ATEX dust explosions
Zones classifications

Criteria to define ATEX zoning regarding dust explosion risks

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Section summary
Zones and criteria of classifications
Examples of zoning

Bulk Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust

1. Introduction

The european ATEX directives 94/9/CE and 1999/92/CE, revised in 2014, have introduced the notion of zoning in order to identify and classify areas within a factory that present a risk of presence of powder cloud in explosive conditions.

The factory operator must carry out a dust explosion risk analysis, often called ATEX study, in order to classify the areas according to the probability of presence of a dust cloud (zoning). On the other side, the equipment supplier must supply an equipment that is compliant for the zone defined by the process operator.

2. Zoning definition and classification

ATEX zone Definition
Zone 20 Location where an explosive atmosphere is permanently present or during long periods or frequently

Order of magnitude : Dust in explosive concentration is present 1000 h or more / y (= normal process conditions)
Zone 21 Location where an explosive atmosphere is likely to be present occasionnaly in normal working conditions

Order of magnitude : Dust in explosive concentration is present 10 h 1000 h / y (= in certain process conditions)
Zone 22 Location where an explosive atmosphere is not likely to be present in normal working conditions or, if it is present, it is only for short duration

Order of magnitude : Dust in explosive concentration is present from 1h to 10h / y (= very rare process conditions or abnormal conditions - leaks)

The zoning is fundamental to help the process operator to put in place the measures necessary to reduce the risks of inflammation of a dust cloud, or to put in place protections in case of explosions.

Note that layers and deposits of powder must be considered as well as it could start burning in case of high temperature or be put in suspension and form an explosive dust cloud.

3. Examples of zoning

[Laurent] is giving some common examples of equipment often falling in one category or another. WARNING : this is only a general indication, only the specific risk assessment of a process will be able to confirm if one equipment belongs to one zone or another depending on a particular context.

ATEX zone Example
Zone 20 Inside silos, hoppers, mixers, dryers, cyclones, filters
Inside transport systems of bulk solids...etc...
Zone 21 Immediate surrounding of silos, hoppers, mixers, dryers, cyclones, filters and system of transport...etc...
Found also around tipping station, when powder is poured into a container or vessel
Zone 22 Occasional leaks at manholes, filters, in case of failure of flexible...etc..

The zoning definition and the reasons justifying a particular zoning, or the absence of zoning, must be described in the ATEX risk analysis and the measures to put in place explained. Conclusions of the risks analysis must be implemented by the factory.

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