q. Why does AutoPIPE Vessel not use an interpolated allowable stress value for ASME BPVC.II.D.C?


Example:

Report value used = 157 instead of 157.4, why?

Material data:

Answer: 

AutoPIPE Vessel refers to the Stress limit table and returns an allowable value based the temperature setting. When a temperature setting is between table values, the program will linearly interpolate the stress value (ex. 157.4 MPa). 

Afterwards, the linear interpolated value is rounded according to the general note (b) of the code (as referenced below) (e.g., 157 MPa). The value is then converted to the appropriate units when necessary, after which it is reported and utilized by the program for subsequent calculations.

Code Reference:

Please review ASME BPVC.II.D.C (2007 to present) material table notes:  

Avoidance:

Consider using one or more of the following suggestions. 

      1. Petition the code committee to change the code by forcing the use of linear interpolated values.
      2. Manually update AutoPIPE Vessel Material values as needed. Or, input the value in the dialog.
      3. Post a new IDEA for AutoPIPE vessel to give the user the option of using linear interpolated values.

Scenario #2:

Report value = 113.07 MPa,  how is this allowable value calculated?

Material data:

 

 

Answer: 

Similar to scenario #1, AutoPIPE Vessel refers to the Stress limit table and returns an allowable value based the temperature setting. When a temperature setting is between table values, the program will linearly interpolate the stress value (ex. 16.364 ksi). 

Afterwards, the linear interpolated value is rounded according to the general note (b) of the code (as referenced above) (e.g., 16.4 ksi). The value is then converted to the appropriate units when necessary (16.4 ksi = 113.07 MPa) , after which it is reported and utilized by the program for subsequent calculations.

Allowable Stress Calculation:

1. Use linear interpolation to calculate the Stress Limit:

Condition #1: 950 Deg F  - 16.4 ksi

Condition #2: 1000 Deg F - 16.2 ksi

Interpolated stress = 951 Deg F - 16.364 ksi

2. Apply round of rule: 16.364 ksi becomes 16.4 ksi

3. Convert ksi to MPa:

1 ksi = 6.8947572932 MPa.

16.4 ksi = 113.07 MPa

Conclusion:

The code clearly establishes particular guidelines for managing interpolated values that may be utilized. As such, AutoPIPE Vessel accurately interpolates the material information value, adheres to the code regulations for rounding, and converts the value into the appropriate units.