Assessment of ignition probabilities - Theoretical foundations and simplified approaches

Many industrial facilities store, synthesize or transfer flammable products. Depending on their flammability characteristics, these products are likely to form flammable mixtures in air. In the event of ignition, dangerous phenomena such as fire, flames or explosions can occur.

As part of a quantified risk assessment, it is necessary to be able to evaluate the probability of such hazardous phenomena. To do this, it is possible to use a methodology such as the quantified bow-tie [SE 4 055]. In fact, the bow-tie will make it possible to :

• identify combinations of causes leading to feared events (FEs) that could result in the formation of a flammable mixture (spillage of flammable liquid, venting of flammable gas, etc.);

• identify the secondary dreaded events (SDEs) that may occur following the occurrence of the dreaded event, depending on whether or not the risk control measures (RCMs) are fulfilling their safety functions (leakage of flammable product over a limited period or not) [SE 4 057].

Once the probabilities of occurrence of the HRAs have been calculated, a probability of ignition must then be assessed in order to estimate those of the hazardous phenomena. As we shall see in this article, there are in fact two ignition probabilities to be assessed: the probability of immediate ignition and the probability of delayed ignition.

After a brief review of the phenomenon of inflammation, the article reviews two approaches to assessing the probability of inflammation:

• quantification using values from databases ;

• quantification using a semi-quantified method.

Mathematical models will not be discussed in this article, as a dedicated article will be devoted to them, due to their complexity compared to other approaches.