Introduction

The determination of Safety Integrity Level (SIL) for safety instrumented functions (SIF) is a basic concept of performance based safety standards. The safety integrity or performance of a SIF must increase with higher SIL levels. There are a number of factors that influence the performance of any safety instrumented function. Some of the key factors are:

• Quality of the components used - total failure rate

• Quality of instrument manufacturer to reduce systematic failures

• Safe versus dangerous failure rate of components

• Automatic diagnostic capability of components

• Automatic diagnostics within the SIF

• Proof testability of the components and the SIF

  • Quality of testing, i.e., what % of the component is actually being tested

  • Portion of each component being tested vs. the portion not being tested

• Redundancy of components

• Common cause strength

  • Diversity of redundant components

  • Physical separation of redundant components

• Use of energize to trip vs. de-energize to trip systems.

• Response time of components.

• Time to repair instrument.

• Systematic failures, e.g. failures that relate to the inherent design of the system rather than random hardware failures.

• Safety Lifecycle activities including audits, assessments, and verifications.

In the opinion of committee members on functional safety standards, some of the above factors cannot be practically quantified, e.g., systematic faults like software bugs or procedural errors. Hence functional safety standards provide requirements for protection against systematic faults as well as requirements to do probabilistic calculations to protect against random failures. For the typical SIF solutions being reviewed in this chapter the results of probabilistic SIL...