Contents of a Hazardous Area Classification Report - Dust

Published 11 Nov '19
Read Time: 5 mins

Combustible Dusts - What is a Hazardous Area?

Referring to AS/NZS 60079.10.2 Classification of Areas – Explosive Dust Atmospheres” – an explosive atmosphere can be defined collectively as:

“A mixture with air, under atmospheric conditions, of flammable substances in the form of gas, vapour, dust, fibres, or flyings which, after ignition, permits self-sustaining propagation.”

Dusts Ain’t Gases”

Flammable Gases and Vapours	Combustible Dusts

Flammable Liquids and Vapours require their presence with sufficient oxygen (in air) and an ignition source (hot enough or with enough energy) to ignite them. The limit at which this will occur is the Lower Explosion Limit. Combustible Dusts have two additional requirements – Dispersion and Confinement. Dispersion is the process of having sufficient dust airborne to form an ignitable cloud given the correct concentration of dust (the Lower Explosive Limit) and sufficient heat (either cloud or layer ignition temperature – which ever is least) or ignition energy (sparking or arcing). Confinement is required for the off gases from the deflagration of the combustible dust to build up pressure, to the extent that the vessel/room/building/tunnel that this occurs in finally fails or relieves, known as the explosion pressure.

Why do we Perform Area Classifications?

It is important to delineate the levels and types of risks within a given hazardous area, and to determine the appropriate Equipment Group and Temperature Classification in a given area. This process is defined as hazardous area classification. This then allows the correct selection of equipment (in terms of protection technique, Group and Temperature Classification) for a given hazardous area.

Types of Hazardous Zones – Dusts

Zone 20: A place in which an explosive dust atmosphere, in the form of a cloud of dust in air, is present continuously, or for long periods or frequently

Zone 21: A place in which an explosive dust atmosphere, in the form of a cloud of dust in air, is likely to occur in normal operation occasionally

Zone 22: An area in which an explosive dust atmosphere, in the form of a cloud of combustible dust in air, is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Electrical legalisation in all states refers to AS/NZS 3000 Electrical Installations “Wiring Rules”. Within this mandatory standard, hazardous area requirements can be found in Section 7.7 “Hazardous Areas”, with “Classification of Hazardous Areas” in subsection 7.7.2. The responsibility for classification of a hazardous area rests with the persons or parties in control of the installation.

Zone Examples:

What should be in an Area Classification Report

When we arrive on site to perform an inspection, we will ask to see the Area Classification Report and Area Classification Drawings. This will indicate what types of hazardous areas exist and what their extent is, as well indicating relevant Group and Temperature Classification/Surface Temperature for the combustible dust being processed or generated. Within AS/NZS 60079.10.2 there is a procedure outlined in Section 4.2 “Area Classification Procedure” which contains the following dot point parts.

“Area classification is based on a number of factors and may require informed input from a number of sources. These factors include:

• A description of the site where the plant or equipment located
• A description of the mechanical process that produces the combustible dusts, flyings, etc.
• Whether the dust is combustible or not. Dust combustibility can be confirmed by laboratory tests.
• Material characteristics for the dusts that are present. These may be obtained from a variety of published sources, a process specialist or by testing. The following parameters should be confirmed:
  • Cloud Ignition Temperature (°C)
  • Layer Ignition Temperature (°C)
  • Ignition Energy (J)
  • Lower Explosive Limit (g/m³) (combustible dusts have no Upper Explosive Limit)
  • Kind of Material:
    • Fibres and Flyings – Group IIIA (paper trimmings, wool or cotton fibres, etc)
    • Non-conductive Dusts – Group IIIB (from coal, plastic, wood finishing, etc)
    • Conductive Dusts – Group IIIC (all metallic dusts)
• Nature of dust releases from process sources – which can then lead to the allocation of hazardous area Zones. That is Continuous, Primary or Secondary sources of release.
• Operational and maintenance procedures for the plant, including housekeeping.
• Other equipment and safety information

Note: Combustible dust Zones are formed by CLOUDS of dust. NOT LAYERS. An area covered in dust that is not disturbed is not a hazardous area but is potentially hazardous. Dust layers can be Sources of Release if disturbed. They are also ignition hazards with respect to surface temperature ignition.

A summary of the reported classifications is made in a Source of Release Table which covers all items required. This is the data input for drafting the Area Classification Drawing.

In addition to the above, AS/NZS 60079.14, Section 4.2 Documentation contains the same information although presented in a different way, YET this time you will see that it clearly states Areas Classification Documents WITH plans – otherwise known as Area Classification Drawings. See example below:

The requirement for the above two items, the Source of Release Table and the area classification drawing is listed within AS/NZS 60079.10.2:

• area classification documents with plans showing the classification and extent of the hazardous areas including hazardous zones
• optional assessment of consequences of ignition (basically, a risk assessment)
• external influences and ambient temperature at the site.
• Combustible dust classification in relation to the group or subgroup of the electrical equipment (e.g., IIIA, IIIB, IIIC);
• temperature class or ignition temperature of the combustible dust (layer and cloud values);
• a list of the properties of the combustible dusts generated by the site process on site in a separate table to the Source of Release Table.

The above requirements represent the MINIMUM requirements for a hazardous area classification study.

As the person responsible for site

Whether you are looking at having your site area classified for the first time or looking at reviewing the original classification, this may be done “in house” (with someone who has the correct competencies and requisite experience over multiple sites and years) or you can outsource this work. Either way you have to own the need for area classification and enforce the outcomes. By following the above guidelines, you will be able to answer any questions relating to the combustability hazards of the site – for example: “why is that a hazardous area”, or “why this equipment is installed here” or “what standard was used”.

Hazardous area classification drawings alone will not provide answers to all questions. The report complete with the Source of Release Table are also vital tools which demonstrate what hazardous areas exist, why they exist and how far they extend. Example of a poor classification – a list of hazardous zones in a table like the source of release table example above, with no further evidence to clarify or substantiate any claims.

Checklist when Engaging a Hazardous Area Classifier

• What standard or standards will be applied and at what versions – always insist on the latest version. In Australia, AS/NZS 60079.10.2 is the starting point for any hazardous area classification. Other standards are referred to in this document and may be applicable in certain situations.
• What are the deliverables? Ensure that all the above elements discussed above are included in the report, and that you receive a hazardous area classification report.
• No one can complete an accurate classification report without at least one site visit, and probably at least two. This is not a desk top exercise.
• Always feel free to disagree with the results. After all, its your plant and you might know more about its operation than the classifier.
• Be prepared to offer the following documents to ensure that your classification is completed in the least time possible:
  • All P&IDs (Piping and Instrumentation Diagrams)
  • All PFDs (Process Flow Diagrams)
  • Mechanical arrangement drawings of the plant – level by level – with elevation and plant views
  • Mechanical general arrangement drawings of each process vessel, which details all inlets and outlets (listed in a legend on the drawing).
  • SDS (Safety Data Sheets) for all products used within the plant and created by the plant.

This material is produced by HAZ. The information is of a general nature only and should not be relied upon.  Each set of circumstances associated with compliance of hazardous areas and Type B compliance is unique and expert advice on your particular circumstances should be sought.  HAZ accepts no liability for any reliance on the information published on this website.