Allowable Combined Stress in WRC-107 analysis
Allowable Combined Stress in WRC-107 analysis
(OP)
What should be the combined allowable stress for an nozzle external force?
I used two softwares to check nozzles subjecting external load. Two softwares have different allowable combined stress. Compress says 3*S is allowable combined stress. And in the manual, they explain that the factor 3 is normally recommended because S is normally less than two third of yield strength and allowable combined stress can be two times of yield strength. On the other hand, Advanced Pressure Vessel takes the lesser of (Material allowable stress * Stress Multiplier, yield strendth) as combined allowable stress. APV's value was roughly half of Compress' value for my nozzles.
Can anyone explain about this point?
I used two softwares to check nozzles subjecting external load. Two softwares have different allowable combined stress. Compress says 3*S is allowable combined stress. And in the manual, they explain that the factor 3 is normally recommended because S is normally less than two third of yield strength and allowable combined stress can be two times of yield strength. On the other hand, Advanced Pressure Vessel takes the lesser of (Material allowable stress * Stress Multiplier, yield strendth) as combined allowable stress. APV's value was roughly half of Compress' value for my nozzles.
Can anyone explain about this point?





RE: Allowable Combined Stress in WRC-107 analysis
- 1.5*S for membrane stresses, as they are to be considered as local primary stresses according to App.4 of ASME VIII Div.2
- 3.0*S for membrane+bending stresses, as bending stresses are to be considered as secondary stresses
So both softwares might be correct, if those limits are used for the applicable stresses!
prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design
RE: Allowable Combined Stress in WRC-107 analysis
For permanent loads stresses I consider design pressure and temperature and 1.5S as allowable stress, meanwhile for alltogether I use operation conditions (that means operating pressure and temp.) and 3.S as allowable ones.
Please, correct me if I´m wrong, but so far, I don´t know any official code that clearly states which are the proper allowable stresses in the classical nozzle to shell junction problem.
Regards
RE: Allowable Combined Stress in WRC-107 analysis
Concerning the code I find the provisions of ASME VIII Div.2 App.4 very exhaustive on the subject (see Table 4-120.1 for classification of stresses in a shell or head near a nozzle).
prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design
RE: Allowable Combined Stress in WRC-107 analysis
RE: Allowable Combined Stress in WRC-107 analysis
I think thermal stress is generally regarded as secondary stress. Am I wrong?
Kyong
RE: Allowable Combined Stress in WRC-107 analysis
From a pure code wording standpoint, look at the Table in Appendix 4 of Div 2. It describes stresses in nozzles due to external loads, and does not differentiate between those due to thermal expansion of the piping system, or those due to weight, pressure and other sustained loads.
RE: Allowable Combined Stress in WRC-107 analysis
I'll try to give my interpretation for the classification of thermal stresses on the nozzles of a vessel, owing also to the (correct) comments of Chuck.
What you think is not completely false. Thermal stresses indeed are generally to be classified as secondary, but a condition must be met, as specified by ASME VIII-2-App.4: they must be associated with the distortion of the structure in which they occur, not a different structure!
So, if as it is currently done, a vessel is analyzed separately from the piping system attached to a nozzle (and for the latter the vessel will normally be considered a fixed point), then all the trasmitted loads, including thermal loads, should, for the sake of safety, be considered all the same, as pointed out by Chuck.
However this doesn't exclude the possibility of analyzing the vessel and the piping together, allowing the classification of stresses due to thermal expansion of piping (and vessel) as secondary. This appears as a quite theoretical situation, but might become of actual interest when a fixed point in the piping is placed close to the vessel (probably to avoid excessive expansion loads), so the coupled analysis may be limited to a short piece of pipe.
prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design
RE: Allowable Combined Stress in WRC-107 analysis
RE: Allowable Combined Stress in WRC-107 analysis
Thank you for letting me understand stress in nozzles better than before.
Now I read all replies again and I found that prex said bending stresses are considered as sencondary stress. But doesn't bending stress also depend on the circumstance to be classified into primary stress or secondary stress. For example, if a bending stress was generated by weight of a pipe connected to a horizontal nozzle, in my opinion, the stress won't be relieved by deformation. If so, this bending stress is primary stress. Isn't it?