Explosion Protection Is Atex Inadequate to Counter the Threat of Dust Explosions?

Author / Editor: Rolf K. Eckhoff / Marcel Dröttboom

The European 'Atex' Directives 94/9/EC and 999/92/EC exhibit a lack of differentiation between explosive gas mixtures and explosive dust clouds. Consequently, some very substantial differences between gases/vapours and dusts have either been neglected or addressed inadequately.

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(Picture: From BSH)

A new approach for standardization of design of electrical apparatuses for 'explosive atmospheres' has been adopted by the International Electrotechnical Commission (IEC) as well as in the European Union (CEN and Cenelec). A central concern is to combine and align standards for combustible dusts with the corresponding ones for combustible gases/vapours.

The European 'Atex' Directives 94/9/EC [1] and 1999/92/EC [2] appear to be a main source of inspiration in this effort, due to the lack of differentiation between explosive gas mixtures and explosive dust clouds in the directives. Consequently, in producing the new aligned/combined standards some very substantial differences between gases/vapours and dusts have either been neglected or addressed inadequately.

Basic Differences between Dust and Gases

Explosive gas mixtures and explosive dust clouds, once created, do exhibit very similar ignition and combustion properties. This may give rise to the misconception that they can be considered more or less identical in all respects. However, as discussed by Eckhoff [3], some very basic differences between gases and dusts need careful attention when designing electrical apparatuses for use in the presence of combustible dusts.

Firstly, the physics of generation, up-keeping and migration of dust clouds and premixed gas/vapour clouds are substantially different. Once a quiescent gas mixture has been created, it will for all practical purposes stay homogeneous because of the random molecular motion.

In dust clouds, however, the fuel particles are generally so much larger than the molecules of the air (in most cases between 1 and 100 mm), that their movement within the air is controlled by inertial forces, including gravity, rather than by random molecular motion. Therefore, in most situations where accidental explosive gas/vapour clouds may form and persist quite readily, the generation and persistence of explosive dust clouds would be highly unlikely.

Fig. 2 illustrates this, and the question is: "Is the explosion hazards created by the butane and the maize starch by opening the valve and the paper box respectively similar or different?" The answer is obvious. Opening the butane container creates a very significant gas explosion hazard, whereas the dust explosion hazard created by just opening the maize starch box is zero. Figs. 3 and 4 illustrate the difference.

The locations of primary dust explosions are practically exclusively inside process equipment where explosive dust clouds can be created as an inherent consequence of the dynamic nature of the process itself. Examples of such processes equipment include mills, powder mixers and dryers, dust collectors and connecting ducts, pneumatic powder transportation pipes, bucket elevators, and silos during filling.

Fig. 5 shows a full scale dust explosion experiment in Norway for studying the propagation of dust explosions in inside a typical integrated process system where explosive dust clouds are generated as part of the process.

Because dust particles are so much bigger than gas molecules they are not likely to travel through narrow holes and slits of the order of 1 mm diameter and smaller in the same way as gas molecules will do. Furthermore, dust particles entering an enclosure in this way will not remain suspended in the air and eventually make up an explosive cloud inside the enclosure, as gas molecules would do, but settle out as layers on the internal surfaces of the enclosure.

It is interesting to note that NFPA 499 [4] contains the following paragraph: "Walls are much more important in separating hazardous and non-hazardous zones in the case of combustible dusts than in the case of combustible gases. Only completely non-perforated solid walls make satisfactory barriers in the case of gases, whereas closed doors, light-weight partitions, and even partial partitions could make satisfactory barriers between hazardous and non-hazardous zones in the case of dusts."

For the purpose of the context of electrical apparatuses, the paragraph may be slightly re-phrased by replacing the first word 'walls' by 'enclosures'.

Well-proven Design Principles

Generally, irrespective of any fire and explosion hazards, the presence of dusts, whether combustible or non-combustible, is for a number of reasons incompatible with delicate mechanisms and components of electrical apparatus. Therefore IEC has produced a standard defining various 'degrees of protection' against ingress of solid objects, including ingress of dust particles.

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Before the appearance of the gas/dust 'alignment'/ 'combination' standards, IEC as well as European standardization based safe design of electrical apparatus for use in areas with combustible dusts, on the two basic principles:

  • Isolation of potential ignition sources by means of dust tight or dust protected enclosures as defined in the IEC standard mentioned above.
  • Exclusion of the possibility that the dust of concern, whether as layer or cloud, could be ignited by the external enclosure surface (excessive surface temperatures, electrostatic discharges, or mechanically generated metal-particle sparks).

This well-proven enclosure philosophy was the basis of two central CENELEC dust standards issued in 1998, and IEC seriously considered adopting these standards. However, probably encouraged by the lack of adequate distinction between gases/vapours and dusts in the European Directive 94/9/EC, they decided to depart from the sound and well proven common European/American approach.

Instead they initiated the production of the new series of dust standards 'aligned' or 'combined' with existing gas/vapour standards. Fortunately IEC has more recently issued a new standard for 'dust ignition protection by enclosures', which revitalizes the well proven basic design basic principles mentioned above.

No Clarification of Gas/Dust Differences

Directive 94/9/EC, which addresses the design of equipment/apparatus, has no doubt been produced largely from the point of view of gas/vapour explosions. Only modest attention has been paid to the special aspects of explosive dust clouds. The same applies to the dualistic appearance of dusts, as either clouds or layers.

