The most common technique used for SIL determination is layer of protection analysis (LOPA). See “what is a LOPA?” for more information.
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 Hazard & Risk Assessment (usually a HazOp) . 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, as follows:
| SIL | PFDavg | Risk Reduction Factor (RRF) | ||
| From | From | To | To | |
| 1 | 0.1 | 0.01 | 10 | 100 |
| 2 | 0.01 | 0.001 | 100 | 1,000 |
| 3 | 0.001 | 0.0001 | 1,000 | 10,000 |
| 4 | 0.0001 | 0.000001 | 10,000 | 100,000 |
A key task during “allocation” is therefore to carry out “SIL determination”; in other words to decide what SIL is required from each SIF.
Provide experienced LOPA study facilitators
Train and give mentoring support to aspiring LOPA study leaders
=Method provides expert LOPA facilitators and scribes who use proven tools and templates. Our approach recognises and supports downstream activities, including the Safety Requirements Specification and functional safety assessment.
Discuss your LOPA needs with us nowLOPA following earlier HazOp to establish the effectiveness of existing and intended protection layers and safety functions at a hydro electricity facility.
Industry: Water + WasteLayer of Protection Analysis for Hydrogen Energy Management System (EMS).
Industry: OtherProject to undertake a Layer of Protection Analysis (LOPA) for three scenarios related to the overfill of the gasoline tanks at an oil Terminal.
Industry: Water + WasteLayer of Protection Analysis (LOPA) study fora distillery expansion project.
Industry: Brewing and DistillingUpdate of the existing Hazard & Operability (HazOp) and Layer of Protection Analysis (LOPA) Studies of new tanker loading control facilities to be installed to take account of the proposed addition of new tanker loading control valves.
Industry: Brewing and Distilling
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See also Functional Safety Lifecycle
Process Safety Lifecycle
See also OT Cyber Security Lifecycle
