WHITEPAPER: ATEX IN PRACTICE (EXPLOSION PROTECTION)

Transcription

1 WHITEPAPER: ATEX IN PRACTICE (EXPLOSION PROTECTION)

2 It is essential that both machine manufacturers and owners of process installations have the necessary knowledge about the Atex directives in order to be able to correctly handle explosion risks in practice that are linked to the use of machines and installations. So they need to devote constant attention to all developments in this field. On 20 April 2016 new Atex directives came into force. Atex 95 was replaced by Atex 114 (Directive 2014/34/EU) and Atex 137 was replaced by Atex 153 (Directive 1999/92/EG). This has consequences mainly for manufacturers of explosion-proof equipment and safety systems. For instance, equipment that complies with the regulations of Atex 114 has to be visibly marked with the X symbol. Installations and environment The difference between the two new directives is that Atex 114 applies to the manufacture of installations and products (CE Directive for equipment) whereas Atex 153 (the social Directive) applies to the environment, procedures and employee instructions. An explosion protection document (EVD) is also required as part of Atex 153. This contains the identification and assessment of explosion risks as well as a classification of danger zones based on the frequency and duration of the occurrence of a hazardous atmosphere. Finally, this document also indicates what protective measures are required for a process installation. To this end the danger has first of all to be analysed and charted, then measures can be taken.

3 Measures Two questions In order to exclude a dust explosion measures should be taken to prevent a combustible substance in combination with oxygen (air from the environment) being exposed to an ignition source. This means that at least one of these three elements always has to be removed from the explosion triangle. This is a primary measure. This is often not achievable because of technical process considerations. A secondary solution is to render potential ignition sources harmless by using equipment with special housings or intrinsically safe designs. Good earthing of process installations is also important. It is also advisable to use safety systems such as spark detection, CO detection, temperature monitoring and misalignment protection. The purpose of tertiary measures is to limit the consequences of an unforeseen explosion as much as possible, for example explosion-resistant constructions, rupture discs and flame arrestors. This category also includes measures such as compartmentalisation and fast-closing valves. A good knowledge of the material and an insight into the danger of each situation contribute to the making of the correct choices in order to achieve a safe, economically feasible installation. Latent danger Two questions need to be asked when it comes to evaluating the risk of a dust explosion: what is the chance of a dust explosion, and what is the severity of such an explosion. The answer to the first question depends on, among other things, the smoulder temperature of the dust layer, the self-ignition temperature of the dust layer / dust cloud and the minimum ignition energy. The severity of the explosion is determined by, among other things, the maximum explosion pressure and the speed of pressure rise as well as the Kst value of the dust concerned. These factors are in turn influenced by, for example, the size and shape of the dust particles: the smaller the particles, the greater the contact surface with the oxygen from the air. The concentration and the moisture content of the dust also play a role. An important concept in the case of the concentration is the lower and upper explosion limit (LEL and UEL). A combustible dust cloud can, however, explode only if there is a sufficiently powerful ignition source. This is determined by the MIE value. The number of types of potential ignition sources is very large and varies from frictional heat to mechanical sparks, from open flame to static electricity, and from hot surfaces to chemical reactions. For a good answer to be found for both questions an accurate risk analysis is essential. The bulk-goods processing industry is above all aware of the risks of a dust explosion. In comparison with gases and vapours dust particles are less easy to remove using ventilation systems. The particles can precipitate and form a danger as a dust layer. In the case of turbulence the dust particles will mix with air. If these are combustible particles, a hazardous dust cloud may form. The ignition energy for this kind of cloud is, however, usually a lot higher compared with gas or vapour clouds. So the formation, restriction and ability to prevent dust are important aspects and part of the Atex evaluation. The X symbol in the hexagon identifies machines that comply with the Atex standard. The X symbol in the triangle warns of a possible explosive atmosphere (an unsafe area).

