Mr.Delaforce,

I don't think API has a problem only with "frangibility pressure" but also with "1.5*pf" criterion when the tank is enough internal pressurized.

By note a. in table Table 5.21, this case is valid for special cases: "Failure pressure applies to tanks falling under F.1.3 only. The failure pressure shall be calculated using nominal thicknesses."

F.1.3 stipulates that "F.1.3 Internal pressures that exceed the nominal weight of the shell, roof, and framing but do not exceed 18 kPa (21/2 lbf/in.2) gauge when the shell is anchored to a counterbalancing weight, such as a concrete ringwall, are covered in F.2 and F.7."

In my understanding, F.1.3 refers to tanks having uplift in operational and/or design pressure. By API approach of frangibility, a frangible tank has the have no uplift in case of roof failure. It appears that a tank which has uplift in operational/design pressure (before roof failure) cannot be frangible, because the tank shall be certainly subject to uplift at roof failure pressure.
I would conclude that F.1.3 refers to non-frangible tanks, however the current interpretation is that the uplift case with 1.5×Pf refers to frangible tanks, isn't it?

And second issue: it is hard to explain which is the meaning of the uplift case with 1.5 × Pf.
I expect to have roof failure at pf, but still I can apply 1.5×Pf as internal pressure? That means to apply a 50% overdesign to a failure event...