Safety and SIS · Glossary

What is Layers of Protection Analysis?

Symbol: LOPAsemi quantitative risk methodSafety and SIS

Layers of protection analysis estimates the frequency of a hazardous event and checks whether independent protection layers reduce it enough.

Layers of Protection Analysis Explained

LOPA starts with an initiating event frequency and multiplies by the PFD of each independent protection layer. The result is compared with the tolerable frequency; any gap sets the SIL of the SIF.

It usually follows a HAZOP and is documented scenario by scenario.

Layers of Protection Analysis Formula

f = f_initiating × Π PFD_IPL and PFD_SIF ≤ f_tolerable ÷ f

f_initiating	initiating event frequency (per year)
PFD_IPL	PFD of each independent protection layer

Example: 0.2 per year × 0.1 × 0.1 = 2 × 10⁻³ per year. Tolerable 10⁻⁵: PFD ≤ 5 × 10⁻³, RRF 200, SIL 2.

Calculators for Layers of Protection Analysis

CalculatorLOPA SIL CalculatorFree online calculator with formula and worked example.CalculatorRisk Reduction Factor CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadSIL PFDavg Calculation Excel SheetFree download in Excel and PDF.

Frequently Asked Questions

What is Layers of Protection Analysis?

Layers of protection analysis estimates the frequency of a hazardous event and checks whether independent protection layers reduce it enough.

What PFD is usually claimed for a BPCS loop?

No better than 0.1, as limited by IEC 61511.

Is LOPA quantitative?

Semi quantitative: it uses order of magnitude values, not detailed modelling.

Learn more on the blog

What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on layers of protection analysis and how it is applied on real plants.

Read Full Article

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