Aspects regarding assessment of protection by control of sources at belt conveyors for bulk materials MIHAELA PARAIAN, EMILIAN GHICOI, CONSTANTIN LUPU, SORIN BURIAN Department for Safety of Installations and Explosion Protected Equipment INCD INSEMEX - The National Institute of Research and Development for Safety in Mines and Explosion Protection 32-34, G-ral Vasile Milea street, Petrosani, jud. Hunedoara ROMANIA insemex@insemex.ro http://www.insemex.ro Abstract: - Transportation of granular materials in bulk with the means of conveyor belts rise a series of problems related to the explosion hazard if the materials are flammable and may generate an explosive atmosphere, as many times is the case in industry (Chemical, Sugar, Biomass, Food, Grain and Coal Industries). Every year, many serious fires and explosions occur in industrial plants as a result of flammable dust. Dust explosions lead to considerable material damage and long production shutdowns. This is the reason why when manufacturing and using belt conveyors for bulk materials adequate protective measures must be applied, in order to prevent of the explosive atmosphere and/or limit the effects of possible explosions. Key-Words: - ATEx, explosion, conveyor belts, assessment, risk 1 Introduction An explosive atmosphere is a mixture of flammable gases, vapors, mists or dusts with air, under specific atmospheric conditions in which, after has occurred, combustion propagates to the flammable mixture. Many factors have an effect on the violence of a dust explosion. First and most obvious is the dust material itself. Kst (explosion constant for dust) is a measure of the explosivity value of the combustible dust- measured in bar.m/sec. Table 1 Dust Explosion Kst Characteristic Class St 0 0 No explosion St 1 >0 <200 Weak explosion St 2 >200 < 300 Strong explosion St 3 >300 Very strong explosion In order for a potentially explosive atmosphere to form, the flammable substances must be present in a certain concentration. If the concentration is too low or too high, no explosion occurs; instead there is just a slow combustion reaction or no reaction at all. Thus the explosion can occur only in the presence of an source and when the concentration is within the explosive range of the substances, i.e., between the lower explosive limit (LEL) and upper explosive limit (UEL). A concentration of 20 g/m 3 is below the minimum explosive limit for most type of dusts. Obviously, concentrations of dust above the minimum explosive limit usually do not occur in occupied areas. However, these concentrations frequently exist in bucket elevators and conveyor housings. Several aspects of the cloud and particle physiology play important roles in the efficiency of the propagation process, including particle shape, particle size distribution, chemistry of the combustion, presence of contaminants (such as vapors, gases, or inert particulates), atmospheric turbulence, and particle moisture content. The size of the dust particles plays an important role in determining the severity of an explosion. A solid fuel only burns at its surface, where it is exposed to air. A cloud of very fine dust particles has a much greater surface area than a cloud of coarser particles. In addition, fine particles weigh less and tend to stay suspended in air longer. In order to cause an explosion, the combustible mixture of air and dust must be contained in some type of vessel. The fireball from even an unconstrained dust "explosion" results in a flash fire ISBN: 978-960-474-318-6 130
that can cause severe burns to personnel caught in its path, and minor pressure effects. Dust layers on floors or walls can be thrown into the air by a dust explosion, thus providing fuel for secondary explosions (the cascade effect). The likelihood of a layer causing a fire can be controlled by the correct selection of equipment and effective housekeeping. As a matter of fact, five components have to simultaneously be present in order for a dust explosion to take place: fuel, confinement, suspension, source and oxidant. All these define the so-called explosion pentagon. Fig. 1 - The explosion pentagon for dust ilustrates the components thed must be present in order for an to occur A basis of safety is a safeguard philosophy that involves specific devices, equipment, and procedures that are intended to negate or greatly weaken one or more of the pentagon components to the extent that a dust explosion cannot occur or, if it does occur, then personnel and facilities are protected from adverse effects. 