API 650 Design Wind Pressure 2

[xema77]

There appears to be a discrepancy in the design wind pressure for API 650 tanks.

-Section 5.2.1 defines the wind pressure on vertical surfaces to be 0.86(V/190)^2 or 18(V/120)^2 which is used in the wind overturning (Section 5.11) and Appendix F calculations.

-When calculating the stiffener requirements for wind (Section 5.9.7) and for external pressure in App V, it indicates that the wind design pressure is 1.48(V/190)^2 or 31(V/120)^2

Can anyone explain why two different wind pressures are used? Thanks.

 

[JStephen]

It looks to me like one factor is the average pressure over the projected shell area and includes a drag coefficient; the other is the peak pressure.

 

[IFRs]

The 31 psf is the classical stagnation pressure of air hitting a flat plate using the modification factors shown. I believe that the 18 psf used for wind stability includes an additional shape factor for cylindrical shells. Using the full stagnation pressure for shell buckling is reasonable because buckling is a local phenomenon, over a small distance the shell would feel the full stagnation pressure and the buckling pressure is perpendicular to the shell. For wind stability, using a shape factor is reasonable because the total overturning moment from wind is a global net pressure and round surfaces shed some of the wind. Note also that API uses a different net pressure for the cone roof which is a different shape.

 

[IFRs]

For what it's worth, AWWA uses similar factors, more explicitly though - 0.60 for cylinders, 1.0 for flat plates and 0.50 for conical. The form of the equation is the same, using 0.00256 (density of air) times wind velocity squared times modification factors.