Abstract
A Safety Instrumented Function (SIF) is a set of equipment designed to reduce the risk of harm from a specified type of incident, by taking an automatic action when required. A number of SIFs may be implemented together in a Safety Instrumented System (SIS), which is the sum of all equipment required to implement the constituent SIFs. The amount of risk reduction required of each SIF is expressed as a Safety Integrity Level (SIL). A SIF is implemented using one or more sensors, a logic solver, and one or more final elements (devices acting directly on the process, such as a shutdown valve). To demonstrate that the SIF achieves its SIL target, a calculation known as SIL verification is executed. This requires an understanding of the failure modes of the hardware used to implement the SIF, the way failure rates are expressed in terms of λ values, and the concept of hardware fault tolerance.
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