LIKELIHOOD OF IGNITION OF RELEASED FLAMMABLES

摘要

Abstract: When a flammable material is released from process equipment, the result may be a fire, anexplosion, or the release may simply dissipate with no apparent effect other than a minor environmentalimpact. Depending on the circumstances, the probability of ignition can range from 0 to 1. For anyoneperforming a risk-based analysis of any type (QRAs, LOPAs, risk-based facility siting studies, or evenapplication of a PHA risk matrix) two things are of importance: the probability the event will occur andthe likely consequences if it does. Of the two, consequences have been the subject of much moreextensive development work. Likelihood, however, is an equally critical input that requires objectiveevaluation and, to date, has had less technologically sound approaches developed and accepted in theuser community. The result is that most of the methods for estimating the likelihood of ignition arerather crude, and in many or most cases not based on process industry data.As a result, it is not difficult to find situations where the probability of ignition for a given set ofconditions varies widely from source to source in the existing literature. There are also some variablesthat are known to be important to the likelihood of ignition that have not, as of yet, been quantifiedrigorously. This results in uncertainty in risk-based studies that limits the ability of the risk manager tojustify spending to reduce the likelihood of ignition. Alternatively, the likelihood of ignition is oftenassumed to be “Yes—will ignite” and consequences are estimated using the worst case scenario. Whilethis is usually the most conservative approach, it may be impossible to justify funding and secure seniorexecutive support. Often, the end result is that nothing is done because the belief is that those worst caseconditions will not actually occur or that some minor adjustment at low cost can interrupt the sequenceof events to cause the worst case.For these reasons, CCPS has undertaken the task of consolidating the available information on thissubject, developing new sources of information where gaps exist, and preparing a book and associatedCD that will advance the art and provide specific algorithms that analysts can use to better estimatethese probabilities. The intended audience for the book will be Process Safety Subject Matter Experts(SMEs) involved in risk assessments and hazard evaluations. The timing of the book’s release (thirdquarter of 2011) is intended to be coincident with the release of the revision to the CCPS book“Guidelines for Evaluating Process Plant Buildings for External Fires, Explosions, and Toxic Releases”in order to support risk-based analyses that will be described in that reference. Using both sources,SMEs will have the tools to make repeatable and defendable risk-based decisions for siting.Note: - To avoid confusion, the following terms are used as described for the purposes of this paper: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 that a certain event will occur, given a stated starting point (e.g. the probabilitythat 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 bothin the context of this paper.