Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Piping codes Subarea: Original Author: Bentley Technical Support Group Comments, Questions, and Answers on CAN/CSA-Z662 Piping Code Calculations General notes: Combination name: Equiv1 1. AutoPIPE calculates the Equivalent Tensile stress 1 (CSA Z662-07 clause 4.7.1 Combined hoop and longitudinal stresses) meant for restrained portions of a pipeline (exclude bending, torsion and shear stresses) 2. In order to obtain consistent results for Equivalent Tensile 1 , a pressure extension analysis should be performed (enable the pressure extension analysis option on the Analysis Set dialog for the analysis set). This type of static analysis insures that the axial pressure forces, which can be substantial in axially restrained system, is included in the stress calculations. 3. AutoPIPE uses the built-in category maximum case (EXP) which instructs the program to use the forces and moments from the expansion case with maximum temperature (which generally will result in maximum forces and moments) in an analysis set. 4. Longitudinal pressure calculations always use P*AP/AS formula, and the pressure area term AP always used corroded inside diameter (where AP = Pressure area of pipe, AS = Cross sectional area of pipe, P = Design internal pressure). 5. Internally, AutoPIPE uses Tresca criterion with no bending, shear or torsional stress to calculate Equivalent Tensile 1 stresses. 6. AutoPIPE considers compressive axial pipe force as negative, for example if the axial pipe forces due to gravity and expansion are compressive, then the stresses due to gravity and expansion will be added to the Longitudinal Pressure stress (causing compressive axial pipe force). Otherwise, if the axial stress due to gravity and expansion forces is tensile in nature, then this stress will be subtracted from the Longitudinal Pressure Stress. 6. When pressure extension analysis is performed, AutoPIPE will subtract the stress due to axial pressure force from the longitudinal pressure stress Item #1: For CAN-Z662 (2007), When reviewing the code equation Equiv1, appears that the value is exactly the same as Hoop. Why is AutoPipe not considering the longitudinal term of the equation in 4.7.1. In some cases there isn't any stress in the expansion cases, so this makes sense, however various nodes do have significant expansion stress, which is as much as or more than the hoop stress, however AutoPipe is still only reporting the hoop stress for Equiv1 results. Answer: Please see the following AutoPIPE help section: Help Contents Contents Tab Reference Information Code Compliance Calculations CAN-Z662 (2007) From the online help: For those portions of restrained pipelines that are fully restrained axially, the equivalent tensile stress (seq) is calculated per the code in Section 4.7.1 as the maximum Tresca effective stress (refer to the discussions on Principal Stress and Maximum Shear Stress in the General Stress Calculations section of the Appendix), excluding bending stresses and shear stresses Principal Stress: The principal stresses are calculated considering the longitudinal stress (Sigma a), the hoop stress (Sigma h), and the torsional shear stress (Tau). from the following equations: In case of Z662 - Equivalent Tensile stress, the shear stress (Tau) is set to zero and there is no bending stress considered for longitudinal stress. If you enter shear stress as zero in equation above, you would get Where Sigma (a) is the longitudinal stress from pressure (P*Ap/As). AutoPIPE reports the maximum stress as: In cases where the net longitudinal stress comes out as a positive value (tension), the maximum stress would be either Hoop or Longitudinal Stress and not (Sigma 1 - Sigma 2). We can relate this to the note provided by the code: In cases where the axial stress due to axial forces by Gravity, Pressure and thermal expansion are high, the net longitudinal stress would become negative and then we will see a value of equivalent tensile stress which is greater than the hoop stress. Item #2: The longitudinal stress calculated following the AutoPipe documentation does not match the results provided by AutoPipe. Can you please provide hand calculations which match the AutoPipe results for a simple system (straight pipe)? How are does Autopipe calculate the longitudinal stress in the general output file when a SIF is applied. Answer: Please see the following AutoPIPE help section: Help Contents Contents Tab Reference Information Code Compliance Calculations CAN-Z662 (2007) Stresses Due to Sustained Loads (CSA-Z662 - 2007) Case with Pressure Extension Cases and with Pcase in Sustained The longitudinal pressure term in the above equation highlighted in yellow is can vary from P= PD/4t, or A = AxF/area, or M = minimum of both, or set to N = 0.00. This is controlled by the following setting: Tools Model option Results Long Pressure (P/A/M/N) option. Confirm that the correct form of this setting is used in the above equation. Item #3: The main load case that we are concerned is Equiv2 or Tensile 2 which captures the compliance to CSA Z662 Clause 4.7.2.1. it is assumed that it is only capturing the bending stresses due to thermal expansion. Answer: We have investigated the issue and the user's observations are correct. AutoPIPE only adds the axial force due to pressure when pressure extension is enabled. The other forces and moments do not include the effects from pressure extension in the default code combination Equiv2 for CSA Z662 code. This is also true for other codes, and is by design. AutoPIPE B31.8 code may be an exception as it always includes a pressure case in the default code combination. To be able to include the effects of pressure extension other than the axial force, the user can include the desired pressure case in the default code combination for Equiv2. This will include the pressure extension effects to moments as well. Another follow up question may be the difference observed between the General Stress Report and Equiv2 stress results (different for the user model). These can be matched by setting the Result Model Option "Total stress (OCT/Max)" to M, which represents Max Shear Stress. AutoPIPE calculates Equiv2 stresses using Tresca stresses, and for General Stress Report the value displayed is Max Shear Stress = Tresca Stress / 2.0. Item #4: What is the reasoning for taking only gravity and thermal expansion bending moments into account for Equiv2 and not pressure? If you have thermal expansion you have pressure as well. You can't just have thermal expansion without pressure. In your competitor's program they are taking bending moments from all cases into account (pressure, gravity and thermal). Answer: Pressure cases contain pressure extension/elongation forces and moments, which piping codes do not clearly require to be included in calculations. Longitudinal Pressure Stress component (P*Ap/As) is always included regardless of Pressure case being a part of the combination or not. Note: As = Cross sectional area of pipe, in2 Ap = Pressure area of pipe, in2 P = Design internal pressure, psi 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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