1. Safety Instrumented Systems
Chapter 45
Safety Instrumented Systems
Evaluation of Processes • Risks • Integrity Levels • Technology Options • Hardwired Systems • Programmable Electronic Systems • Safety PLC

2. A combination of safety layers reduces the inherent process risk to a small residual risk level.
Any process involving a hazardous material has an inherent level of risk. See Figure Risk is a measure of the probability and severity of adverse effects of a process failure. Risk is used to evaluate how often a failure can happen and the consequences if a failure does happen. One common theme in all safety systems is to have multiple levels of protection, which increase the integrity level and reduce the residual risk.

3. A required SIS integrity level can be determined from an evaluation of event severity and event likelihood.
A determination of the required SIS integrity level is obtained from a chart of hazardous event severity and event likelihood. See Figure For example, a process that has a hazardous event severity rating of “severe” and an event likelihood rating of “occasional” has a required SIS integrity level of 3. The IEC, ISA, and AIChE define integrity levels in terms of safety availability, probability of failure on demand (PFD), and risk reduction factor (RRF). The RRF is equal to 1/PFD. The levels of protection and the SIS integrity level determine the type of technology required.

4. An alternative qualitative risk ranking evaluates event consequences, frequency, probability of avoidance, and probability of occurrence to determine a required SIS integrity level.
An alternate qualitative risk ranking is also available. This risk ranking determines the SIS level required based on the consequences, frequency and exposure, possibility of avoidance, and probability of occurrence. See Figure For example, a failure event with a consequence of “several deaths” (Cc), frequency of “rare to frequent” (Fa), possibility of avoidance of “almost impossible” (Pb), and a probability of occurrence of “slight” (W2) requires an SIS integrity level of 2.

5. The choice of safety PLC architecture depends on the application.
A safety PLC is a highly reliable PLC that includes fail-safe designs, built-in self-diagnostics, and a fault-tolerant architecture. Safety PLCs are also designed to meet third-party approval criteria because of the special hardware and software. Safety PLC architectures can be simple, low-cost, safe single systems (1oo1), or they can be more complex, with dual (1oo2D) or TMR system architectures, to satisfy the SIS integrity level needed. See Figure 45-4.