The SIL level of a product is determined by three things: The Systematic Capability Rating, The Architectural Constraints for the element, The PFDavg calculation for the product.
The Systematic Capability
Systematic Capability is established by having your quality management system audited per IEC 61508. If the QMS meets the requirements of 61508 a SIL Capability rating is issued. The rating achieved depends on the effectiveness of your QMS. The certificate is for the systematic capability of a product.
The Architectural Constraints
Architectural constraints are established by following Route 1H or Route 2H. Route 1H involves calculating the Safe Failure Fraction for the element. A valve is typically one component of the final element of a safety instrumented function (SIF).
Architectural Constraints from FMEDA Results
Route 1H – Safe Failure Fraction (SFF) according to 7.4.4.2 of IEC 61508.
Safe Failures
Safe + Dangerous Failures
Route 2H – Assessment of the reliability data for the entire element according to 7.4.4.3.3 of IEC 61508.
Route 1H Table
| Safe Failure Fraction | Hardware Fault Tolerance | ||
| 0 | 1 | 2 | |
| < 60% | SIL1 | SIL2 | SIL3 |
| 60% < 90% | SIL2 | SIL3 | SIL4 |
| 90% < 99% | SIL3 | SIL4 | SIL4 |
| >99% | SIL3 | SIL4 | SIL4 |
Route 2H Table
| Hardware Fault Tolerance | ||
| 0 | 1 | 2 |
| SIL 2 | SIL 3 | SIL 4 |
Type B elements using Route 2H shall have a diagnostic coverage not less than 60%.
The PFDavg calculation
The PFDavg is based on the dangerous failure rate, system diagnostics, proof test coverage and test intervals. Typically, a final element assembly will have a PFDavg the only meets SIL 1. However, there are things that can be done with the diagnostics and proof test that would improve the PFDavg to SIL 2.
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