It’s very interesting to me to find that people are still using excel to do PFD calculations.  Whereas this is probably okay for simple single element architectures (i.e. single sensor, single logic solver and single final element), it becomes extremely complicated when dealing with redundant architectures.  Moreover, for performing SIL verification, it’s not just a simple case of calculating PFD (for continuous applications) or PFDavg (for demand applications) because compliance with IEC 61511 requires the end user to account for systematic capability as well as architectural constraints, when considering the Safety Integrity Level (SIL) of a Safety Instrumented Function (SIF).  When referring to redundant systems, common cause (β factor) also needs to be considered, along with Proof Test Coverage, Proof Test Interval and Mean Time To Restore (MTTR).  Spurious Trip Rate also needs to be accounted for as part of the assessment.

It’s also interesting reading some of the online forums that deal with functional safety and the discussions regarding calculating the SIL of a SIF using PFDavg.  The simple λt/2 is not sufficient for PFDavg.


Not to mention the fact that you have to find the failure rate data for each device used in the SIF(s), including the logic solver(s).  There are SIL calculation software tools on the market that can help with this but they too need good, accurate, failure rate data. 

There are products on the market, including exida’s exSILentia suite of tools that provide a very easy and efficient way of being able to perform SIL determination, SIL calculations, and SIL verification.  However, exSILentia contains a database of failure rate information for field devices, logic solvers and final elements, a feature that differentiates it from other products.  Selecting the right tool will provide the Safety Instrumented System (SIS) designer with a quick and easy way of verifying that the SIF design can meet its target SIL.  exSILentia provides the ability to perform SIL determination for SIFs, based upon either a Risk Graph, Frequency based or Hazard Matrix approach.  If you want to know more then why not check out our webinars on SIL Selection using exSILentia and register for our upcoming webinar SIL Verification using exSILentia.

Tagged as:     Steve Gandy     SIL verification     SIL calculation     SIL     Safety Integrity Level     PFD     MTTR     Mean Time To Restore     IEC 61511     exSILentia  

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