ATEX zones explained

In which ATEX zones can explosive areas be classified?

A hazardous area is an area where such a dangerous explosive atmosphere may occur that measures to protect workers are required.

Explosive areas are divided into zones based on the probability of a hazardous explosive atmosphere occurring. This classification is then used to determine the necessary precautions.

In the Netherlands, the NPR 7910-1 is used for zone classification of gas environments and the NPR 7910-2 for dust environments. Zoning should only be done when a minimum amount of flammable substance is exceeded. The following definitions are used.

ATEX Zone 0 and 20:

An area where an explosive atmosphere is present continuously or for a long time (> 1000 hours / year or > 10 % of operating time).

• ATEX Zone 0 and zone 20 generally only occur inside installations (evaporators, reaction vessels, mills, dryers) and in the vicinity of ventilation holes and other openings.

ATEX Zone 1 and 21:

An area in where an explosive atmosphere is expected or occasionally present during normal operation (between 10 and 1000 hours / year or between 0.1 and 10% of the operating time).

• ATEX Zone 1 occurs next to filling openings, around fragile vessels or glass pipes and around leaking sealing glands, pumps and slides.
• ATEX Zone 21 occurs around dust removal or filling stations and in places with dust accumulation. 

ATEX Zone 2 and 22:

An area where an explosive atmosphere is unlikely to occur during normal operation and wheresuch an atmosphere, if present, will exist only rarely and for a short time (less than 10 hours / year or less than 0.1% of the operating time).

• ATEX Zone 2 occurs around zones 0 and 1.
• ATEX Zone 22 occurs in the vicinity of dust-containing installations if it can leak out, causing a dangerous accumulation of dust.

ATEX Zone 0, 1 and 2 apply to explosive gas atmospheres and zones 20, 21 and 22 to explosive dust atmospheres.

Non (explosive) hazardous area:

An area where an explosive atmosphere is unlikely to such an extent that no special provisions regarding sources of ignition are necessary.

Areas near technically secure closed pipelines transporting flammable substances are not considered hazardous places.

Special area: 

An area in which, due to the presence of an ignition source (e.g., furnace burner), zoning is irrelevant.

The zone ultimately determines the minimum safety level required for equipment used in this environment. The moment that an ATEX zone is present, all equipment in that zone must be certified according to the correct safety level. No ignition sources may be present in the zone while an explosive atmosphere can occur. This also applies while working in a zone and for any portable equipment!

Example of zoning for a tank with flammable liquid

ATEX Zones classifications

The figure above shows an example of a tank with flammable liquids. The tank is set up in the open air, is filled and emptied regularly and connected to the outside air with an opening (pressure equalization). The flash point of the flammable liquid is around the average annual temperature and the density of the vapors formed is greater than that of air. Therefore, the constant presence of a hazardous explosive atmosphere inside the tank must be assumed. The inside of the tank is consequentlyclassified as ATEX zone 0.

Occasionally vapors may be released from the pressure equalization opening, formingexplosive mixtures. The area around the opening is therefore classified as ATEX zone 1. Infrequent adverse weather conditions may cause vapors to run down the outside of the tank wall, creating a hazardous explosive atmosphere. So, the area directly surrounding the tank is classified as ATEX zone 2.

The size of the zones outside the tank is based on the expected amount of vapor released. This depends on the properties of the liquid, the size of the opening, the frequency of filling and emptying and the average level changes inside the tank. Furthermore, the size of the explosion hazard areas mainly depends on the presence of natural ventilation.

The following figure shows a situation where a dusty, flammable product is manually poured from a container into a hopper. During this process, an explosive mixture of dust and air may occasionally form in the discharge area of the container. This area should be classified as ATEX zone 21. In the area around the hopper dust accumulation occurs. This may occasionally and briefly whirl up forming a hazardous explosive atmosphere. This area is classified as ATEX zone 22.

Mill with hopper, product feed, product discharge and filter

Inside the mill itself dust is present in the form of a dust cloud during operation. Cleaning filter hoses may also cause a dust cloud at regular intervals. The inside of the mill and the filter are therefore classified as ATEX zone 20.

From the machine there is a continuous discharge of the milled product. Consequently, an explosive mixture in the form of a dust cloud will form inside the drain tank during operation. Hence, the drain pan is classified as ATEX zone 20. In the drain area dustaccumulates due to leaks. This area is therefore classified as ATEX zone 22. The size of zones 21 and 22 is based on the extent to which the used product tends to whirl up.

The extent of preventive measures depends on the likelihood of hazardous explosive atmospheres arising (zoning). The information in Table 4.1 must therefore be taken into account when determining the extent of preventive measures.