4 Risk analysis To keep the risks of a dust explosion to a minimum suitable measures should be taken (based both on legislation and regulations and on economic criteria). In a risk analysis the focus is on the formation of dust clouds and primary or secondary sources of danger. For example, it can be ascertained whether, for instance, there is a continuous dust cloud: a place where a dust cloud constantly hangs or is frequently present for short periods, e.g. over 10% of the operating period of an installation or the duration of an activity (on average > 1,000 hours a year). A primary source of danger is a place from which in the case of normal operations a combustible substance is expected to be released now and again, e.g % of the operating period of an installation or the duration of an activity or work (on average > 10-1,000 hours a year). A secondary source of danger is a place from which in the case of normal operations no combustible substance is expected to be released; and, if a combustible substance is released, this is only rarely and for a short time, e.g. less than 0.1 % of the operating period of an installation or the duration of an activity or work (on average < 10 hours a year). The above evaluation forms the basis for the zone classification of the situation on site. Zone classification We base this only on solid materials and concentrate on the dangerous area. The non-dangerous area (NGG) is an area for which the amount of dust is too small to cause a dangerous explosion. The dangerous areas are classified into zones. The aim of the dangerous zone classification is to evaluate the permissibility of ignition sources in an area in which combustible substances are present.

5 Dangerous zone classification of solid materials This classification is based on the likelihood of the presence of an explosive atmosphere. The zone class is determined by: the nature of the sources of danger, i.e. the frequency and duration with which, and the circumstances under which, a combustible substance is released or is present the nature of the dust layers, i.e. the frequency and duration with which, the circumstances under which, and the quantities in which combustible dust layers are present local ventilation conditions / meteorological conditions in the area surrounding the sources of danger the degree of cleaning (no footprints to be seen). This means that the zone classification depends in part on the quantity of combustible substance present, whether dust is extracted locally and the removal of dust layers on process installations. The dimensions of the zones depend on the migration of the dust in the surrounding areas. In determining the zone classification a distinction is made between dust clouds (table 1) and dust layers (table 2). Table 1 Zone classification for dust clouds Zone classification with areas Presence of combustible dust with dust clouds Continuous dust cloud 20 Primary source of danger 21 Secondary source of danger 22 A non-limited zone 21 is usually surrounded by a zone 22 with a distance of 3 metres around the zone 21 The explosion triangle. If one of the three elements fuel, oxygen or ignition source (heat) is removed, no explosion can occur. Table 2 Zone classification for dust layers Cleaning level Thickness of the dust layer * Duration of a dust layer Zone classification Good Negligible N.a. NGG Adequate Not negligible < 8 consecutive hours 22 Poor Not negligible > 8 consecutive hours 21 * Negligible thickness of the dust layer is a thickness of < 0.1 mm. NGG = Non-hazardous area.

6 Legislation In the case of zone classification the relevant legislation should be taken into account. Explosion-proof equipment is classified into groups and categories. Group I relates to equipment for underground work and to surface mining where mine gas can be a danger. Group II relates to equipment for surface environments that can come into contact with an explosive atmosphere. This equipment is then subdivided into three categories (D stands for dust here): Cat. 1 D: very high protection level, even in the case of extraordinary problems with the equipment Cat. 2 D: high protection level, even in the case of frequent problems and faults in the operation of the equipment Cat. 3 D: normal protection level, in the case of normal operation. Table 3 Zone classification with equipment categories Zone Category of equipment 20 1 D 21 1 D or 2 D 22 1 D or 2 D or for non-conductive dust 3 D What are Kst and Pmax? The Kst value (maximum speed of pressure rise) and Pmax (maximum explosion pressure) are values that provide information about the severity of a dust explosion: the force of the explosion and the speed at which an explosion travels. Both values indicate the hazard group (dust class) to which the dust in question belongs. In the BIA report 13/97 these values are ascertained for most raw materials using l aboratory tests. The Kst value can give an indication of the explosion speed and therefore also the extent to which protection is required. The Pmax is an indication for the design of the installation. For a lot of raw materials the Pmax is many times greater than what an installation can withstand. So prevention and/or pressure relief are required in order to remain below the Pmax of 0.5 bar, for which pressure structures can indeed be built. Economic criteria Finally, there are also always economic arguments that play a role in the achievement of an Atex situation (table 4). So there should always be a balance between the cost price and the effectiveness of the application. For new installations it is often simpler to be able to comply with the Atex directives. In the case of existing installations sometimes technical and economic restrictions have to be taken into account. In this case we need to look at the situation within and outside the equipment. These situations can differ between plants, installations and applications and focus on the following aspects: preventing avoiding restricting suppressing. There are various solutions and techniques available for this, which can best be achieved via personal advice. One example is to ensure, using ventilation, that the situation remains below the LEL value. For advice go to: Table 4 Economic criteria for technical solutions in new and existing installations (1 = most favourable / 5 = least favourable) System Reliability Installation costs (new) Installation costs (existing) Yield Insulation Containment (pressure (shock) resistant) Pressure relief Suppression Inertisation (dispensing of gas)

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