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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

RE: Frangible Roof

It depends.
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

If they are API 650 Annex H tanks they have circulation vents and do not need fragible joints.

RE: Frangible Roof

(OP)
If they are API 650 Annex H tanks they have circulation vents and do not need fragible joints..

Is there any reference standard, guide, etc that corroborates the above statement?

RE: Frangible Roof

See API 650 5.8.5.1 ?

RE: Frangible Roof

(OP)
The item 5.8.5.1 of API 650 is only for external fire. What about internal fire/explosion?

RE: Frangible Roof

"5.8.5.1 Tanks designed in accordance with this standard and having a fixed roof shall be vented for both normal
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 item 5.8.5.1 of API 650 is only for external fire. What about internal fire/explosion?"

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

(OP)
Thank you all for the responses!

Exactly, API 650 does not address internal explosion. One good reference is the EEMUA 180 which offers some good guidance.

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