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SAFETY INTEGRITY LEVEL ASSESSMENT (SIL)

The Safety Integrity Level (SIL) Study is required to assess the discrete level for specifying the safety integrity requirements of the safety instrumented functions (SIF) to be allocated to the safety instrumented systems. Layer of Protection Analysis (LOPA) methodology will be followed which accounts the protective layers available to prevent and to mitigate hazardous events with inherently safe design concept, to prevent the occurrence / mitigate the consequences from such hazardous event.

Technical Challenge:

There is an increasing dependence on Safety Instrumented Systems (SIS) to achieve tolerable risk levels in the process sector.  In addition there is an increasing need to justify that the risk levels that have been achieved are tolerable.

SIFs are one of the layers of protection which are designed into process plants to prevent the occurrence of major hazard scenarios; this means that the failure or absence of a SIF increases the probability of a hazardous scenario occurring and escalating into a major accident. It is then necessary to estimate the probability of the SIF failing to act on a demand, in order for the users to determine if the target risk levels of their organization or country are being achieved.

What are the benefits of conducting SIL Study?

Functional safety is one of the cornerstones of the safety strategy for Process Safety Management (PSM) program of any process plant. It is therefore of great importance that the lifecycle of Safety Instrumented Functions (SIFs), including specification, design, validation, commissioning and operation & maintenance, is in accordance to the requirements of the recognised functional safety standard: IEC 61508 [1] or its sector-specific derivative.

When SIL should be done?

Safety performance criteria for SIS is defined by Safety Integrity Level (SIL) of the identified loop. SIL is an instrumented safety system that is designed in accordance with the IEC 61508 and IEC 61511 standards.

SIL study is also one of the safety studies which is carried out to prevent hazard in process industries. Generally SIL study is carried out after the HAZOP study is completed for a project.

HAZOP study ensures safety and reliability at process level and SIL study ensures availability or robustness at component level in a process industry.

A SIL level applies to an entire SIS loop under consideration. SIL levels are used when implementing safety instrumented function that must reduce the existing intolerable process risk to tolerable risk level.

As we can see from the figure below, SIS is part of Prevention as well as mitigation of hazard.

Layers of Protection (Ref. BS IEC 61511- part-3)

SIL LOPA Layers of Protection

Approach / Methodology

SIL assessment is a risk based approach to identify the required safety integrity levels (SIL) for safety instrumented functions (SIFs) in accordance with IEC 61508 / 61511 or ANSI/ISA S84.01.

The SIL assessment is performed by a multidisciplinary team led by a SIL Facilitator. Determination of the SIL of a SIF can be achieved using different qualitative and quantitative approaches.

  • Risk Graphs – qualitative method, proposed in IEC 61508
  • Layers of protection analysis (LOPA) – alternative qualitative method, widely used in the process industry
  • Fault tree analysis (FTA) / Event tree analysis (ETA) – quantitative methods

Generally a combination of the methods is employed. For example, performing an initial qualitative assessment for all safety functions ("screening process") and performing a detailed quantitative assessment for the higher criticality or less well understood functions.

Layer of Protection Analysis is a simplified form of quantitative risk assessment. In a typical process plant, various protection layers are in place to lower the frequency of undesired consequences: the process design (including inherently safer concepts); the basic process control system; safety instrumented systems; passive devices (such as dikes and blast walls); active devices (such as relief valves); human intervention; etc

There is a close relationship between HAZOP & LOPA

close relationship between HAZOP & LOPA

The sequential steps used to conduct the SIL classifications ( LOPA method) are shown as follows:

SIL Classification

The SIL classification proceeding will be recorded on SIL classification worksheets

After completion of SIL classification, SIL Verification calculations will be performed using the exSILentia software Version 2.0 by Exida. This is a tool to determine the SIL rating for each SIF by using Exida database for failure rates of system components. The software also provides for evaluation of architectural constraints to IEC 61508/IEC 61511.

Typical SIL Study team will include:

  • Following will be the typical team composition for the SIL Study:
  • SIL Study Chairman (Full Time)
  • SIL Study Scribe (Full Time)
  • Project Manager (Part Time)
  • Loss Prevention Engineer (Full Time or Part Time)
  • Process Engineer (Full Time)
  • Instrumentation & Control Engineer (Full Time)
  • Operations Representative (Full Time)
  • Other Engineers (Electrical / Mechanical / Piping etc) on Call Basis

Standards & Software Requirement

The software to be used for the SIL study is LOPA Excel Sheet (for Assessment)

                                                For SIL Verification – ExSILentia- ( Exida)   

International Standards:

•          IEC 61508 – 2010 Edition – Functional Safety of Electrical / Electronics / Programmable Electronic safety related systems

•          IEC 61511 – 2004 Edition – Functional Safety: Safety Instrumented Systems for the Process Industry Sector – all 3 parts.

The SIL Study is generally classified under three stages as

  • SIL Identification
  • SIL Verification
  • SIL Validation

Typical Worksheet

Typical SIL Worksheet

iFluids Engineering has in-house capability, softwares and expertise to support client needs on SIL identification as well as SIL verification studies…

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