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API-650 Tank External Pressure

API-650 Tank External Pressure

(OP)
I'm evaluating an API-650 tank for external load, snow and external pressure.

The equations (V.7) are

DL + (Lr or S) + 0.4 Pe or DL + Pe + 0.4 (Lr or S)

My tank has a continuous 6" w.c. vacuum applied to it. When looking at the above loading conditions I'm replacing the 0.4 factor with 1.0 since in my mind the tank is always operated at the maximum external pressure, and could potentially see maximum snow loading in conjunction with the maximum external pressure.

Looking for a peer check,agree?

Thank you

RE: API-650 Tank External Pressure

Adjusting the factor upwards if necessary make sense.
However, if it is always operated at that external pressure, then the design external pressure should be some higher value.
There would normally be some margin between operating pressure/vacuum and set points on the relief valves, and then some additional margin beyond the set points to generate the required flow rates. Check the definitions and see which of those values is the "design" value.

You could potentially have situations where the tank is never under vacuum when completely empty, or never empty at maximum wind speed, etc., and those situations may not be addressed by the standard.

RE: API-650 Tank External Pressure

(OP)
JStephen, Thanks for the feedback. Just looked through Addenda 2 of API. Seems like they caught this too. The 0.4 factor got replaced with a variable Fpe which

"is a modifier for the design external pressure when used in load combinations with other variable loads. Value equals normal operating external pressure/design external pressure or a minimum of 0.4. Manufacturer to use 0.4 when not specified. (See 5.2.2.)"

Note: I think they mean that the value of Fpe*Pe shall be equal to the normal operating external pressure. Since Fpe is just a coefficient

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