Calculations

Overview of TM Calculations

A Corrosion Analysis includes one or more assets and all the TM-specific records that are linked to them. As you conduct a Corrosion Analysis, various calculations and validations are performed, and the results are stored in various records involved in the Corrosion Analysis.

In this section of the documentation, we provide descriptions of the calculations and validations that are performed within a Corrosion Analysis. These descriptions are classified according to the type of record for which the calculation or validation is performed:

Asset Corrosion Analysis: Stores summary information and calculations that are performed using the information in the related Thickness Measurement Location and Thickness Measurement records.
TML Corrosion Analysis: Stores summary information and calculations performed using the information in related Thickness Measurement Location and Thickness Measurement records.
Thickness Measurement: Stores measurement data and performs calculations. The values calculated in Thickness Measurement records are used for calculating values in the associated TML Corrosion Analysis record.

Thickness Monitoring uses specific formulas, equations, and estimation methods to arrive at the values that are stored in these records. In order to understand the information that is displayed to you for a given analysis, you must understand how the values were determined.

Measurement Calculations

Measurement data is stored in three fields in Thickness Measurements:

Readings: Stores one or more values representing thickness measurement data collected at a specific TML.
Uncorrected Measurement: Stores the unadjusted measurement value.
Measurement Value: Stores the final, adjusted measurement value.

Measurement data is stored in three fields in Thickness Measurements:

Readings: Stores one or more values representing thickness measurement data collected at a specific TML.
Uncorrected Measurement: Stores the unadjusted measurement value.
Measurement Value: Stores the final, adjusted measurement value.

Measurement data is recorded in these fields according to the following workflow:

When a Thickness Measurement is created, you enter data into the Readings field.
Note: Readings can be added by accessing the Thickness Measurement directly, using the Measurement Data Entry Workspace, or using a datalogger.
A calculation is performed against the values in the Readings field, and the result is used to populate the Uncorrected Measurement field automatically.
The Measurement Value field is populated automatically with the value in the Uncorrected Measurement field.
Note: The Uncorrected Measurement field is meant to serve as an intermediate storage location between the Readings field and the Measurement Value field. In baseline Thickness Monitoring, the Measurement Value is simply populated with the value in the Uncorrected Measurement field. You can create custom rules to perform additional adjustment or correction calculations.

The type of calculation that is performed in step 2 is determined by the settings defined for measurement readings in the TM Admin Preferences. The baseline TM product offers three calculation options: Average, Minimum, and Maximum.

Note: Alternatively, you can define a custom formula that will be used to calculate measurement data. In this topic, however, we describe only the baseline options.

Consider a Thickness Measurement that contains the following values in the Readings field:

0.3
0.31
0.317

In this case:

Using the Average calculation method, the Uncorrected Measurement value would be calculated as the average of the three values in the Readings field, 0.309.
Using the Minimum calculation method, the Uncorrected Measurement value would be set to 0.31.
Using the Maximum calculation method, the Uncorrected Measurement value would be set to 0.317.

Datasheet Calculations

Asset Corrosion Analysis > Average Corrosion Rate

Asset Corrosion Analysis > Statistical Corrosion Rate

TML Corrosion Analysis/Corrosion Analysis Settings > Short Term Corrosion Rate

TML Corrosion Analysis > Factor Remaining Life Date

About Interpolation

Thickness Monitoring (TM) can determine which Allowable Stress value to use for calculating T-Min and Maximum Allowable Working Pressure by looking in the Piping Stress reference table or the PV Stress reference table and finding a record where the Material Specification, Material Grade, and Design Code match exactly the values in the Thickness Measurement Location (TML).

Details

Multiple records with varying temperatures and stress values may exist for a given combination of Material Specification, Material Grade, and Design Code values. So, after a match is found on Material Specification, Material Grade, and Design Code, Thickness Monitoring (TM) evaluates Design Temperature.

