Beyond PFD: Mastering SIF Performance for High and Continuous Demand Modes
When process safety engineers discuss Safety Instrumented Systems (SIS), the conversation often centers on the Probability of Failure on Demand (PFD). This makes sense, as the vast majority of Safety Instrumented Functions (SIFs) operate in a low demand mode, where they are called upon to act infrequently, much less frequently than they are tested. Industry-leading software, like our Vertigo™ tool, excels at these PFD calculations, providing the essential reliability data needed for most applications. However, a critical question arises: what is the correct approach when a SIF operates in a high demand or continuous demand mode? In these scenarios, the entire paradigm for performance analysis shifts from probability to frequency.
For a low demand SIF, a dangerous failure may lie dormant and undetected until a process demand occurs. In the high demand or continuous regime, the system is always, or very frequently, in a state of demand. This means a dangerous failure of the SIS is expected to result in a hazardous consequence either immediately or, at the very least, well before the next scheduled proof test. As a result, engineers can no longer take credit for the risk reduction provided by periodic testing or, in many cases, even diagnostics. The focus must shift from surviving until the next test to preventing the failure from occurring in the first place.
If the benefits of testing are negated, how can we achieve the low failure rates required for these critical SIFs? The answer lies in fault tolerance. Instead of relying on diagnostics and intervention, safety is achieved through robust architectural design, such as 1oo2 or 2oo3 voting arrangements, that can withstand component failures while remaining operational. This design philosophy requires a different calculation method. Instead of PFD, we must calculate the dangerous failure rate. For complex systems, the most effective and thorough method for determining this failure frequency is Fault Tree Analysis (FTA).
Performing these essential failure rate calculations for complex redundant subsystems requires a powerful and specialized tool. The Kenexis integrated safety suite includes the Arbor™ Fault Tree Analysis tool, which is optimally designed for these exact scenarios. To demonstrate its power and provide you with the skills to confidently model high and continuous demand SIFs, Kenexis is pleased to invite you to our upcoming webinar. In this session, we will present how to perform dangerous failure rate calculations for complex systems using Arbor™.
Join us to learn how to move beyond PFD and ensure the safety and reliability of all your safety functions, regardless of their operational demand. Register for the webinar below.
This webinar is part of Kenexis’ ongoing commitment to providing the industry with the tools and knowledge needed to ensure the highest standards of safety and compliance.