Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: calculcations Subarea: Original Author: Bentley Technical Support Group ASME B31.8 Piping Code Calcualtion Issues Comments, Questions, and Answers Item #1, How to calculate B31.8 code allowable of per code equation: Given the following example information: Answer: Sc = 0.33 Su T at minimum installed or operating temperature = 0.33 * 455.05 = 150.16 N/mm2 Sh = 0.33 Su T at maximum installed or operating temperature = 0.33 * 455.05 = 150.16 N/mm2 SL = longitudinal stress calculated according to para 833.6.(a) From AutoPIPE on-line help, refer to the sustained stress calculation for B31.8: Note: this value is already calculated in AutoPIPE as stress results for Gr + MaxP loadcase SL = 327.53 N/mm2 f = Range factor = 1.0 (setting from Tool model Options Results) Sa = 1.0 * [ 1.25 * (150.16 + 150.16) - 327.53] Sa = 47.886 N/mm2 as shown in the output report above. Note: The limit of SL = 0.75 S is for sustained stress acceptance. The actual stress calculated is used by AutoPIPE when calculating the thermal allowable SA. Item #2, Exactly how is AutoPIPE calculating the combined stress for a node point, equation used and individual values being applied in the equation? Answer : For Tresca stresses, AutoPIPE calculates three values for longitudinal stress as follows: SL1 = Axial (Neutral axis) SL2 = Axial + Bending (Max Long if Axial is positive) SL3 = Axial - Bending These three values are used to calculate Tresca stresses using: Similar approach is taken for Von Mises calculation. Item #3, The hoop stress uses the D/t ratio to decide which formula to use, thin or thick, However which is being used in combined stress equation, does the D/t ratio still apply for the hoop stress values used in the combined equation? Answer : D/t ratio is used to decide hoop stress formula to be used in combined stress equation. A good discussion on a similar issue is available in our Pipe Stress Analysis Forum which can be accessed by the link below: http://communities.bentley.com/products/pipe_stress_analysis/f/275801/p/70679/186018.aspx#186018 Item #4, How can I set the RESTRAINED pipe option for B31.8 2007 onshore / underground piping Answer: Please see the following AutoPIPE help section: Help Contents Contents Tab Reference Information Code Compliance Calculations ASME B31.8-2007 Code Combinations Here you will see 2 charts displayed, restrained and unrestrained. Note that the combination's category will determine which calculators are used, i.e. restrained or unrestrained equations. Under the charts are a number of notes that you should review for a better understanding of AutoPIPE's analysis. The calculations for restrained piping is conservative and assumes a small amount of contribution from bending, even on straight piping. The more restrained the piping is with more supports or higher soil properties the less contributions from bending and the closer to code calculations for St = E x alpha x (delta T). Item #5, I am working on a buried 10 inch pipeline ( ASME B31.8) , I have noticed that Auto pipe does not consider the bending stresses in the pipe due to large diameter elastic bends ( 1200 m). it only reports the stressses due to pressure , temperature and gravity . can any one help ? Answer: from Karim Rinawi AutoPIPE forum response: https://communities.bentley.com/products/pipe_stress_analysis/f/275801/t/93651.aspx In most cases these initial stresses are ignored as are circumferential weld stresses. Similarly, for buried pipe the overburden soil load will add stresses which often are analyzed separately. These overburden stresses were recently supported in AutoPIPE for some codes in which case they are added to other sustained stresses In addition to additional checks to insure that the pipe will not buckle under the soil load alone. It is worth pointing out that the overburden stresses act against the initial cold bending stresses. The way to account for the elastic bending stresses is to calculate them manually and add them to the sustained stress. The moment created by bending can be calculated by E.I/R where E is the elastic pipe material modulus, I is the pipe moment of inertia and R is the bend curvature radius. You can build a simple cantilever pipe model and apply this moment to the free end in a User case to get the cold bending stress. This bending stress can then be manually added to the maximum sustained stress in the long radius bend and the results compared to the sustained allowable. Note that you may not need to add it to expansion stress range since this is a one time event and would not reverse. However it can be added to operating cases as in B31.3 Appendix P, if that option is being used. You can also calculate the cold bend stress using a pipe with two rigid anchors on both sides and apply a rotation equal to pipe span length times divided by bend radius or theta = L/R in radians which can be converted to degrees. This rotation is applied to one of the end anchors under U1 user case. See Also Bentley AutoPIPE External Links Bentley Technical Support KnowledgeBase Bentley LEARN Server Comments or Corrections? Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article to the "Comments" area below. THANK YOU!
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