En-Light-ening ATEX – Part 1

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ATEX explosive atmosphere

In this upcoming series of articles, Tristan Pulford discusses ATEX, the hierarchy of controls and what you need to be aware of. This first part takes a look at ATEX, what it is, and how to know if it applies to you and your company.

The ATEX directives are a European set of directives aimed at the management of explosive atmospheres to ensure they are classified, and the appropriate equipment is installed in potentially explosive atmospheres. Mostly associated with the petrochemical world, ATEX is relevant for any substance which is potentially explosive in a dust, gas, vapour or mist form, noting that some fluids with given flashpoints also fall under ATEX.

ATEX is actually the name for a group of two European directives which are:

  • Directive 99/92/EC (also known as ‘ATEX 137’ or the ‘ATEX Workplace Directive’) covers workplaces and the responsibility of employers who have a relevant substance. This typically covers the hazardous area classification (HAC), and other organizational methods.
  • Directive 94/9/EC (also known as ‘ATEX 95’ or ‘the ATEX Equipment Directive’) which relates to equipment that is installed within a potentially hazardous area (zone) and the associated protective systems.

For the majority of companies, ATEX 137 is more relevant and will be the main focus of this article, but when lighting is discussed in more detail the equipment directive will also be discussed.

Both these ATEX Directives however aim to reduce and control the risk as low as reasonably practical (ALARP) through a risk assessment (which the hazardous area classification is part of) and associated controls (such as the equipment within the zone).

Do I need to consider ATEX?

This question crops up a lot, in most industries, it is obvious when ATEX is required as they deal with potentially flammable substances, such as fuels, starches or other organic dust. Within the directive the official definition of an explosive atmosphere is:

a mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts in which, after ignition has occurred, combustion spreads to the entire unburned mixture

So practically what does that mean? It means that if we look at the Material Safety Data Sheet (MSDS) for the substance, we should be able to find this information out. Within the MSDS there are two important sections that show whether the equipment falls under ATEX, namely Section 2 and Section 9.

Section 2 covers the Hazardous identification, and within this section, if the substance falls under any of the categories of flammability, and it can be in a gas, vapour, mist, or dust it will be covered under the requirements of the directive. In this case, dust, generally, must have a particle size of fewer than 500 microns (0.5mm) to be considered as part of the standard, as if it is larger the combustion is likely to stop before being spread to the entire unburnt mixture. However, there is a classification for larger size particles referred to as ‘flyings’ which needs to be considered for certain substances.

A typical example (for methane in this case) of section 2 from an MSDS is shown here:

ATEX graphic
Figure 1: Section 2 of a MSDS for Methane

 

As can be seen from the example, methane is an extremely flammable gas, and hence would fall under the ATEX directive.

As previously mentioned, one important area to be aware of is that when the particle size is larger than 500 microns, section 2.3 may make mention that the substance may be explosive when in a dust form, these are referred to as ‘flying’, which is common in, for example, polyethene beads used in various plastic processes. If, in that case, it is possible that dust could be created (which it frequently is at the bottom of silos or within the process) then ATEX would need to be considered.

Section 9 of the MSDS covers the physical properties of the material, which is important with regards to ATEX as it helps specify the equipment that will be installed in the zone. An example of Section 9, is included below (again for methane):

ATEX graphic 2
Figure 2: Section 9 of a MSDS for Methane

 

This should provide important information:

  • Flash Point: This is the point at which a fluid starts to turn to vapour. This is important as any fluid with a flashpoint under 85oC must be considered, but if the flashpoint is much higher than operating conditions within the process, anything above 85oC would not need to be zoned under the ATEX directive (controls to prevent the temperature being approached would be required).
  • Flammability: This is the type of flammable substance.
  • Lower Explosion Limit (LEL): This is the lowest concentration of the substance with air that will create a potentially explosive mixture. This is usually measured as percentage volume for gases, but g/m3 for dust.
  • Upper Explosion Limit (UEL): This is the highest concentration of the substance with air that will create a potentially explosive mixture. This is usually measured as percentage volume, but g/m3 for dust.
  • Auto-ignition Temperature/ *Minimum ignition Temperature: Is the temperature that the substance will self-ignite when in a cloud mixture with air, potentially causing an explosion.
  • *Layer Ignition Temperature (LIT): The temperature at which a layer of 5mm dust will ignite, which is important for the maximum surface temperature of equipment.
  • Minimum Ignition Energy (MIE): The minimum energy required to ignite a cloud of gas, vapour, or dust.
  • *Dust Explosion Class (St0 – 3): The strength of any potential explosion ranging from 0 (no explosion) to 3 (very strong explosion), which is based on how rapidly the pressure increases over time.

*Specifically for dust

In our experience, the MSDS for vapours and gases are much more informative with many of the items being complete, while for dust these items are either missing or listed as “Information not available”. In either case, it is the responsibility of the organisation to ensure that the correct equipment is installed within any zones identified, which means that this information is required. Typically, this means that a sample of dust must be sent to a test house, which will carry out tests to gather this information for you.

In the next instalment in this series, we consider the hierarchy of controls and hazardous area classification.

For any more information on ATEX or anything else you’ve read here please contact [email protected].

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