Tab: Corr Rates IF

The Corr Rate IF tab of the Calculation Summary contains Corrosion Rates and Interval Factors.

The Corrosion Rates (CRs) are described in Corrosion Rate Calculations – Details. Below you will find more information on the Interval Factors (IFs).

Total Interval Factor (TIF)

Remember that MII = RL x TIF. This Total Interval Factor (TIF), also called the Governing IF, is made up of multiple parameters: Circuit IF, DSCF IF and PERF IF. The selected Calculation Method will determine which parameters are used in the calculation.

If Calc Method 1 is used:

If Calc Method 2 is used:

Since there are two DSCF IFs (see below), there are actually two possible TIFs for Calc Method 1:

IMS selects the lower value between TIF Long and TIF Short as the TIF for the MII calculation. If the RL is based on either the Maximum Long CR or the Circuit Average Long CR, then TIF Long will be used. If the RL is based on either the Maximum Short CR or the Circuit Average Short CR, then TIF Short will be used.

The PERF IF is used for both Calculation Methods. Normally only if the Inspection Reason for the Circuit is set to “CLASS 4”, will the PERF IF be ignored.

Data Scatter Compensation Factor (DSCF) IF

A corrosion Circuit should be defined as such that all points included are corroding at about the same CR. Having part of a Circuit corroding at 0.304 mm/y (12 mpy) and another part at 0.025 mm/y (1 mpy) is not the ideal Circuit.

IMS does a check on how much Data Scatter is in the CRs for the Circuit. It compares the maximum CRs (Long & Short) to the Circuit Average CR (Long & Short).

Note: The DSCF IF is not a factor until the Max/Average Ratio is greater than 5:1. As the data scatter increases and causes the ratio to exceed 5:1, the DSCF IF will decrease, resulting in a shortened MII. The DSCF IF will never be greater than 1.0 or negative.

A DSCF IF = 1 is an indication the data scatter in neglectable and the IF will not be adjusted. If the data scatter is high, i.e. the DSCF IF is less than 1, you should consider the need for additional inspection (UT scan or RT). Also, you may need to create a new Circuit for the points with a different CR.

Circuit IF

A specific IF called the Circuit IF is assigned at the Circuit. This IF generally has the most impact on the calculations.

The Circuit’s IF Strategy determines whether this IF is inherited from the RBI component (SRBI) or not (IMS). When the IF Strategy is set to “IMS”, the IF must be set in the Circuit Details.

To set the IF Strategy and Circuit IF:

• Go to the Circuit Details section and click Edit. See PEI Circuits - Details Page.
• Select the IF Strategy from the drop-down: "SRBI" / "IMS".
  • If you selected “SRBI”, select the Inherit Component. Note: The RBI Interval Factor will now be used in the Circuit’s CR calculations. See Relationship between CLs, Components and Circuits.
  • If you selected “IMS”, select the Default value, or specify another appropriate value for the Interval Factor. Note: This IF will now be used in the Circuit’s CR calculations.
• Click Save when done.

Setting the IF Strategy.

 

PERF IF

The Percent Examination Required Factor (PERF) IF is included to provide assurance that enough CMLs are being checked. PERF does a check on the number of CML's examined vs. the number required to be examined.

As the number of CMLs examined decreases, the PERF IF decreases resulting in a shortened interval. 

The PERF IF is not allowed to be greater than 1.0 or negative. If the number of CMLs examined equals or exceeds the number required, then the PERF IF becomes 1:

The % of required CMLs can differ per site. This value (Required % PERF) is defined in Settings \ Maintenance \ Configuration \ Configuration per Hierarchy Type according to Equipment Group, Inspection Reason and Special Emphasis.

Required % PERF is defined in settings. 

Example: Assume there are 20 CMLs in the Circuit on a CLASS 1 PIPE Circuit. Three readings have been taken. 

The above configuration shows 25% CMLs are required for a CLASS 1 PIPE. Thus the # of required CMLs are 5 and the PERF IF becomes: