How to obtain correct foundation load when there is a negative Straight Flange input in hemispherical head Component Properties field when using AutoPIPE Vessel?
The hemispherical head in a pressure vessel is usually used for high pressure design and the designed thickness of the head is usually less than the thickness of the shell.
Due to different thicknesses, the hemispherical head is welded to the tapered shell section, at a level below (or to the left of) the reference tangent line (see image below), hence a negative value of the straight flange portion of head is specified.
In AutoPIPE Vessel (versions 40.03.00.08 and before), in the above scenario, where there is a negative value of the Straight Flange in the hemispherical head, the foundation load calculated is non-conservative, the value of foundation load reported by the program is less than the actual value. This is because the program inadvertently ignores the weight of the shell attached to the first hemispherical head in the foundation load calculations.
The vessel calculations are unaffected and only foundation load is affected. Also only the shell attached to the first head is affected and this issue occurs only in case of hemispherical head and not for any other type of head eg. ellipsoidal. This particular issue has been already fixed and the fix would be implemented for the upcoming release of the product (Connect Edition Update 4), scheduled in second quarter 2017.
Shown below is the discrepancy, explained by the images:
Foundation load with the defect (TFS-D640871)
Foundation load with the defect resolved
As can be seen above, the actual foundation load value (second image where the defect was fixed) is higher than the ones computed in the existing version of the software.
There are following workarounds to obtain the correct foundation load.
Workaround 1 –No need to remove the straight flange portion, insert dummy shells of length = 0 between each hemispherical head and shell as shown below (in the image below the changes are made to both head and shell junctions for consistency, however it is sufficient to make the change to only the affected junction which in this case is the lower one). It leads to have a minor issue on the weight corresponding to the weight of the straight flange which is not considered.
Note - this does not affect the vessel calculations or geometry)
Correct values of foundation load obtained:
Workaround 2– remove the straight flange of the heads, the foundation loads are very close (just a bit lower).
Note - this method requires to make change in the vessel geometry ie. removing the straight flange, so it is advised that this model is created separately only for the foundation load, and after the actual design being modeled in a separate model, (in the image below the changes are made to both head and shell junctions for consistency, however it is sufficient to make the change to only the affected junction which in this case is the lower one).
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