A Frequency Based Approach to Hole Size Selection for Consequence Analysis

Global Congress on Process Safety
2011 Global Congress on Process Safety
Global Congress on Process Safety
March 15, 2011 - 8:00pm

When performing a consequence based facility siting study, it is often necessary to choose a criteria for the maximum credible hole size that is the source for a hazardous release. Many companies choose a variable hole size using a fraction of the pipe size to determine the source hole. Other companies choose a single hole size criteria that all pipes larger than this diameter will be analyzed using this diameter of a hole. These methods are often based on studies that are decades old at best or the whim of the company at worst.

An alternative approach is for the facility to determine an unacceptable event frequency and the hole size would be determined using this frequency. The frequency would be evenly apportioned to the contributing hazardous process streams in the facility. For each stream, a piping and equipment count will be taken and failure frequencies for each item will be attributed accordingly and summed to generate a frequency vs. hole size curve. The apportioned event frequency will be compared to this curve to determine the unacceptable credible hole size.

This approach produces scenarios whose hole size is based on the complexity of the hazardous stream and all frequencies are equal. This is in contrast to current methods which do not take into account the complexity of the system and can produce scenarios which have orders of magnitude spread in their likelihoods of occurrence.

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