API 12F vs. API 650 and anchors

[Skipdeiter]

Using a 210 BBL, design pressure of 16 oz. from Table 1 of API 12F, OD is 120" and height is 15'. Floor is 1/4" thick, shells are 3/16" and roof is 1:12 slope and also 3/16" thick. I chose a 210 BBL as the floor and shell thicknesses would be equivalent to an App. J tank of API 650.

I am well aware the that the roof geometry b/n these codes are different, but my issue is with F.4.2, which is independant of any roof compression areas and slope. Applying F.4.2 to a 12F tank provides a Pmax of around 0.5 ounces. API 650 wants this tank anchored.

Can anybody shed light on the mentality behind the formula of F.4.2? A positive pressure (increase in design P) is available due to shell weight, roof weight (0.08 * th) and is decreased due to the wind, which leads to anchorage.

Assuming the tank is fixed at the floor, call it liquid level which stops any deformation of the floor, the tank is essentially a cantilever with the wind as a load. Stress due to wind on the shell are minimal in this scenario.

Does J.3.9 intend to ignore F.4.2 and simply worry about overturning moment due to wind?

This issue pertains to purchasers specifying API 650 tanks when API 12F will work.

 

[IFRs]

If there is net uplift due to pressure against the roof balanced by the roof and shell weight only (ignoring for a moment some donut of liquid), the shell will lift off the foundation while the bottom is held there by the liquid weight. This leads to bad stresses and strains at the corner weld. Ergo, anchors. You can use a thicker annular bottom ring so a larger donut of liquid is available to resist the uplift. Wind and seismic add additional overturning loads that anchors must resist if the tank is unstable resisting these loads. Note that wind acts horizontally and also vertically. The vertical component adds to the internal pressure and tends to pick the shell off the foundation while leaving the bottom there. Appendix J is not a design code - it is more of a cookbook for low stress, small tanks of very specific sizes. Good engineering judgment should be used when these tanks are placed where wind and seismic forces could affect the tank.

 

[JStephen]

API-12F tanks are not designed per API-650. What it really boils down to is that the API 12F/12D tanks are intended for oilfield service, and if one blows over, it's not the end of the world, whereas API-650 tanks are designed for more permanent use in more critical facilities. If you check overturning on a 55 gallon drum, it'll probably calculate out to be inadequate also.