SIL Verification (PFDavg) Calculator
Verify a safety instrumented function: enter each subsystem (sensors, logic solver, final elements) with its voting, failure rate, proof test interval and common cause factor. The workbook adds the PFDavg values and shows the achieved SIL against the target.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 16 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Subsystems
| Subsystem | Voting | λDU per channel (FIT) | Proof test interval (months) | β (%) | PFDavg | Share of total (%) | Remarks |
|---|---|---|---|---|---|---|---|
| Pressure transmitters PT 1101 A/B/C | 2oo3 | 150 | 12 | 5 | 3.46 × 10⁻⁵ | 2 | |
| Safety PLC (from certificate) | 1oo1 | 20 | 120 | 8.76 × 10⁻⁴ | 49 | ||
| Fuel gas SDVs XV 1101 / 1102 | 1oo2 | 1,500 | 12 | 10 | 7.15 × 10⁻⁴ | 40 | |
| Solenoid valves | 1oo2 | 400 | 12 | 10 | 1.79 × 10⁻⁴ | 10 |
About this Calculator
A SIL calculation shows whether a safety function achieves the risk reduction required by the hazard analysis. Final elements usually dominate the total, so the share column shows at a glance where to spend effort: valve redundancy, partial stroke testing or shorter test intervals.
Basis: Simplified IEC 61508 / ISA TR84.00.02 PFDavg equations (no diagnostics or repair time): 1oo1 λTI/2, 2oo2 λTI, 1oo2 (λTI)²/3 + βλTI/2, 2oo3 (λTI)² + βλTI/2.
How to use
- Enter the SIF and its target SIL from the safety requirements specification.
- List each subsystem with its voting, λDU per channel in FIT (failures per 10⁹ h), proof test interval and β for redundant groups.
- Read the PFDavg of each subsystem and its share of the total; the largest share is where improvement pays most.
- Check the achieved SIL against the target.
- Shorten test intervals, add redundancy or choose better devices if the target is not met.
Formulas used
- λ = λDU (FIT) × 10⁻⁹ per hour; TI in hours = months × 730.
- 1oo1: λTI/2; 2oo2: λTI; 1oo2: (λTI)²/3 + βλTI/2; 2oo3: (λTI)² + βλTI/2.
- Total PFDavg = sum of subsystems; RRF = 1 ÷ PFDavg.
Related tools
Frequently Asked Questions
What is PFDavg?
The average probability that a safety function fails to act when demanded, over its proof test interval.
Why do final elements dominate?
Valves and actuators have much higher dangerous undetected failure rates than transmitters and safety PLCs.
Is this enough for SIL verification?
It is a simplified estimate. Formal verification also includes diagnostic coverage, repair times, test coverage, architectural constraints and systematic capability.
Learn more on the blog
What is SIL (safety integrity level)
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article