What is a Safety Instrumented System?
A Safety Instrumented System or SIS is a system that is designed to automatically take the process or equipment to a safe state when specific safety critical conditions are detected. Some key characteristics of a typical SIS include:
- It is completely independent of the basic process control system. This avoids any interference or complications from process control loops.
- High integrity components are used which are specially rated for use in safety applications. This includes sensors, logic solvers and final elements like valves.
- It is designed to achieve and maintain the process unit or plant in a defined safe state if a hazardous operation scenario(or HOS) occurs.
- A safety logic solver runs automated safety logic to detect pre-defined abnormal situations and take the necessary trip actions. This is based on input from field mounted safety level sensors and other devices.
- Final elements like emergency shutdown valves are used to isolate, bleed, vent, divert or remove any hazardous materials from the process to achieve a safe state.
- Overall reliability targets are set much higher than a normal control or instrumentation system. Common figures targeted are 99% per year.
Why is SIS needed in process plants?
Process plants handle complex physical and chemical processes involving hazardous materials like flammable, toxic substances and high pressures and temperatures. Some key risks include fires, explosions, and dangerous material releases. An Safety Instrumented System is needed to:
- Automatically detect any process deviations from safe operating limits and take appropriate trip actions to avoid a hazardous event escalating into an accident.
- Shutdown equipment and isolate/remove inventory in case of identified emergency scenarios like loss of containment, overpressure/overfilling, uncontrolled reaction, etc.
- Place the process unit or plant in a safe, stable endpoint condition even if the basic process control fails to function properly or operators do not respond on time.
- Meet international functional safety standards like IEC 61508/61511, which mandate independent hardwired safety systems be installed for processes handling hazardous substances above threshold quantities.
- Help process plants achieve their safety targets as loss of containment incidents can severely impact lives, the environment, assets, and company reputation/finances. Reliable SIS is one of the critical layers of protection.
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Design and Implementation of an Effective SIS**
The key aspects include:
- Conducting a formal process hazard analysis and risk assessment to identify all Hazardous Operating Situations (HOS) that need to be safeguarded by the SIS.
- Defining the safety instrumented functions (SIFs) needed, with each SIF addressing a specific HOS through logical safety trips.
- Selecting suitable smart field mounted sensors, logic solvers and final control elements rated for use in safety applications with required hardware fault tolerance, diagnostics and fast response times.
- Developing detailed cause and effect matrix and safety logic diagrams based on the required SIFs.
- Rigorous design reviews and hazard/operability studies to validate functional requirements are fully met.
- Establishing a safety requirements specification documenting all aspects of target SIL levels, safety integrity targets, proof test intervals etc.
- Following international standards for lifecycle management including detailed planning, documenting, commissioning, validation and ongoing maintenance/proof testing to sustain reliable operation.
- Regular audits and functional safety assessments to ensure any changes or modifications do not compromise the safety integrity.
A properly engineered SIS designed and implemented through the above steps can deliver the as low as reasonably practicable (ALARP) level of risk reduction for major accident hazards.
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