2 Elimination or reducing at a minimum the explosion risk The situation of a simultaneous presence of the explosive atmosphere and the efficient source, as well as the foreseen effects of an explosion, leads directly to the three base principles in explosion prevention and protection(see EN 1127-2). a) prevention: - ing the explosive atmospheres. This objective may be reached mainly by altering the flammable substance concentration up to o value outside the explosion range, or by modifying the oxygen concentration up to a value under the limiting oxygen concentration (LOC); - ing all the possible efficient sources. This can be accomplished through a proper design of equipment, protective systems and components; - de-energizing the equipment containing sources when an explosive concentration of the flammable substance is present. b) protection: - limiting the explosion effects to an acceptable level. This can be accomplished in a certain way by protective constructive measures. Unlike the above described measures, in this case explosion occurrence is assumed; - risk elimination or mitigation is achieved by applying one or more of the above mentioned principles of prevention and protection; - the first option has to be always explosive atmosphere ance; - the higher the probability of occurrence of explosive atmosphere, the higher is the extent of the measures adopted against efficient sources, and the opposite. In order to allow suitable precautions selection, an explosion safety concept has to be developed for each case in part. 2.1 Zoning and warning signs The European Directive 1999/92 asks the employers that the places where explosive atmospheres can occur must be classified into zones by the employer. These zones are the same as in use for the Directive 94/9/EC (ATEX 95). At locations where explosive atmospheres may occur, the "Ex" warning sign must be displayed at the point of entry. These signs need to conform to the shape, colours and proportions laid down in the Directive and can display further information if required to clarify risks or actions to be taken. 2.2 Dust classification The classification of the areas with risk of explosion due to the presence of dust is based on the probability of the presence of an explosive atmosphere. The standard SR EN 60079-10-2 / CEI 60079-10-2 Explosive atmospheres - Part 10-2: Classification of areas - Combustible dust atmospheres defines three zones: Zone 20: a place in which an explosive atmosphere under the form of combustible dust in the air is present continuously, frequently or for long periods. Zone 21: a place in which an explosive atmosphere under the form of combustible dust in the air is likely to occur under occasionally. ISBN: 978-960-474-318-6 131
Zone 22: a place in which an explosive atmosphere under the form of combustible dust in the air is not likely to occur under but, if it does occur, it will persist for only a short period of time. Although there is no fixed rule regarding the presence (duration and probability) of explosive atmosphere in relation to the zones, the following table represents a useful reference. Table 2 Classification criteria occurrence Frequently likely or for long periods or continuously Occasionally likely in Unlikely in and only for short periods Area classification for dusts Probability of explosive atmosphere Overall duration of explosive atmosphere 20 P > 10-1 > 1000 h / year 21 10-1 > P > 10-3 from 10 to 1000 h / year 22 10-3 > P > < 10 h / 10-5 year Classification of hazardous areas at belt conveyors Table 3 ATEX RATING BELT CONVEYORS FOR BULK MATERIALS inside outside Zone 20 Zone 21 or 22 2.3 Prevention of source Combustible dusts and fibers can be ignited by several electrical or mechanical sources such as: Hot surfaces Arcing or sparking in switches, contacts, brushes, etc. Electrostatic discharge Thermite sparks Mechanical sparking or friction In order to effective sources or mitigate their effect, a number of explosion protection measures can be applied. 2.3.1 Types of Protection for electrical equipment for Dust Hazard Locations Electrical equipment for use in hazardous areas needs to be designed and constructed in such a way that it will not provide a source of. There are four recognized types of protection for hazardous area electrical apparatus for dust. Each type of protection achieves its safety from in different ways and not all are equally safe. In addition to the equipment being suitable for the Group and the Temperature Class required, the type of protection must be suitable for the zone in which it is to be installed. The different types of protection and the zones for which they are suitable are as follows: Table 4 Equipment cod Description Zone where it can be employed tda20 20, 21, 22 tdb20 20, 21, 22 iad Intrinsic Safety 20, 21, 22 mad Encapsulation 20, 21, 22 tda21 21, 22 tdb21 21, 22 ibd Intrinsic Safety 21, 22 mbd Encapsulation 21, 22 pd Pressurization 21, 22 tda22 22 tdb22 22 icd Intrinsic Safety 22 2.3.2 Types of Protection for non-electrical Equipment for Dust Hazard Locations ISO 80079-36 (superseding EN 13463-1) specifies the basic method and requirements for design, construction, testing and marking of non-electrical equipment and Ex Components intended for explosive atmospheres This standard is supplemented or modified by the standards concerning specific types of protection: ISBN: 978-960-474-318-6 132
