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# API 650 Appendix F - max design pressure

## API 650 Appendix F - max design pressure

(OP)
I am checking the allowable internal pressure for a tank designed to API 650. The plant wants to add a nitrogen blanket and the process engineer wants this information in order to design the pressure-vacuum relief valve. I was using the equation in F.4.1, which includes the tank diameter, but does not give units for this variable. If I use 'feet' I come up with a pressure greater than allowable for API 650 tank construction. If I use 'inches' I get a rather small value (around 1 in-H20) which seems too low. So, what are the units for this calculation?

### RE: API 650 Appendix F - max design pressure

D is feet, P is in inches of water. Pressure will also be limited by F.4.2 if applicable. For a small tank, you may calculate a pressure higher than the 2.5 psi, but it's limited to the 2.5 psi to use API-650.

### RE: API 650 Appendix F - max design pressure

Don't forget to check the need for or adequacy of anchors when adding internal pressure, especially if the tank is not small in diameter...

### RE: API 650 Appendix F - max design pressure

(OP)
JStephen, IFRs,

Thanks for the responses.

Another question, in F.4.2 when calculating the weight of shell and structural attachments (for evaluating WDL), do you include the floor plate as well? I assume the intent is the total weight of the tank, minus the roof and any roof structure.

### RE: API 650 Appendix F - max design pressure

Floor plate is not included.

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