This is evident right from the definition of 'explosive atmosphere' in Article 1, item 3: 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". It is clear from the context in the directive that this definition addresses explosive clouds only.

However, dust layers/deposits are also "mixtures of air and dust in which combustion can spread to the entire unburned mixture". Therefore, the definition in Directive 94/9/EC of 'explosive atmosphere' also embraces dust layers/deposits. But this fact is totally neglected in the existing standards based on the directive.

Directive 1999/92/EC addresses the general safety inside of a factory or plant. Also in this context it is essential to account for the dualistic flame propagation regime of dusts, viz. in clouds and layers. However, neither the directive nor the IEC and European area classification standards based on the directive address this fact.

The logical approach would be to include combustible dust layers/deposits that can propagate open or smouldering fires in the definition of 'hazardous areas' alongside with explosive dust clouds, irrespective of whether the layers/deposits may become dispersed into explosive dust clouds.

Another shortcoming of this directive is that the term 'source of release', which is used throughout the directive for 'explosive atmospheres' at large, does not apply to dusts.

With dusts this term is generally inappropriate and confusing and should be replaced by a more relevant term e.g. 'area of dust cloud generation'. This is because, as opposed to gas explosions, dust explosions in the process industries are practically always initiated in dust clouds generated inside process equipment, and very seldom in dust clouds that have been unintentionally 'released' to the outside.

Unfortunate Alignment of Standards

The absence of clarification of the basic differences between gas and dust in Atex directives 94/9/EC and 1999/92/EC has resulted in some unfortunate alignments and combinations of dust and gas standards for the design of electrical apparatus.

Pressurized Enclosures and Rooms

Neither the enclosure standard nor the room standard make much sense in the case of combustible dusts. This is because they rest on the erratic assumption that particles in a dust cloud that embrace an enclosure, by entering the enclosure though unintentional narrow holes and gaps, can accumulate as an explosive cloud inside the enclosure.

In industries being faced with potential dust explosion and fire hazards, attempts at applying these standards must inevitably cause considerable confusion and frustration.

Encapsulation by Moulding

Moulding is a type of protection by which electrical parts that can ignite an explosive gas/vapour atmosphere are moulded into a compound material in such a way that any external explosive atmosphere surrounding the moulded unit cannot make contact with these parts. The combined IEC moulding standard for gases and dusts is essentially a gas standard and is far too complicated and sophisticated to be relevant for dusts.

Intrinsically Safe Apparatus

Intrinsically safe electrical apparatus for use in areas with combustible dusts is covered by a voluminous IEC standard, which is again more or less a direct copy of the corresponding gas standard, with little relevance for dusts.

As discussed above, electrical circuits, switches, etc., to be used in areas containing combustible and/or electrically conductive dusts should always be kept inside dust tight or dust protected enclosures, which makes intrinsic safety superfluous.

However, in some very special cases there is a genuine need for intrinsically safe apparatus even in areas containing combustible powders/dusts, e.g. open capacitive level indicators for solid bulk materials stored in silos and bins.

One striking deficiency in the intrinsically safe apparatus standard for dust is that it does not at all differentiate between dusts of different ignition sensitivities by introducing 'dust groups' corresponding to the 'gas groups' in the gas standards. In stead, the gas group IIB requirements (minimum ignition energy, MIE = 0.06 mJ) are imposed on all dusts.

New Dust Standard for Non-electrical Apparatus

Even this standard clearly demonstrates the substantial difficulties encountered when assuming that protection principles for gases and vapours also apply to dusts. When the basic principle of the flameproof concept is explained in the introduction to the standard only gases and vapours are mentioned.

When writing the standard it was most probably realized that it would not make much sense to include dusts and mists/sprays in this explanation.

Conclusions

  • The physics of generation and sustainment of dust clouds and premixed gas/vapour clouds are substantially different. Hence, in most situations where accidental explosive gas clouds may be produced readily, generation of explosive dust clouds would be highly unlikely.
  • As opposed to the flame propagation of premixed gases, the propagation of flames in mixtures of dust and air is not only limited to the flammable dust concentration range of clouds. The state of stagnant layers and deposits in fact offers an additional possibility of flame propagation, which is also covered by the definition of 'explosive atmosphere' in the two European Directives 94/9/EC (1994) and 1999/92/EC (1999). However, the two directives primarily address gases/vapours, whereas the particular properties of dusts are treated rather inadequately.
  • As a consequence several unfortunate IEC and European dust standards, resulting from this deficiency, have been issued.

The need for revising or re-interpreting the two European directives and the corresponding need for revising some inadequate dust standards resulting from the deficiencies in the directives are discussed. n

References

[1] European Directive 94/9 EC: Directive 94/9/EC of the European Parliament and the Council, of 23 March 1994, on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres. 1994.

[2] European Directive 1999/92/EC: Directive 1999/92/EC of the European Parliament and of the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres (15th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC). 1999.

[3] Eckhoff, R.K.: Dust explosions in the process industries. 3rd ed., Gulf Professional Publishing/Elsevier Science, Amsterdam/Boston/Heidelberg 2003, ISBN 0-7506-7602-7.

[4] NFPA 499: Recommended practice for the classification of combustible dusts and of hazardous (classified) locations for electrical installations in chemical process areas. Publication No. 499, National Fire Protection Association (NFPA), Quincy 1997.

* R.K. Eckhoff, The author is Professor emeritus of the University of Bergen, Norway, Tel. +47 5558/2858, E-Mail: rolf.eckhoff@ift.uib.no

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