The Safety Integrety Level (SIL) of a product is determined by three things:

1. The Systematic Capability Rating
2. The  Architectural Constraints for the element
3. The PFDavg Calculation for the product

The Systematic Capability Rating

Systematic Capability is established by having your quality management system audited per IEC 61508. If the QMS meets the requirements of IEC 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 for the element

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).

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.

The PFDavg Calculation for the product

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. 

SIL complience happens when the all three areas are met fully.  You cannot have a SIL for a product that only meets 2 of the 3 areas.   The lowest SIL achieved for each area will be the entire SIL for the product.