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. However, more interesting in this context is the question of what in fact is to be understood by “appropriate” explosion protection measures?
“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 – as it is legally required – discussion about the sense or nonsense of explosion protection, the introductory approval of the writer particularly with regards to 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 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” Fig. 1.
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 × 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 × ∑ Pw.Z. (2)
the following central requirement results:
RREx ~ Pg.e.A × ∑ 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. 2) 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).
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