(126c) Systematic Errors That Compromise Integrity of Safety Instrumented Systems

Safety Instrumented Systems (SIS) are an important Independent Protection Layer (IPL) to mitigate risk. Where many safeguards, such as a Basic Process Control System (BPCS), are credited with one order of magnitude risk reduction (RRF 10), a SIS is capable of mitigating one, two or three orders of magnitude (RRF 10, 100 or 1000). Proper specification, design and maintenance of a SIS are critical to ensure that the SIS delivers the assigned risk reduction. Issues with SIS lifecycle management can lead to systematic errors such as underestimating the Safety Integrity Level (SIL) target, overestimating the risk reduction provided by design, and inadequate maintenance, testing or replacement. This paper will discuss some of these systematic errors and provide tips to avoid them.