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 test reports, several variables* have been identified as key to a realistic PFDavg calculation.

• * The Key Variables Needed for PFDavg Calculation, White Paper, exida 2015, http://www.exida.com/Resources/Whitepapers/the-key-variables-needed-for-pfdavg-calculation

The key variables in a PFDavg calculation are impacted by site operational practices and performing a SSU audit has many benefits.

• A model can show this impact and could be used to show the benefit of improved operational processes.
• Life cycle cost comparisons can be done to justify improved operational processes.

SO how is the SSI performed? What are the ways one can find out where they stand?

There are 2 different approaches to a SSI evaluation (audit).

1. Site Audit: the best method to establish a SSI ranking would be a site audit against an expanded version of IEC 61511.  An experienced safety process engineer will ask questions, review procedures, audit documentation produced, and score the results in a detailed report along with recommendations for improvement.
2. A simple questionnaire can provide initial results with less accuracy.

If you would like further detailed information please click the below links. 

Modeling site specific safety and reliability – The Site Safety Index

Site Safety Index ImpactTM on Failure Rates and PFDAVG