Hardware Safety Integrity the SIL Way

Wednesday, February 15, 2012 @ 06:02 PM gHale

By Nicholas Sheble
“Two instruments failed at BP’s Texas City refinery resulting in an explosion. They measured and alarmed level of liquid. Should we rely on operators or automatic systems to prevent such accidents?” Karl Watson asked during his webinar Tuesday.

Watson is Process Safety Consultant at ABB Consulting and he presented “Design of SIL (safety integrity level) Systems.”

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“Automatic systems are ‘functional safety’ or IEC 61511 and its American interpretation ISA 84,” Watson said. Automatic systems are the way industry has decided to proceed rather than fight and risk human error.

The broadcast provided an overview of the considerations in designing a SIL system including design calculations, the different failure modes, and the sources of failure rate data.

IEC 61511 sets out practices in the engineering of systems that ensure the safety of an industrial process with instrumentation. Such systems are Safety Instrumented Systems (SIS). The title of the standard is “Functional safety — Safety instrumented systems for the process industry sector.”

“Process industries rely on Safety Instrumented Systems to protect and maintain operating assets,” Watson said. “Target levels of safety must be maintained, without jeopardizing operating effectiveness with spurious trips.”

The SIL of a safety system is the assessment of the risk reduction required through that system to give a tolerable level of risk. Crucial steps in operating effective safety systems include determining the target SIL using LOPA.

LOPA is layers of protection analysis. It’s an analytical tool for assessing the adequacy of protection layers in use to mitigate process risk. LOPA builds upon well-known process hazards analysis techniques and the frequency of potential incidents and the probability of failure of the protection layers.

“The point is to design the system to achieve the required SIL, and to maintain that system through constant oversight and testing to sustain the required SIL,” declared Watson.

Different SILs of low demand operations have these numbers for PFD (Probability of Failure on Demand) and RRF (Risk Reduction Factor).
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For continuous operations, matters change.

Here is the webinar – “Design of SIL Systems.”

Nicholas Sheble (nsheble@isssource.com) is an engineering writer and technical editor in Raleigh, NC.

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