Types of Protection for nonelectrical Equipment Standards flameproof (dh) IEC 60079-1 protection by (th) IEC 60079-31 pressurization s (ph) IEC 60079-2 protection by constructional safety (ch) liquid immersion (kh) ISO 80079-37 protection by control of sources(bh) According this standards, the manufacturer must carry out the following for the risk assessment of non-electrical equipment: 1. All types of potential sources must be determined systematically and then evaluated. 2. The frequency of potential sources must be considered, i.e. for - - a fault condition - a fault condition not likely to occur 3. Measures must be defined that will prevent of the potentially explosive atmosphere: - Requirements for equipment manufacturing - Stipulations for the operator provided in the operating instructions 2.4 Directives relevant to dust explosion The explosion protection requirements are regulated through two European Directives: - 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. - 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. Dangerous areas classification, as well as equipment installation/selection is the employer's, respectively users' responsibility, as regulated by the European Directive 1999/92/EC (HG 1058/2006). On the other hand, the manufacturers of equipment and protective systems intended for use in Ex classified areas have to place on the market products with levels of protection adequate to the foreseen intended use, as regulated through European Directive 94/9/EC (HG 752/2004, HG 461/2006). The employer must ensure that work equipment and all installation materials are suitable for use in hazardous places. In doing so, he must take account of the possible ambient conditions at the workplace in question. The work equipment must be so assembled, installed and operated that it cannot cause an explosion. 2.5 Equipment selection The equipment must be selected based on the hazardous zone, the substances present, the temperatures and the environmental characteristics of the installation locations. The selection must be made considering the equipment category (ATEX 94/9/EC) which must be SUITABLE for the type of zone (ATEX 99/92/EC), according to the criteria given in table 5. Table 5 Zone explosive Presence of atmospheres (explosion hazard) 22 Infrequently and for a short period only 21 Likely to occur 20 Continuously, for long periods or frequently Users Directive 1999/92/EC (HG 1058/2006) Avoidance of effective sources ( hazard) During Also during (single fault) Also during rare (two independent faults) Level of Protection Required Group II Category Normal 3 High 2 Very high Manufacturers Directive 94/9/EC (HG 752/2004, HG 461/2006). 2.6 Equipment protection level EPL The ATEX Directive 94/9/CE defines the categories 1,2,3 (group II) based on the level of protection. The new standards EN/IEC introduce the concept of EPL (Equipment protection level). Levels of protection required for Group IID (new Group III) equipment according to the intended use are shown in the following table. 1 ISBN: 978-960-474-318-6 133
The following table shows the relationship between EPL and zones, and the EPL awarded to each type of protection. Table 6 Equipment protection (EPL) Da Db level Zone 20 (and 21 and 22) 21 (and 22) ATEX Category 1D 2D Dc 22 3D The news standard EN/IEC introduces group III regarding equipment for dusts. The dusts are divided into 3 subgroups: (IIIA, IIIB, III C). Table 7 Group IIIA IIIB IIIC Hazardous substances Combustible volatile substances (fibers) Non-conductive dusts Conductive dusts Table 8 Normal temperature rise of bearings frictional heat from moving parts inside the gearbox frictional heat from the brakes dust entering the brake housing frictional heat from the belt idler rollers dust deposits on the gearbox static electricity discharge surface temperature of all moving parts Expected malfunction bearing failure due to loss of lubrication belt rubbing on spilled product ingress of stones or metal fragments to gearbox unacceptable oil loss from the gearbox brakes left on too long after the drive motor has started