API 650 APP F

[sme75]

Appendix F gives guidelines for calculating the participating area of the roof and shell used to resist an internal pressure. The detail I am working on is similar to detail a shown in figure F-1. My question is if the vertical leg of the angle that is welded to the shell is longer than the distance wc, can more of an area be used to calculate the max allowable pressure? It seems that if the angle is attached to the shell at the bottom of the leg, that it would be reasonable to allow some portion of the shell adjacent to the attachment to be used. The angle can't fail without taking the shell with it. In my case the angle leg is 6" and the tank diameter is 52' so I am not trying to use a section of the shell far from the top, just between 6" - 9" (16t from the attachment point) I know the code seems pretty clear on this, I am just looking for some thoughts on the matter. Thanks

 

[JeffcoatSmitt]

Like you said the code IS very clear, but your argument does sound reasonable. There are a lot of 100' - 200' tanks in service and I'm sure their compression rings are bigger than the area of the shell that you are allowed to take credit for, so the API-650 committee must have it's reasons. The formulas for Wh & Wc have been used by both API-650 & API-620 for a long time. I don't know if I've seen the derivation of these formulas, but you might want to check Brownell & Young's "Design of Process Equipment" or API Publication 927 "Evaluation of the Design Criteria for Storage Tanks with Frangible Roofs" as a starting point for some insight.