Pipe stress analysis : Stress intensification factor question 7

[jistre]

I'm running up against a problem here. I'm doing some piping stress analysis using B31.3. I was wondering if any of you knew of a place to find information regarding SIFs of dummy leg applications. Dummy leg to tee, bend, straight, etc. Any help would be much appreciated.

Thanks

 

[BigInch]

Are you looking for something that is not in B31.3 Appendix D?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[jistre]

Pretty much, BigInch. What I'm looking for is what SIF, if any, is needed on a tee which has the branch pointing up and has a trunion support welded on the bottom of it.

 

[JohnBreen]

Hi jistre,

It is a very good question and it has been discussed for quite some time. The SIF's that are found in Appendix "D" of B31.3 were adapted from the original work of A.R.C. Markl and a team of his colleagues at Tube Turns in the late 1940's and early 1950's. There are no Code provided SIF's for the application that you describe and what most of us do is use Finite Element method (FEA) to determine what B31 applicable SIF's we use with beam theory piping software. Tony Paulin's Pipe/FE software is a good FEA product for doing this sort of thing.

BTW there was a methodology for sizing dummy legs, base elbows, support trunnions and the like developed by Kellogg and used in their design offices years ago. That document is out on the Internet somewhere. It has been discussed in other piping forums lately and there seems to be some question about its validity. But it is still used.

http://www.coade.com/ubbthreads/ubb...&Words=dummy+leg&topic=0&Search=true#Post5450

Regards, John.

 

[jistre]

Thanks, John. That summary pretty much is what I was dreading would be the answer. I knew that the 31.3 SIFs were established way back, but I had a small hope that a more comprehensive set of them would be somewhere. Oh well, off to talk to the more experienced guys to learn how to determine a suitable SIF using the software.

jistre

 

[stanier]

With all due respect to Paulin and other FEA software providers and their sales I suggest that the ASME Committee license the software, do some modelling and extend the Table in ASME B31.3 for SIFs.

That way it would negate the need for the average piping engineer to buy, learn to use and deploy results of FEA. After all FEA can go horribly wrong if the boundary conditions are not set up correctly.

Wouldnt this be more effcient for industry?

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[JohnBreen]

Hi Stanier,

Well you know what they say about our Code committees: they are as steady as the rock of Gibraltar and they move just about as fast. I feel your frustration.

Actually, some good work was done by the companies who make weld-on branch connection fittings to extend the data. Lately Glynn Woods and Ev Rodabaugh have been doing some additional testing of full size components to do the same.
Some results of this testing are being evaluated by B31 MDC.

I think the Committees generally favor the actual test data as in some cases (some types of fabricated branch connections) the "perfect cylinder meeting perfect cylinder with perfect homogeneous welds and no HAZ" software "fabrication" solutions do not capture the real world and the SIF's are not exactly credible. So it is left to the individual piping engineer to develop his/her own SIF data. If it ever happens again that some piping component manufacturer wants to burn some excess profit I have a few suggestions as to how they could do it. And, if I ever will the Power Ball....


Regards, John

 

[proinwv2]

Am I missing something? SIF's are only of concern in fatigue or with brittle materials. Non-brittle materials with static or non-cyclic loading elastically redistribute stresses.

Paul Ostand

http://www.ostand.com

 

[unclesyd]

Here is tidbit on piping SIF's that you might want to take a look at.

http://www.sstusa.com/00julsep.htm#sep00

 

[JohnBreen]

ostand wrote:

“Am I missing something? SIF's are only of concern in fatigue or with brittle materials. Non-brittle materials with static or non-cyclic loading elastically redistribute stresses.”

Well, yes and no. Since unclesyd has gotten Ron Haupt into this discussion, read what Ron has to say about SIF’s used for non-cyclical sustained loadings:

B31.3 Interpretation Re. SIF Use in Sustained Stress Calculation

Should one use SIFs for SL calculation since the code does not address the issue directly? Someone had asked this question a long time ago and the reply was published as a code interpretation (see below). However, there are engineers who have interpreted the interpretation to mean that SIFs should not be used at all in the SL equation. Ron Haupt, our NQA Manager (code committee member), clarifies:

B31.3 Interpretation 6-03R, Question (2)

Question (2) of B31.3 Interpretation 6-03R, issued May 24, 1988 states:

Question: In accordance with ANSI/ASE B31.3, para. 302.3.5(c) when calculating the longitudinal bending stresses due to sustained loads, what stress intensification factors should be applied?

Reply: ANSI/ASME B31.3 does not address the application of stress intensification factors for longitudinal stress due to sustained loads; but see ANSI/ASME B31.3, paras. 300(c)(3) and (5).

The reply should not be construed to mean that B31.3 does not require stress multipliers of some sort in calculating sustained load stresses, only that B31.3 does not currently say anything about them. A careful reading of the B31.3 paras. referred to provides the only guidance in the matter that can be provided because an interpretation should only reflect what is currently in the Code, not what the B31.3 committee wishes was in the Code. Para. 300(c)(3) essentially states that the Code generally provides a simplified design approach. This is why finite element methods, inelastic analysis, or fracture mechanics approaches are not explicitly incorporated in the Code. Para. 300(c)(5) further states:

The engineering design shall specify any unusual requirements for a particular service. Where the service requirements necessitate measures beyond those required by this Code, such measures shall be specified by the engineering design. Where so specified, the Code requires that they be accomplished.

In the case of longitudinal stresses due to pressure, weight, and other sustained loadings (para. 302.3.5(c)), the Code uses a simplified approach to assure that a piping system will not collapse. Using competent engineering judgement (see the B31.3 Introduction) it is obvious that a bend or elbow will collapse more readily than a straight pipe and, thus, some sort of nominal stress multiplier should be used to penalize the bend or elbow relative to straight pipe. What that stress multiplier should be, the B31.3 committee has not yet agreed upon. In other words, at the present time it would be considered an unusual requirement, in B31.3 terms. (It is not unusual in B31.1 terms, which uses a stress multiplier of 0.75i in B31.1 sustained stress calculations to penalize components relative to straight pipe). Thick-wall piping systems, well supported, and containing few components susceptible to collapse may, by inspection, not require nominal stress multipliers to help in identifying locations where a collapse concern could exist. In general, however, the designer may just wish to automatically use some nominal stress multiplier to remove the need to apply judgement, which at times may be difficult to defend.

It should be seen then, in an indirect manner and the only way available to the B31.3 committee, that the reply to Interpretation 6-03R, Question (2), by referencing paras. 300(c)(3) and (5), requires more than just a nominal stress check. That is, it infers that a stress multiplier, if using a consistent simplified approach, or some other method to evaluate the collapse potential of the piping system should be used. Para. 302.3.5(c) is a required, but simplified guard against collapse; alternatively more rigorous methods (see para. 300(c)(3)) or competent engineering judgement (see the B31.3 Introduction) must be used to provide an adequate margin of safety against collapse. For example, if pressure plus weight (being the only sustained loading) stress calculations for a thick-walled, well supported piping system disclosed very low nominal stresses, further collapse considerations may not be necessary. On the other hand, if high nominal stresses were calculated, some further effort would be necessary to evaluate components that are susceptible to collapse to meet B31.3 Code requirements. This further effort could be anything from component testing, to the use of a nominal stress multiplier for components that relates to the collapse potential of such components, to the use of elasto-plastic finite element analysis methods.

Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (

http://www.ppea.net)

is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.