SIF and SIL Simulator for Beginners | SIS Explained
A SIF is a safety job (sensor, logic solver, valve). A SIL is the grade that says how reliably that job must work. Many SIFs together make the SIS. Learn it in 5 short steps: 1 Concept, 2 Build a SIF, 3 Test it with 1000 hazards, 4 Find the SIL you need, 5 Quiz.
Procedure map (click a step)
Hint
1 Concept: SIS, SIF, SIL
Your SIF summary
| Sensor | |
| Logic solver | |
| Final element | |
| PFDavg (chance of failing when needed) | |
| Risk reduction factor (1 / PFDavg) | |
| SIL achieved | |
| SIL required (step 4) | |
| Verdict |
SIS vs SIF vs SIL at a glance
| Term | What it is | Example |
|---|---|---|
| SIS | The whole safety system of the plant | All trip loops in the control room |
| SIF | One safety job against one hazard (sensor + logic + valve) | High pressure: close the inlet valve |
| SIL | A grade 1 to 4 for how reliable one SIF must be | This SIF needs SIL 2 |
SIL levels (low demand mode)
| SIL | PFDavg | Risk reduction | Fails in about |
|---|---|---|---|
| SIL 1 | 0.1 to 0.01 | 10 to 100 | 1 in 10 to 1 in 100 hazards |
| SIL 2 | 0.01 to 0.001 | 100 to 1000 | 1 in 100 to 1 in 1000 |
| SIL 3 | 0.001 to 0.0001 | 1000 to 10 000 | 1 in 1000 to 1 in 10 000 |
| SIL 4 | 0.0001 to 0.00001 | 10 000 to 100 000 | rare in process plants |
Confused about the difference between SIF, SIL and SIS? This interactive simulator helps you understand these three important concepts in process safety engineering.
You can build a simple Safety Instrumented Function (SIF), select sensors and valves, change proof-test intervals, and see how these choices affect the estimated probability of failure. You can also explore how other protection layers help reduce process risk.
What Is the Difference Between SIS, SIF and SIL?
These terms are closely related, but they mean different things.
- SIS (Safety Instrumented System): The complete system used to implement one or more safety instrumented functions.
- SIF (Safety Instrumented Function): A specific safety action designed to bring a process to a safe state when a defined hazardous condition occurs.
- SIL (Safety Integrity Level): A level of safety integrity assigned to a SIF based on the required risk reduction.
For example, if the pressure inside a vessel becomes too high, a pressure transmitter can detect the condition, a safety logic solver can process the signal, and an emergency shutdown valve can close the inlet line. This complete safety action is a SIF.
The SIL requirement tells the engineering team how much risk reduction that safety function must provide.
How to Use the SIF and SIL Simulator
Step 1: Understand the basic concepts
Select SIS, SIF and SIL in the diagram to learn what each term means. This is a good starting point if you are new to safety instrumented systems.
Step 2: Build a Safety Instrumented Function
Choose a sensor, voting arrangement, logic solver and final element. You can also change the proof-test interval and observe how the simulator’s estimated PFDavg changes.
Step 3: Test the safety function
Create a simulated process hazard and observe whether the SIF responds. Run 1,000 simulated demands to see how many failures occur in the model.
Step 4: Find the required SIL
Change the hazard frequency, consequence level and credited protection layers. Compare the resulting simplified risk estimate with the performance of the SIF you built.
Step 5: Test your knowledge
Answer the quiz questions to check your understanding of SIF, SIL and SIS.
What Is PFDavg?
PFDavg stands for Average Probability of Failure on Demand. It describes the average probability that a safety function will fail when it is required to operate in low-demand mode.
A lower PFDavg generally means a lower probability of failure on demand and a higher risk reduction factor.
The risk reduction factor is commonly expressed as:
Risk Reduction Factor = 1 / PFDavg
For low-demand operation, the standard SIL bands are:
- SIL 1: PFDavg from 0.1 to 0.01
- SIL 2: PFDavg from 0.01 to 0.001
- SIL 3: PFDavg from 0.001 to 0.0001
- SIL 4: PFDavg from 0.0001 to 0.00001
These ranges help explain the relationship between SIL and probability of failure. Actual SIL verification requires an appropriate reliability calculation and assessment of the complete safety function.
Why Does Proof Testing Matter?
Safety instruments can develop hidden faults that remain undetected during normal operation. Proof testing helps reveal these faults and confirms that the safety function can perform as intended.
In general, reducing the proof-test interval can reduce the contribution of undetected dangerous failures to PFDavg. However, the actual result depends on the equipment, test coverage, repair time, failure rates and the assumptions used in the calculation.
Why Must the Complete SIF Be Evaluated?
A safety function includes the sensor subsystem, logic solver and final element. Looking at only one component is not enough to establish the reliability of the complete function.
The sensor may detect the hazard correctly, but the shutdown valve could still fail to close. Redundancy, diagnostics, common-cause failures, proof testing and systematic capability also affect the final result.
Who Should Use This Simulator?
This simulator is useful for instrumentation engineers, maintenance engineers, control and instrumentation professionals, SIS engineers, engineering students and anyone preparing for process safety or instrumentation interviews.
Use it to build a basic understanding of safety instrumented functions before studying detailed SIL verification and safety lifecycle requirements.
Important: This is an educational simulator, not a certified SIL calculation tool. Its component failure rates, voting assumptions and simplified risk model are illustrative. Do not use its results to assign or verify SIL for a real plant. Actual safety decisions require a documented risk assessment, appropriate calculations and qualified engineering review in accordance with applicable standards, including IEC 61511.
