exida explains Blog

Entries tagged with: PFDavg

Super-Duper Logic Solver Data; So what?

The Architectural Constraint Blind Side
  • by Denise Chastain Knight, P.E., CFSE, CCPSC
  • Thursday, August 27, 2015
  • Certification

The Architectural Constraint Blind Side

I did my homework, purchased certified devices, and specified physical redundancy. I expected an uneventful SIL Verification but the assessor is telling me that I have functions failing Architectural Constraints in the sensor and final element groups. How can that be? 

Low demand mode Safety Instrumented Function (SIF)…

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  • by Dr. William Goble, CFSE
  • Wednesday, February 27, 2013
  • Software

The Real PFDavg Variables

I was recently asked how the PFDavg value in an exida FMEDA report was calculated. Those PFDavg values are calculated using the exSILentiaTM program. The person who posed the question was using the equations from Part 6 of IEC 61508 and got a different value. Why?

exSILentia

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  • by Dr. William Goble, CFSE
  • Tuesday, June 07, 2011
  • Certification

The Weakest Link

One of the objectives of new performance based standards like IEC 61508 is to allow performance metrics (like PFDavg for low demand systems) to identify the weak links in any proposed design.  Obviously, it is not necessary to put effort into design and spend money on areas…

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Two Billion Hours

When I studied Reliability Engineering in graduate school, I was taught that failure rate data came from field failure studies.  I assumed that operating companies always kept accurate records when a piece of equipment failed.  I also assumed that someone would investigate the failures and assign a root cause.…

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Using SILSafeData to Check Your Failure Data Source
  • by Dr. William Goble, CFSE
  • Monday, October 29, 2018
  • Certification

Using SILSafeData to Check Your Failure Data Source

www.SILSafeData.com is a complimentary resource that contains the upper and lower bounds failure rates for many categories of automation equipment, as well as the methodology used by exida to derive the numbers.  

Many of you have asked how to…

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What is SIL compliance?

What is SIL compliance?

The Safety Integrety Level (SIL) of a product is determined by three things:

  1. The Systematic Capability Rating
  2. The  Architectural Constraints for the element
  3. The PFDavg Calculation for the product

The Systematic Capability Rating

Systematic Capability is established by having your quality management system audited per IEC 61508.…

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WHY are Failures Occurring at a Different Rate from Our Other Site?
  • by Ted Stewart, CFSP, exidaCSP
  • Thursday, January 14, 2016
  • Certification

WHY are Failures Occurring at a Different Rate from Our Other Site?

Site audits have shown many differences in how things are done from site to site. This impacts many variables in a safety instrumented system!

A key metric for process industry designs is called average Probability of Failure on Demand (PFDavg).  After several studies of many field failure and proof…

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Why Run-to-Fail Is Not A Good Maintenance Strategy For SIS

Why Run-to-Fail Is Not A Good Maintenance Strategy For SIS