A Safety Instrumented Function (SIF) is a function implemented by means of instruments and is provided to reduce the likelihood or magnitude of an accident to personnel or the facility itself in terms of asset, environmental and production loss. A SIF is designed to achieve or maintain a safe state of the process or mitigate consequences, in response to a specific upset scenario. A SIF may consist of one or more initiators (sensors, transmitters), the logic solver (trip amplifiers, safety interlocks, fail safe output), and one or more final elements (valves, motors, etc.) and utilities such as power and instrument air supply required to perform the function. The reliability of each SIF in preventing/ mitigating the hazards is reflected in the performance requirement placed upon each SIF. This performance requirement is defined in the form of a Safety Integrity Level (SIL). SIL, which is numerically defined as a Probability of Failure on Demand (PFD), indicates the degree of reliability of the function.
SIL classification: The SIL classification is to gather a SIL team to assess every safety loop. The assessment is guided by the likelihood or magnitude of an accident to personnel or the facility itself in terms of asset, environmental and production loss. When each SIL loop is classified, it is assumed that all other loops function correctly. As a result, only final element failure is considered where on initiator activates more than one function. The SIL team will discuss and agree on the health and safety, economic and environmental consequences of the loop failing to operate on demand. Each consequence will then have an assigned SIL according to the respective risks matrixes.
SIL Verification: A Safety Instrumented Function SIF may consist of one or more initiators (sensors, transmitters), the logic solver (PLC) and one or more final elements (valves, motors, etc.) and utilities such as power and instrument air supply required to perform the function. The reliability of each SIF in preventing/ mitigating the hazards is reflected in the performance requirement placed upon each SIF. This performance requirement is defined in the form of a Safety Integrity Level (SIL). SIL, which is numerically defined as a Probability of Failure on Demand (PFD), indicates the degree of reliability of the function.
