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Methodologies in Reducing Systematic Failures of Wired IPLs

by Richard E. Hanner & Tab Vestal

The history of high consequence incidents in industry reveals that most accidents were the result of systematic failures, not hardware failures. However, a higher degree of focus in engineering is often on the quantifiable failures of hardware. Process Safety risk gaps are often closed or reduced by several types of Independent Protective Layers (IPLs). Two common types are Safety Instrumented Functions (SIFs) and Basic Process Control System (BPCS) functions. The SIFs typically reside within a SIL-rated programmable logic controller, and their achieved quantitative performance is calculated based on random hardware failures of the SIF hardware components. Conversely, BPCS protective layers are assigned generic industry-accepted probability of failure credits. The BPCS generic industry-accepted probabilities of failure are conservatively assigned and consider unquantifiable human-induced systematic failures.

In either case, the likelihood of systematic failures can be reduced by recognizing design, specification, maintenance, and operations activities that are potential sources, and applying measures to prevent or reduce them. By reducing systematic failures, you reduce the risk in the industrial process and increase confidence in meeting the intended integrity requirements. This technical paper will discuss the common sources of systematic failures and preventative or mitigative measures to prevent their occurrence.

Keywords: Systematic failure, random hardware failure, Independent Protective Layer, IPL, SIF, SIS, BPCS, common cause, Human Factor Analysis, SIL Verification


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