Actually in Caesar, no matter what the SIF Mulitpier is set to in the Configuration file, the full SIF is always used by Caesar for the Expansion cases and the SIF Multiplier is used only for the Sustained and Occasional cases for B31.3.
NozzleTwister,
Are you saying that for a fabricated tee, for example, that there is no stress intensification for Primary loads? This does not seem to be correct due to gross geometric discontinuities.
Primary loads are considered to be constant and not cyclic so ASME B31.3 allows you to ignore them when calculating your primary stresses since the SIFs really only apply for cyclic or fatigue loading such as thermal.
Of course if a system's contents are constantly loaded and unloaded then those primary loads become cyclic and you would want to use the SIFs and possibly review the system for fatigue.
NozzleTwister,
I fully understand about cyclic fatigue and that primary loads are generally not cyclic BUT your post seemed to infer that there is no stress intensication/concentraction for primary loads. With a tee the stress levels will be higher around the actual tee (in the crotch) than in the general pipe due to the structural discontinuity.
DSB123 it is important to NOT confuse our compeers who read these discussions. We don't want to confuse SIF's with "stress intensifiers", "stress risers", or "stress concentration factors" and the like ("Dave Peterson's factors"). SIF's were developed for the B31 Codes by Markl's cyclic loading experiments (loadings that produced cyclic bending stresses). The first time the SIF term entered the B31.1. Code for Pressure Piping was in the 1955 edition. It was intended to address cyclic secondary stresses due to thermal expansion/contraction - our first fatigue based code rules. This edition also introduces the "f" stress range reduction factor for use in calculating the allowable stress range. It was later that (in response to inquiries) that subsequent committees wrote Code Cases that mandated the application of a fraction of the calculated SIF's (but never less than unity) to the sustained pressure and weight stress calculations (longitudinal stresses only). For what it is worth, I personally have never been convinced that SIF's (as we now define them) have any place in calculating primary longitudinal bending stresses due to sustained weight and pressure. If we had cyclic fatigue test data that resulted from cyclic applications of internal pressure I would feel more comfortable with them but I think using the SIF's from cyclic bending test is a bit of a stretch - but hey, that is just one person's opinion.
It is interesting to note that the ASME nuclear piping Codes use stress multiplication factors (B1, B2 and C1, C2) that are roughly twice the magnitudes of the SIF's in the B31 Codes. Of course, the nuclear Code is calculating true elastic bending stresses and B31 Codes are not really doing that. That is the reason stresses calculated to B31 rules must only be compared to the allowable stresses of B31 (i.e., they are intended by the B31 Code writers to be used together).
Are the geometric and metallurgical discontinuities that result in stress concentrations in such weldments as fabricated branch connections (I reserve the term "Tee" for B16.9 factory manufactured fittings) important? Yes, of course they are. But they are addressed in the rules for pressure design and specifically in the rules for area replacement in FBC's. These must be applied along with the Code(s) minimum requirements for fabrication and examination. Of course, stresses in B16.9 fittings are not addressed by B31 Codes - that issues is left to B16.9 and the requirement that the fitting must be able to accommodate higher pressures than the matching straight piece of pipe (as demonstrated by "proof tests").
The stress calculation equations in the B31.1 and B31.3 Codes (YES!!! we finally have an explicit sustained pressure plus weight equation in B31.3 as a result of a recent Code Case - but look at it closely, it is "interestingly" different) really only address longitudinal pressure stress ((P * D) / 4t)). Your point is well taken regarding the higher stresses in the crotch area of the B16.9 Tee's (and you will notice that we (fairly) recently put new rules into B31.3 to address the differences in needed material thickness in bends at the extrados and intrados) but SIF's were never intended to address this issue. But then again, the B31 SIF's in appendices "D" are those of only one manufacturer's Tees and the crotch radii of other products (and their resulting SIF's) will vary. We say in the notes of Appendices "D", if you have better data, then use it - but document it.
It is also interesting to note that if you were to apply the B31 pressure design area replacement rules to B16.9 Tee's those rule would indicate that additional reinforcement is needed. B16.9 Tee's benefit from metal grain continuity and "blended" surface geometry and so there is less concentration (fewer areas with "stress risers").
I think that Nozzle Twister's point is that the stress concentrations do of course exist in the cited examples, but B31 Codes have not addressed them by SIF's per se.
And.........getting back to the original topic, for cryogenic systems all the standard B31.3 rules apply including the application od SIF's). The original B31.10 Cryogenic Piping Code Committee recognized early on that the only real difference in design was in material selection and that could be addressed by allowable stresses and other rules.
