Suppose that the SIL analysis team is conducting a LOPA to investigate the risk scenario illustrated in the following diagram, where each box represents a part of the scenario, and each label indicates the family that stores the relevant information:
When Valve A-1001 fails, flammable gas is released into the atmosphere. If the flammable gas reaches a potential ignition source (e.g., electric switch, motor), causing a vapor cloud explosion in the vicinity of the operator, it could cause a fatal injury.
In this risk scenario, the fatal injury is a consequence of the valve failure, and the following events or actions are the conditional modifiers:
- The flame igniting
- The vapor cloud exploding
- The operator being in the vicinity of the explosion
Since these actions and events appear within the risk scenario, the probability associated with the consequence occurring is increased exponentially. In other words, if the operator was not in the vicinity of the blast, the probability of fatal injury would be less. By examining the granular events that are associated with a risk, the SIL analysis team can more accurately assess the SIL value for the safety system.
In this example, consider the following values:
- Probability of the operator being present in the vicinity of blast = .5 (i.e., 50%)
- Frequency of failure of Valve A-1001 = .2
For the above values, the Unmitigated consequence frequency of the scenario is calculated as follows:
- Probability of the operator being present in the vicinity of blast x Frequency of failure of Valve A-1001 = 0.5 x 0.2 = 0.1