API 650 Tank Question 1

[minorchord2000]

I am tasked with designing a tank to hold acid (sg = 1.84) and one of the design criteria I have from the owner says: "Tank will be rated for 6" water pressure and -.4 oz vacuum." Can anyone shed some light on what this means? I am a structural engineer, not a mechanical engineer. The tank is 8 ft diameter and 17 feet high. Thanks.

 

[JStephen]

6" water means the vapor space over the acid is pressurized to 6" water column = 0.2167 psig.
-0.4oz vacuum means the vapor space over the acid is subject to a partial vacuum of 0.4 oz/square inch = 0.025 psig.
See the chart in the start of Annex F for pressure design. It shouldn't make much difference in the design.
The standard vacuum is 1" water column, and you won't need to consider vacuum design in any detail.

 

[minorchord2000]

Thanks so much for that clarification.