Hi JohnBreen,
I always like to read your posts with interest and thanks for the post. You may be slightly mis-understanding me or perhaps I was wrong to talk about tees. I was really talking about fabricated "tees".(i.e. Fabricated set-on branches). I still beleive that there is a stress intensification which should be considered for the sustained stress condition as there does not seem to have been sufficient work/investigation on this aspect. If a piping system contains fluid and it subject to both pressure fluctuations as well as emptying and filling then not only is the pressure stress cyclic but also the bending moments caused by the fluid weight cyclic.(Obviously more important on thinner walled systems) Granted this is not the general case but all I was pointing out is that you cannot generalise with the design of piping systems.
I note your point about the fact that the B31 Codes do not calculate the true bending stress levels (due to comparison with the Code allowables) I have come across a those using the results of a flexibility analysis (i.e. stress results for B31.1) to perform an assessment of the creep life used for a HP steam system.
As for the assessment of Cryogenic pipework. Another point worthy of note if the material is carbon steel.Normally the allowable stress range is reduced to 50% of that calculated in B31.3 to ensure that there is no chance of brittle fracture in the welds on the first cool down to the sub-zero temperature.
Yes, your opinion is also that of the majority of voting B31.1 Committee Members. It is interesting to note that whereas many of us would like to see some cyclic pressure fatigue test results, that was not (contrary to what I may have implied) the issue when the B31.1 and B1.3 "book" Committees adopted rules to include the SIF's to the calculation of stresses due to sustained pressure and weight. This from B31.1 is typical:
B31.1 Interpretation 1-27, February 23, 1981 (I still had hair then!!!)
Subject: Appendix D on Stress Intensification Factors
Inquiry: Is it the intent that the stress intensification factors shown in Appendix D of each respective Code be applied for sustained and occasional loads as well as for expansion loads?
Reply: Stress intensification factors (SIF's) listed in Appendix D of the various ASME B31 Code Sections are intended for design against fatigue failure and are to a large extent developed from cyclic bending tests of piping components. Therefore, the application of the SIF for cyclic bending and torsion loads is appropriate. Sustained and occasional loads may not be cyclic; however, it is the intent of the various Codes to provide adequate protection from component collapse. It has been shown that the SIF of 0.75i (but not less than one) found in ANSI/ASME B31.1 applied to sustained and occasional bending and torsional loads provides a conservative margin against component collapse.
What I read in Nozzle Twister's response is that he understands the intent of the various B31 Code Committees. The SIF shall be applied to the branch connection - 0.75 times the SIF (but not less than a product of 1.0 as a moment multiplier) for calculation of stresses due to sustained and occasional loadings, and the full 1.0 times the SIF for calculation of stresses due to displacement (e.g., thermal expansion-contraction) loadings. I think Nozzle Twiser was just commenting on how C2 handles the situation - C2 will always apply the full 1.0 times the SIF in the stress calculations for the displacement stress.
I was concerned because the original question seemed to ask if the rules for applying SIF's would be different for cryogenic systems. And, of course, the answer to that is "no". So, anyway, I think that we all three are in agreement with what the Codes have prescribed (albeit, I "muddied the waters" with my aside regarding cyclic pressure). Other areas (not SIF's) of the B31.3 Code were written to address cryogenic applications. Of course, it really point out how important it is to read all the rules - design (including material selection), fabrication, erection and testing - to get the "big picture" on the Code's requirements.
Well, methinks our colleague Essex57, got a comprehensive answer. Since this is a C2 discussion, he now understands how C2 will handle the displacement stress calculations, and he has been "warned off" not applying the correct SIF to the calculation of the sustained P + W stresses. More to the point I think we have provided the motivation to "do the right thing" buy replaying the "background" for our friend.
Interestingly, a companion inquiry (1-28, March 23, 1981) to the one cited above, asked if the Code intended to not include longitudinal stresses caused by effects other than pressure in the assessment of sustained and occasional loadings and the answer (interpretation) was "no". The Committee intended that ALL longitudinal forces and bending moments be included in this assessment. This is good to remember when looking at the new and "interesting" Code Case from B31.3 that gives us an explicit equation for calculating these stresses. The Committee followed that answer with their typical expression of frustration: "The ANSI/ASME B31.1 Code is not a design handbook that can be used without competent engineering judgement with regard to the design analysis of a piping system" to which I (finally) say "Amen"
Thanks DSB123 for your thoughtful participation. I hope we all stimulated some thought processes in our "community".
