A risk graph reaches a SIL by choosing four parameters: consequence (C), exposure or occupancy (F), possibility of avoiding the hazard (P) and demand rate without the SIF (W). It is quick and good for screening many functions, but only valid when calibrated to the company's tolerable risk.
The parameters
- C – consequence: from minor injury to many fatalities.
- F – exposure: fraction of time a person is in the hazardous zone.
- P – avoidance: whether people can notice and escape (warning, time, alternative route).
- W – demand rate: how often the hazardous situation would occur without the SIF, after other layers.
Calibration process
- Start from the tolerable frequency of the worst consequence in each C category.
- Define each F, P and W level numerically (for example, W1 below 0.03 per year).
- For each path, compute the unmitigated risk (demand rate × exposure × avoidance failure) and the risk reduction needed to reach the tolerable frequency; place the resulting SIL in the graph.
- Check the calibrated graph against a few LOPA cases; they should agree within one SIL.
Where risk graphs mislead
- Uncalibrated graphs copied from a textbook or another company.
- W chosen without considering the other layers consistently.
- F and P used to argue down every function.
- Scenarios with large consequences or SIL 3 outcomes: use LOPA or quantitative analysis instead.
In First Out AI
The Risk calibration tab holds your definitions for every C, F, P, W level and the outcome of each path; the "Assess" selectors give the result for a chosen path. Load the example calibration to see the structure, then replace it with your company's.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.