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Layer Of Protection Analysis Calculator

Work out layer of protection analysis instantly with clear inputs, formula shown and shareable results.

About Layer Of Protection Analysis Calculator

Layer of protection analysis multiplies the initiating event frequency by the probability of failure on demand of each independent protection layer. Comparing the mitigated frequency with the tolerable frequency shows the additional risk reduction needed, and that factor maps directly to the safety integrity level a new instrumented function would require.

Formulas

Mitigated frequency

f mitigated = initiating frequency x PFD of each independent layer

SIL from risk reduction

RRF up to 10 is SIL 1, to 100 SIL 2, to 1000 SIL 3, beyond that SIL 4

Worked example

With the default values below, Layer Of Protection Analysis Calculator returns Mitigated event frequency of 0.00050000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Initiating event frequencyminimum 0 per year
0.1 per year
Probability of failure of the first protection layeraccepts 0–1
0.1
Probability of failure of the second protection layeraccepts 0–1
0.05
Tolerable event frequencyminimum 0 per year
0.0001 per year

Results

Mitigated event frequency
0.00050000 per year
Risk reduction achieved
200.0
Additional risk reduction needed
5.00
Safety integrity level implied
SIL 1 required

Step by step

  1. 1Values used
  2. 2Mitigated frequency
  3. 3SIL from risk reduction
  4. 4Mitigated event frequency= 0.00050000 per year
  5. 5Risk reduction achieved= 200.0
  6. 6Additional risk reduction needed= 5.00
  7. 7Safety integrity level implied= SIL 1 required

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 8 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Layer of protection analysis and SIL determination must be carried out by a competent process safety team to the applicable functional safety standard.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Layer of protection analysis and SIL determination must be carried out by a competent process safety team to the applicable functional safety standard.

Frequently Asked Questions

How is the Layer Of Protection Analysis figure worked out?

f mitigated = initiating frequency x PFD of each independent layer. Layer of protection analysis multiplies the initiating event frequency by the probability of failure on demand of each independent protection layer.

What makes a protection layer independent?

It must be separate in sensor, logic and final element from the initiating cause and from other layers, and be auditable and specifically designed to prevent the consequence. Shared instruments break independence.

What values do I need to enter?

This calculator takes 4 inputs: Initiating event frequency, Probability of failure of the first protection layer, Probability of failure of the second protection layer, Tolerable event frequency. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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