The SIF Analysis Steps

The SIF Analysis Details Page is used to record the SIF Analysis details. The SIF design itself mainly takes place on the Design Tab.

The Aim

The aim is to design the Subsystems in terms of their Equipment Actuators and Test Intervals, such that the Achieved PFD is equal to or lower than the Target PFD, while ensuring that the Success Criteria, Fault Tolerance, and Performance are all OK.

Step 1: Define the SIF

The SIF design process can start after the Target PFDs were established and the SIFs have been defined.

The Target PFDs are typically established during the LOPA study. In some cases, a SIF Analysis can be started independently from a LOPA, e.g., when you have an emergency button. In this case, a static Target SIL must be specified during the SIF Analysis.

The SIF is defined in terms of its Subsystems – the Sensor, Logic Solvers, and Final Element. The SIF type can be a “full” or “sensor only” or “final element only”. For each Subsystem, Equipment must be assigned. This is done on the SIF Analysis Details Page and specifically on the General Tab. Some additional Assessment Details, e.g., Design intent, Consequence of failure on demand, Consequence of spurious trip, etc. should also be defined here before the design can start.

If verification of the function response time is required, the Process Safety Time (PST) is required as well.

Step 2: Design the SIF

• (Further) design the Subsystems in terms of Equipment Actuators.
• Define the Success Criteria.
• Tune and set the Test Intervals.
• Verify integrity, i.e., that the Achieved PFD is less than the Target PFD.
• Verify the Fault Tolerance.
• Verify the Performance (sensor Accuracy, TSO requirement)
• Verify the overall functional Response Time vs PST.
• Approve the SIF Design. 

For more information see SIF Design Tab - Overview.