Sample SIL Calculation

As a PES, the Tri-GP controller is designed to minimize its contribution to the SIL, thereby allowing greater flexibility in the SIS design.

Figure: Comparison of Percent Availability and PFD

Tri-GP controller module failure rates, PFDavg, Spurious Trip Rate, and Safe Failure Fraction (SFF) calculation methods have been independently reviewed by TÜV Rheinland. The numbers presented here (and in the following tables) are typical. Exact numbers should be calculated for each specific system configuration. Contact the Invensys Global Customer Support (GCS) center for details on calculation methods and options related to the Tri-GP controller.

Figure: Simplified Diagram of Key Elements

This table provides simplified equations for calculating the PFDavg for the key elements in an SIS. Once the PFDavg for each element is known, an SIL can be determined.

Table 1 Simplified Equations for Calculating PFDavg

Note Equations are approximate

To determine the SIL, compare the calculated PFDavg to the figure on page 5. In this example, the system is acceptable as an SIS for use in SIL2 applications.

Table 2 Determining the SIL Using the Equations

For additional information on SIL assignment and SIL verification, visit the Premier Consulting
Services web site at

Reference: Triconex