The Safety Requirements Specification (SRS) serves as a critical document in the design and implementation of Safety Instrumented Systems (SIS). It outlines all the requirements of Safety Instrumented Functions (SIFs) necessary to achieve functional safety. The purpose of the SRS is to provide a comprehensive guide for designing, implementing, and validating the SIS in compliance with safety standards.
Importance of the SRS in the SIS Lifecycle
The SRS plays a vital role in the SIS safety lifecycle by:
- Defining how each SIF is designed and integrated into the SIS.
- Guiding the design team to meet the SIF safety integrity requirements.
- Supporting the validation process for the SIS to ensure compliance with functional safety objectives.
Key Requirements for Developing the SRS
The development of an SRS is guided by the standards outlined in IEC 61511-1/2 (Clauses 10 & 12). The SRS must include clear, precise, verifiable, maintainable, and feasible specifications to ensure usability throughout all phases of the safety lifecycle.
Core Elements of the SRS
- SIF Description and Functional Safety Requirements
- A detailed description of all SIFs, including cause-and-effect diagrams and logic narratives.
- Identification of plant input/output devices associated with each SIF (e.g., field tags).
- Safe State Definition
- Clear definitions of safe states for each SIF, ensuring hazardous events are mitigated.
- SIL Determination
- Safety Integrity Levels (SIL) determined through Risk Graph, LOPA, or Risk Matrix studies.
- Process Requirements
- Sources of demand and demand rate for each SIF.
- Process measurements, range, accuracy, and trip points.
- Operational Requirements
- Response times to achieve safe states within process safety time.
- Proof test intervals and implementation procedures.
- Criteria for manual shutdown and resetting after shutdown.
- Failure Modes and SIS Response
- Failure modes for each SIF and corresponding SIS responses (e.g., alarms or shutdowns).
- Maximum allowable spurious trip rates.
- Environmental and Operational Conditions
- Specifications for extreme environmental conditions, including temperature, humidity, EMI/RFI, and other factors.
- Interfaces and Integration
- Interfaces between the SIS, BPCS, and other systems.
- Modes of operation for plant processes (e.g., startup, shutdown, emergency).
- Maintenance and Overrides
- Requirements for maintenance overrides, bypasses, and administrative controls for clearing them.
- Mean time to repair (MTTR) considerations, including spares and service contracts.
- Validation and Documentation
- Target proof test intervals and validation criteria for SIF implementation.
- Procedures for startup, restarting, and fault handling.
Structure of the SRS
A well-structured SRS includes:
- A description of hazards and their consequences.
- Reference to relevant P&ID drawings.
- Relationship between process measurements and outputs, including logic and required permissives.
- Environmental considerations during normal and emergency conditions.
- Procedures for handling faults and ensuring safe states during SIS operation.
