For a vacuum tank with external half-pipe jacket: Can I credit the moment of inertia that comes from external spiral half-pipe jacket instead of adding stiffening rings on a cylindrical shell section (for UG-29)?

Shmulik,

In my opinion "No". My opinion is based on the following interpretation.

Interpretation: VIII-1-83-308
Subject: Section VIII-1, UG-28, UG-101, and Appendix 9, Spiral Welded Pipe
Date Issued: October 1, 1984
File Number: BC84-156

Question (1): Are the rules in 9-5(c)(2) applicable to half pipe jackets that spiral around the vessel?

Reply (1): No.

Question (2): Are the rules of UG-28 applicable in determining the required thickness of the inner vessel wall due to the pressure in the spiral half pipe jacket?

Reply (2): No.

In addition to that I also believe that rules of appendix 9 are not applicable where the jacket is made from half pipe. Rules of appendix EE may be used (in your case) only if both jacket and shell contain positive pressure.
Comments from any other member are requested.

_________________
Thanks & Regards,
Nasir
Welding Engineer
DESCON Engineering Limited.
Lahore, Pakistan.
Mobile # 0092-336-4145402