API-650 Frangible roof

[Encho]

Hi everybody,

When I am determining the frangible roof area (A), I use the formula in 5.10.2.6.
Checking the roof to top angle compression ring. It is limited to details a-e in fig. F-2. Based on these details, I can determine the participating area for the shell (A_s) and the participating area for the roof (A_r).
Having these values I can subtract A-A_s-A_r and the result will be the area for the angle (A_a).

My question arises when the result for the A_r is negative. What does it mean?
I wouldn´t need angle for this kind of configuration?
For this case the participating area for the shell plus the participating area for the roof is exceeding the limit value for frangible joint?

Thanks,

 

[IFRs]

Whenever the total area exceeds the maximum area for frangibility, your joint is not frangible. If the area of just the roof and shell exceed the limit for frangibility, you can stop there. This does not mean you don't need a top angle, just that you won't have a frangible joint up there. The need for an angle or other structural member up there is from other sections of the code.

 

[Encho]

IFRs thanks for your answer.

Anyway I correct a mistake in my post:
My question arises when the result for the A_a is negative. What does it mean?

 

[fegenbush]

Encho,

Start with the definition of A_a which is the maximum allowable cross sectional area of the angle. If that value is negative, it is not possible to have a frangible roof for the tank and emergency venting must be provided by other means is required by the customer.

 

[JStephen]

Note that if a tank is anchored, the requirements are somewhat different.