Para 3.3 of API 650 12th Edition : - Allowable Stress limits

[Some Curious Guy]

Hi All,

I am relative new to Storage tanks. Why does API-650 Para P 3.3 limits local primary membrane stress and local bending stress to 1.1 Sd while allowing a higher limit of 1.5 Sd for membrane stress. Shouldn't it be the other way round. In pressure vessel we usually limit general primary membrane stress to S and local membrane stress to 1.5 S. Am I reading Para P 3.3 incorrectly. Please tell if I am missing something.

 

[HTURKAK]

You are reading Para P 3.3 correctly !!!!

Have a look to Para P 2.1

P.2.1 Scope
This Annex establishes requirements for the design of storage-tank openings that conform to Table 5.6a and
Table 5.6b and will be subjected to external piping loads. The requirements of this Annex represent accepted
practice for the design of shell openings in the lower half of the bottom shell course that have a minimum
elevation from the tank bottom and meet the requirements of Table 5.6a and Table 5.6b.....

And the following figure copy and paste from Structural Shell Analysis ( by Johan Blaauwendraad • Jeroen H. Hoefakker )

You see that , the membrane hoop stress due to liquid content height overestimated at the lower half of the bottom shell course...











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