Safety Integrity Level (SIL)

The Safety Integrity Level (SIL) Study is required to assess the distinct level for specifying the security integrity needs of the security instrumented functions (SIF) to be allotted to the security instrumented systems. Layer of Protection Analysis (LOPA) methodology are followed that accounts the protective layers to stop to forestall and to mitigate dangerous events with inherently safe design conception, to prevent the incidence / mitigate the results from such unsafe event.

Technical Challenge:

There is an increasing dependence on Safety Instrumented Systems (SIS) to achieve tolerable risk levels within the method sector. Additionally there’s an increasing ought to justify that the chance levels that are achieved are tolerable.

SIFs are one amongst the layers of protection that are designed into method plants mitigate the incidence of major hazard scenarios; this suggests that the failure or absence of a SIF will increase the likelihood of a unsafe scenario occurring and escalating into a significant accident. It’s then necessary to estimate the chance of the Sif failing to act on a requirement, 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 among the cornerstones of the security strategy for process Safety Management (PSM) program of any method plant. it is therefore of great importance that the life cycle of Safety Instrumented Functions (SIFs), together with specification, design, validation, commissioning and operation & maintenance, is in accordance to the necessities of the recognized practical safety standard: IEC 61508 [1] or its sector-specific spin off.

When SIL should be done?

  • SIL study is also one among the safety studies that is carried out to prevent hazard in method industries. Usually SIL study is carried out once the HAZOP study is completed for a project.
  • Safety performance criteria for SIS is defined by Safety Integrity Level (SIL) of the known loop. SIL is AN instrumented safety system that’s designed in accordance with the IEC 61508 and IEC 61511 standards.
  • SIL study is also one among the safety studies that is carried out to prevent hazard in method industries. Usually SIL study is carried out once the HAZOP study is completed for a project.
  • HAZOP study ensures safety and reliability at process level and SIL study ensures accessibility or robustness at component level in a process industry.
  • A SIL level applies to a whole SIS loop under consideration. SIL levels are used when implementing safety instrumented function that must scale back the existing intolerable process risk to tolerable risk level.

As we are able to see from the figure below, SIS is an element of prevention as well as mitigation of hazard.

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

This image shows the different layers of protection in an SIL assessment.

Approach / Methodology

SIL assessment could be a risk based approach to spot the desired 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 may be achieved using completely different qualitative and quantitative approaches.

  • Risk Graphs – qualitative technique, projected in IEC 61508
  • Layers of protection analysis (LOPA) – various qualitative methodology, wide employed in the method trade
  • Fault tree analysis (FTA) / Event tree analysis (ETA) – quantitative strategies

Generally a mixture of the strategies is used. As an example, performing arts 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 could be a simplified form of quantitative risk assessment. in a very typical process plant, varied protection layers are in situ to lower the frequency of unwanted consequences: {the process the method} design (including inherently safer concepts); the basic process management 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

The diagram signifies hazop to identify hazards and llopa to provide that extra layer of protection.

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

The flow chart depicts the identified SIF loops and their consequences level and the severity category.

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 often a tool to determine the SIL rating for each SIF by using Exida database for failure rates of system parts. The software additionally provides for analysis of study constraints to IEC 61508/IEC 61511.

Typical SIL Study team will include:

Following are the everyday 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

iFluids Engineering has in-house capability, softwares and expertise to support client desires on SIL identification in addition as SIL verification studies.