Calculating Facility Siting Study Leak Sizes - Applications of the Maximum Design Leak (MDL) Approach

Consequence-based Facility Siting Studies (FSSs) typically requires the user assume a credible leak size to use in the evaluation of potential releases, which is often up to a 2” diameter leak. Many facilities tend to be less complex in comparison to large refineries or petrochemical plants, leading operators at the less complex facilities to ask why they should assume the same leak sizes as more complex facilities. Other facilities have unique processes with safety systems and factors they would like to quantify in a FSS. One solution would be to perform a Quantitative Risk Assessment (QRA) to capture the risks from all potential release locations and release sizes. However, many companies have not defined risk tolerance criteria and are resistant to do so for various reasons or do not want to invest in a QRA, which are more costly than a consequence-based study. A unique approach developed by ABS Consulting and first presented in 2011 is called the Maximum Design Leak approach [1]. This approach calculates frequency-based leak sizes and then applies the leak size that exceeds a frequency criterion (events/year) in a consequence-based FSS instead of assuming a given leak size as credible. This avoids having to establish risk criteria in terms of fatalities/year and having to model a plethora of scenarios yet takes advantage of many features in a QRA. This paper presents case studies as examples of how the MDL has been applied to several situations.