(57i) Method for Uncertainty Evaluation of Safety Integrity Level (SIL) Calculations

Advanced control and safety instrumentation systems are now widely used to control the risk of process operations. The IEC 61511 standard requires a verification calculation that a proposed design for a safety instrumented function (SIF) achieves the desired safety integrity level (SIL). The evaluation of the safety integrity level of a new or existing safety instrumented system requires detailed calculations based on the failure rates of the device and the planned maintenance/testing cycle for the system. The failure rates of the devices are often taken from standard failure rate tabulations of equipment. All of the data used in the SIL calculations are uncertain. This paper develops a general method for uncertainty analysis of the SIL calculations based on the application of probability theory - variance contribution analysis (VCA) - to the equations presented in ISA TR 84.00.02-2115. In addition, special design cases not covered in the ISA TR84 are also presented. Such design cases include the currently popular 1oo2D where the instrumentation is used to cross check itself to identify instrumentation failures. An example is worked to demonstrate the methodology.