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.
- 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 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. 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. 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. 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: email@example.com