Procedure to Change SIS Voting from 1oo1 to 2oo2 for SIF & SIL 2: Comprehensive Guide
Safety Instrumented Systems (SIS) are fundamental to the safe operation of modern industrial facilities. They protect personnel, equipment, and the environment from hazardous events by reliably executing Safety Instrumented Functions (SIF). Voting logic, such as 1oo1 (one-out-of-one) or 2oo2 (two-out-of-two), is a critical aspect of these systems, directly influencing reliability, availability, and safety integrity levels (SIL).
This blog post will explain in detail the procedure to change SIS voting logic from 1oo1 to 2oo2 for a SIL 2-rated SIF, including all the necessary steps, documentation, and considerations required to successfully accomplish this critical modification.
Understanding SIS Voting Logic
What Is SIS Voting Logic?
Voting logic refers to the method used by SIS to determine when to execute a safety action based on input signals from multiple sensors or devices.
Common Voting Configurations:
| Configuration | Description | Reliability |
|---|---|---|
| 1oo1 | Action triggered by a single sensor signal | High safety, lower availability |
| 2oo2 | Both sensors must agree before action | High availability, balanced safety |
Why Change from 1oo1 to 2oo2 Voting?
Changing from 1oo1 to 2oo2 voting logic is typically driven by the need to:
- Increase system availability by reducing false trips.
- Maintain compliance with higher safety integrity requirements.
- Enhance reliability in critical processes.
This change ensures continuous process operation unless both sensors detect a hazardous condition simultaneously, minimizing unnecessary shutdowns.
Step-by-Step Procedure to Change Voting from 1oo1 to 2oo2
Step 1: Conduct a Comprehensive Risk Assessment
Before modifying SIS, perform a thorough risk assessment, such as a Hazard and Operability (HAZOP) or Layer of Protection Analysis (LOPA):
- Confirm SIL 2 requirements.
- Document the risk reduction achieved by changing the voting logic.
Step 2: Update Functional Safety Documentation
Adjust the Safety Requirement Specification (SRS) documentation clearly indicating the new voting logic:
- Define sensor type, configuration, and response.
- Clarify the revised safety logic in functional safety descriptions.
Step 3: Select Appropriate Sensors and Devices
Choose suitable sensors and transmitters for a 2oo2 configuration:
- Sensors must meet SIL 2 capability.
- Sensors should support diagnostic coverage and redundancy.
- Use certified equipment conforming to IEC 61508/IEC 61511 standards.
Step 4: Modify the Safety Instrumented System Logic Solver
Modify logic solver programming and configuration:
- Program the SIS logic solver or safety PLC to recognize and implement 2oo2 voting logic.
- Perform logic solver diagnostics and test scenarios to confirm correct implementation.
Step 5: Update Wiring and Instrumentation Infrastructure
Implement necessary wiring changes:
- Ensure redundant sensor wiring is independently routed to minimize common-mode failures.
- Install junction boxes and marshaling cabinets to support new sensor installation.
Step 6: Conduct Functional Testing and Validation
Carry out comprehensive validation and testing procedures:
- Execute thorough factory acceptance testing (FAT).
- Perform site acceptance testing (SAT).
- Document all test results confirming functionality and compliance with SIL 2.
Step 7: Revise and Enhance Maintenance Procedures
Update existing maintenance routines:
- Establish routine checks for sensor calibration and diagnostics.
- Train maintenance personnel on updated configurations and new procedures.
Step 8: Document and Communicate Changes
Thoroughly document all changes:
- Update technical manuals, process control documents, and training materials.
- Ensure clear communication to operational and maintenance teams.
Necessary Documentation and Approvals
Changing SIS voting logic is a significant modification that requires rigorous documentation and approvals:
Required Documents:
- Updated Safety Requirement Specification (SRS)
- Revised Piping and Instrumentation Diagrams (P&IDs)
- Instrument datasheets and certificates
- Functional safety assessment (FSA) reports
- Detailed project change management documents
Required Approvals:
- Internal safety committee or management approval
- Independent third-party review or certification
- Regulatory compliance verification, if applicable
Real-World Example: Voting Logic Change in Petrochemical Plant
Scenario:
A petrochemical facility faced repeated nuisance shutdowns due to false trips in a temperature monitoring SIF with 1oo1 voting. The management decided to switch to a 2oo2 voting logic configuration.
Implementation Highlights:
- Conducted LOPA and confirmed SIL 2 requirements.
- Selected SIL-certified temperature transmitters.
- Updated the SIS logic solver (Honeywell SMSC) with 2oo2 logic.
- Installed redundant sensors with independent wiring routes.
- Successfully tested system integrity through comprehensive SAT and FAT.
Results:
- Reduced false shutdowns by 75%.
- Enhanced plant availability significantly.
- Maintained compliance with safety and regulatory standards.
Preventing Common Pitfalls in SIS Voting Logic Change
Pitfall 1: Insufficient Risk Assessment
- Solution: Perform detailed risk assessments (HAZOP, LOPA) before implementation.
Pitfall 2: Incorrect Sensor Selection
- Solution: Use only SIL-certified equipment and confirm compatibility with existing infrastructure.
Pitfall 3: Poor Implementation Documentation
- Solution: Ensure thorough documentation updates and training for all relevant staff.
Future Considerations and Continuous Improvement
Continuous improvement and regular reviews of SIS and voting logic:
- Regularly reassess SIS performance and availability.
- Periodically verify alignment with changing process conditions or regulatory standards.
- Encourage feedback loops from operational and maintenance personnel to refine procedures.
Conclusion: Ensuring Optimal Safety and Operational Performance
Changing SIS voting from 1oo1 to 2oo2 logic is a strategic decision that improves system availability and reliability without compromising safety integrity. However, it requires meticulous planning, rigorous assessment, and thorough documentation to ensure compliance with SIL 2 requirements.
By systematically following this guide, implementing necessary documentation, and rigorously validating changes, industries can successfully enhance their safety and operational performance, maintaining regulatory compliance and safeguarding people, assets, and the environment.