Thickness Required Due to External pressure of LIMPET COIL

[c8829]

Hello Everyone,
Why thickness required due to external pressure for shell on which limpet coil welded has vast difference between two different cross section 1) Half circular 2) Box type as in App.9 ,Fig 9-5 (f-1).
In first case where Half coil limpet is considered gives very less required thickness as per Appendix.EE.
In second case with similar cross section area, as per App.9,Fig 9-5 (f-1) gives very higher thickness required.
Any explaination?
Thanks.

 

[CuMo]

The most simple explanation for this "phenomena" is that
round shapes "handle" stresses caused by internal or external pressures way better than flat shapes.
Hence why all pressure vessels have "round" shapes: cylinders, spheres, ellipses, etc.

 

[c8829]

For 3" Pipe
for first case required thk is 10mm
for second case required thk is 22mm