Understanding Probability of Failure on Demand (PFD)
An Interactive Visual Guide based on IEC 61511 for Low Demand Mode
What is PFD?
Probability of Failure on Demand (PFD) is a measure of the reliability of a safety function. It represents the likelihood that a safety system (like an emergency shutdown valve) will fail to perform its required function when a demand occurs.
In simple terms: If a dangerous situation happens, what's the chance the safety system won't work? A lower PFD means a more reliable system.
Adjust PFD Level
Use the slider to change the PFD value and see how it affects the number of failures.
(Less Safe) Low PFD
(More Safe)
Current PFDavg is
This corresponds to
Legend
Visualizing Failures per 10,000 Demands
Each square represents one demand on the safety system. Watch how many turn red (fail) as you adjust the PFD.
Safety Integrity Level (SIL) Table (IEC 61511)
| SIL | PFDavg Range | Risk Reduction Factor (RRF) |
|---|---|---|
| 4 | ≥ 10-5 to < 10-4 | 100,000 to 10,000 |
| 3 | ≥ 10-4 to < 10-3 | 10,000 to 1,000 |
| 2 | ≥ 10-3 to < 10-2 | 1,000 to 100 |
| 1 | ≥ 10-2 to < 10-1 | 100 to 10 |
Very well explained, thanks