PFDavg & Safety Integrity Level Verification
Should not exceed useful life of equipment (typically 10-20 years)
Understand PFDavg calculation methods, architectural constraints, common cause failures, and IEC 61508/61511 requirements
PFDavg (average Probability of Failure on Demand) is the average probability that a Safety Instrumented Function will fail to perform its intended safety action when a process demand occurs. It is calculated over the proof test interval and is the primary metric for SIL verification in low-demand mode per IEC 61511.
For a 1oo2 (one-out-of-two) voting architecture, PFDavg accounts for both independent failures and common cause failures: PFDavg = (lambda_DU x TI)^2 / 3 + beta x lambda_DU x TI/2, where lambda_DU is the dangerous undetected failure rate, TI is the proof test interval, and beta is the common cause failure factor (typically 2-10%).
Architectural constraints define the minimum Hardware Fault Tolerance (HFT) required for each SIL level. For SIL 1, HFT of 0 (1oo1) is acceptable. For SIL 2 with Type B devices, HFT of 1 (e.g., 1oo2) is required. For SIL 3, HFT of 2 (e.g., 2oo3) is typically required for Type B devices. These constraints ensure that no single component failure can prevent the safety function.
SIL (Safety Integrity Level) is a discrete classification (1-4) that defines the required reliability of a safety function. PFDavg is the calculated probability of failure that determines the achieved SIL. SIL 1 corresponds to PFDavg of 0.01-0.1, SIL 2 to 0.001-0.01, SIL 3 to 0.0001-0.001, and SIL 4 to 0.00001-0.0001.