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Likelihood Evaluation

Figure 15-7 Example event tree

15.11.2 Domino Effects

We seldom evaluate accidents within one process unit that may affect another unit as the likelihood is very small.
However, several situations with potentially catastrophic impacts are drawing more attention including:
 Secondary Dust Explosions
 Impact of Pool Fire on Nearby Equipment
 Potential for Physical Damage to Nearby Equipment from Explosion Debris

15.12 Pool Fire Frequency Analysis using Simplified Fault Tree

There are typically many sources for leaks or spills that could ignite and cause a pool fire within the area (or nearby)
where the equipment being analyzed is located (Figure 15-8). An estimate of the quantity of fuel present should be made
to determine if heat up of vessel contents can occur in a reasonable time period. This time will be different for different
vessel fill levels. For Reactive Cases, low Temperature of No Return (TNR) means a smaller, shorter fire could create a
problem. Next, fire scenarios for nearby equipment based on the leak sources and ignition probabilities are performed. To
determine the overall frequency, each source must be identified and its contribution to the overall frequency for pool fire
estimated and summed to a total.

In a simple Fault Tree analysis, each leak scenario is analyzed as a series of “and” gates and the total frequency is
estimated by the summation of all scenarios. As a simplification, common-cause failures are not included, so the final result
for low frequency scenarios (less than 0.0001/year) may be overly optimistic and should use more advance Fault Tree
software. Common-cause failures are a single event that may affect several branches in the Fault Tree. Examples of
common-cause failure would include power loss disabling several electrical systems simultaneously or a maintenance error
resulting in mis-calibration of multiple sensors.

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