Is a Two-Inch Hole Adequate for a Siting Study?

For several decades now, many consequence-based siting assessments (e.g., API RP 752) have relied upon a two-inch hole to represent what is termed a “credible release size” or a “maximum credible event” (MCE).  Alternatively, a study may evaluate siting by use of a quantitative risk analysis (QRA), which would apply a full range of hole sizes.  Due to QRA complexity, the MCE analysis has often been the approach for siting studies in the US, and the two-inch hole has been one of the more common choices for the MCE.  Hole size choices range from small leaks (1/4-inch hole) through one-, two-, four-, and even six-inch holes.  Why a two-inch hole?  Does a two-inch hole provide a sufficient accurate qualitative evaluation of risk?  When a single hole size is chosen for a given purpose in a consequence analysis, that hole size should not be chosen based solely on engineering judgement, but should be based on more defensible information.   This paper seeks to explain the basis for selecting the two-inch hole, including a literature review, concepts from consequence modeling, data from failure frequencies, and some engineering judgement.  Examples comparing the use of a single hole size (with emphasis on the 2-inch hole) to QRA results will also demonstrate some of the potential benefits and problems of an MCE analysis.