What is SIL? Should we select SIL 4 for our Safety Instrumented Function?

Jenny is a young engineer assigned to a project to install a new Safety Instrumented System for a plant. Eddie is her supervisor, who is in charge of the project. Jenny wonders why have different SILs for different SIFs. Eddie explains the reasons. Watch the video below to know more.

What is SIL?

Safety Integrity Level (SIL) is a measure used to describe the integrity of a Safety Instrumented Function (SIF). It is a measure of how well the SIF will perform. A Safety Instrumented System will have many safety instrumented functions and each of them can have a different SIL.

 

SIL for Low Demand Systems and High Demand Systems

For both low demand systems and High Demand Systems/ Continuous Demand systems, the international functional safety standard IEC 61508, lists four Safety Integrity Levels, starting from SIL 1 (the lowest one) to SIL 4 (the highest one). For  process industries, we use the standard IEC 61511 (almost identical to ISA S84 in the US) for Safety Instrumented Systems.

 

Is a SIL 4 system the best?

The criteria for these levels is different for High Demand systems (depends on PFH of the safety function)  and Low demand systems (PFDavg of the Safety Function).

 

Is SIL 4 really the best ? Should our SIF be SIL 4?

In the short video above, we are referring to a Safety Instrumented System being installed in a process plant (a low demand system). Many engineers and managers, who are new to Functional Safety automatically assume that SIL 4 is the best option for any SIF. Is it really so?

Short answer: No, not really. Your SIL should be appropriately selected based on the risk reduction that you seek to achieve using the SIF.Based on this you can select your SIF to be SIL1, SIL 2, SIL 3 or SIL 4.

Also, there is a big hidden disadvantage of SIL 4, which some people may not know. Please see the video  to know about it.

Where can I learn more about SIL? How can I become a certified Safety Instrumented Systems professional?

That’s easy. Simply take the Abhisam Safety Instrumented Systems course today, learn everything via easy to understand animations, simulations, videos, exercises, examples and more. Take the associated exam and on passing you earn the title of certified Safety Instrumented Systems Professional.

More information on Functional Safety can be found here.

Contact us to know more.