What Else is Needed Besides a Sil Evaluation?

摘要

The concept of the Safety Instrumented Function (SIF) associated with a Safety Instrumented System (SIS) is that the instrumentation will provide an interlock actuation, relief of pressure action, or some other function to prevent the process from becoming unstable or dangerous, or that it will provide an alarm so that compensatory actions may be taken prior to the process reaching a Safety / Design Limit. A Safety Integrity Level (SIL) evaluation performed in accordance with ANSI/ISA-84.00.01 / IEC 61511 determines the probability that the SIS will take action when commanded to do so. However, there is another element that must be considered, beyond the ability of the SIS to operate acceptably on demand. This element is the uncertainty associated with the SIS equipment, which will determine whether the SIF will or will not occur at the desired process value. The ability of the SIS to respond at the correct process conditions or with sufficient time to perform the required SIF depends on the SIS equipment uncertainties. These uncertainties are inherent in the instrumentation, in its operation, in its application, in its environment, in the effects imposed by the process itself, in its setup and maintenance, and in its supporting equipment. This paper will address the performance characteristics of the SIS equipment and their impact on the ability of the SIS to perform its designated SIF before the process approaches the Safety / Design Limit.