Dears,

We have large bore thin piping subjected to relatively low pressure and high temperature. These are highest category stress critical piping requiring detailed Caesar-II modelling.

Due to D/T <100 limitation in the use of B31.3:App-D SIF & Flexibility factors for mitre bend and branches, the following four approaches are presented below. Please choose the right approach & justify with reason & practise followed.

1. Raise the thickness of the whole piping - straight pipe, elbow, mitre bend - all to meet D/T <100, although B31.3 pressure thickness calculation require much lesser temperature.
This is a costly, but fool-proof solution as material cost of piping and welding rises while providing a rule-based reliable piping.

2. Just raise the exact length of mitre elbow portion and branch -run and branch piping portion up to their zone of reinforcement in area replacement method to thickness meeting D/T <100, keeping the straight length just after these mitre bends and branch portions to thickness required for pressure thickness calculation. This is a pragmatic solution, knowingly violating the intent of the code with the grandfather clause that the piping designed with this method are remaining safe for decades.

3. To use FEATOOLS & FEPIPE software (FEPIPE for mitre elbow and FEATOOLS for branch SIF and flexibility), we have definite straight length requirements. So, use pressure design thickness all along the piping having D/T >100 & use user SIF & flexibility factor /branch stiffness derived from FEA in Caesar-II analysis. This is an approach which is technically safe and sound and economical in material and welding cost.

4. For spendthrift A/E firms not ready to buy licenses of FEA software, there is a hybrid approach where D/T <100 thickness can be extended to 3.5/4.0 D straight length after mitre bend and run/branch portions of branch connection and use B31.3 App-D SIF & flexibility factor. Here, A/E firm will save cost, causing somewhat high material and welding cost. Also, the basis is hybrid in nature - in between design by rule & analysis.

Now, what should be right way from the stand point of the owner ? I need an economical & code conforming approach from you all! I am sure that I will not be disappointed.

regards,
sam

This is not what you wanted but I figured you should get some response.

Hire a consultant. Let the consultant convince you of their approach.

Here's what B31.3 paragraph 300(c)(3) says:
A designer capable of applying a more rigorous analysis shall have the latitude to do so; however, the approach must be documented in the engineering design and its validity accepted by the owner. The approach used shall provide details of design, construction, examination, inspection, and testing for the design conditions of para. 301, with calculations consistent with the design criteria of this Code.

It comes down to owner acceptance.

When browsing this history post, I would like to know whether there was anybody that had experienced practical projects applying the Approach No.4 mentioned above, because by this means the thinner wall thickness (satisfying pressure design formula) can be applied for the main straight-run header, with only branches and fittings , and their adjacent straight pipes using thicker wall thickness (satisfying D/T<100). By this means the cost of large-diameter pipes (e. g the very long-run large-sized unloading pipeline in LNG Terminal, for jetty head to storage tank) can be significantly saved.

Thank you.

I think use FEA method is the best way to solve this problem. Found related articles
https://www.whatispiping.com/how-to-use-...stress-analysis
https://www.whatispiping.com/stress-analysis-of-lines-having-dt-ratio-more-than-100

I personally recommend utilizing NozzlePro for calculating SIFs and flexibilities for fittings, and inputting that back into CAESAR.

I would also recommend looking at local forces and check for buckling at high axial load locations.