Corrosion Rates in Thickness Monitoring

A Controlling Corrosion Rate for each TML is calculated in the TML Corrosion Analyses, and then, based on the method selected in the Corrosion Analysis Settings linked to the asset or TML Group, the Controlling Corrosion Rate will be determined for the asset or TML Group based on the corrosion rates of the linked TMLs.

Asset Corrosion Analyses and Corrosion Rates

You can use one of three methods to determine the Controlling Corrosion Rate for an asset or TML Group:

• Average Corrosion Rate
• Formula Corrosion Rate
• Maximum Corrosion Rate

Average Corrosion Rate

The Average Corrosion Rate for an asset or TML Group is calculated based on the Controlling Corrosion Rate for every linked TML, the number of TMLs, and the Safety Factor defined in the Corrosion Analysis Settings. There are three options that may affect how the value of the Average Corrosion Rate is calculated:

• Safety Factor
• Minimum Number of TMLs
• Percentile

By default, these options are configured in such a way that all Active Thickness Measurement Location records included in a TM Analysis will be used with a Safety Factor of 1. In other words, if you want the Average Corrosion Rate to be calculated as described in this topic, you do not need to modify these options.

Formula Corrosion Rate

The Formula Corrosion Rate is referred to as the Statistical Corrosion Rate, both on the Asset Corrosion Analysis datasheet and in this documentation.

The Statistical Corrosion Rate is determined by performing a statistical analysis of the TML Controlling Corrosion Rates, eliminating outliers, and determining the average corrosion rate from the remaining TMLs. Several calculations are performed to calculate the Statistical Corrosion Rate. The calculated value is contingent on the Standard Deviation Factor for the Corrosion Analysis, set in the Std Deviation Factor box in the Corrosion Analysis Settings workspace for the asset or TML Group.

First, an upper and lower limit for TML Controlling Corrosion Rates is calculated. Then, all active TMLs with a Controlling Corrosion Rate greater than the upper limit or less than the lower limit are excluded. The Controlling Corrosion Rates of the remaining active TMLs are averaged to determine the final Controlling Corrosion Rate for the asset or TML Group.

Maximum Corrosion Rate

The Maximum Corrosion Rate for an asset or TML Group is determined by evaluating the Controlling Corrosion Rates of all linked, active TMLs. The Maximum Corrosion Rate in the Asset Corrosion Analysis will be set to the highest (or fastest) corrosion rate among these TMLs.

TML Corrosion Analyses and Corrosion Rates

The following corrosion rates can be calculated for a given TML, if they are selected in the linked Corrosion Analysis Settings. These values are stored in the TML Corrosion Analysis:

• Short Term Corrosion Rate (STCR)
• Long Term Corrosion Rate (LTCR)
• Least Squares Corrosion Rate (LSCR)

Additionally, up to two custom corrosion rates can be defined by your organization. This documentation focuses on the baseline corrosion rates provided in the Thickness Monitoring module.

TML Controlling Corrosion Rate

The Controlling Corrosion Rate for a TML is determined by evaluating the corrosion rates that have been calculated for the TML. The TML Controlling Corrosion Rate is set to the highest (i.e., most conservative) of these corrosion rates.

If a corrosion rate cannot be calculated, either because the TML does not have enough measurement data or because other criteria dictate that the corrosion rate should not be calculated, the TML Controlling Corrosion Rate will be set to the Minimum Corrosion Rate defined in the Corrosion Analysis Settings.

If all of the calculated corrosion rates are less than the specified minimum value and the Use Minimum Corrosion Rate check box is selected in the Corrosion Analysis Settings, the TML Controlling Corrosion Rate will be set to the Minimum Corrosion Rate defined in the Corrosion Analysis Settings.

Thickness Monitoring includes several baseline TML-level corrosion rates that you can choose to calculate for TML Corrosion Analyses. In addition to using these baseline corrosion rates, you can define custom corrosion rate calculations to use within Thickness Monitoring. Using the options in the Global Preferences workspace, you can define up to two custom calculations.

When custom TML corrosion rates are in place, you will see various changes throughout Thickness Monitoring. The following sections describe the changes you will encounter.

Corrosion Analysis Settings

When custom corrosion rate calculations are defined, they will appear in the Corrosion Rate Options section of the Corrosion Analysis Settings workspace, using the labels that you defined via the Global Preferences workspace.

TML Corrosion Analyses

When custom corrosion rate calculations have been defined, and the associated Corrosion Analysis Settings for an asset or TML Group specify that they should be calculated, the following fields will be populated in all associated TML Corrosion Analyses:

• Custom Calculation A Corrosion Rate
• Custom Calculation A Loss
• Custom Calculation B Corrosion Rate
• Custom Calculation B Loss

These fields always exist in TML Corrosion Analyses and are included by default on the TML Corrosion Analysis datasheet.

