How is the SIL level of the SIS system itself determined?
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Is it based on the highest SIL rating of the instruments on site? Even if there is just one instrument with a SIL3 rating and all the others are SIL2, then the SIS system will also require a SIL3 rating.
The requirements for the SIL level apply to SIF, where SIF = field sensor + logic solver + final elements.
After HAZOP (Hazard and Operability Analysis) and LOPA (Layer of Protection Analysis), the SIL (Safety Integrity Level) requirements for each individual circuit of the safety instrumented system SIS are determined. Then, specific instruments, control systems, and final control elements are selected to achieve the SIL level. Once the SIS system is defined in accordance with the SRS (Safety Technical Requirements), SIS verification calculations and testing must be carried out.
The SIL level depends on many factors, including the type of technology used, the number of system components, the component failure probability PFD, the system redundancy, the inspection and testing cycle, and so on
Tables without a SIL level can also form circuits that meet a SIL level. It’s all calculated based on probability. . .
To add more information about the second floor: if all components in a circuit system are of SIL3 level except one which is of SIL2 level, and if the circuits are connected in series, then the overall SIL level of that circuit will not exceed SIL2. Therefore, whenever it is required that a circuit have a SIL2 level, it is necessary to perform PFD calculations for all the components in that circuit, and select the appropriate circuit components based on those results.
First, the event outcomes are determined based on HAZOP analysis; subsequently, the SIF values are determined through LOPA analysis, and the SIL level for each circuit is determined corresponding to those SIF values. After determining each SIL level, the measuring instruments, sensors, controllers, etc. that make up each circuit have their own SIL levels and failure rates; the sum of these values must not exceed the corresponding SIL level specified earlier. That is, if the SIL level is determined to be 2 through LOPA, then the selection of instruments, sensors, and controllers in each circuit will need to be made in accordance with SIL2 or even SIL3 standards. After purchasing, it is still necessary to verify each one again.