Methodology for selecting hole sizes for consequence studies

Abstract

Predicting release rates is the first step, and a crucial step, in consequence analysis. When the release is from an isolated volume of vessel and/or piping, the release rate decreases with time. There is often debate about what equivalent hole sizes should be used for a consequence study, and usually a range of hole sizes (3–4 values) is examined.This paper shows the effect of hole size on the ultimate impact of hydrocarbon releases for several scenarios and the methodology to select them. The impact depends on the intensity and exposure time. The intensity for a fire is the thermal radiation level, and that for an unignited release is the gas concentration. As the release rate decreases with time, so does the intensity. Probit functions describe the probability of a given impact based on the time-varying intensity. For a number of example scenarios, the predictions show that the worst-case hole size is an intermediate hole size, i.e., the impact goes through a maximum with increasing hole size. For smaller holes, the event is small enough that its impact is low even though the duration is long. For larger holes, the initial event is large but decreases so rapidly that the impact is low. For the intermediate hole, the event is large enough, and the duration long enough, to cause the greatest impact.This consequence study was made evaluating hole sizes with diameters between 5 and 400 mm in a fixed volume upstream process vessel. Worst case scenario consequence predictions for fire damages, effects on people and toxic releases were determined to be somewhat different for different hole sizes. However, hole sizes in the 30 mm–90 mm range seems to have the highest impact in dry gas service.