TML Trends Graph

When custom corrosion rate calculations have been defined, and the associated Corrosion Analysis Settings specify that they should be calculated, the custom corrosion rates will be displayed on the Trends graph, except when:

• The values in the Custom Calculation A Corrosion Rate and Custom Calculation B Corrosion Rate fields are set to 0, and the value in the Minimum Corrosion Rate field in the TML Corrosion Rate section of the linked Corrosion Analysis Settings is set to 0. In this case, no corrosion rate line will be displayed.
• The value in one Custom Calculation Corrosion Rate field is set to 0, the other Custom Calculation Corrosion Rate field is set to a value greater than 0, and the value in the Minimum Corrosion Rate field in the TML Corrosion Rate section of the linked Corrosion Analysis Settings is set to 0. In this case, only the corrosion rate line corresponding to the Custom Calculation Corrosion Rate with a non-zero value will be plotted. For example, if you set the value in the Custom Calculation A Corrosion Rate field to 0 and the Custom Calculation B Corrosion Rate field to any value greater than 0, and assuming the Minimum Corrosion Rate field in the TML Corrosion Rate section of the linked Corrosion Analysis Settings is set to 0, then only one corrosion rate line, corresponding to the value in Custom Calculation B Corrosion Rate, will be plotted. Points corresponding to measurements will be plotted regardless of whether any corrosion rate line is displayed.

Long Term Corrosion Rate is a calculation that indicates the rate of corrosion over the full length of time for which measurements have been recorded. The LTCR will be calculated for a given TML if the analysis settings for the related Equipment or TML Group specify that LTCR should be used.

The LTCR is calculated using the Long Term Loss and Long Term Interval, where:

LTCR = Long Term Loss/Long Term Interval

...where:

Long Term Loss = Base Measurement Value - Last Measurement Value

...and:

Long Term Interval = Last Measurement Date - Base Measurement Date

Example: Long Term Corrosion Rate Reading

LTCR = .01 inches / 10 years = 1 MILS/year

Because the LTCR is based on the base measurement and the last measurement, if only one measurement has been recorded, LTCR cannot be calculated. If two or more measurements have been recorded, the LTCR can be calculated. If only two measurements have been recorded, all corrosion rates can be calculated.

The following diagram shows a graphical representation of the LTCR.

Short Term Corrosion Rate is the rate of corrosion as determined by the most recently recorded values. The STCR will be calculated for a given Thickness Measurement Location record only if the Analysis Settings for the related asset specify that STCR should be used and there are two more Thickness Measurements for the Thickness Measurement Location record.

The STCR is calculated using the Short Term Loss and Short Term Interval, where:

STCR = Short Term Loss / Short Term Interval

...where:

Short Term Loss = Near Measurement Value - Last Measurement Value

...and:

Short Term Interval = Near Measurement Date - Last Measurement Date

Example: Short Term Corrosion Rate Reading

Suppose you have the following readings.

In this case:

STCR = .005 inches / 5 years

STCR = .001 inches/year = 1 MILS/year

The rate of corrosion is determined by the most recently recorded values. Because the STCR is based on the Near Measurement and the Last Measurement values, it can be calculated only when two or more measurements have been recorded for a given TML.

The is determined using the least squares linear regression method, which is a mathematical method of finding the best-fit line for a set of datapoints by minimizing the squares of the offsets.

Example: Least Squares Corrosion Rate

Suppose you have the following measurement data:

Represented graphically, GE Digital APM calculates the datapoints to look like this:

The Maximum Historical Corrosion Rate (MHCR) is determined by the calculated values for the three calculated TML corrosion rates: Short Term Corrosion Rate (STCR), Long Term Corrosion Rate (LTCR), and Least Squares Corrosion Rate (LSCR). The MHCR is set to the highest corrosion rate that has been calculated for any of the three corrosion rates over the life of the TML.

The MHCR can be calculated as long as at least one corrosion rate has been calculated for the TML.

Example: Maximum Historical Corrosion Rate

Suppose you have the following values:

• STCR = 12.3 MILS/year
• LTCR = 11.2 MILS/year
• LSCR = 11 MILS/year

The MHCR would be set to 12.3 MILS/year. If additional Thickness Measurements were added to the TML, the corrosion rates would then be calculated as:

• STCR = 11.5 MILS/year
• LTCR = 14 MILS/year
• LSCR = 13.5 MILS/year

The MHCR would be reset to 14 MILS/year. If additional adjustments were made to Thickness Measurements that caused the calculations to be reset to:

• STCR = 13.5 MILS/year
• LTCR = 12 MILS/year
• LSCR = 12 MILS/year

Then the MHCR would remain 14 MILS/year.