Understanding the risks, post the risk assessment, is an area where S-Series Solutions adds significant value. Whether we facilitate, participate or get involved after a completed PHA, we can help close the defined gaps. Our staff has the skill set to take the PHA report and propose solutions from the action items and recommendations.
Safety Integrity Level (SIL) Verification
Safety Integrity Level (SIL) Validation
Safety Requirement Specification (SRS) Development
We perform SIL verification calculations using Isograph’s Reliability Workbench (FaultTree+) using our “in-house” failure rate data or a facility’s failure rate data, if available. Following international best practices, S-Series has a TÜV Rheinland Functional Safety Engineer on staff.
SIL Certification vs. SIL Verification
As more and more vendors are providing ‘SIL Certificates’ for their products it’s important to clearly understand the difference between this and the SIL capability of a SIF. SIL Certification is specific to one component of the SIF. In most cases, a third-party company performs a Failure Modes, Effects, and Diagnostic Analysis (FMEDA) to evaluate a device’s failure behavior. Dangerous and safe failures and their failure rates are determined in order to quantify the component’s safe failure fraction – one of the performance parameters required to determine architectural constraints for its use in a SIF. Other industry-standard performance requirements are also evaluated. The final result is a determination of not only the highest SIF SIL rating the device can support but also a list of requirements that must also be met. This is not a SIL verification calculation.</p>
<p>A SIL verification calculation is performed on the entire SIF. It is based on component failure rates (for all SIF components), hardware fault tolerance, logic solver capability, common mode failures, and proof-test coverage and intervals. Other parameters also used in the SIL verification should be fully detailed in the SIF’s safety requirements specification. The final result of this calculation is a determination of the SIL rating of the SIF.</p>
<p><b>Bottom line:</b> A SIL 2 certified field device does not ensure that its associated SIF will meet a SIL 2 performance requirement. Only an overall SIF SIL verification can do that.