“Explosion protection is expensive!” - granted: In light of the considerably lower likelihood of occurrence of explosions in comparison with fires, the question of the meaningfulness of what are often more cost-intensive investments in appropriate explosion protection measures is understandable.
Irrespective of the already superfluous (Read More)
- as it is legally required - discussion about the sense or nonsense of explosion protection, the introductory approval of the writer particularly with regards the often catastrophic scale of such events is put into perspective. However, more interesting in this context is the question of what in fact is to be understood by “appropriate” explosion protection measures? This article is
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intended to address this question on the basis of practical examples from the field of dust handling facilities.
According to TRGS 720 / TRBS 2152 “Hazardous explosive atmospheres”, the employer must determine and assess the risk of his employees as part of his obligations in accordance with the German Occupational Safety and Health Act [Arbeitsschutzgesetz] (including the Ordinance on Hazardous Substances [Gefahrstoffverordnung] and the Ordinance on Industrial Safety and Health [Betriebssicherheitsverordnung]) and implement the necessary safety measures. In accordance with this, he must check in the first stages of the hazard analysis whether there exist combustible materials and whether the formation of explosive atmospheres in hazardous quantities should be anticipated.
Explosion Prevention versus Explosion Protection
Although the legislative authority gives precedence explicitly to safety measures to avoid hazardous explosive atmospheres through substitute combustible materials, the experienced reader knows of the practical relevance of this preferred preventative measure. A baker simply needs flour and sugar to bake, a power station burns coal and sawdust naturally arises in chipboard factories. All these materials are capable of causes dust explosive atmospheres. As a result, the explosion danger is essentially a given in all of the above examples.
So if hazardous explosive atmospheres cannot be safely prevented, the employer must assess the probability and duration of the occurrence of hazardous explosive atmospheres and the probability of the existence or arising of effective ignition sources. This stage of the assessment is commonly known in practice as “zoning” (see Table 1 below).
But what is frequently forgotten when implementing explosion safety measures in dust handling facilities, such as dust collectors, is the fact that the classification of hazardous places in terms of zones in accordance with TRGS 720 (1) 7. is ultimately only down to the so-called prevention of ignition sources.
Digression: Risk-based, probabilistic approach
In principle, the ignition prevention measures to be taken should make ignition sources ineffective or reduce the probability of it being effective. Consequently, the scope of explosion prevention measures complies with the probability of the occurrence of hazardous explosive atmospheres (zone). This probabilistic concept is based on the comparative assessment of the generally accepted residual risk (RREx), which arises from a combination of the severity (AS) and the probability of an explosion (PEx):
RREx = AS x PEx (1)
In the case of an explosion, an unaccepted measure of damage is essentially anticipated. In consideration of the fact that the probability of an explosion is characterised by the probability of the existence of a hazardous explosive atmosphere (Pg.e.A) and the probability of the occurrence of an effective (of the thirteen in accordance with EN 1127) ignition source(s) (Pw.Z.),
PEx = Pg.e.A. x ∑ Pw.Z. (2)
the following central requirement results:
RREx ~ Pg.e.A x ∑ Pw.Z. = const. (3)
For this reason, in the practice of explosion protection, when applying the preventative measure of “Avoidance of Ignition Sources”, hazardous areas are only categorised into zones from these previous contexts in order to avoid ignition sources as follows:
• In zone 2 and 22: Ignition sources which can constantly or frequently occur.
• In zone 1 and 21: As well as the ignition sources stated for zone 2 and 22, ignition sources which can occur occasionally, e.g. in foreseeable disturbances to a working material.
• In zone 0 and 20: As well as the ignition sources stated for zone 1 and 21, ignition sources which can occur rarely.
By implication, this emphasises that the zoning is completely irrelevant in the case of the application of explosion protection measures, which reduce the effects of an explosion to an uncritical degree. The effects of an explosion in zone 20 are ultimately no more or less hazardous than those in zone 22.
In practice, for the aforementioned example of a dust collector system (see Fig. 1) which is protected with a flameless venting device and an explosion isolation flap valve, only measures to avoid ignition sources, but not to prevent ignition sources are obligatory. In the raw gas / dirty air section of the filter, which is normally classified as an hazardous place zone 20, also a rotary air lock of equipment category 3D could be used if this was also inspected and approved to be pressure shock resistant and flameproof. (Author’s comment: In all likelihood, most cases deal with identical devices, which are then only put onto the market with a different label).
