Annular Bottom Plate Thickness of API 650 storage tanks

[TheEverest]

Can anybody please help me find the annular bottom plate thickness as per API 650? Could not visualize Table 5.1. How do we calculate stress'b' and what exactly is the meaning of corroded t?

 

[JStephen]

First off, many tanks do not require annular bottom plates, if so, the table is not applicable.
Secondly, where annular rings are required, the thickness may be controlled by requirements of Annex E, Annex M, etc.
The "b" in "stress-b" is referring to the footnote "b" below.
I believe "t" is used for the actual as-built thickness, "corroded t" is that thickness minus corrosion allowance.
And t-d is the required thickness for product, t-t is the required thickness for hydrotest.
If the plate is ordered to exact required thickness, t and t-d would be the same.

Example: You calculate required thickness including CA as 0.95" including 0.06" CA, and calculate hydrotest thickness as 0.82".
Due to material availability, you actually use 1.00" plate.
Then nominal t = 1.00, corroded t = 1.00 -0.06 = 0.94", td=0.95", tt=0.82".
Product stress = (0.95-0.06)/0.94 x Sd, Hydrostatic stress = 0.82/1.00 x St, where Sd and St are the allowable stresses used to calculate shell thickness in the first place.
Then "Stress in first shell course" is the greater of those two quantities.

 

[TheEverest]

Thank you JStephen. I got the proper understanding of the product stress b of this table. But still I wonder why Annular plates are used only for tanks exceeding diameter 100 ft according to Annex M. Is the first shell attached directly to the foundation or via other bottom plates in tanks with diameter less than 100 ft?

 

[JStephen]

I haven't checked into the details. But the restraint of the bottom of the shell introduces some stress into the bottom plate. I assume that for smaller tanks, the lap-welded bottom plate is strong enough to resist this stress, and for larger, higher-stressed tanks, it is not.

 

[TheEverest]

Yeah, you might be true. Lap welded bottom plate shall be used for smaller tanks.