Frangible Roof
Frangible Roof
(OP)
Dear Srs.,
Lets say I have a project with the following characteristics:
1 - 5 equal lined tanks with diameters of 16.3 meters and 26.4 meters in height.
2 - Type of roof = Dome Self-Supported Radius = 0,8 D
3 - Roof Thickness = 3/8 9,5 mm
4 - The tanks are connected at the top by a platform structure.
5 - All the tanks with internal floating roofs
6 - The tanks were designed with 0,8D dome roof with 3/8 thickness to adequately withstand the self-weigth and weights of the platform structure / internal floating roof.
Questions:
a Is frangible joint roof mandatory?
b Is it possible to design a frangible roof considering the project above?
c One way to obtain a frangible joint is when the roof plates are welded to the compression ring with a 3mm seal weld only, and when the roof plates are not fixed to any roof supporting structure. Considering the project above, the roof thickness is 9,5 mm and AWS recommends a minimum weld size of 3/16 4,8mm and also there is the platform structure which stiffens the roof. Then we conclude that considering a 3mm weld wont be effective, right?
Thank you.
Roberto
roberto@unitecnica.com.br
Lets say I have a project with the following characteristics:
1 - 5 equal lined tanks with diameters of 16.3 meters and 26.4 meters in height.
2 - Type of roof = Dome Self-Supported Radius = 0,8 D
3 - Roof Thickness = 3/8 9,5 mm
4 - The tanks are connected at the top by a platform structure.
5 - All the tanks with internal floating roofs
6 - The tanks were designed with 0,8D dome roof with 3/8 thickness to adequately withstand the self-weigth and weights of the platform structure / internal floating roof.
Questions:
a Is frangible joint roof mandatory?
b Is it possible to design a frangible roof considering the project above?
c One way to obtain a frangible joint is when the roof plates are welded to the compression ring with a 3mm seal weld only, and when the roof plates are not fixed to any roof supporting structure. Considering the project above, the roof thickness is 9,5 mm and AWS recommends a minimum weld size of 3/16 4,8mm and also there is the platform structure which stiffens the roof. Then we conclude that considering a 3mm weld wont be effective, right?
Thank you.
Roberto
roberto@unitecnica.com.br





RE: Frangible Roof
Generally, API-650 and the fire codes require either a frangible roof or emergency venting, but not both.
API-650 has specific requirements for a roof to be considered frangible, and you would need to go through the details.
It may be possible to show that a roof is frangible even when it does not meet the API criteria, but it would depend on the circumstances as to whether that was helpful or possible.
RE: Frangible Roof
RE: Frangible Roof
Is there any reference standard, guide, etc that corroborates the above statement?
RE: Frangible Roof
RE: Frangible Roof
RE: Frangible Roof
conditions (resulting from operational requirements, including maximum filling and emptying rates, and atmospheric
temperature changes) and emergency conditions (resulting from exposure to an external fire). Tanks with both a fixed
roof and a floating roof satisfy these requirements when they comply with the circulation venting requirements of
Annex H. All other tanks designed in accordance with this standard and having a fixed roof shall meet the venting
requirements of 5.8.5.2 and 5.8.5.3."
Err it seems to cover a lot more than just external fire....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Frangible Roof
The emergency venting and frangible roof provisions in API-650 are for external fire only. (And, the requirements for a frangible roof or emergency venting in NFPA 30, etc., anticipate the same condition.) The condition anticipated is something like in this video:
https://www.youtube.com/watch?v=6qcrwNM74sg
API-650 simply doesn't address venting or frangible roofs for the cases of a fire in a tank, vapor explosion in a tank, or detonation of the product in a tank, and that is not normally a consideration in tank design.
As to why, I'm not so sure. Perhaps there is too much variation in the circumstances to address, perhaps it would require a dynamic analysis rather than static to confirm proper operation. Also note that roof failure for that outside-fire case is going to be localized. The one case I'm aware of, of roof failure due to internal combustion, basically flipped the roof like a pancake on a 50' tank and is not something desirable as a "design condition".
RE: Frangible Roof
Exactly, API 650 does not address internal explosion. One good reference is the EEMUA 180 which offers some good guidance.