WHAT IS SIL (SAFETY INTEGRITY LEVEL)?
SIL certification specifies the SIF (Safety Instrumented Function) reliability of a system. SIF is the function of detecting and preventing a dangerous situation that may occur during any operation. For example, SIF is the process of detecting an overheating engine temperature with a thermal sensor and stopping the engine. The SIF function controls a specific process. A separate SIF function is required for each process. However, all SIF functions form the SIS (Safety Instrumented System) system. SIS is the control system that controls the entire system and brings the system to a safe state in dangerous situations. In addition, the term “Functional Safety” refers to the successful realization of all SIF functions in a system and as a result, the risk that occurs during operation is reduced to an acceptable level.
SIL certification is applied within the scope of SIF. The SIL certificate specifies the reliability range of the SIF function. The reliability range is expressed in terms of “Probability of Failure on Demand” (Probability of Failure on Demand, PFT) or its inverse “Risk Reduction Factor” (Risk Reduction Factor, RRD).
| SIL LEVEL | RELIABILITY | POSSIBILITY OF FAILURE ON DEMAND | RISK REDUCTION FACTOR |
| SIL 4 | > %99.99 | between 0.01% and 0.001% | Between 10000 and 100000 times |
| SIL 3 | Between 99.9% and 99.99% | Between 0.1% and 0.01% | Between 1000 and 10000 times |
| SIL 2 | Between 99% and 99.9% | Between 1% and 0.1% | Between 100 and 1000 times |
| SIL 1 | Between 90% and 99 | Between 10% and 1% | Between 10 and 100 times |
SIL certification is defined at four different levels according to the probability of the system to fail on demand. The higher the SIL level, the less likely the system is to fail and the higher the system performance. However, as the SIL level increases, the cost and maintenance costs of the system increase and the system becomes more complex.
The required SIL (Safety Integrity Level) level in a system is determined by the magnitude of the risk and the level of protection the system needs when an undesired event occurs. In the case of industrial applications, systems with SIL 4 level are very complex and not cost-effective, making them unsuitable for realization. In addition, if the system needs to be SIL 4 certified in order for the risk magnitude in a system to reach an acceptable level, this may be due to an error in the design of the system.
SIL certification is a safety level related to the whole system. Any element or component in the system does not have a SIL level. These parts are classified as suitable or not suitable for use in a system with SIL certification. In addition, the fact that only the elements or components used in the system comply with the SIL level does not make the system SIL compliant. In order for a system to be suitable for any SIL level, the whole system must be examined.