If the Design Temperature in the TML is an exact match to a temperature value in a Piping Stress or PV Stress record, TM will simply use the corresponding Allowable Stress value. For example, consider a TML that contains the following values:

Material Specification = SA-216
Material Grade = B
Design Code = ASME B31.3
Design Temperature = 100

Now, consider a Piping Stress record that contains the following values:

Material Specification = SA-216
Material Grade = B
Design Code = ASME B31.3
Design Temperature = 100
Allowable Stress = 20000

In this case, the values in the TML are an exact match to the values in the Piping Stress record. Therefore, TM will use the Allowable Stress value of 20000 to calculate Maximum Allowable Working Pressure and T-Min.

If the Piping Stress or PV Stress reference table does not contain an exact match on the Design Temperature defined in the TML, TM will use one the following methods for determining Allowable Stress, depending upon whether or not the Application Settings specify that interpolation should be used:

If interpolation should not be used, TM will use the Allowable Stress value for the lower bound temperature value.
If interpolation should be used, TM will determine the interpolated stress value using the values that exist in the Piping Stress table.

When Interpolation Is Used

If TM cannot find an exact match in the Piping Stress or PV Stress reference table, and the Application Settings specify that interpolation should be used, TM will calculate the interpolated Allowable Stress value using the temperature and stress values that make up a range that includes the TML Design Temperature.

For example, consider a Piping Stress reference table that contains the following two records.


Field	Record #1	Record #2
Material Specification	SA-403	SA-403
Material Grade	WP316	WP316
Design Code	ASME B31.3	ASME B31.3
Design Temperature	100	200
Allowable Stress	20000	18000

Now, consider a TML that contains the following values:

Material Specification = SA-403
Material Grade = WP316
Design Code = ASME B31.3
Design Temperature = 150

The TML Design Temperature falls within the range established by the two Piping Stress records in the table. Linear interpolation assumes that stress varies linearly as a function of temperature within the defined range. Using this assumption, the Allowable Stress at 150 degrees can be calculated using the following formula:

Sx = S1 - (T1 - Tx) / (T1 - T2) * (S1 - S2)

Where:

Sx = Allowable Stress for the TML
S1 = Allowable Stress at Lower Boundary
S2 = Allowable Stress at Upper Boundary
T1 = Temperature at Lower Boundary
Tx = Temperature defined for TML
T2 = Temperature at Upper Boundary

Given the values from our example, we calculate Sx as:

Sx = 20000 - ((100 - 150) / (100 - 200) * (20000 - 18000))

So:

Sx = 19000

This interpolated Allowable Stress value will then be used to populate the Allowable Stress field in the TML.

Note: For TM to determine Allowable Stress in this way, the Piping Stress and PV Stress reference tables must contain records that establish the range that includes the TML Design Temperature.

When Interpolation Is Not Used

If TM cannot find an exact match in the Piping Stress or PV Stress reference table, and the Application Settings specify that interpolation should not be used, TM will use an Allowable Stress value that corresponds to the lowest temperature value in the temperature range that includes the TML Design Temperature.

For example, consider a Piping Stress reference table that contains the following two records.


Field	Record #1	Record #2
Material Specification	SA-403	SA-403
Material Grade	WP316	WP316
Design Code	ASME B31.3	ASME B31.3
Design Temperature	100	200
Allowable Stress	20000	18000

Now, consider a TML that contains the following values:

Material Specification = SA-403
Material Grade = WP316
Design Code = ASME B31.3
Design Temperature = 150

In this case, the Design Temperature in the TML is not an exact match to a Piping Stress record. The TML Design Temperature, however, does fall within the temperature range established by the two Piping Stress records in the table. So, in this case, TM will use the record that contains the most conservative (i.e., lowest) Allowable Stress value. Therefore, in this case, an Allowable Stress value of 18,000 will be used in the TML.

Note: For TM to determine Allowable Stress in this way, the Piping Stress and PV Stress reference tables must contain records that establish the range that includes the TML Design Temperature.