SIL Verification Study: Ensuring Safety and Compliance in Critical Systems

Introduction

In safety-critical industries such as chemical processing, oil and gas, aerospace, and manufacturing, ensuring system safety is essential to protect human lives and the environment. Functional safety relies heavily on determining and verifying the Safety Integrity Level (SIL) of safety systems.

A SIL Verification Study is a structured process that confirms a safety system meets the required SIL. This evaluation ensures that safety functions perform as intended under real-world conditions, playing a vital role in mitigating risks and ensuring compliance with standards like IEC 61508 and IEC 61511

What is SIL Verification?

SIL Verification validates that a Safety Instrumented System (SIS) or safety function meets the target SIL defined during risk assessment. SIL levels, ranging from 1 to 4, determine system reliability based on the Probability of Failure on Demand (PFD) or failure frequency:

  • SIL 4: Highest level of safety integrity.
  • SIL 1: Lowest level of safety integrity.

This study assesses the system’s design, including hardware, software, and operational processes, to ensure compliance with the target SIL.

SIL Verification Procedure

The SIL Verification process involves the following steps:

1. Define the Safety Instrumented Function (SIF)

  • Identify and define each Safety Instrumented Function (SIF) to be assessed.
  • Document all identified SIFs along with their target SILs.
  • Identify all components of the SIF, including sensors, logic solvers, and final elements.

2. Design the Safety System Architecture

  • Ensure the design meets reliability and fault tolerance requirements of the target SIL.
  • Define system components and interactions to achieve specified SIL.
  • Ensure compliance with standards like IEC 61508 or IEC 61511.

3. Select Reliability Data

  • Gather failure rate data for sensors, logic solvers, and final elements from reliable sources.
  • Ensure data is up-to-date and relevant.

4. Define Test Intervals and Proof Test Strategies

  • Establish test intervals for Proof Tests (PST), Functional Tests (FST), or Partial Valve Stroke Tests (PVST).
  • Align test intervals with operational data and target SIL.

5. Calculate Probability of Failure on Demand (PFD)

  • Calculate PFD for each SIF component (sensor, logic solver, final element).
  • Ensure total PFD meets the requirements for the target SIL.

6. Validate System Performance (PFD and HFT)

  • Check overall PFD and hardware fault tolerance (HFT).
  • Ensure performance aligns with SIL criteria.

7. Analyze Results and Identify Gaps

  • Identify areas for improvement if PFD does not meet targets.
  • Address gaps through adjustments in system design or parameters.

8. Optimization (If Required)

  • Adjust test intervals or system architecture as needed.
  • Optimize components to meet SIL requirements.

9. Documentation and Reporting

  • Document all verification activities, results, and recommendations in a comprehensive SIL Verification report.

10. Final Review and Approval

  • Review results with stakeholders and obtain approval for implementation.

11. Implementation and Ongoing Monitoring

  • Implement the verified system and monitor its performance.
  • Update the verification as needed based on operational changes.

Probability of Failure on Demand (PFD) Calculation

The PFD for each SIF is determined by:

  • PFDS: Probability of Failure on Demand of the Sensor
  • PFDLS: Probability of Failure on Demand of the Logic Solver
  • PFDFE: Probability of Failure on Demand of the Final Element
A table displaying Safety Integrity Levels (SIL) criteria with PFDavg for low-demand mode and PFH for high-demand/continuous mode. It includes ranges for SIL 1 to SIL 4, based on IEC 61511 standards
SIL Performance Criteria IEC 61511

Hardware Fault Tolerance (HFT)

To meet SIL requirements, each SIS subsystem (including sensors, logic solvers, and final elements) must fulfill minimum fault tolerance criteria.

A table outlining the minimum Hardware Fault Tolerance (HFT) requirements for Safety Integrity Levels (SIL) from SIL 1 to SIL 4, categorized by low demand and high demand/continuous modes based on IEC 61511 guidelines
Architectural Constraints IEC 61511

Benefits of SIL Verification

  • Ensures compliance with industry standards like IEC 61508/61511.
  • Reduces risks and prevents catastrophic events.
  • Improves system reliability and operational safety.
  • Provides a structured approach to continuous improvement.

Conclusion

SIL Verification is a critical process to ensure that Safety Instrumented Functions (SIFs) reliably meet the safety requirements necessary to protect against hazards. By thoroughly assessing the performance, reliability, and fault tolerance of each system component—such as sensors, logic solvers, and final elements—SIL Verification guarantees that safety systems operate effectively under demand conditions. This process reinforces safety, compliance, and operational efficiency across industries that rely on high-integrity safety systems.