In the following series of blogs, we'll go back to basics and run down everything you need to know to get started in functional safety.  We'll start with some more general terms and descriptions and make our way to more advanced material.

3. SIF

SIF – Safety Instrumented Function – is a set of equipment intended to reduce the risk due to a specific hazard safety loop. Its purpose is to 1. Automatically take an industrial process to a safe state when specified conditions are violated; 2. Permit a process to move forward in a safe manner when specified conditions allow (permissive functions); or 3. Take action to mitigate the consequences of an industrial hazard. 

A SIF is composed of sensor(s), logic solver(s), and final element(s). It includes elements that detect an accident is imminent, decides to take action, and then carries out the action needed to bring the process to a safe state. Its ability to detect, decide, and act is designated by the safety integrity level (SIL) of the function. 

An individual Safety Instrumented Function (SIF) is designed to first identify the need and then act to bring the system to a safe state for each hazard scenario. It is a specific singleset of actions and the corresponding equipment needed to identify a singlehazard and act to bring the system to a safe state.

The effectiveness of the SIF is typically measured by its (often expressed as a SIL or target).

Can you think of any examples of SIFs? 

• On detecting high temperature, prevent column rupture by shutting off steam flow to the re-boiler.
• On detecting high pressure, prevent tank rupture by opening valve to relief system.
• On detecting high level, open drain valve to direct excess liquid to waste sump to reduce environmental damage.
• Stop motor by disconnecting power or activating brake when severe over-speed is detected.

(What about this one: on detecting a fire, issue alarms to minimize damage and possible injury? This is not a complete SIF since it does not achieve a safe state. The final actions must be included!)