A Risk Graph is used to determine reliability requirements of the instrumented safety functions based on the severity of the hazards, in combination with other risk reducing measures. The risk graph method is a less comprehensive analysis than a LOPA and may yield less accurate results.
The risk graph method utilizes four risk parameters;
- Consequence of the hazardous event (C)
- Frequency of presence in the hazardous zone × exposure time (F)
- Possibility of avoiding the consequences of the hazardous event (P)
- Probability of the unwanted occurrence (W)
The combination of the risk parameters above enables a risk graph as shown in the figure below. Following a path given by the parameters determine the reliability requirement.