However, a look into systems which are protected in practice shows that all (possible) stops are pulled out to apply preventative measures such as avoiding ignition sources, despite the existence of consequence-limiting measures.
In exaggerated terms, for dust collecting systems for example, in which often the (comparatively higher probability of) external ignition sources require measures of explosion protective measures, operators purchase and install any little explosion-proof equipment, even though the burst panel is fitted at the enclosure and already offers the legal safety level required. With regards to the comparably low probability of ignition within the design parameters of working equipment (see for example EN 13463-1 introduction), such “concepts” are reduced to absurdity. For example, a manufacturer recently applied for his silo discharge screws of equipment category 1D to be considered a unique selling point, although most of today’s silos are already protected by using explosion venting devices. So who does it surprise when the introductory cost-benefit issue of explosion safety is presented in light of such upwardly-forced investments?
It is beyond any question that only an “appropriate” mix of preventative and protective measures can lead to a consistent explosion safety concept. According to the interpretation of the author, the “freedom” of the designs of this “appropriate” explosion protection mix is meant in TRGS 720 / TRBS 2152, when the legislative authority speaks of “suitable combinations of preventative and constructive measures in accordance with expert judgement”. This interpretation is supported in the more precise interpretation of the European Directives 94/9/EC (ATEX 114) and 1999/92/EC (ATEX 153). According to these, all necessary measures must be taken to ensure that the workplace, the work equipment and the relevant connection devices are designed, constructed, assembled, installed, maintained and operated in a way to minimize the risk of explosions:
In view of equation 1, if the effects of an explosion are limited to an uncritical degree using explosion protective measures, an acceptable residual risk arises virtually independently of the probability of occurrence, with reference to the risk matrix, recognised by the professional industry and tried-and-tested in operational practice, of the VDI series of guidelines 2263 “Dust fires and dust explosions: Hazards, assessment, protective measures” (see Fig. 2).
What explosion protection can learn from explosion prevention
Although an explosion could essentially lead to catastrophic effects and death in any “zone”, similar to preventative explosion measure, in which the scope of measures is aligned as described to the “probability” (frequency and duration) of the occurrence of hazardous explosive atmospheres, the question of the requirement of a risk-oriented approach is raised in conclusion for protective explosion measures as well. The example of an system protected using explosion suppression, but the protective system of which was deactivated at the point of explosion, illustrates - if only in the approach - the necessity of such a reliability concept.
In the context, it becomes clear that a risk-oriented categorisation of protective explosion measures must also consequently occur with regards to the “probability” (frequency and duration) of the occurrence of effective ignition sources. In comparison with preventative explosion safety measures, with which an explosion is not permitted in principle, an impact-related categorisation must also take place, which considers the expected measure of damage.
A first approach to this is already stated by TRGS 721 / TRBS 2152-1, whereby the affected measures in “areas with explosion impacts exceeding the usual degree” in scope and type must be taken into account.
In areas, in which meeting places, corridors with dense traffic, residential buildings and larger office premises are in the hazardous area, only non-manipulatable or non-deactivatable, protective systems should be allowed to be used. Furthermore, with passive explosion protective systems, which are not normally installed and checked by the manufacturer, operators should consider the compliance with test requirements as per §§ 14 and 15 in connection with Appendix 4 Section A Number 3.8 of the BetrSichV.
On the basis of the experience of the author in the relevant expert committee activities, the development, coordination and validation of suitable assessment standards within the bodies of experts requires a considerable degree of work and time. For this reason, details of assessment standards for the categorisation of constructive protective measures and autonomous protective systems has not been entered into.
Summary
In this article, the contexts of preventative and protective explosion safety measures could be shown clearly, transparently and with regard to German and European legislation. It was comprehensively shown that an appropriate explosion safety concept, which is based predominantly on the use of protective measures (most-common example: explosion venting in connection with explosion isolated decoupling), permits the forgoing of additional preventative measures that become more cost-intensive. If ignition sources in explosion-prone systems cannot be avoided in operational practice with sufficient safety, then a safety-technical and economically reasonable combination of preventative and protective measures can be used according to professional discretion. In doing so, it is the operator’s responsibility to adjust the scope of preventative safety measures which purely reduce the probability of occurrence to their own requirements for a reliable yield and trouble-free value added.
© February 2013
Dr.-Ing. Johannes Lottermann
B. A. Mariana Becker
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