In the Functional Safety Lifecycle, the next step after hazard and risk assessment is “allocation of safety functions to protection layers”. This is a process whereby suitable control measures are specified for each of the potential hazardous events found during the H&RA. This includes mechanical safeguards such as pressure relief valves, and also Safety Instrumented Systems (SIS).
The process of “allocation” is deciding which protective measures will be put in place, and how reliable they need to be. If a safety instrumented function (SIF) is to be used, the reliability required from it will be expressed as a safety integrity level, or SIL. There are four SILs – 1, 2, 3 and 4, which give increasing levels of reliability. The next task during “allocation” is to carry out “SIL determination”; in other words to decide what SIL is required from each SIF.
The most common technique used for SIL determination is layer of protection analysis (LOPA). See “what is a LOPA?” for more information.
77 LOPA scenarios were reviewed as part of an ongoing functional safety project.Industry: Agrochemical
A LOPA “Layer of Protection Analysis” study for three events relating to a hydro-power system: flood gates opened not in accordance with procedures, flood gates not opened when required and insufficient flow. The purpose of the LOPA study was to establish the effectiveness of existing and intended protection layers and safety functions. The LOPA study considered four scenarios derived from the three events and identified a set of actions where there were uncertainties in quantifying the initiating causes and frequency modifiers.Industry: Power
A Layer of Protection Analysis of a drinks industry demonstration area and laboratory, following an earlier HazOp (P-6164).Industry: Brewing and Distilling
LOPA of oil storage site (based on typicals approach)Industry: Water + Waste
LOPA of continuous annealing process in steel industryIndustry: Mining / Metal / Cement
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