Insights of a Dust Hazard Assessment (DHA)

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We have discussed a lot about dust explosion hazards and the risk to plant. The next question which stands is "How to carry out Dust Hazard Assessment (DHA)?" The intent of this post is for making you aware of how you can carry out a dust hazard assessment or if you have a third party doing it for you, then how to review the DHA report?



If you are handling combustible dust in your plant, there is a risk of dust explosion. Hence, in order to make our operations safer, it is recommended to carry out Dust Hazard Assessment in your facility.

An explosion including a Dust Explosion is characterised with a Pentagon which includes a combination of Fuel, Oxidant, Mixing/ Dispersion, Ignition Source and Confinement as shown in the image below:

All risk assessments shall follow the hierarchical Three Rules approach:

1. Do not have a flammable atmosphere, but if you do…
2. Do not ignite it, but if you do…
3. Do not hurt anyone.

DHA study is carried out using the following steps:

Step 1: Review of Dust characteristics

This is the first step before initiating a DHA study which includes identification of the following:

• Dust must be flammable/combustible in nature.
• Particle size distribution should be capable of propagating flame. Based on thumb rule and based on standards, a particle size of < 500µ is the major cause of risk. However, a possibility of attrition generating fines cannot be discredited with larger size material.

If both conditions mentioned above are met, we move on to Step 2.

Step 2: Identification of Flammable Atmosphere

Next step is to establish where the identified combustible dust can form a flammable atmosphere (i.e. when the dust cloud is dispersed in oxidant environment).

• The atmosphere needs to support combustion such that oxidant must be present in concentration beyond Limiting Oxidant Concentration (LOC).
• The concentration of dust must be beyond Minimum Explosible Concentration.

Keeping in mind the above points, a site round/ discussion session is conducted with the plant in order to understand operations at individual location and location of flammable atmosphere generation are identified. If a flammable atmosphere exists, we move on to step 3.

Step 3: Review the availability of an Ignition Source

Once it has been established that a flammable atmosphere can exist, the next step remains is the identification of ignition sources. It has to be understood here, that even if one ignition source exists in the area, we move on with the assessment. 

EN1127-1 gives 13 potential ignition sources to be considered in a flammable atmosphere, out of which the most common in a facility includes Naked Flames, Hot Surfaces, Unsuitable/ Malfunctioning Electrical Fittings, Mechanical Sparks, Electrostatic Discharges, Lightning and spontaneous combustion/ exothermic reaction. 

Other ignition sources include Stray electric currents within installations, Radiofrequency electromagnetic radiation, Visible and HF electromagnetic radiation, Ionising radiation, Ultrasonic sound waves and Adiabatic compression and shock waves.

Step 4: Hazardous Area Classification (HAC)

In step 4, Hazardous Area Classification is carried out to divide areas in plant among Zone 20, 21 or 22 based on the probability of the existence of a flammable atmosphere. 

• Zone 20: Explosive dust atmosphere is continuously present, or present for long periods.
• Zone 21: Explosive dust atmosphere is likely to occur during normal operation.
• Zone 22: Explosive dust atmosphere is not likely to occur in normal operation, and if it occurs it will only exist for a short time.

Step 5: Risk Assessment

If there is a potential for a flammable atmosphere and an ignition source, then the next step is to determine the risk to people and identify the actions required to ensure the risk is As Low As Reasonably Practicable. This will include measures to avoid the existence of flammable atmosphere, to remove ignition sources, to remove oxidant or to protect people and equipment against explosion.

Step 6: Identification and implementation of Basis of Safety

The "Basis of Safety" for an operation/ area is the combination of measures relied upon to ensure safety.

1. Explosion Prevention:
• Avoidance of Flammable Atmosphere, such as the use of an adequately sized Dust collector suction hood, housekeeping, etc.
• Avoidance of Ignition Sources, for which ignition sensitivity test data is required
• Avoidance of Oxidant, such as Absolute Inerting or Partial Inerting
2. Explosion Protection with Explosion Isolation
• Explosion Containment 
• Explosion Suppression
• Explosion Venting 

Then the recommendations are suggested so as to ensure that the Basis of Safety is implemented and maintained in site. 

Hopefully the next time you will be able to conduct DHA or review the DHA report using the above-mentioned methodology.

That is all for this post. Until next post stay safe and keep sharing!

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