How To Calculate The Thickness of Miter Elbow (SS316)

Hi?

I would like to know the related codes & standards or calculation method to decide the Thickness of Miter Elbow to be fabricated by SS316 inside biological aeration water tank as shown on the figure below.

At first, I've calculated the Thickness of Miter Elbow by using ASME B31.3 Par. 304.1.2 Straight pipe under internal pressure and 304.2.3 miter bend.

However someone challenged that the applied ASME B31.3 is wrong application since the external force created by water flow to the miter elbow will be existed.

I've tried to find the calculation method to consider both the external force and internal pressure in miter elbow. But I've failed to find it.

So I request your help to this matter as follows.

1. How to decide the thickness of miter elbow? [see No.2 in figure above]

2. Please advise whether the stiffener at neck of miter elbow is necessary. And also please inform me the related stiffener sizing method. [see No.3 in figure above]

Thanks