ATEX – principles and practice
08 December 2015
For many ATEX, the EU’s explosive atmospheres legislation, is about Ex rated equipment in a strictly EU context. This would not be a correct assumption and it is important to understand the ‘principles and practice’ behind ATEX, which are more in-depth. There are two ATEX directives with regard to ‘equipment’ (94/9/EC) and ‘worker protection’ (1999/92/EC), but EU directives, for which there are now many hundreds, cannot be seen in isolation of one another, as they not only interact, but also give ‘legal effect’ to a range of supporting guidance and technical standards.
Furthermore, the two ATEX directives have different legislative roots. The ‘equipment’ directive, which is to be updated in 2016 by Directive 2014/34/EC, is one of the ‘new approach’ directives, which has its origins in the free movement of goods and conformity assessment, i.e. CE marking. This ‘new approach’ restricted the content of legislation to ‘essential requirements’ leaving the technical details to European harmonised standards.
Standards now increasingly internationalised
In turn this led to the development of European standardisation policy by CEN (non-electrical) and CENELEC (electrical) to support this legislation. However, these standards are now increasingly internationalised. The CENELEC standards for ATEX electrical compliance are now the International Electrotechnical Commission’s IEC 60079 series of standards, forming part of their IECEx scheme, while CEN’s range of non-electrical ATEX standards are now in the process of joint adoption by the IEC and the International Organization for Standardization (ISO).
For instance, neither China nor Russia have adopted the EU’s ATEX directives, but they have adopted the IEC 60079 series and the CEN non-electrical ATEX standards as mandatory GB codes and GOST standards respectively. This then comes to the hub of the matter, no matter where you operate in the globe, legislation requires you to have a safe plant and you will be benchmarked against the recognised technical standards and guidance. Plus these days it’s very difficult to find a corner of the globe not affiliated to the IEC and ISO; even North Korea is.
In Ireland and the UK the relevant acts on health and safety place an obligation on the employer related to “so far as is reasonably practical”, while in German that obligation is defined as “Stand der Technik”, i.e. to operate to state-of-the-art technology. The message is clear: one ignores the ‘soft law’ associated with these standards at one’s peril.
In Europe, the Framework Safety Directive 89/391/EEC defined the nine ‘principles of prevention’, which in order of priority went from the principles of inherent safety of avoiding risk and combating risk at source, through the principle of adapting to technical progress, to the least desirable principle of relying on appropriate instructions to the workforce.
Chemical agents, construction activities and occupational noise
There are 19 ‘daughter’ directives to this framework safety directive, which elaborate on specific details associated with chemical agents, construction activities, occupational noise, etc. The ATEX ‘worker protection’ directive is one of these ‘daughter’ directives and simply expands on these core principles.
In particular in its Article 3, it requires that appropriate technical and organisational measures be taken in the following order of priority:
- ‘The prevention of the formation of explosive atmospheres, or where the nature of the activity does not allow that;
- The avoidance of the ignition of explosive atmospheres, and;
- The mitigation of the detrimental effects of an explosion so as to ensure the health and safety of workers.
These measures shall where necessary be combined and/or supplemented with measures against the propagation of explosions’.
A compliance strategy, which is solely focused on Ex equipment simply doesn’t ‘cut the mustard’. There is an overarching obligation to prevent the formation of explosive atmospheres in the first place, i.e. inherent safety.
The technical progress, to which we are required to adapt to, has reached a level where it is actually possible, with sealed equipment and adequate ventilation, to essentially process a flammable solvent without having to designate areas external to the equipment as hazardous. Similarly, powder processing in sealed equipment is such that for both ATEX compliance and hygienic reasons, persistent layers of dust should not be occurring on surfaces external to equipment.
This overarching obligation to prevent the formation of explosive atmospheres in the first place can also be seen in the IEC standards, which prescribe the methodology for hazardous area classification, i.e. specifying the zones. However, the extent and shape of these zones has limited definition in these IEC standards, this issue being left to national and industry guidance. In the UK there is good such guidance from the Institute of Energy for petroleum installations, while the domestic gas industry (IGEM) also has good guidance.
