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