Allowable Combined Stress in WRC-107 analysis 1

[kyong]

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?

 

[prex]

The limits I normally adopt (for stresses in vessel wall) are:
- 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
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[juancito]

I agree with prex. But it has to be pointed out that you may have permanent (sustained) loads and thermal loads.
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

 

[prex]

Thank you juancito for correctly pointing out that thermal stresses will be secondary in (almost) all cases: it never occurred to me to include such stresses in a vessel calculation, but of course this may be a quite normal condition.
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
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From the point of view of the nozzle, external loads from piping due to thermal expansion and sustained loads such as weight and pressure are all treated the same. In gneeral, local deformation of the nozzle will not relieve the loads due to thermal expansion of a piping system, and membrane stresses in the nozzle and nozzle/shell junction should be treated as primary or local primary.

 

[kyong]

Chuck,

I think thermal stress is generally regarded as secondary stress. Am I wrong?

Kyong

 

It depends upon the circumstances. A secondary stress is deformation or strain controlled, self relieving. From the standpoint of stresses in the nozzle, local plastic deformation will not relieve the loads imposed on the nozzle due to thermal expansion of the piping system. From the standpoint of the piping system, the thermal expansion stresses are treated as secondary, although the word secondary is not used in B31 Codes.

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.

 

[prex]

kyong
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
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If you couple the vessel and piping analysis, make sure you consider the potential for elastic follow-up/plastic strain concentration.

 

[kyong]

Chuck and prex,
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?