mechanical sparks could ignite the material.

The MIE is affected by factors such as environmental conditions (temperature, pressure, and airflow), the physical and chemical characteristics of the dust (particle size, moisture levels, and composition), and testing setups like electrode configurations.

MIE ranges from extremely low values, such as 0.01 mJ for very reactive dust like sulphur, to over 1000 J for less sensitive substances (detailed overview). Low-MIE dusts are particularly prone to ignition by static discharge or sparks, while high-MIE dusts are prone to ignition by grinding or welding sparks (figure).

The main application of MIE is in assessing and preventing electrostatic hazards, ensuring materials are handled and processed safely to reduce ignition risks.

Minimum Ignition Temperature (MIT/Tig)

MIT is the lowest temperature at which a dust cloud ignites when briefly exposed to a heated surface. The value depends on factors such as the material properties of the dust, particle size, and the design of the testing vessel, including its shape and surface material.

This parameter is not used for zone classification but is crucial for eliminating ignition risks.

MIT helps ensure that equipment operates below certain temperatures, reducing the likelihood of ignition from hot surfaces. It is also a consideration in explosion suppression system design and in specifying safe operating limits for equipment.

Layer Ignition Temperature (LIT/Ts)

 LIT is the minimum temperature at which a dust layer of 5 mm thickness ignites on a heated surface. This value decreases with thicker dust layers, as reduced heat dissipation increases the likelihood of ignition. The thermal history of the material can also influence the test results.

Although not used for zone classification, LIT helps understanding risks associated with smoldering or glowing dust layers. It is valuable in the selection of equipment that operates below the ignition temperature of accumulated dust.

Maximum Explosion Pressure (Pmax)

Pmax is the peak pressure produced during a dust explosion within a confined space, determined under ideal conditions with the optimum dust concentration. Dusts that generate a pressure rise below 0.5 bar across all concentrations are considered non-explosible and categorized as Group B materials.

For most explosible dusts, Pmax typically ranges between 6 and 10 bar. While this parameter is not relevant for zone classification, it is used to assess eplosion risks. In addition, the Pmax parameter can provide insights in how to reduce the consequences of explosions, if they can not be avoided.

Maximum Rate of Pressure Rise (Kst)

The Kst value measures the maximum rate of pressure rise during a dust explosion and categorizes dust into explosibility classes (St 0 to St 3). It reflects the speed at which pressure builds. Similar to the Pmax, the value of the Kst lies in understanding explosion risks and controlling explosions when they are unavoidable.

Powder Volume Resistivity

This parameter measures how well a dust retains electrostatic charge, which can act as an ignition source. High resistivity increases the risk of charge accumulation, particularly in environments where dust is handled or transported. Controlling resistivity through material handling systems or environmental adjustments (e.g., humidity control) is helpful in reducing electrostatic risks.

Prevention by housekeeping

Logically, housekeeping is not a direct aspect of flammability of dust, but it plays an essential role in minimizing explosion risks. Maintaining clean process areas and preventing dust accumulation is the first step in maintaining effective anti-explosion measures.

A high housekeeping standard can reduce the extent of hazardous zones and significantly lower the likelihood of secondary explosions. Housekeeping is one of the best strategies to achieve a lower Ex zone classification, reducing risks and saving money.