Head with nozzle - high stress in FESU

[tomislav.begic]

Hello,

I have to check existing vessel's nozzle with respect to additional loads from pipe statics.
It seems that the nozzle is overstressed just from the internal pressure/temperature loads, excluding loads from the pipeline.

This (I believe) have to be wrong since this is existing pressure vessel.

1) Can someone check the FESU model, substructure "sub_003" and tell me if I did something wrong in definition of the model?

2) Is there a way to make FESU model more accurately since a head is connected to a shell with flange connection, therefore we have high rigidity on the boundary?

2022-01-13 11_59_58-EQ_191234_Apparatezeichnung.png

Thank you for your time!

[rohr2admin]

Dear Tomislav,

Have a look at the EN 13445-3, Table C2 for stress classification. The high stresses around the intersection area can be classified as "local membrane stresses", this is covered by the equation 03/04 SPL in ROHR2fesu. The utilization in this area is ~63%, so absolutly within allowable values. The global membrane stresses are <100%, so from the internal pressure you shouldn't have any trouble.
Regardind the SP-equation: Here Table C2 gives the information, that in the case of a formed head/ nozzle intersection loads from internal pressure and global loads, as well as stresses from thermal restraints can be cassified as "Q", which is covered by the euqation "SPQ" in fesu.
In the stress analysis for the substructures you can enable this checkbox to consider this:

Pb.png

If you do this, you also won't have any trouble in this equation.
Basically you already consider a rigid structure due to the boundary conditions of the substructure:

transition.png

On the vessel side I therefore would reccomend to extrent the sustructure a bit, as this boundary conditions might have an influence onto the deformation of the nuckle area of the head, better keep a distance of at least 1 or 2x Da, to avoid influencing on the results due to boundary conditions. In this case it probably won't change a lot, but still better. If you scale the results up in the result mode, you can see that this might be a unplausible deformation:

res1.png

Here a picture with extended substructre:

res2.png

Best regards
ROHR2support

[tomislav.begic]

Thank you very much for a fast and expert answer. It was very helpful.

However, I want to ask another question regarding your comment on extending the substructure.
Isn't it more correct (only in this case) not to extend the substructure further since I really do have a boundary condition here - a head is directly welded to the flange ring which is pretty thick?

[rohr2admin]

Hi,

yes you are right, I didn't look at that side of the vessel. I would agree in such case.

Best regards
ROHR2support