Minimum thickness for annular plate ring water tank 1

Hello Creed2023,

Table 4.5 of API 653 is for products with a density less than 1.
For density greater than or equal to 1 you must take into account paragraph 4.4.6.3.

In your case according to table 4.5 (density less than 1) for a stress <27000 and thickness of the first course <0.75". The minimum admissible thickness is 0.2".

kind regards

Since we are talking about water SG=1, then Creed2023 you are correct of the minimum thickness of 0.236" is required.

That's how it is. For density equal to or greater than 1, you must use table 5.1 of API 650.
In your case 0.236 is the thickness

Was an annular ring used because of the grade of your bottom ring shell steel, seismic considerations or for another reason in API 650? If it is not required, don't call it an annular ring but sketch plates cut into annular shapes. This will save you a lot of time, trouble and cost since they will be considered bottom plates. See API 650 5.5.1 and 3.1. If you are needing an annular ring per API 650, the thickness and width of new annular plates are related, see API 650 5.5.2, this can sometimes trip you up.

Note that API 653 4.4.6.2 and 4.4.6.3 are for tanks with annular rings for other than seismic loading. Was your tank needing an annular ring for seismic loading? If not seismic but for other API 650 reasons, as noted above you have to go back to API 650 table 5.1, which is why many annular rings are 5/16" or 3/8" - so that upon the first inspection there is more than 0.014" of corrosion allowed on 1/4" plate before you hit 0.236" thickness.

Note that API 653 minimum thicknesses are the retirement thickness, where the material needs to be replaced, while in API 650 they are minimum design thickness for new construction.[ul]
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