Cylinder-to-cone Reinforcement

[VTechie79]

A reinforcing plate has been added to the outside periphery of the bottom cone head of an existing API 620 storage tank. See attached sketch. The client has asked me to verify that the modification shown in the sketch is good for a given operating temperature and pressure. I am trying to justify using the cone thickness and reinforcing plate thickness together as the available thickness. The cone thickness beyond the reinforcement will be calculated separately.

Here is my thought process...
[ol 1]
[li]When computing required thickness using unit forces calculations, it shouldn't matter if the thickness is a single layer or multiple layers assuming the resulting unit force is in tension and each layer is at least 3/16".[/li]
[li]The top layer (reinforcing plate) would be equivalent to a double fillet weld joint efficiency (E or JE) of 0.70 and the bottom layer (existing cone) has a 0.85 JE. Even though the combined joint configuration is not specifically covered in API 620, Table 5-2, taking the lower of the two JE (0.70) as the combined JE should be a safe practice. [/li]
[/ol]

My questions are...
[ol a]
[li]Given the aforementioned thought process, am I violating the letter or intent of API 620?[/li]
[li]What suggestions/comments/advice can be offered?[/li]
[li]Am I completely off base with this line of reasoning?[/li]
[/ol]

 

[JStephen]

You have two separate issues. One, is it adequate. Two, does it meet the API-620 standard. You could potentially come up with all sorts of arrangements that were structurally suitable that don't comply with API-620. That is, they're just not covered by the standard.
That being said, I'd be inclined to treat the lapped part like the lower plate wasn't there, figuring you'd never get them fit up where both plates had identical tension.
One of the API-620 requirements relates to the centroid location of the composite section, and when that extra plate is added, it may throw that off.