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# Spherical API-620 Tank Free Body Diagram

## Spherical API-620 Tank Free Body Diagram

(OP)
thread809-140665: Need equation clarification - API 620

Hello,

I'm tasked with re-rating some API-620 tanks. I would love some clarification in regards to the free body diagram and force (Pg,Pl,W) sign conventions.

Can someone show a better sketch than what is already in the code for a sphere that is ground supported and full nearly to the top with liquid ?

### RE: Spherical API-620 Tank Free Body Diagram

Since you've pulled the top portion of the sphere up in your diagrams I assume that's the free-body diagram you're interested in.

As shown in API 620 Figure 5-4 for "Panel b", the pressure P will be upwards. Since gravity is downwards, W/At will have the opposite sign to P.

PS : Think carefully about the shell just above the ground. There will be additional loads/stresses in this area not calculated by the simple formulas in clause 5.10.

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