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Navigating the SIL Assessment Maze: The Risk Graph Method Explained

28 Feb

Safety Integrity Level (SIL) assessment is the process of assigning risk reduction magnitude to safety instrumented functions (SIFs). There are four Safety Integrity Levels (SIL) for Safety Instrumented Functions: SIL 1 to SIL 4, of which SIL 4 has the highest level of safety integrity and SIL 1 has the lowest level of safety integrity.

AIE has adopted a structured approach to execute SIL assessment by using the Risk Graph/ Layer of Protection Assessment (LOPA) methods in accordance with IEC-61508 & IEC-61511 standards. SIL assessment through the Risk Graph approach is a qualitative method, and it can generally be utilized to screen out the safety systems that require a low safety integrity level. A more detailed assessment can be conducted by using the LOPA method for the SIFs with a high-consequence severity which require a more detailed assessment.

The risk graph assessment process starts with the identification of hazardous events and their potential consequences. These are often taken directly from HAZID/HAZOP study outputs. Our experienced SIL team will identify the safety systems or components which serve to mitigate the hazardous events identified in the HAZOP. The risk graph method determines the SIL level based on the following parameters:

  • C: The consequences of the hazardous situation for Personnel Safety, Environment and Commercial Loss (production & asset)
  • F: The Exposure Time, also expressed as “Occupancy”
  • P: The probability of avoiding the hazardous situation
  • W: The Demand Rate

By combining these four risk parameters, a risk graph can be formed, which results in embedded SIL numbers (as given in below Fig 1). This risk graph gives the SIL level for people, assets, and environmental risk, and the most conservative SIL number should be used as an overall SIL of the SIF under consideration.

SIL Assessment

Fig 1: Risk Graph Assessment




Advantages of the risk graph approach include the following:

  • The risk graph can be utilized as a screening tool to easily assess a large number of functions, eliminating those with lower or no safety levels, and highlighting those with larger safety levels.
  • The risk graph assessment can be performed as a team exercise involving a range of disciplines/expertise.
  • The assessment can be adjusted in various ways to take account of other risk mitigation measures such as alarms and mechanical protection devices.




Our technical consultancy services include all aspects required to conduct a successful SIL assessment. Our experts can support in:

  • Verifying the inputs to the assessments by providing third party review of hazard assessments.
  • Preparing a SIL assessment using industry compliant methodologies.
  • Providing professional advice on reliability and maintenance of safety instrumented systems.
  • Developing Performance Standards.
  • Conducting failure analysis and providing recommendations for the improvement of SIF reliability.

Contact us at info@aiegroup.org for more information.




  • Functional Safety of Electrical/Electronic/Programmable Electronic Safety Related Systems (E/E/PES): IEC-61508.
  • Functional Safety – Safety Instrumented systems for the process industry sector: IEC61511-Part1 to Part3.

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