What is an SIS?
SIS stands for Safety Instrumented System — an automated system designed to bring a process installation safely to a halt or hold it in a safe state during a (threatening) hazardous situation.
An SIS intervenes automatically when a process threatens to move outside safe limits.
SIS systems are designed in accordance with the IEC 61511 standard (process industry) and use the concept of SIL (Safety Integrity Level).
🎯 What does an SIS do?
An SIS:
- Detects a hazardous process condition
- Evaluates via logic whether a limit value has been exceeded
- Automatically performs an action to reduce the risk e.g. shut down a pump, close a valve, stop an oven
So an SIS is not an alarm system — it takes action automatically, regardless of operator response.
🔧 Structure of an SIS loop
An SIS consists of the following components:
| Component | Role |
|---|---|
| Sensor | Measures a critical variable (pressure, temperature) |
| Logic Solver | Safety PLC or system that decides and drives |
| Final Element | Actuator (e.g. valve, relay, shutdown) |
📌 This is referred to as a SIF (Safety Instrumented Function) — a functional safety loop.
🏭 Examples of SIS functions
| Application | SIS action |
|---|---|
| Overpressure in a reactor vessel | Emergency valve opens to relieve pressure |
| Flame failure in a gas burner | Shutting off the gas supply |
| Excessive temperature in an oven | Switching off the heating |
| Excessive level in a tank | Closing the inlet valve or stopping the pump |
🔢 SIS and SIL
Each Safety Instrumented Function (SIF) is assigned a SIL classification (SIL 1–4), based on the risk analysis (such as LOPA or HAZOP):
- SIL 1 → Low risk reduction required
- SIL 2–3 → Medium to high risk
- SIL 4 → Very high safety requirement (rarely applied)
A higher SIL requires stricter design, test, and redundancy requirements.
🔗 SIS vs. other systems
| System | Goal | Takes action? |
|---|---|---|
| SCADA / DCS | Process monitoring and operation | Manual or semi-automatic |
| SIS | Functional safety | ✅ Automatic |
| ESD (Emergency Shutdown) | Subset or application of an SIS | ✅ |
| Alarm system | Warning the operator | ❌ Notification only |
✅ Advantages of an SIS
- Direct safety action without operator
- Traceable and validatable operation
- Compliant with international standards (IEC 61511 / IEC 61508)
- Prevents accidents, damage, and production losses
- Essential for risk-driven industries (oil & gas, chemicals, pharma)
📌 In summary
An SIS is an automated safety system that prevents or limits hazardous situations by intervening directly — without operator action. It consists of Sensor, logic, and Actuator and operates in accordance with strict SIL requirements.