The international functional safety standard IEC 61511 provides the safety lifecycle as a steadfast guideline to assess and mitigate risk for manufacturing processes including refineries, chemical, petrochemical, pulp and paper, and power plants. To achieve a functionally safe system, it is essential to follow each requirement in the standard. However, consistent execution is difficult to achieve and often depends on the tools used to perform analysis and specification of the safety instrumented system. For the functional safety consultants at exida, the need for a consistent work process was fulfilled with the creation of the exSILentia software suite. exSILentia includes a module for each stage of the safety lifecycle. Use of the tool ensures quality assessment and execution of a safety instrumented system, as well as compliance to the safety standard. exSILentia also streamlines these tasks, easily transferring data from one module to another to save the user time and money. 

The Safety Lifecycle (SLC) includes the following key tasks:

Analysis Phase:

  • Scope Definition and Process Design
  • Process Hazard Analysis (PHA)
  • Layer of Protection Analysis (LOPA)
  • Safety Integrity Level (SIL) Selection
  • Safety Requirement Specification (SRS)

Realization Phase:

  • Safety Integrity Level (SIL) Verification
  • Detailed Design Safety Requirement Specification (Design SRS)
  • Programming of the PLC
  • Specification of Proof Tests

Operation Phase:

  • Configuring Safety Instrumented System (SIS) into field collection database
  • Field Failure and Proof Test Recording
  • Standard Compliance, Audit Preparedness

Use of excel or an in-house tool may seem like the cheapest solution to support these SLC tasks and to design and operate a safety instrumented system. However, with each phase of the lifecycle comes a hefty to-do list that requires hours of preparation, discussion and documentation. As hours add up, the cost of the project increases. Use of exSILentia reduces the hours required for each task significantly by organizing and transferring inputs from one step to the next, providing built-in failure rate data, performing design calculations and generating necessary reports.

In this cost analysis, an estimated time to complete the tasks using excel versus using exSILentia is provided. The time estimate for each task assumes 10 nodes are analyzed, each resulting in 5 safety instrumented functions (SIF). To attribute a cost range to the hours spent, an hourly rate of $75 is assumed, as well as a burdened rate of $150 per hour.

In the end, analyzing 10 nodes and subsequently analyzing, implementing, and maintaining 50 SIFs using excel or an in-house tool will take a grand total of approximately 2,000 hours. For users of exSILentia these same tasks should take about 600 hours. Depending on the hourly rate of the engineers assigned to each task, exSILentia will save $120K-$240K per 10 nodes and 50 SIFs. It is possible for a system in the process industry to have hundreds of nodes and SIFs. Based on the analysis documented in this paper, we can assume that use of excel or an in-house tool is nearly 4 times more expensive then use of the complete exSILentia suite.


Hours Spent - Using Excel

Hours Spent - Using exSILentia

Time/Cost Delta

SLC Analysis Phase




SLC Realization Phase




SLC Operation & Maintenance Phase




Grand Total




Cost (Hourly Rate: $75/hour)




Cost (Burdened Rate: $150/hour)




Tagged as:     safety lifecycle     safety instrumented system     Kate Hildenbrandt     IEC 61511     exSILentia  

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