CASE
STUDY
WHEN SIL 2 WAS NOT SIL 2:
VERIFICATION FOR A SPECIALTY CHEMICAL MANUFACTURER
A North American specialty chemical manufacturer believed its safety instrumented functions met their assigned Safety Integrity Levels. Verification calculations showed several did not. This is how the shortfall was identified, quantified and closed inside the existing maintenance cycle.
| Challenge | Solution | Result |
|---|---|---|
| SIL assignments inherited from original design, never verified by calculation. | Full SIL verification across all safety instrumented functions, with shortfall analysis. | Verified SIL for every function, and a remediation plan scoped to the existing turnaround. |
Project Background
The operator runs a multi-unit specialty chemical facility with safety instrumented systems installed across two decades of capital projects. Each project assigned Safety Integrity Levels to its safety instrumented functions, and those assignments were recorded and carried forward. What had never happened was verification: confirming by calculation that the installed architecture, the selected devices, the proof test intervals and the failure data actually delivered the SIL claimed on paper. An insurer question during a renewal review prompted the operator to ask whether the claims could be substantiated. They could not, and nobody internally could say how large the gap might be.
The core problem was that SIL assignment and SIL verification had been treated as the same activity. A target had been set during design, documented, and then assumed to be achieved. No probability of failure on demand calculation existed for any function in the facility, so the operator had a register of claims and no evidence behind any of them.
Several factors made the gap likely to be real rather than theoretical. Devices had been replaced over the years with functionally equivalent but not identical instruments, often without any review of the effect on the safety function. Proof test intervals had drifted to suit maintenance scheduling rather than to satisfy the interval the original SIL claim depended on. Some final elements were shared between safety and basic process control duty, which affects the architecture in ways the original documentation did not capture.
The operator also faced a practical constraint. Any remediation requiring process downtime had to fit the planned turnaround, which was less than a year away. A verification exercise that identified problems but delivered its findings too late to scope into that window would leave the facility carrying the shortfall for another operating cycle.
Arista Cyber delivered a verification program structured to produce actionable findings early, so that remediation requiring downtime could be scoped into the turnaround while the remaining analysis continued.
Safety Function Inventory
Established a complete register of safety instrumented functions, their assigned SIL targets and the documentation supporting each assignment.
As-Installed Verification
Confirmed the actual installed devices, architecture and voting arrangements against the design documentation for each function.
Failure Data Consolidation
Assembled device failure rate data from manufacturer certificates and recognized sources, with the data source recorded for each device.
PFDavg Calculation
Calculated average probability of failure on demand for every function, using the as-installed architecture and the actual proof test intervals in use.
Shortfall Analysis
Identified every function failing to achieve its assigned SIL, and quantified the margin by which each fell short.
Remediation Option Development
Developed options for each shortfall, distinguishing changes requiring downtime from those achievable during normal operation.
Turnaround Scoping
Packaged the downtime-dependent remediation into a defined scope the operator could integrate into its turnaround planning.
Every safety instrumented function in the facility now carries a verified Safety Integrity Level supported by a documented calculation, with the device data sources, architecture assumptions and proof test intervals recorded alongside it. Where a function achieves its target, the operator can demonstrate why. Where it does not, the margin is quantified.
The shortfall analysis changed how the operator thought about the problem. Several functions failed their target not because the original design was inadequate but because proof test intervals had lengthened over time to fit maintenance scheduling. Those were correctable without any process interruption, by restoring the interval the SIL claim depended on. Separating that category from the functions requiring hardware change meant a significant portion of the gap closed without touching the turnaround scope at all.
The remainder was packaged as a defined turnaround scope, with each item traceable to the specific function and the specific shortfall it addressed. That traceability is what allowed the work to survive the turnaround prioritization process, because every line could be defended on its own terms rather than as part of an undifferentiated safety request. The operator also now has the calculation baseline it needs to assess future device replacements before they happen, rather than discovering their effect at the next verification.
Who Should Engage Arista Cyber?
Operators who hold SIL assignments they cannot substantiate by calculation, who have replaced safety system devices without assessing the effect on the safety function, or whose proof test intervals have drifted from the intervals their SIL claims depend on.
What's the Next Step?
Ready to substantiate the Safety Integrity Levels your safety case depends on? Contact Arista Cyber to schedule SIL verification.
- Commission SIL verification for your safety instrumented functions
- Request a shortfall analysis against your existing SIL assignments
- Schedule a functional safety lifecycle assessment