Safety Integrity Level (SIL) is a quantitative target used to measure the level of performance required for a safety function to achieve tolerable risk for a process hazard. Defined in standards like IEC 61508 and IEC 61511, SIL provides a framework for assessing and mitigating risks through safety instrumented Function (SIF). The objective in selecting SIL for a SIF is to reduce the process risk to tolerable level.
A qualitative method may be used as a first pass to determine the required SIL of all SIFs. Those which are assigned a SIL 3 or 4 by this method should then be considered in greater detail using a quantitative method to gain a more rigorous understanding of their required safety integrity.
The concept of Safety Integrity Levels (SIL) emerged in the 1990s as a structured approach to classify safety integrity into four discrete levels. Each level specifies the requirements needed to ensure safety integrity in systems designed to prevent hazardous events.
The four SIL levels are:
- SIL 1: Lowest level of safety integrity. The SIL 1 SIFs are normally implemented with a single sensor, a single SIS logic
- solver and a single final control element.
- SIL 2: A Moderate level of safety integrity. SIL2, SIFs are typically fully redundant from the sensor through the SIS logic solver to the final control element.
- SIL 3: High level of safety integrity. SIL3, SIFs are typically fully redundant from sensor through the SIS logic solver to the final control element and require careful design and frequent proof tests to achieve low PFD figures. Many companies find that they have a limited number of SIL 3 SIFs due to the high cost normally associated with this architecture.
- SIL 4: Highest level of safety integrity (rarely implemented in process industries). These SIFs are included in the IEC 61508 and 61511 standards, but such SIFs are difficult to design and maintain and are not used in LOPA.
The higher the SIL level, the more stringent the requirements and the lower the Probability of Failure on Demand (PFDavg). Consequently, higher SIL levels also involve greater complexity, higher costs, and more rigorous testing.
Key Features of SIL
- Discrete Levels: SIL levels range from 1 to 4. SIL 4 offers the highest safety integrity, while SIL 1 provides the lowest.
- Application in Process Industries: While SIL 1 to SIL 3 are commonly implemented in the process industry, SIL 4 systems are often deemed uneconomical due to their high cost and complexity. If a process requires SIL 4 for safety, it may indicate a fundamental issue in the process design itself.
- Standard Guidelines:
- IEC 61508: This standard covers SIL 1 to SIL 4 and provides comprehensive details.
- IEC 61511: Focuses on SIL 1 to SIL 3, as these are the most relevant to the process industry, excluding sectors like nuclear energy. SIL is directly associated with the likelihood of system failure. Higher SIL levels correspond to lower PFDavg values and increased risk reduction.
Challenges in Implementing SIL
- Economic Constraints: SIL 4 systems are rare in the process industry due to their cost and complexity. For most processes, SIL 3 is the practical maximum defined by IEC 61511.
- Misconceptions About SIL Ratings: It’s crucial to understand that SIL levels apply to systems, not individual components. Proper implementation and compliance with standards are key to achieving the desired safety integrity.
- Process Design: If a process requires SIL 4 for safety, it may indicate deeper issues in the process design that should be addressed through non-instrumented methods or design changes.
Why SIL Matters in Process Safety
SIL is fundamental to ensuring safety in industries like oil and gas, chemical manufacturing, and pharmaceuticals. By defining clear safety performance targets, SIL helps:
- Mitigate risks effectively
- Enhance system reliability
- Reduce the likelihood of hazardous events
Adopting SIL-compliant systems ensures compliance with international safety standards and protects both personnel and assets.
A common misconception is that individual products or components are assigned specific SIL ratings. In reality, these items are deemed suitable for use within a particular SIL environment but are not individually SIL-rated. SIL levels are applied to safety functions (SIFs) and safety systems (SISs) as a whole. Components such as logic solvers, sensors, and final elements are designed for specific SIL environments, but their proper implementation depends on the end user. Achieving the desired risk reduction level requires that the equipment or system is used as intended. Simply purchasing SIL 2 or SIL 3-rated components does not guarantee a SIL 2 or SIL 3 system; the entire system must be correctly designed, integrated, and tested.
Explain the terms SIL ‘a’, SIL ‘b’, and ‘No SIF’ (or ‘SIL 0’)
SIL ‘a’
A safety function is required to meet the tolerable risk target, but the Risk Reduction Factor (RRF) requirement is not more than 10. The safety function, therefore, does not need to be a SIF and need not comply with IEC 61508 and IEC 61511.
SIL ‘b’
The RRF requirement of the SIF is too high to be realistically achievable. When the analysis shows a SIL ‘b’ result, it usually indicates that the risk has been overestimated due to a mistake in the analysis.
‘No SIF’ or ‘SIL 0’
means that the tolerable risk target is met already, and the safety function is not required (from a risk evaluation point of view; it may be required for other reasons, such as prescriptive requirements).
Top References
- https://www.exida.com
- Reliability, Maintainability and Risk by Dr. Davud J. smith
- https://blog.msasafety.com
- IEC 61511
- Safety Instrumented Systems Verification: Practical Probabilistic Calculations by William M. Goble and Harry Cheddie
- Layer of Protection Analysis, Simplified Risk Assessment by CCPS.
- Functional Safety from Scratch by Peter Clarke
Certified Functional Safety Professional (FSP, TÜV SÜD), Certified HAZOP & PHA Leader, LOPA Practitioner, and Specialist in SIL Verification & Functional Safety Lifecycle, with 18 years of professional experience in Plant Operations and Process Safety across Petroleum Refining and Fertilizer Complexes.
