Is frangible roof possible for tanks on f.7 of api 650

[Inamullah Khan]

hi,

I am designing 15.5 m cone roof tank with 5.6 kpa internal pressure. Tank is going on F.7, and client is asking for supported roof Below are my queries:

1- IF tank is on F.7, that means tank is uplifting, can roof be made frangible in this case.
2- Can roof be rafter supported type in case it is going on F.7.
3- F.7 means you also have to check roof on API 620, SO in that case can the roof be supported??
4- Are the formulas from F.4 and F.6 applicale if tank is on F.7,because the decision tree of Annex F, does not say so. In my case failure pressure(F.6) comes out to be less the design pressure specified by client, but in F.7 you need meet Failure pressure requiremnt??
5- as per my understanding compression area cannot be less than calculated in F.5, SO if maximum frangible area comes out to be less than area of F.5, the frangibility is not possible.


Regards,
Inam

[marian_g]

By API definition of frangibility, a frangible tank must have no uplift in case of roof failure.
It appears that a tank which has uplift in operational/design pressure (i.e. uplift before roof failure) cannot be frangible, because the tank shall be certainly subject to uplift at roof failure pressure.

For the rest of questions probably you'll get answers on eng-tips forum where you posted the same thread.