"Use nominal thickness" setting in AutoPIPE


Comments, Questions, and Answers related to Tools>Model Options> Results> "Use nominal thickness" setting in AutoPIPE:

Q1. Why does mill tolerance not effect sustained stresses in AutoPIPE calcs for ASME B31.3?

Answer:

Select Tools> model options> Results> and press the "Help" button in the lower right corner of the dialog. From this point, select the hyperlink for "Use nominal thickness ". On this page is a chart that shows what wall thickness is used for which code calculations. For ASME B31.3, per Stress Category = Sustained, when this feature is checked On; Tnom is used, when this feature is checked Off; Tcor is used. If you go to the Definition of Terms  page in AutoPIPE to look up these definitions:

tnom  =  Nominal wall thickness of the pipe. 
 
tcor  =  Corroded wall thickness of the pipe (= tnom - c).

As you can see the mill tolerance does not come into play for the calculation of Sustain stress code calculation.

Furthermore, take a moment to search the actual code for "Mill Tolerance" to see exactly where it is being used.

Q2.  How corrosion allowance is considered and treated under ASME NB Code?

Answer:

AutoPIPE provides a setting under Results → Model Options called "Use Nominal Thickness". This option determines whether corrosion allowance is applied in calculations involving:

Because design codes evolve over time, this setting ensures flexibility in analysis based on your project requirements.

Behavior of the Setting:

Enabled (Use Nominal Thickness = ON):
AutoPIPE uses the nominal wall thickness for all calculations, ignoring corrosion allowance.

Disabled (Use Nominal Thickness = OFF):
AutoPIPE uses the corroded wall thickness (nominal thickness minus corrosion allowance) in calculations.

Confirm for your self by performing a simple test:

    1. Create a model with corrosion values assigned to all Pipe IDs.
    2. Enable "Use Nominal Thickness" under Results → Model Options.
    3. Analyze the model and generate a full output report (including a complete Model input listing).
    4. Save the model.
    5. Save a copy of the model with a new name (e.g., Disabled.DAT).
    6. In the newly saved model, disable "Use Nominal Thickness".
    7. Re-analyze and generate the same output report.
    8. Compare both reports using a file comparison application (e.g., Beyond Compare, UltraEdit, or likeness).
One report will show all analysis results based on nominal thickness. Meanwhile, the other report will show analysis results based on corroded thickness. The comparison application will highlight all differences between the two files. This would be exactly how this setting, "Use Nominal Thickness", affects the entire model and all analysis calculations.

As an example comparison for ASME NB - 2023, differences were observed in the following sub-reports

Note: As of Dec 2025, AutoPIPE help for "Use Nominal Thickness" is missing a table on ASME NB. One will be adding in a future version of the application. 

Q3. For Stoomwezen this setting is off by default and for EN13480 it is on by default. Is this difference in default settings a code requirements or not?

Answer:

Each piping code has requirements for what wall thickness would be used in a specific calculation. The CAE development team automatically adjust many default setting based the piping code selected. In this case, please see the following AutoPIPE help section:  Help > Contents> Contents Tab> Reference Information> Code Compliance Calculations> En1380 and Stoomwezen,

Stoomwezen: 

Stresses Due to Sustained Loads (STOOMWEZEN)

Noticed that the value tred or corroded thickness is used. Another words "Use nominal wall thickness" by default should be OFF .

En13480: 

Stresses due to Sustained Loads (En13480 - 2012)

Notice that the value en or nominal wall thickness is used. Another words "Use nominal wall thickness" by default should be ON

Again default setting are usually based on code requirements. 

See Also

Results Model Options

Bentley AutoPIPE