However, the most comprehensive guidance for gases, dusts and liquids is to be found in the long-established German collection of worked examples, formerly called BGR104 and now designated DGUV R 113-001. In the US the relevant NFPA codes also provide good generic examples.
Ex certified equipment, both electrical and non-electrical
Once an area has been zoned then only Ex certified equipment, both electrical and non-electrical, can be installed in that area. Prior to introduction of ATEX in 2003, there were well-developed national standards for Ex rated electrical equipment, which with little change developed into the IEC 60079 series of standards. While the ‘rules of the road’ were well understood for electrical equipment, prior to ATEX no such compliance requirements and certification existed for non-electrical equipment. As such then, there was much confusion, as the relevant standards had first to be developed.
However, while electrical equipment is inherently connected with active ignition sources, non-electrical equipment is not. Much non-electrical equipment, with little or no modification, can be successfully utilised in all but the most hazardous of zones. Compliance is therefore based around an ignition hazard assessment, in which it is demonstrated that none of the thirteen recognised ignition sources can become effective.
In many cases, except for the most hazardous Zones 0 and 20, self-compliance can be completed by the manufacturer. Indeed, an operating company can ‘manufacture for own-use’ and complete themselves the necessary CE and ATEX compliance.
In certain circumstances, such as a high speed mill or a spray dryer, it is not possible to demonstrate that avoidance of ignition sources can suffice as the sole protective measures, i.e. one or more of the 13 ignition sources could be active.
Tramp metal ingress
For example, in a high speed mill, mechanical impact could occur from tramp metal ingress or a component breaking loose, while in a spray dryer self-ignition of a powder layer at elevated temperatures is often an unavoidable residual risk. It is therefore necessary under these circumstances to apply additional constructive explosion measures to mitigate the detrimental effects of an explosion.
These are based on explosion resistant construction, explosion suppression or explosion venting. However, it is important to note that if an explosion vent is activated, it is a very violent event and may seriously injure or damage any personnel or structures in its path. The exception to this being the more advanced ‘flameless’ vents, where the explosion is quenched and this is a design, which can thus be used in indoor areas.
Then there is a risk that an explosion can propagate between interconnected equipment, not only igniting flammable material in an interconnected plant, but also causing pressure piling. The latter is where an explosion in one of the process units travels through the interconnecting pipework into the next process unit. The pressure of the ‘primary’ explosion together with the pressure from the ‘secondary’ explosion in the other unit produces one larger explosion pressure that the downstream equipment potentially cannot handle.
As such then it is necessary to install appropriate explosion chokes, which for flammable gases and liquids are flame arrestors, while for combustible dust more complex rotary valves, slam shut valves, suppression barriers or explosion isolation flap valves are generally required. Note: All of these explosion chokes have to be certified as Ex rated protection devices.
In conclusion, therefore, ATEX is no longer solely EU centric and a proper ATEX compliance strategy starts with inherent safety, i.e. reducing the risk at source and consequently the number and extent of hazardous areas. It may initially appear to cost more to install higher specification piping and seals, but the cost of installing and maintaining an Ex rated electrical installation, for a production room compliant with the typical gas group IIB T4, is more than four times that of a standard electrical installation.
In a similar fashion a reactor processing flammable liquid can have its internal zoning reduced from Zone 0 to Zone 2 by the application of a high quality inerting system. The non-electrical compliance of the agitator then changes from a situation, where ignition sources in rare malfunctions have to be considered, to the situation where only ignition sources in normal operation have to be considered.
This greatly simplifies the cost of the agitator, which can now be self-certified by the manufacturer. Therefore, incorporating such inherent safety into the design not only delivers a safer plant, but also has a longer-term financial benefit.
Author: Pat Swords BE CEng PPSE FIChemE CEnv MIEA, principal process and environment, health and safety consultant, PM Group
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