Stress analysis load conditions essential for typical piping systems
Thread Content
The main purpose of stress analysis is to ensure: A. structural integrity (meeting the design pressure of the fluid it carries, resisting failure due to various loads throughout its lifetime, and ensuring that the system’s maximum stresses are below the allowable values specified in the codes). ) B. Operational integrity (the load at the device connections is kept within the allowable values for pipe end loads, leakage at the joints is prevented, and the deflection and displacement of the pipelines are kept within permitted limits). ) C. Optimal design (to avoid excessive flexibility in the piping system and high loads on the support structures, seeking the best design for both the pipes and the structure. ) WW = the weight of water filled in the piping system, HP = the pressure used for hydraulic testing, W = the weight of the piping, including the fluid inside it and the insulation layer, P1 = the design internal pressure, T1 = the operating temperature, T2 = the design high temperature, T3 = the design low temperature. WIN1, WIN2, WIN3, WIN4: wind loads acting in certain specific directions; U1, U2, U3, U4: uniformly distributed (seismic) loads acting in certain specific directions. When conducting analysis, it is necessary to verify at least the following load conditions: a. Water pressure testing condition: Pipelines are usually tested under pressure before actual operation to ensure there are no leaks; water is typically used as the medium for this testing. Therefore, in this case, the pipe will bear the weight of the water medium as well as the pressure from the hydrostatic test. Based on this, we adopt the first operating condition in CAESAR II as follows: 1. WW+HP HYDb. Operating condition: At the start of operation, the working fluid will flow through the pipes at a certain temperature and pressure. Based on this, the operating conditions of the system are as follows: 2. W+T1+P1 OPE – operating temperature condition; 3. W+T2+P1 OPE – designed high-temperature condition; 4. W+T3+P1 OPE – designed low-temperature condition. c. Continuous conditions: There will be continuous loads – weight and pressure – throughout the operation of the entire plant. Therefore, the continuous load conditions are as follows: 5. W+P1 SUSd. Random conditions: The pipeline may be subjected to random wind and seismic loads. Therefore, to verify the stresses under these operating conditions, the following load cases must be established: 6. W+T1+P1+WIN1 OPE – considering wind load in the +X direction; 7. W+T1+P1+WIN2 OPE – considering wind load in the -X direction; 8. W+T1+P1+WIN3 OPE – considering wind load in the +Z direction; 9. W+T1+P1+WIN4 OPE – considering wind load in the -Z direction; 10. W+T1+P1+U1 OPE – considering seismic load in the +X direction; 11. W+T1+P1-U1 OPE – considering seismic load in the -X direction; 12. W+T1+P1+U2 OPE – considering seismic load in the +Z direction; 13. W+T1+P1-U2 OPE – considering seismic load in the -Z direction. When performing stress analysis using these loads, load cases 6 through 13 are used solely to check the load values at the joints. To obtain accidental stresses, we also need to add the sustained load to the pure accidental conditions and then compare it with the values permitted by the codes. To this end, the following load cases were established: 14. L6-L2 OCC + pure wind load in the +X direction; 15. L7-L2 OCC + pure wind load in the -X direction; 16. L8-L2 OCC + pure wind load in the +Z direction; 17. L9-L2 OCC + pure wind load in the -Z direction; 18. L10-L2 OCC + pure seismic load in the +X direction; 19. L11-L2 OCC + pure seismic load in the -X direction; 20. L12-L2 OCC + pure seismic load in the +Z direction; 21. L13-L2 OCC + pure seismic load in the -Z direction; 22. L14+L5 OCC + pure wind load + continuous; 23. L15+L5 OCC + pure wind load + continuous; 24. L16+L5 OCC + pure wind load + continuous; 25. L17+L5 OCC + pure wind load + continuous; 26. L18+L5 OCC + pure seismic load + continuous; 27. L19+L5 OCC + pure seismic load + continuous; 28. L20+L5 OCC + pure seismic load + continuous; 29. L21+L5 OCC + pure seismic load + continuous. Load cases 22 through 29 will be used to verify accidental stresses based on the allowable values specified in Code B31.3 (which is 1.33 times the Sh value given in the code). For load cases 22 to 29 mentioned above, scalar combinations are used, while algebraic combinations are used for the other cases. Expansion cases: As required by the specifications, the following load cases will be used to verify the expansion stress: 30. L2-L5 EXP 31. L3-L5 EXP 32. L4-L5 EXP 33. L3-L4 EXP For the entire stress range, the aforementioned load cases (from case 30 to 33) are used to verify the expansion stress. The aforementioned loading conditions are the most fundamental ones required to analyze any stress system. In the aforementioned loading conditions, the loading conditions specified in numbers 1, 5, and 22-33 are used for stress verification. The load conditions numbered 1 to 13 are used to check the constraint reactions, displacements, and nozzle loads. The PSV connection system and the rotating equipment connection system may also require certain additional load conditions. It is not necessary to conduct seismic and wind load analyses every time. Therefore, if the piping system is not within the scope of the seismic and wind load analyses specified for the project, then these load conditions can be removed. However, to perform wind load and seismic analysis, the appropriate relevant data must be entered into the CAESAR II data table. If the stress system involves the use of applied displacement (D) and force (F), then D1, D2 or F1, F2 (as applicable) must be added to the aforementioned loading conditions. The calculation results should preferably meet (but are not limited to) the following requirements: 1. The testing pressure and sustained stresses should be below 60% of the values permitted by the specifications. 2. The expansion stresses and accidental stresses should be below 80% of the values permitted by the specifications. 3. For process pipelines, the deflection under sustained loads should be less than 10 mm ； For steam pipes, two-phase flow pipes, or flare lines, the deflection should be less than 3 mm. For plant pipes, the maximum displacement should be less than 75 mm, while for pipe tray pipes it should be less than 200 mm
Can seismic loads and wind loads be directly superimposed on W+P1?
We generally don’t consider these loads acting in combination, as that would make the approach too conservative
I didn’t express myself clearly; now I usually use a simpler format like W+P1+WIN1 OCC W+TI+P1+WIN1 OPE, which gives an immediate result.
The pure wind load or pure seismic load is no longer recalculated.
They’re not the same; what you do isn’t accurate, and the difference is quite large. As for why, it requires a lot of explanation – I’ll post another thread when I have time
I just calculated the wind load using a 12-meter-high vertical pipe model, with two different combinations of wind loads applied; the stress values obtained were identical. One is the method you mentioned: first calculate the stress due to the pure wind load, and then add it to the stress under continuous loading, using a scalar value. In one case, I simply added WIN1 to the sustained stress, and in the end the calculation results for both were the same.
I didn’t see it clearly; the direct addition of stresses gives the same result, but for quadratic stresses, if subtraction is not used, the result will be different. These operating conditions are designed because when there are many of them, combining them makes things more convenient and reduces the risk of making mistakes.