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Guidelines for determining the probability of ignition of a released flammable mass


  • This article was originally presented under the title “Guidelines for Evaluating the Likelihood of Ignition of Released Flammable Masses” at the 9th Global Congress on Process Safety, San Antonio, TX, 29 April to 1 May 2013.

  • Note: To avoid confusion, definitions of the following terms as used for the purposes of this article are provided below.

  • Frequency: The rate at which a certain event occurs, expressed in terms of events per unit time (e.g., “explosions per year”), Probability: The odds or expectations that a certain event will occur, given a stated starting point (e.g., the probability that I roll a “3” on a single die is 1/6). The units of probability are dimensionless, Likelihood: A term that can be used in place of either “frequency” or “probability,” and generally both in the context of this paper.


It is not difficult to find situations in which the probability of ignition for a given set of conditions varies widely from source to source. There are also some variables that are known to be important to the probability of ignition that have not yet been rigorously quantified. This results in uncertainty in risk-based studies, limiting the ability of the risk manager to justify spending for appropriate projects that would reduce the probability of ignition as a way of managing risk.

For this reason, CCPS commissioned a new book for release in 2013 that consolidates the available information on this subject and proposes ignition probability algorithms that advance the state of the art. Purchasing the book also allows access to software that codifies (and automates some of) the methods discussed in the book. The intended audiences for the book are Process Safety Subject Matter Experts (SMEs) involved in risk assessments and hazard evaluations. The timing of the new book's release is after the release of the revision to the CCPS book, “Guidelines for Evaluating Process Plant Buildings for External Fires, Explosions, and Toxic Releases,” and near the release time of another CCPS book, “Guidelines for Enabling Conditions and Conditional Modifiers for Layer of Protection Analysis” in order to support the risk-based analyses described in those publications. © 2013 American Institute of Chemical Engineers Process Saf Prog 33: 19–25, 2014

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