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What Is Safety Instrumented System


Safety Instrumented System (SIS) is a distinct, reliable system used to safeguard a process to prevent a catastrophic release of toxic, flammable, or explosive chemicals. It take an automated action to keep a plant in a safe state or put it in a safe state when abnormal conditions are present.

The safety instrumented system consists of an engineered set of hardware and software controls which are especially used on critical process systems.


The system is composed of:

  • Sensors: Monitors the process to detect any abnormal condition.
  • Logic solvers: Receives signal from the sensor, then assess if the condition is hazardous; if it is,it send signal to take action.
  • Final control: Will implement the action sent from the logic solver. It may be an automatic shutdown signal.

A typical critical processes which requires the SIS is the operation of a steam boiler.

The Safety instrumented system is designed at different integrity level; this is called Safety integrity level (SIL). The integrity level use, depends on the level of hazard posed by the process.

The SIS is designed around individual functions in the plant, called Safety Instrumented Functions (SIF). The SIF are set of equipment intended to reduce the risk due to a specific hazard. They detect an imminent accident, decide to take action, and then carry out the action needed to bring the process to a safe state.


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When To Install A Safety Instrumented System

Before a Safety Instrumented System is designed, the design team must do a detailed risk analysis, identifying all of the potential risks and deciding which of the risks require a Safety Instrumented Function to be defined.

Risk matrix can be used to identify the level of risk that is tolerable, and at what point a function require as a SIF to be defined.

Read more on SIS here


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SIS is just one layer of protection installed in a plant to safeguard the process, equipment, personnel, and the community.

All SIS techniques are mentioned in IEC 61511 and IEC 61508.

Safety Instrumented System Training

If you want to be knowledgeable in the design of the SIS, you need to enroll for training.

Check for trainers near you.

The training will cover:

  • Introduction: What is a Safety System? | Danger of Overconfidence and Complacency  | Lessons Learned from Past Accidents
  • Guidelines and Standards: ANSI/ISA | IEC
  • General SIS Design Considerations: Design Life Cycle | Independent Safety Layers
  • Hazard and Risk Assessment: Hazard Identification | Risk Assessment | Determining SILs | Layer of Protection Analysis (LOPA)
  • Failure Modes: Safe vs. Dangerous | Redundancy Issues
  • System Technologies: Relay | Microprocessor | Field Devices | Certification vs. Prior Use
  • Operation and Maintenance: Installation | Testing | Management of Change

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