Consideration of Escalation Risks in Facility Siting
Dennis Ngai, Venkatesh Sourirajan, Jason Fung
A small fire when uncontrolled, can potentially lead to failure of equipment containing a large inventory of hydrocarbon or a structure holding hydrocarbon bearing equipment within the same process unit, resulting in an escalation of a minor event to a much larger event with catastrophic consequences. Similarly, a major fire in a process unit can escalate to cause failure of equipment and piping in adjoining units or facilities, thereby causing severe damage. Escalation risks can be assessed following a consequence-based approach or a risk-based approach. A risk-based approach incorporates both frequency and consequence estimation. While consequence-based analysis is a simpler method to use, the selection of credible release scenarios such as 1-inch, 2-inch or larger hole size can become a subject of debate. The alternative is to follow a risk-based approach wherein all leak sizes are modelled together with mitigation system failure, directionality and ignition probability to determine the cumulative likelihood of a damaging fire event. Such an approach would typically consider a target frequency threshold of 1 x 10-4 per year. This method is subject to the usual uncertainties associated with a frequency assessment with respect to both failure frequency data and the various event probabilities. This paper presents a case study based on two representative process units in a refinery to determine the appropriate separation distance between the units considering both risk-based and consequence-based approaches for comparison. A risk-based approach is more appropriate for modelling the time dependent and spatial characteristics of a jet fire and provides a more realistic result including the requirement for jet fire protection for individual equipment and structures, both within the unit and in adjoining areas but will require detailed modelling. For initial design and layout development, the results show that a 30 m separate distance between the units can be considered.