SIF Analysis Overview

A Safety Instrumented System (SIS) typically monitors the same variables as the Process Control System (BPCS) and acts when a variable is outside its normal range, e.g., when the BPCS fails. Each SIS performs one or more Safety Instrumented Functions (SIF). SIFs comprise out of three elements: Sensors (e.g., a flowmeter), Logic Solvers (e.g., a safety PLC) that detect dangerous conditions, and Final Control Elements (e.g., a valve) that are manipulated to achieve a safe state.

The three elements of a SIF.

SIFs respond to specific, defined hazards, by implementing specific actions to put the equipment into a safe state to provide a defined degree of risk reduction. The risk reduction required from a SIF is characterized by the Safety Integrity Level (SIL).

SIL indicates the degree of risk reduction, provided by a SIF, implemented by a SIS, within a given process. It is a measure of the SIF’s performance, in terms of Probability of Failure on Demand (PFD). When designing a SIF, the appropriate SIL is crucial for achieving the required level of safety. 

IEC 61508 defines four SIL levels, with SIL 4 providing the highest level of safety performance. The table below shows the associated PFD upper and lower limits and RRF (Risk Reduction Factors) upper and lower limits for each SIL. It differs depending on the mode (Demand vs Continuous mode – see Assessment Summary).

SIL Table (Demand Mode):

SIL Table (Continuous mode): 

Normally, a higher SIL level means a more complex system and higher installation and maintenance costs. Process plants typically only require SIL 1 and SIL 2 SIFs.

To determine a SIF’s SIL level, the SIF’s overall PFD must be calculated. This SIL calculation combines the Failure Rate data for each of the individual SIF elements and accounts for test frequency, redundancy, voting arrangements, etc.