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Safety and SIS · Glossary

What is PFDavg?

average probability of failure on demandSafety and SIS

PFDavg is the average probability that a safety function fails to act when a demand occurs, over the proof test interval.

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PFDavg Explained

Dangerous undetected failures build up between proof tests, so the probability of being failed rises over time. PFDavg averages it.

It is calculated from failure rates, voting, proof test interval and coverage, and compared with the range for the target SIL.

PFDavg Formula

PFDavg ≈ λDU × TI ÷ 2   (1oo1)
λDUdangerous undetected failure rate (per hour)
TIproof test interval (hours)
Example: λDU = 2 × 10⁻⁶ /h, TI = 8760 h: PFDavg ≈ 8.76 × 10⁻³, within SIL 2.
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Calculators for PFDavg

CalculatorPFDavg CalculatorFree online calculator with formula and worked example.CalculatorProof Test Interval CalculatorFree online calculator with formula and worked example.CalculatorPartial Stroke Test CalculatorFree online calculator with formula and worked example.

Free Downloads

Free downloadSIL PFDavg Calculation Excel SheetFree download in Excel and PDF.Free downloadSIL Proof Test Record Template in ExcelFree download in Excel and PDF.

Frequently Asked Questions

What is PFDavg?

PFDavg is the average probability that a safety function fails to act when a demand occurs, over the proof test interval.

Why does PFDavg depend on test interval?

Hidden failures accumulate until found; testing more often finds them sooner.

Which part of a SIF usually dominates PFDavg?

Often the final element, the valve and its solenoid.

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What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on PFDavg and how it is applied on real plants.

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Related Terms

Safety termSafety Integrity LevelSafety integrity level is a target level of risk reduction for a safety instrumented function, from SIL 1…Safety termProof Test IntervalProof test interval is the time between periodic tests that reveal dangerous undetected failures in a…Safety termFailure RateFailure rate is the number of failures per unit time of a device; in safety it is split into safe and…

More Safety and SIS Terms

Safety termPFHPFH is the average frequency of dangerous failures per hour, used for safety functions in high demand or…Safety termRisk Reduction FactorRisk reduction factor is how many times a safety function reduces the frequency of a hazardous event; it…Safety termSafe Failure FractionSafe failure fraction is the share of all failures that are either safe or dangerous but detected.Safety termDiagnostic CoverageDiagnostic coverage is the fraction of dangerous failures that automatic diagnostics detect.Safety termHardware Fault ToleranceHardware fault tolerance is the number of hardware faults a subsystem can have while still performing its…Safety termVoting ArchitectureVoting architecture describes how many of N redundant channels must act to trip, such as 1oo2 or 2oo3.

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