brake disengagement fails clutch slippage belt idler roller seizes and is rubbed by the moving conveyor belt slippage of conveyor belt on the driving drum due to loss of tension or stalling of the belt belt driven at overspeed friction between the belt and fixed parts moving parts close together, gap filled with dust 3 Explosion risk assessment for belt conveyors for bulk materials Explosion risk assessment is not an easy endeavor. First of all, the sources have to be identified and it has to be checked if the adopted protective measures do prevent effective sources, as follows: - during, if the source is located in the Zone 22; - also, during (single fault), if the source is located in Zone 21; - also, during rare (two independent faults) if the source is located in Zone 20. A belt conveyor is a more complex installation, which includes a number of electrical and nonelectrical devices. As regards electric equipment, the issues are solved by using only products conforming with the requirements in the European Directive ATEX 94/9/CE. An analysis of the non-electric equipment in belt conveyor shows the following possible sources: If the need is, for example, to assess the risk due to slippage of the conveyor belt on the driving drum, due to loss of tension or stalling of the belt, the type of belt has to be taken into consideration, according to its resistance to drum friction. The standard EN 12882 Conveyor belts for general purpose use - Electrical and flammability safety requirements classifies the conveyor belts in categories, according to their protective performances, respectively resistance to burning, static electricity and drum friction. Certainly, using a suitable belt may be considered as the proper measure adopted to prevent, but if the endangered area classification as Zone 20 is considered, two protective measures are required to be employed. In this case these are either a system for monitoring the tension in the belt or comparing the relative speeds of the drive roller and the belt. According to the requirements in ISO 80079-37 (EN 13463-6) regarding protection by control of sources represents a suitable solution to this situation. Control of source "bh" is defined as: protection where mechanical or electrical devices are used in conjunction with ISBN: 978-960-474-318-6 134
non-electrical equipment to manually or automatically prevent a potential source from becoming an effective source Ignition prevention devices/systems is defined as: arrangement that converts signals from one or more sensors into an action, or indication, to prevent a potential source from becoming an effective source Fig. 2 shows several solutions for controlling the sources for an enclosed belt conveyor. Fig. 2 - Enclosed belt conveyor sensor locations Source: Hazard Monitoring Equipment Selection, Installation and Maintenance, Johnny Wheat, 4B Components Ltd, East Peoria, IL, USA If protection by control of sources type of protection is applied, an assessment of the employed equipment is required, equipment which in their turn shall not be able to generate an source. Moreover, an assessment of the system s safety in has to be performed. In this regard, pren ISO 80079-37 defines two prevention systems: type b1 and type b2 that shall be used as appropriate for the EPL, as shown in table 8. Table 8 Intend ed EPL of the equipment Gc, Dc Result of the hazard assessment for the existing equipment: Effective source during Ex "bh" control system necessary: Ignition prevention type A single sources during None Igniti on prevention type b1 (m,a) Gb, Db Ga, Da during Effecti ve source during during during and during during and An independent or fail -safe sources during and A single sources in None An independent or fail -safe sources during and rare A single sources in rare None b2 or two b1devi ces (b1(a), b1(m,a )) b1(m,a ) b2 or two b1 devices b1 (a), b1 (m,a) b1(a) ISBN: 978-960-474-318-6 135
during, and rare In order to assess conformity of prevention systems with the requirements of EN 80079-37 may be used either the Performance Level (PL) of ISO 13849 or the Safety Integrity Level (SIL) of EN 62061. The standard to be used depends on the choice of technology, experience and customer requirements. PL (Performance Level) is a technology-neutral concept that can be used for electrical, mechanical, pneumatic and hydraulic safety solutions. SIL (Safety Integrity Level) can, however, only be used for electrical, electronic or programmable safety solutions. According to EN ISO 13849-1 the risk is estimated using three factors: injury severity (S, severity), frequency of exposure to the risk (F, frequency) and the possibility of ing or limiting