From Belts and Suspenders - What are “Appropriate” Explosion Protection measures?

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

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?

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(Picture: Rembe/ars nova crossmedia design)

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 Eq. 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. 3).

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.

* Dr.-Ing. Johannes Lottermann, Head of Projects & Expansions at Rembe GmbH, Germany.

* * B.A. Mariana Becker, Rembe GmbH, Germany.

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