Is SIL verification required for all SIF circuits rated at SIL Class A?
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I would like to ask: in our company’s SIL classification (LOPA) analysis report, all SIF circuits have been classified as SIL Class A. Some experts have suggested that we haven’t carried out SIL verification and that such verification is necessary. If it’s not on SIS, is SIL verification required? Are there any corresponding specifications?
None of the existing specifications and standards mention the term SILa; it is merely an informal designation used within the industry. If the classification result is SILa, DCS interlocks can be used as an additional independent protection layer, provided that DCS has not been utilized in the existing independent protection layers. The SIL verification scope theoretically includes interlock circuits of SIL1 and above. In a situation like yours, either have the grading authority make a change and assign it as SIL0 ； Or go to SIS.
Since LOPA requires a SIF circuit, it indicates that the DCS protection mechanisms have been exhausted; the DCS can no longer be used, and a SIF circuit must be installed. However, this is because the RRF might be relatively low and not meet the SIL1 criteria; in fact, this relationship can entirely be handled by setting it according to SIL1 standards. Because the lowest SIL level is 1; I’ve never heard of a level of 0.
The official statement is that there is no such thing as SILA; this is purely an attempt to create trouble. Logically, if an LOPA analysis shows that a system does not meet the SIL1 standard, then it is not necessary to use a SIS. If the requirements of Document No. 116 are applicable, and there are two key aspects and one major aspect involved, it is still necessary to input them into the SIS system
SIL classification can be carried out using methods such as LOPA, risk diagrams, and risk matrices. The LOPA analysis method does not apply to SIL levels below SIL1; if a level lower than SIL1 cannot be achieved, the SILA method from the risk diagram analysis in IEC61508-5 standard can be used as a reference. SILA indicates that a circuit needs to be implemented in the SIS, but no requirements are placed on its PFD (i.e., verification is not necessary).