Safety Integrity Level (SIL) Estimator

IEC 61511 Risk Graph · Consequence · Occupancy · Demand

When to use: Use this during a process hazard analysis to determine the Safety Integrity Level a Safety Instrumented Function (SIF) must achieve. The calibrated IEC 61511 risk graph combines consequence severity (C), exposure/occupancy (F), possibility of avoidance (P), and demand rate (W) to yield the required risk reduction. The resulting SIL fixes the target average probability of failure on demand (PFDavg) for the safety loop. Confirm critical functions with a quantitative LOPA where required.

Risk Graph Parameters

C — Consequence Severity

F — Frequency / Exposure (Occupancy)

P — Possibility of Avoiding Hazard

W — Demand Rate (Probability of Unwanted Occurrence)

No SIL Required

Required Safety Integrity Level

Results

Consequence (C)B — Serious permanent injury / one death

Exposure / Occupancy (F)B — Frequent to continuous exposure (>10% of time)

Avoidance (P)A — Possible under certain conditions

Demand Rate (W)2 — Low (once per 10 yr)

Determined SILNo SIL (other measures)

Required PFD (avg)Not required

Risk Reduction FactorNot required

Standards & References

IEC 61511 / IEC 61508 — Functional safety

Calibrated risk graph (C-F-P-W parameters)

SIL set by required risk reduction (PFDavg)

Verify with quantitative LOPA where required