Nozzle 2D shell analysis acc. to EN 13445-3

[TomoB]

Hello all,

I have a trouble assessing the FEA results of a set-in nozzle on a dished end.
Design pressure of a vessel p = 40 atu = 40,53 bar; temperature T = 100°C.

Due to changed pipe routing the existing vessel's nozzle should be checked against the loads from piping, but high stresses due to internal pressure lead me to further questions.

Analysis is done with ROHR2fesu module according to EN 13445-3 standard.
The method is based on 2D shell elements.

I do not understand how exactly the junction of a nozzle with a shell is defined in the FE model.
In my specific case/example, I have a nozzle/jacket according to Figure 1.

Figure 1

Question
From the cross section (Figure 2) it can be seen that the junction is made in the median point of a shell wall. That is why defined pressure of element No. 4998 is 0 bar(g). However, the nozzle element above, which is still located in the shell wall have pressure 40 bar(g).
Is this a wrong simplification and can the calculation be verified this way? What about welds on both sides of the jacket and the elements that are still located in the shell wall?

If I consider only internal pressure the nozzle is already overloaded, even if I classify these elements as local.
I am thinking about ignoring internal pressure loads for the area of the nozzle located in the dished end wall. Would this be the correct apporach?

Thank you in advance!

 

[FEA way]

I don’t know the capabilities of this software but in such cases, it might be a good idea to use solid elements for the junction itself - either with mixed mesh or with submodeling approach. Then you won’t have to worry about the inaccuracies caused by assumptions introduced with shell elements and you will still be able to use them for less critical regions of the model.

 

[TomoB]

Thanks for the answer.
Unfortunately, the software is very limited. This is an add-on to the base software which is intended for the pipe stress analysis.
It is mostly used for piping elements - circular, elliptic and oval shapes using shell elements - only shell elements.

 

[TGS4]

Then you have the wrong tool to perform the work. You will need either a different tool or you may need to contract out the work. If the latter, please ensure that the consultant/engineer is well-versed in the EN-13445 methodologies.

 

[TomoB]

I wouldn't say it is the wrong tool. The software is designed for this type of problems and it is common to use shell theory for this verifications.
I think it is more the question of an interpretation, knowledge and experience of a 13445-3 standard and of shell theory for this type of calculations (pressure vessel design).
Perhaps I should move this thread to another forum, Boiler and Pressure Vessel engineering.