Safety Instrumented Systems (SIS) are installed in Process Plants to mitigate process hazards by taking the process to a “safe state” when predetermined set points have been exceeded or when safe operating conditions have been transgressed.The SIS is one Protection Layer in a multi-layered safety approach since no single safety measure alone can eliminate risk. A Layer of Protection Analysis (LOPA) is a method whereby all known process hazards and all known layers of protection are closely scrutinized. For each process hazard where the LOPA study concludes that existing protection cannot reduce risk to an acceptable or tolerable level, a Safety Instrumented System is required. Not all process hazards will require the use of a SIS. Each hazard that requires the use of an SIS must be assigned a target SIL level. This article serves as a high-level summary as to how SIL levels are determined for process applications. What Are SIL Levels?SIL is an acronym for “Safety Integrity Level” that comes from two voluntary standards used by plant owners/operators to quantify safety performance requirements for hazardous operations:.
Safety Integrity Level (SIL) is a measure of safety system performance – not a measure of process risk. The higher the level of risk, the greater the system performance required.
IEC 61508: Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems. IEC 61511: Safety Instrumented Systems for the Process Industry SectorAs defined in the IEC standards, there are four SIL Levels (1-4). A higher SIL Level means a greater process hazard and a higher level of protection required from the SIS. To generalize how SIL Level is determined, see Figure 1. SIL Level is a function of hazard frequency and hazard severity.
Hazards that can occur more frequently or that have more severe consequences will have higher SIL Levels. Figure 1: General chart demonstrating how SIL Level is a function of hazard frequency and hazard consequence. Used with permission from. To determine SIL Levels of process hazards, it is helpful to understand the Safety Life Cycle. Safety Life CycleThe IEC standards define a concept known as the Safety Life Cycle, see Figure 2.
The Safety Life Cycle provides a repeatable framework whereby all process hazards are identified and analyzed to understand which hazards require the use of a SIS for mitigation. By design, this is a cyclic process, not a linear process with an endpoint. Any changes in process design, operating conditions, or equipment requires cycling back to the beginning to ensure any changes are properly implemented.
Figure 2: Safety Life Cycle model. Adapted from IEC 61511.For the remainder of this article, we will focus on the steps to follow to determine SIL Level, starting with the Process Hazard Analysis. Process Hazard AnalysisA Process Hazard Analysis (PHA) is a systematic assessment of all potential hazards associated with an industrial process. Figure 3: General view of plant safety protection layers. Used with permission from Magnetrol.Some specific examples of Protection Layers include:.
Fire suppression systems. Leak containment systems (dikes or double walls). Pressure relief valves. Gas detection/warning systemsThe general steps of a LOPA are as follows.
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( April 2015) Safety integrity level ( SIL) is defined as a relative level of risk-reduction provided by a, or to specify a target level of reduction. In simple terms, SIL is a measurement of performance required for a (SIF).The requirements for a given SIL are not consistent among all of the functional safety standards. In the functional safety standards based on the standard, four SILs are defined, with SIL 4 the most dependable and SIL 1 the least. The applicable SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. Contents.Assignment Assignment of SIL is an exercise in risk analysis where the risk associated with a specific hazard, that is intended to be protected against by a SIF, is calculated without the beneficial risk reduction effect of the SIF.
That unmitigated risk is then compared against a tolerable risk target. The difference between the unmitigated risk and the tolerable risk, if the unmitigated risk is higher than tolerable, must be addressed through risk reduction of the SIF. This amount of required risk reduction is correlated with the SIL target. In essence, each order of magnitude of risk reduction that is required correlates with an increase in one of the required SIL numbers.There are several methods used to assign a SIL.
These are normally used in combination, and may include:. Risk matrices.
Risk graphs. Layers of protection analysis (LOPA)Of the methods presented above, LOPA is by far the most commonly used by large industrial facilities.The assignment may be tested using both pragmatic and controllability approaches, applying guidance on SIL assignment published by the UK HSE. SIL assignment processes that use the HSE guidance to ratify assignments developed from Risk Matrices have been certified to meet IEC EN 61508.Problems There are several problems inherent in the use of safety integrity levels.
Charlwood, S Turner and N. Worsell, UK Health and Safety Executive Research Report 216, 'A methodology for the assignment of safety integrity levels (SILs) to safety-related control functions implemented by safety-related electrical, electronic and programmable electronic control systems of machines', 2004.
^ Redmill, Felix (2000). Retrieved 16 February 2017. CASS Scheme, Conformity Assessment of Safety Systems,.
Marszal, Edward, 'Safety Integrity Level Selection – Systematic Methods Including Layer of Protection Analysis', The Instrumentation, Systems, and Automation Society, Research Triangle Park, NC, USA, 2002. Mitchell, KJ, Longendelpher, TM, Kuhn, MC, 'Safety Instrumented Systems Engineering Handbook', Kenexis, Columbus, OH, USA, 2010.Textbooks D.
Simpson, 'Safety Critical Systems Handbook – A Straightforward Guide to Functional Safety, IEC 61508 (2010 Edition) and Related Standards' (3rd Edition, 270 Pages).M. Punch, 'Functional Safety for the Mining Industry – An Integrated Approach Using AS(IEC)61508, AS(IEC)62061 and AS4024.1.'
(1st Edition, in A4 paperback, 150 pages).M.J.M. Houtermans, 'SIL and Functional Safety in a Nutshell (Risknowlogy Best Practices Series, 1st Edition, eBook in PDF, ePub, and iBook format, 40 pages). Scharpf, 'Practical SIL Target Selection - Risk Analysis per the IEC 61511 Safety Lifecycle'M.
Faller, 'Functional Safety - An IEC 61508 SIL 3 Compliant Development Process, (Third Edition)' External links.