Year: 2018

Dangerous failures in the SIS

Dangerous failures of the SIS When calculating Probability of Failure of a Safety Instrumented Function (SIF), the most important are the dangerous failures, as we see in the following equations of 1oo1 and 1oo2 architectures.The value Lambda DD is the rate of dangerous detected failures, and Lambda DU corresponds to the undetected ones. We can …

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The diagnostics in the SIS

The importance of diagnostics in the SIS The diagnostics in an Safety Instrumented System (SIS) are of crucial importance because they are the key to reduce the rates of undetected dangerous failures and, therefore, to reduce the probability of failure on demand (PFD / PFH) and increase the SIL. We can see it in the …

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Common cause failures

The importance of common cause failures When designing a control system, we should paid special attention to common cause failures, that is, to the factors that may cause the simultaneous failure of several components or redundant channels. It is an even more important aspect in the case of safety instrumented systems (SIS) and is considered …

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Safety Instrumented Function

What is a Safety Instrumented Function? The safety instrumented function is a control loop in a process or machine whose objective is safety. SIF is its acronym in English. In the following image we see the most common simplified representation of the SIF. The integrity and performance of the safety instrumented function depends on a …

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SIL requirements

SIL requirements – Systematic Capability, Failure Probability and Architectural Constraints. The designer of the safety instrumented function must verify that the 3 SIL requirements of the IEC61508 Standard are met. Each requirement will meet a certain maximum SIL level. The final SIL of the SIF will be the lowest of the three and must be …

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