API 650

In part 5.5 “annular bottom plates” – 5.5.2 there is a formula to calculate the radial width of the annular bottom plates:
At least 600 mm
215*tb/(HG)^0.5

But later on, in 5.11.2 in the second note, they refer to the minimum projection of the annular ring, with L (the greater of 450mm or Lb, not more than 0.035D).

Aren’t those 2 things the same? (With a different formula)

No, they are not the same. The computation in 5.11.2 tells you how much of the annular plate (whose width is computed in 5.5.2) must be outside the tank.

5.5.2 is clearly the inside of the tank (from shell to weld with the bottom plates).

But doesn't the text in 5.11.2 also say "inside":
... the minimum projection of the supplied thicker annular ring inside the tank wall, L, shall be the greater of ...

While you state its the part outside the tank. I thought the part outside the tank is described in 5.4.2.

Sorry, I was not near a hard copy of the Code when I answered (I won't do that again).

You're correct, it does appear that Sections 5.5.2 and 5.11.2 provide two different "minimum" requirements for the same thing. Note though that Section 5.11.2 is a much newer section of the Code when compared to Section 5.5.2.

Some years ago I found two different sections of the Code that computed the same quantity differently. When I questioned this (with someone more experienced), his reply was "Different sections of the Code are written by different people/groups. Why would you expect them to agree?"

You can write to API and ask for clarification on this matter. Perhaps they'll update the Code for the next Addendum.

As I know, this requested clarification is now API Agenda Item 650-711.

Regards.

Dear Gaya,

Referring to your question it is worth to say that 5.11.2 presents formulas valid for the particular case of Wind Load Calculation.
That is why for this calculation the liquid weight (wL) is the weight of a band of liquid at the shell using a specific gravity of 0.7 and a height of one-half the design liquid height H, exactly as 5.11.2 Unanchored Tanks says.
So the numerical coefficients in 5.11.2 formulas are already amended by this assumption.

Best regards.