Safety Instrumented Function (SIF) design is based on performance requirements. During the design of the SIF, engineers make various assumptions, including assumptions concerning SIF equipment failure rates, SIF demand rates, proof test frequencies, and mission time which form the basis for the expected operational SIF performance.
Are those assumptions realistic? Is the Safety Instrumented Function really operating as assumed?
The IEC 61511 functional safety standard requires users to periodically compare actual (operational experience based) performance with assumed (during the design of the SIF) performance.
Key topics covered include:
- Performance Assumptions: The various factors assumed during design, such as equipment failure rates, proof test frequencies, mission time, and demand rates.
- IEC 61511 Requirements: The international standard's expectation that users periodically monitor SIS reliability parameters and demand rates to identify potential systematic failures.
- Operational Challenges: Significant hurdles in interpreting data, such as determining true operating hours, achieving statistically viable sample sizes, and ensuring data homogeneity (avoiding the combination of data from different environments or technologies).
- The Data Philosophy Document (DPD): A proposed solution to manage data collection systems, providing a consistent approach to recording information, defining responsibilities, and ensuring that conclusions drawn from the data are valid.
- Benefits: Beyond regulatory compliance, studying operational data helps identify "bad actors" (faulty devices or locations), ensures designed safety integrity is maintained, and optimizes maintenance strategies by identifying premature wear-out.