Dears,

I have computed a branch SIF by FEA (FESIF); it is found to be much above B31.3 SIF for D/T <100 with run and branch lengths of 5D & 4D where D is the run pipe diameter, as my location of branch is having long lengths around the branch point. Which SIF will I choose - as suggested in B31.3 App-D as D/T <100 or from FEA, which is realistic ?

If I consider first, I will save in initial cost, losing in lifecycle cost, as the second one can give proper fatigue strength.

regards,
sam

Sam,

This is a good question. Here is the response from Tony Paulin from PRG (for those who do not know PRG are developers of FEATools and other FEA software for PVP industry, www.paulin.com).

SIFs are a function of the basic geometric parameters d/D, D/T and t/T, and are a function of the component “type”. We can address the issue more specifically if you can either send or describe the input. Barrel type fittings have t/T problems that can be confusing. Reinforcing pads include additional parameters, one of which (the pad width), isn’t included in the B31.3 SIF.

B31.3 does not really provide a torsional SIF and so the FESIF or FEATools i-factor output for torsional SIFs can be greater than 1.0, the default for the Code, and in this case the B31.3 calculated stress can be too low by the value of the i-factor. This is considered the single largest omission from the B31.3 stress evaluation, especially when the D/T ratio is larger than 50. A rough estimate of the torsional SIF is (io)x(d/D).

The problem may be “perceived” only when the thickness of the branch is larger than the thickness of the run. In this case the B31.3 SIF is somewhat “hidden” in the effective section modulus where the Code requires the section modulus of the branch to be calculated by the section modulus of the run. The approach used in the Code can result in stress estimates that are too low when t/T << 1. The stress estimates are probably reasonable when t/T>1, the actual SIF however should be found from (i)(t/T). (See WRC 329 section 5.0).

FESIF also has an iK calculator that will compare correlation results that give an appropriate value for the B31 Codes for comparison.

Larger D/T junctions can also be sensitive to the weld thickness and the length of the attached pipe. FESif permits the input of both of these parameters that are not considered in the Code SIF.

If you would like to post the branch connection type and geometric properties, a more specific comparison can be made.

In short, if your system is going to cycle more than a few thousand times, or there will be some other loading that can potentially cause fatigue failures, then the proper SIF should be computed and used, and the FEA approach corrects a number of issues identified in the B31.3 code equations for SIFs in Appendix D.

Regards,
Tony

-------

If you want us to review your files, send in your CAESAR II file and FEATool input file to the Intergraph Smart Support at http://smartsupport.intergraph.com/

Dear Mandeep,

Today 29/3/14 5 pm indiatime, I sent the problem with four nozzlepro input files to email "support@paulin.com" for Tony Paulin & You to review the same.

Ours is a large diameter thin pipe (D/T >100) low temperature application. Even for D/T <100, we are finding SIFs much higher in FESIF compared with ASME B31.3 App-D SIF; which one to choose for SIF input in Caesar-II analysis ?

When I asked for opinion from one engineer here in India, he told if you want a democratic answer, go for ASME APP-D - as majority will find it comfortable like a student appearing in IIT entrance test for IITs in India failing miserably passes with flying colors flashing a tribal card.

But, in real life, democracy can't replace the truth! So, we want a true answer - is D/T > 50 or D/T > 100 - which is the real cut-off for FEA derived SIF for a flexible piping application ?
regards,
sam

Sam,

I will ask Tony about the file. Since FEATools is product sold via Intergraph. We are providing 1st line of support. So, it should go through Intergraph Smart Support portal, the link is here.

http://smartsupport.intergraph.com/

I will check with Tony on your item.

Mandeep

Have you been able to get the answer?

Yes; I got the answer from PRG support on 2nd Apr,2014.

For d/D = 0.47 near to 0.5 limit of note 11 of B31.3 App-D & D/T = 83.9, out of plane SIF is much higher than B31.3 App-D,

So, the result is not surprisingly odd! Perhaps, we rely too much on WRC-107, 297, B31.3 App-D even beyond their area of applicability!

regards,
sam

here is the note 11 from B31.3 2012 edition

(11) The out-of-plane stress intensification factor (SIF) for a reducing branch connection with branch-to-run diameter ratio of 0.5 < d /D < 1.0 may be nonconservative. A smooth concave weld contour has been shown to reduce the SIF. Selection of the appropriate SIF is the designer’s responsibility.

This is a goo reminder to be aware of the limits of any analysis that we are performing.

At this time we still have this option for users to submit their CAESAR II models for translation using FEATools (using accurate bend SIF and flexibilities), along with a review, and comments by Paulin Research Group.

http://www.coade.com/caesarii/FEAModels.shtml

This is good way for user to some exposure to the converted model and get to compare the results before and after using the FEA generated SIFs and flexibilities.

For those who many not know what FEATools is:
FEATools (developed by PRG and sold by Intergraph) incorporates FEA generated bend SIF and flexibilities in to CAESAR II model automatically and generates a modified model. You can explore more about FEATools on our website (at this time the link is at http://www.coade.com/products/featools)