Defining SIF Success Criteria

Success Criteria for the SIF Analysis is defined in terms of the Equipment-Actors. The table below summarizes the Success Criteria nomenclature.

Typical Success Criteria nomenclature used:

The of Success Criteria must be defined on two layers:

• High level Success Criteria, per System type

• Lower level Success Criteria, per Subsystem

To define the Success Criteria:

• Click Success Criteria. Note: The top button is for the System and is defined per Subsystem. The lower button is for the Subsystem and is defined per Equipment-Actor.

  

• Click NooM to select the logic. This will automatically select the Subsystems (or Equipment-Actors). Refer to the table above.

• Alternatively, to write your own criteria, click Add to add Subsystems (or Equipment-Actors).

• Click Check to Validate.

• Click Save.

Note 1: If a Subsystem only contains a single Equipment, the Success Criterion will only contain this single Equipment-Actor between square brackets. The same applies when there is only one Subsystem for a System.

Note 2: The validation checks:

• The syntax of the statement.

• That the Subsystems (or Equipment-Actors) referenced in the statement are indeed listed.

• That all listed Subsystems (or Equipment-Actors) are included in the success criterion statement.

If the Success Criterion statement contains one or more errors listed in the above validation checks:

1. The Optimize TI or PFD calculation will generate a warning and will not execute.

2. The Dangerous Fault Tolerance verification will fail.

PFD(A+B) vs PFD(NooN(A,B))

Note that PFD(A+B) and PFD(NooN(A,B)) may yield slightly different Achieved PFD results in IMS, as the common cause (beta) probabilities, PFDc(.) and PFDcc(.), are defined slightly differently for NooN compared to ‘AND’ gates in IMS. When modeling beta for a 2oo2 arrangement (or any NooN structure), the calculated average PFD decreases, which is why it is typically not done in practice. It is important to understand that just because beta modeling is omitted, it does not mean there is an issue with the model or its rationale. Nor does it imply that the model inaccurately represents reality—it is simply a more conservative approach to exclude beta. To provide maximum flexibility for the user, IMS offers both calculation options: the A+B annotation used in the Success Criteria fields provides more conservative PFD calculations by excluding beta, whereas the 2oo2 annotation includes beta in the PFD calculations.