SIL Proof Test Interval Calculator
Work out how often a safety instrumented function must be proof tested to meet its SIL target, from the dangerous undetected failure rate and the voting architecture.
- 1oo1: PFDavg = λDU T ÷ 2 → T = 2 PFD ÷ λDU
- 1oo2: PFDavg = (λDU T)² ÷ 3 → T = √(3 PFD) ÷ λDU
- 2oo3: PFDavg = (λDU T)² → T = √PFD ÷ λDU
Simplified PFDavg Equations
1oo2: PFDavg = (λDU × T)² ÷ 3
2oo3: PFDavg = (λDU × T)²
T is the proof test interval in hours. The design margin keeps the calculated PFDavg below the SIL boundary to allow for uncertainty in failure data.
Worked example
A 1oo1 SIF with λDU = 2 × 10⁻⁶ /h for SIL 2 with 50 % margin (PFDavg 0.005) needs T ≤ 2 × 0.005 ÷ 2 × 10⁻⁶ = 5000 h, about 7 months.
Frequently Asked Questions
How often should a SIF be proof tested?
At the interval assumed in the SIL verification, calculated from λDU and the target PFDavg.
What is λDU?
The dangerous undetected failure rate: failures that stop the function and are not found by diagnostics.
Why add a design margin?
Failure data is uncertain; designing to about half the SIL limit gives headroom.
Learn more on the blog
What is SIL (safety integrity level)
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article