Protection System Based on Risk Assessment

Once the severity and the probability corresponding to a scenario are estimated, that is, the risk is assessed, a decision can be made on the nature of the protection system to be implemented. If a safety instrumented system (SIS) is to be used, consisting of one or more independent protection levels (IPL), the required reliability of the protection system, constituting a so-called Safety Integrated Level (SIL) can be determined by using this risk assessment, respective of the required risk reduction. 

Quantitative failure frequency data are difficult to obtain for multipurpose batch plants in the way that they are often used in the fine chemicals and pharmaceutical industries. Moreover, a quantitative assessment requires detailed knowledge of the control instruments, which may not be available during process development Therefore, a semi-quantitative approach is proposed, providing the required reliability for future plant equipment. 

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