Which factors influence the achieved reliability of a safety instrumented function? (Select all that apply)

The achieved reliability of a Safety Instrumented Function (SIF), often quantified by its Probability of Failure on Demand (PFD), is a composite measure influenced by multiple factors throughout its lifecycle. These include the inherent failure rates of its components (sensor, logic solver, final element), the system architecture (e.g., redundancy) and susceptibility to common-cause failures, the frequency and quality (coverage) of proof testing, and the mean time to repair detected faults. Furthermore, operational factors like the quality of bypass management are critical, as improper bypassing renders the SIF unavailable, directly and negatively impacting its achieved reliability in practice.

(All options were deemed correct as they all directly or indirectly influence the achieved reliability of a SIF over its operational life.)

1. International Electrotechnical Commission. (2016). IEC 61511-1:2016, Functional safety - Safety instrumented systems for the process industry sector - Part 1.

Clause 11.9, "Requirements for quantifying the effect of random failures," outlines the need to consider failure rates, architecture, common cause, diagnostic coverage, and proof tests (A, C, E).

Clause 16.2.3, "Bypassing or placing out of service," mandates procedures to manage bypasses, acknowledging their impact on safety (D).

2. Center for Chemical Process Safety (CCPS). (2007). Guidelines for Safe and Reliable Instrumented Protective Systems. AIChE.

Chapter 11, "SIF-SIL Verification," provides equations and discussion showing PFD is a function of failure rates (λ), test interval (TI), common cause factor (β), and mean time to repair (MTTR) (A, B, C, E).

Chapter 12, "Installation, Commissioning, and Validation," and Chapter 13, "Operation and Maintenance," emphasize that procedures like bypass management are essential to maintaining the designed SIL (D).

3. Gobles, W. M., & Cheddie, H. (2005). Safety Instrumented Systems Verification: Practical Probabilistic Calculation. ISA.

The entire book is dedicated to calculating PFD, detailing the mathematical dependency on failure rates, test intervals, repair rates, common cause failures, and architecture (A, B, C, E).