TYPICAL SIF SOLUTIONS

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  

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 verification calculations, including architecture limitations per lEC 61508 (Ref. 1), will be used to demonstrate whether the design satisfies the SIL requirements. 

It also needs to be emphasized that SIL is a lifecycle issue and although our present focus is in the hardware solutions required to satisfy the safety functions, all phases of the Safety Lifecycle have to be reviewed for final SIL verification. For all the examples and solutions, a low demand mode of operation is assumed since this is the mode that predominantly applies to the process industries. 

The failure rates and other parameters used in the examples are not to be assumed to be typical for all applications. The data used by those who do SIF verification calculations must be evaluated for every application. When stress conditions are above the levels specified in the data reference [for example, (Ref. 2)] then failure rates must be raised. 

Although the solutions in this chapter focus on safety integrity evaluation, it must be remembered that the reliability of the system is also important since this impacts on the nuisance or spurious trips of the plant. TTiese trips can have a large impact on safety and cost. Therefore (Mean Hme To Fail Spurious) is also calculated. 

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The ion is usually made by attack of HF on hydrous silica and is stable even in basic solution. Although the salts that crystallize are normally those of the SiF -ion, the pentafluorosilicate ion is found in compounds such as [Ph4As][SiF5]. Typical reactions producing SiFj salts are... 

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