Localisation of Gas and Dust Ex-Zones


Thread Starter

Samir Gasimov

Hello everybody.

Who must determine the localisation of Gas and Dust Ex zones (IEC 60079) when build new factory?

Thanks all

Bruce Durdle

That will depend very much on your local regulatory system. You need to check the legal requirements for the area where the factory will be situated - they may not recognise the IEC standards, for example.

In general, however, the primary responsibility for defining the extents and degrees of any hazards in a factory will lie with the owner (effectively the CEO or similar elevated personality).

The technical skills needed to do the job include knowledge of the properties of the relevant materials, and how they behave under process operation conditions (pressure, temperature, etc.) They tend to fall into the chemical engineering category rather than electrical.
A detailed list of the competencies required is given in AS/NZS 4761, with similar competency specifications for other areas of Explosive Atmospheres work - I'm not aware of any other similar specifications for other jurisdictions.

Samir Gasimov

Dear Bruce Durdle.

Thank you very much.

If I am an Instrument engineer and I must choose the instrument for measurement. Who must determine the Ex zones? I or customer?

Best regards

Bruce Durdle

To determine the zones, the first step is to draw up a schedule of all the materials used on the site, together with the properties that affect the flammability - LEL, density, minimum ignition energy, ignition temperature etc. These material need to be assigned to equipment groups (IIA, IIB, IIC).

Next, all the points where material could be released from containment need to be identified and put into a schedule. This will need to take into account things like operating patterns (is a drain line used regularly to take samples, for instance). These sources need to be graded as "Continuous", "Primary" or "Secondary" depending on how often a release can occur. Then, depending on the ventilation, they can be associated with Zone 0, Zone 1 or Zone 2 respectively (assuming you are dealing with gases or vapours - for dusts, the equivalents are Zone 20, 21 or 22). The effect of ventilation is not trivial in an enclosed plant.

Finally, to work out how big the zones are, the pressure at each source and the effective cross-section of the release point need to be factored in. This can be done using tables in various documents.
This whole process needs a lot more understanding of the process, the equipment, and of how materials such as gases and vapours behave, that an electrical technician usually has. The electrical guy can be expected to know the details of how equipment can be selected for different zones, but the zoning must be done by someone who has the necessary skills - and the owner has to buy in to the results and know the limitations.

Ideally, the classification has to start with the feasibility study for the plant, as there are financial implications of using flammable materials. It can also feed back into plant design decisions - eg deciding to use dedicated small-diameter sampling points rather than take samples from drains. It should not be left until the electrical designers or installers arrive on site to do their part of the job.