Operating Stress vs Operating Stress Range

Hello,everyone

I have a puzzlement when I read ASME B31.3 Appendix P.

According to Appendix P,

Eq.(P17a) to determine the operating stress,So
Eq.(P17b) to determine the operating stress range,SE



But Eq.(P17a) is the same as Eq.(P17b).

So what is the difference in the calculation of So and SE.

And the Code Stress when choosing "OPE CASE (e.g. W+P+T)" and "Stresses" is the operating stress(So) or the operating stress range(SE)?

B31.3 Appendix P is intended to remove the ambiguity involved in calculating the expansion stress limit SA as listed in equation (1b). In (1b), SA is a function of SL and SL might change because of nonlinear response of the piping system. (Is the +Y support active when evaluating sustained stress?) The allowed limit for expansion (and operating) in Appx. P does not reference SL (see (P1a)).

If you wish to use this Appendix, you must not only consider the usual expansion stress range but also include a check of the stress state for all expected operating conditions. Appx. P checks stress ranges AND stress states. For completeness, Appx. P provides equations for stress states (SO) and stress ranges (SE). You are right, they are identical.

The maximum of all these states and ranges will be SOM and SOM must be less than SOA.

I have interpreted Appendix P slightly differntly. I think "So" & "SE" are different. The formula used for computation for "So" & "SE" are same in appearance, however the magnitude of individual stresses (Sa, Sb & St), which would be used for computation of So and SE will differ. The probable reson may be as below:

"So" is the actual principal stress what is generated in the piping system under an Operating Condition, which is calculated as per formula P17a in Appendix-P. There may be various operating conditions that might be analysed. So you have set of "So"s (So1, So2, ... ).

"SE" is the range of stress between any two operating conditions. For example: SE1 = So1 - SUS, SE2 = So2 - SUS and so on.

Now if you see the magnitude of "Sa", "Sb" & "St" in computation of "So" and "SE", you can understand that the magnitudes would be different. Hence the values of "So" & "SE" will be different for a typical stress analysis.

With this, we have to compare the max. of "So" & "SE" against "SoA" as per Appendix-P.

Regards,
Kaustubh Joshi

Perhaps I see the confusion here...

If I run a load case defined as W+T1+P1 (OPE) using B31.3 in C2, the stresses listed in that stress report will be due to loads from this load case and no others. The formula used to calculate the stress is set by the catagory in parentheses (here, OPE). And the allowable limit for that stress is also set you the stress category. I would have to create the load cases or load combinations for all expected states and ranges to examine all the stresses referenced in Appendix P.

To properly utilize B31.3 Appendix P, a C2 user must confirm that all operating states (OPE) and expansion stress ranges (EXP) are below the Appendix P limits. Strictly speaking, that would also include the installed state of the system but any problems there should be picked up by the sustained stress check.

One aside on kvjoshi's range calculation...

There would also be an SE3, where SE3=So1-So2. This might only be significant if T1 is greater than ambient and T2 is less than ambient, or, when there are cold leg / hot leg situations.

Yes, I agree with Mr. Dave.

There will be sets of "So" & "SE" and user will have to formulate load cases for all the possible operating conditions.

However, this Appendix may be followed if the user opts for the same. Its not the mandatory requirement by the code.

So the SE in appendix P does the same job as displacement stress range calculation as per para 319.4.4 of the base code….

Having said this, we have higher allowable stress if we use appendix P for calculating expansion stress than we normally have by using eqn. 1a of para 302.3.5 viz SA = f (1.25 Sc + 0.25 Sh). Am I interpreting correctly?

Also the So and SE formulas of app. P contains the term Sa. Does this mean we are overriding para 319.2.3 c “the average axial stresses……”. My interpretation is that Sa here includes axial forces due to displacement strains also.

Pl. correct me if I’m wrong…

Eqn. 1a is a simplification of the allowable limit for the expansion stress range. Eqn. 1b, SA = f [1.25 (Sc + Sh)-SL], is a more accurate limit and is the default in C2. (We call it the liberal allowable but it is not really liberal, the other calculation is conservative.) You must know SL to apply Eqn. 1b. In the slide rule days it "cost" too much to use this. Also, Eqn. 1a could be used before supports are placed to see if there is adequate flexibility for expansion.

So, Appendix P is similar to Eqn. 1b, not 1a.

Yes, Appendix P includes axial stress while the base Code suggests it can be ignored in most situations. Once again, the base Code reflects the analysis capabilities of the day (slide rule) while Appendix P gives a nod to the PC.

C2 has a switch to "Add F/A in Stresses". Use this if you wish to include that average axial stress in the expansion stress range calculation. Note that in our next Version - Version 5.10 - C2 will provide additional options for this switch.

I am curious...

The base codes method is default in CAESAR II, Appendix P offers an alternate (hopefully more thorough review of some of theses loads) I wonder why these users are using this approach?

Client Specs?
Company Policy??
Curiousity???

or beats me thought I would try something new????

.......what is a "slide rule"?

Is this an allusion to Code Committees Members being olde guys?

Actually F/A was in the B31.1 Code for many years. It was only "recently" dropped (about 1972ish). Now it has shown up again in B31.3. Sometimes I wonder 'bout them boys.

John,

I am not so sure that Dave Diehl knows how to use a slide rule. I know I don't.

http://www.hpmuseum.org/srinst.htm

(Not verified, so only use it if it works).


Shannon
(who once owned a slide rule because she was pretty sure she was supposed to (and had a free one offered) but has never used it. It appears to have been lost in a move somewhere along the way.

A small observation in KVJoshi's response:

"So is the actual principal stress in the piping system...".

It is "stress intensity" i.e.twice the max. shear stress..