SRS Development
Understanding Safety Requirements Specification (SRS) Development
What is Safety Requirement Specification (SRS)?
Safety Requirement Specification (SRS) plays an essential role in functional safety engineering. It sets the foundation for the design, implementation, and operation of Safety Instrumented Functions (SIF) and Safety Instrumented Systems (SIS). The guidelines for the Safety Requirements Specification (SRS) Development are set in IEC 61511/ISA 84 Clause 10 and focus on five areas of requirements listing 29 elements.
- Hazard Prevention – the hazard each SIF prevents and the function the SIF performs
- Operation Modes – who and when SIFs are put into service and are required to be available, bypass requirements
- SIF Performance – Probability of Failure on Demand (PFD) and Spurious Trip Rate (STR)
- Functional Requirements – SIF and SIS
- Design Requirements – SIF and SIS
- Operation and Maintenance
The Safety Requirement Specification (SRS) in simple terms is a document that provides a mechanism to ensure that safety systems can achieve the set performance goals and mitigate potential risks to a safe state. A Safety Requirements Specification (SRS) Development process should be used as an input and to guide detailed design and subsequently verify the design meets the requirements. A project that does not complete the Safety Requirements Specification (SRS) Development prior to detailed design is not in compliance with RAGAGEP and will likely experience high project cost during the design phase.
The Step-by-Step Process of SRS Development
The methodology for Safety Requirements Specification (SRS) Development involves starting with the Process Hazard Analysis (PHA) and Layer of Protection Analysis (LOPA) to document the risk and conceptual mitigation requirements into a Conceptual Safety Requirement Specification (SRS). The Safety Requirements Specification (SRS) Development is updated once detailed design and SIL Verification is complete to incorporate any changes or validate initial requirements.
Conceptual System SRS
The process starts by understanding the process through a PHA/HAZOP process to identify the risk that need to be mitigated. Safety Integrity Levels (SIL) are determined through a Layers of Protection Analysis (LOPA) required to adequately mitigate the risk to an acceptable level. Safety instrumented functions (SIFs) are then allocated as protection layers that either achieve the SIL or as specified in Prescriptive Owner Requirements Specifications.
The Hardware/ Architecture & Software SIS requirements, general and specific SIF requirements are then documented in the Safety Requirements Specification (SRS) including parameters such as the following:
- Purpose and Scope
- General Project
- Information and Standards used
- Independent protection layer risk reduction allocations.
- Define risk reduction assumptions
- SIF Architecture
- Sensor & transmitter accuracy
- Bypass procedure
- Safe state of the process
- Process inputs and trip points
- Process outputs and actions
- Functional relationships, failure modes
- Manual shutdown and reset requirements
- Maintenance/bypassing requirements
- Response time requirements
- Human machine interface requirements; and
- Cybersecurity
- Process Safety Time
- Test frequency requirments
- Alarming requirements
Design SRS
A SIL Verification process is conducted for the final SIFs Design that is based on the conceptual Safety Requirements Specification (SRS) Development and includes associated functional and operational (ex: proof test procedure interval) parameters. If the Target Safety Integrity level is not achieved then proof test intervals, input-output relationships and redundancy options might have to be adjusted. These changes are updated in the Design Safety Requirements Specification (SRS).
Risk Analysis
A thorough risk assessment is done for each SIF to ascertain its SIL. Important factors considered include the potential process hazards, the likelihood of these hazards occurring, and the effectiveness of current protection layers.
Documentation
Keeping track of every decision, assumption, and rationale during the Safety Requirements Specification (SRS) Development is essential. This thorough documentation aids in transparent communication, and it becomes invaluable during any future system reviews or audits.
Benefits of an Effective Safety Requirements Specification (SRS) Development
A meticulously developed Safety Requirement Specification (SRS) offers multiple advantages:
- Enhanced System Reliability: A well-detailed Safety Requirements Specification (SRS) ensures the development of robust safety systems capable of effectively mitigating process risks.
- Compliance with Standards: A good Safety Requirements Specification (SRS) Development aligns with global safety standards, ensuring operations are both safe and compliant.
- Operational Efficiency: A Safety Requirements Specification (SRS) serves as the foundation for efficient system design, significantly reducing the chances of system failures and operational downtimes.
SRS Development Conclusions
Developing a comprehensive Safety Requirements Specification (SRS) Development is critical in the realm of functional safety engineering. A well-prepared Safety Requirement Specification (SRS) not only safeguards assets and operations but also serves as a guidepost for system designers and operators. It’s an indispensable tool for anyone aiming to achieve high safety standards in their operations.
FAQ
Safety Requirements Specification (SRS) Development defines safety and functional requirements for Safety Instrumented Systems (SIS) and Safety Instrumented Functions (SIF) in Functional Safety Engineering.
A safety requirement specification (SRS) is guided by IEC 61511/ISA 84 standards, which ensure proper design and performance of safety systems.
SRS Development is used after hazard analysis (PHA/LOPA) and before detailed design to guide system development.
Safety Requirements Specification (SRS) Development improves safety by clearly defining system behavior, risk controls, and performance targets like PFD and STR.
SRS is important because it ensures reliable system design, regulatory compliance, and effective risk reduction in Functional Safety Engineering.
