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 are SIS needed in Process Plants?
Process plants handle complex physical and chemical processes involving hazardous materials like flammable, toxic substances and high pressures/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.
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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