Head Straight Flange Thickness

[waskillywabbit]

In designing the same vessel using several different PV softwares a question has arose.

Compress is designing the head straight flange using the total design length. It is just a small piece and it is attached to the head. So, the head will provide support. If you consider the analogy of a cylinder under external pressure it would fail in the middle of the unsupported design length. This is the weakest point.

The straight flange length should be considered when analyzing the attached cylinder for vacuum condition.

The head is also checked for external pressure. The straight flange thickness is more than the thinner formed thickness of the head.

Can you clarify Codeware's interpretation or code reference that justifies the procedure you are implementing?

Thanks.

Brian

 

[TomBarsh]

The fact that the head straight flange is only a small (short) piece is immaterial. The unsupported length "L" is between the lines of support as per Figure UG-28.1. The head straight flange will have a line of support at 1/3 depth "h" of the head and one at some other location (other head, stiffener ring, cone-cylinder junction, etc). Once the vessel is welded up there is no distinction between the head straight flange and other cylinders.

Unfortunately the Code does not seem to define "h" but they refer to the "bendline" in examples in Appendix L (Bednar text uses same term), which is the tangent line.

Consider this, what analysis would you perform on a vessel whose head had no straight flange (the vendor forgot it), a short 1.5" long cylindrical shell, and then several normal shell courses, then another shorty cylinder and misfabricated head. What is the difference if that shorty cylinder is separate or is integral with the formed head? Once it's all welded up there is no difference from the standpoint of structural response to external pressure.



"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support

http://www.codeware.com

 

[TomBarsh]

Actually, UG-33 (heads under ext pressure) defines "h0". This is the height on outside of the head as "measured from the tangent line (bend line)".

Presumably "h" is the corresponding height on the inside surface.



"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support

http://www.codeware.com

 

[TomBarsh]

Note, if the head straight flange had different thickness from the adjoining cylinder (or, for that matter, in any case where the cylinders are of different thickness) then it likely has a different buckling capacity than if both cylinders are of same thickness. However, the ASME Code does not get this sophisticated (currently).

Or if one can justify a "line of support" at a change in thickness (I imagine this would be difficult to do) then in COMPRESS one could specify a "pseudo-stiffener ring" to model that line of support.

"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support

http://www.codeware.com

 

[TomBarsh]

This unrelated issue may address your question as well, see thread1259-219855.




"Although this forum is monitored by Codeware it is not intended as a venue for technical support and should not be used as the primary means of technical support."

Tom Barsh
Codeware Technical Support

http://www.codeware.com