Safety and SIL · Free Calculator
Risk Reduction Factor and SIL Calculator
Convert between risk reduction factor, PFDavg and SIL band, or find the risk reduction a safety function must deliver from the event and tolerable frequencies.
RRFPFDavgSIL bandTolerable risk
per year
per year
Required RRF2,000
Required PFDavg (maximum)0.0005
SIL bandSIL 3
How this result was calculated
- PFDavg ≤ tolerable ÷ unmitigated = 0.0001 ÷ 0.2 = 0.0005
- SIL from the IEC 61511 low demand table
IEC 61511 low demand mode
| SIL | PFDavg | RRF |
|---|---|---|
| 1 | ≥ 10⁻² to < 10⁻¹ | 10 to 100 |
| 2 | ≥ 10⁻³ to < 10⁻² | 100 to 1000 |
| 3 | ≥ 10⁻⁴ to < 10⁻³ | 1000 to 10,000 |
| 4 | ≥ 10⁻⁵ to < 10⁻⁴ | 10,000 to 100,000 |
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RRF and SIL
RRF = 1 ÷ PFDavg
Required PFDavg = tolerable frequency ÷ unmitigated frequency (after other protection layers)
Required PFDavg = tolerable frequency ÷ unmitigated frequency (after other protection layers)
Worked example
An event at 0.2 per year with a tolerable frequency of 10⁻⁴ per year needs an RRF of 2000: PFDavg ≤ 0.0005, which falls in the SIL 3 band.
Key insight: take credit for other independent protection layers first (LOPA); every layer you can justify lowers the SIL the SIS must reach.
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Frequently Asked Questions
What is a risk reduction factor?
How many times a safety function reduces the frequency of a hazardous event: RRF = 1 ÷ PFDavg.
What RRF is SIL 2?
100 to 1000.
Where does the unmitigated frequency come from?
From the hazard analysis (HAZOP, LOPA): initiating event frequency reduced by other independent protection layers.
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