the injury (P, possibility). For each factor two options are given. Where the boundary between the two options lies is not specified in the standard, but the following are common interpretations: S1 bruises, abrasions, puncture wounds and minor crushing injuries S2 skeletal injuries, amputations and death F1 less frequently than every two weeks F2 more often than every two weeks P1 slow machine movements, plenty of space, low power P2 quick machine movements, crowded, high power By setting S, F and P for the risk, the PLr (Performance Level required) that is necessary for the risk source. Finally, the risk assessment includes a risk evaluation where it has to be determined if the risk needs to be reduced or if sufficient safety is ensured. In order to determine the PLr the standard provides a risk graph into which the application factors of severity of injury, frequency of exposure and possibility of ance are input. Figure 3 - Risk Graph from Annex A of EN ISO 13849-1 An prevention system of type b1 complies to the Performance Level (PL b) and type b2 complies with (PL c) of ISO 13849. 4 Conclusion Assessment of the risk, when using equipment and protective systems in environments with flammable substances that could generate fire and explosions, is particularly important for ensuring workers' health and safety. According to the legislation in force, the responsibility for risk assessment and adopting the required protective measures in order to ensure an acceptable safety level is incumbent on the equipment manufacturers and users. For this purpose is required that the belt conveyors used in environments with potentially explosive atmospheres, as well as all the components to be submitted to an official hazard analysis, welldocumented, that would identify and list all the potential applied in order to prevent the potential come efficient. The equipment used must be selected based on the hazardous zone, the substances present, the temperatures and the environmental characteristics of the installation locations. The ATEX Directive 94/9/CE defines the categories 1,2,3 (group II) based on the level of protection. The new EN/IEC standards introduce the concept of EPL (Equipment protection level). The selection of equipment to be employed in potentially explosive atmospheres must be made considering the equipment category (ATEX 94/9/EC) which must be SUITABLE for the type of zone (ATEX 99/92/EC). control of source "b" is one of the types of protection applicable for preventing sources in belt conveyors. For conformity ISBN: 978-960-474-318-6 136
the requirements stipulated in EN 80079-37, in the assessment procedures the new standards EN ISO 13849 and EN 62061, replacing the old standard EN 954-1, have to be implemented. References: [1] ***, 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. Official Journal No. L 023, 2000-01-28, 57-64 [2] ***, 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. Official Journal No. L 100, 1994-04-19 [3] ***, Non-binding Guide of Good Practice for implementing of the European Parliament and Council Directive 1999/92/EC on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres, European Commission, DG Employment and Social Affairs, Brussels, April 2003. [4] ***, EN 13463-1 Non-electrical equipment for use in potentially explosive atmospheres - Part 1: Basic method and requirements. [5] ***, EN 13463-6 Non-electrical equipment for use in potentially explosive atmospheres - Part 6: control of source 'b' [6] ***, ISO 80079-36 Explosive atmospheres Part 36: Non-electrical equipment for use in explosive atmospheres Basic methods and requirements [7] ***, ISO 80079-37 Explosive atmospheres Part 37: Non-electrical equipment for use in explosive atmospheres Non-electrical type of protection constructional safety 'c', control of sources 'b', liquid immersion 'k' [8] ***, EN 954-1 Safety of machinery. Safetyrelated parts of control systems. Part 1: General principles for design [9] ***, EN ISO 13849-1 Safety of machinery - Safety-related parts of control systems - Part 1: General principles for design (ISO 13849-1:2006/Cor 1:2009) [10] ***, CEI 61508 Functional safety of electrical/electronic/ programmable electronic safety-related systems. Part 1: General requirements [11] ***, SR EN 60079-10-2 CEI 60079-10-2 Explosive atmospheres - Part 10-2: Classification of areas - Combustible dust atmospheres [12] ***, EN 620+A1:2011 Continuous handling equipment and systems - Safety and EMC requirements for fixed belt conveyors for bulk materials [13] Johnny Wheat,, Hazard Monitoring Equipment Selection, Installation and Maintenance, 4B Components Ltd, East Peoria, IL, USA ISBN: 978-960-474-318-6 137