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frangible roof tank as per api 650

frangible roof tank as per api 650

frangible roof tank as per api 650

Dear Sir,

I have start working to design a tank of dia 7000mm and height 8000mm. There is frangible roof joint requirement as per data sheet. please suggest the design of tank and if possible please provide any ref. calculation and drawing.

Thanks in advance.

RE: frangible roof tank as per api 650


RE: frangible roof tank as per api 650

Dear JStephen,
I am new in designing tank as per api650. Please also suggest/advice the following points.

1. If material of construction of tank is titanium than how to select the stress values ( Sd, St ), because table 5.2a does not cover this material.
also min. yield strength and min. tensile strength.

2. As per my above question my tank has 7m max dia( given in data sheet)it means its OD is 7m, or we use 7m as ID. Please advice.
3. Since roof is frangible then it falls in clause as dia is 7m. but this clause does not has roof thickness calculation formula, what process we use to design roof. Please advice.

Thanks for your valuable suggestions.

RE: frangible roof tank as per api 650

Titanium tanks are not covered under API-650.
From a design standpoint, it won't make much difference if the nominal diameter is taken as OD, ID, or centerline of plate. That's a detailing issue.

RE: frangible roof tank as per api 650

Dear JStephen,

Thank you for reply. Please also advice or arrange the frangible roof design calculation and drawing.


RE: frangible roof tank as per api 650


I can answer your question regarding frangible roof design. A tank with a diameter of 7m (23'-0") cannot have a frangible roof per You will be required to provide a vent capable of relieving the emergency venting set forth in API 2000.

To illustrate this, calculate the maximum area permitted for shell-to-roof reinforcement per Then, calculate the contributing areas of your shell and roof per Figure F.2. Subtract those two values to determine the maximum cross sectional area of your rim angle. If the value you obtained is negative, it is not possible to have a frangible roof.

As for roof design, you have not specified if you have a self supporting cone roof (See 5.10.5), a rafter supported cone roof (See 5.10.4), or a dome or umbrella roof (See 5.10.6). If you specify where you are having difficulties, we may be able to be of more assistance.

RE: frangible roof tank as per api 650

fegenbush, I've wondered if that isn't a typo in API-650. If you go to paragraph (d), it requires you to design the anchorage for 3 times the failure pressure. But then it requires compliance with paragraph (a). But then item 5 in paragraph (a) that you reference appears to be based on the assumption of an unanchored tank. It looks to me like the intent is that it should comply with paragraph (a), items 1-4 only, but that isn't what it says. Any thoughts?

Static, notice that entire standard is written around carbon steel tanks and the typical behavior of them, and if you're using a material not included in the standard, you don't really know how that affects the design.

Also note that API and/or the welding research council have published some reports on roof frangibility that might be of interest if the issue is really important.

Also notice that in Appendix F, some of those terms that include roof thickness are figuring the weight of the roof plates, although it's not apparent in the equations, and would need adjustment for other densities.

RE: frangible roof tank as per api 650


That's a good question. The answer is that it is not a typo. The intent of frangible roof design is for the roof-to-shell joint to fail preferentially to the shell-to-bottom. This you may have already known. However, in order to achieve this, it is necessary to limit the roof-to-shell strength. So, item 5 was added to accomplish this.

You are correct that these items are based on an self anchored tank. However, the addition of anchorage does not significantly affect the shell-to-bottom joint strength with regards to rupture of the corner weld. The roof-to-shell joint must overcome the weight of the roof, the pitch of the roof adds strength, and there is a rim angle to strengthen this joint further. The shell-to-bottom joint is (reasonably close to) perpendicular, has the weight of the product stressing it, and has no rim stiffener (although the external projection of the bottom plate or "chime" has some stiffening effect). This causes the shell-to-bottom joint to be at a disadvantage.

The rationale behind paragraph (d) is to prevent the failure of the weakest part of the tank, the floor plates in bending. Without sufficient counterbalance weight, the pressure in the tank could cause the foundation of the tank to lift and rupture on of the single sided lap welds in the floor plates. As I mentioned before, these have little to no positive slope (and often have a negative slope) to help them resist any pressure. I do not know the origin of the safety factor of 3 with regards to uplift resistance.

RE: frangible roof tank as per api 650

My thinking goes like this: Suppose you have two identical tanks, except one has anchors for seismic. If the unanchored tank qualifies as a frangible roof, then the anchored tank of identical construction ought to, regardless of the anchor design. At the worst, you'd pop all the anchor bolts off, but if you're blowing the roof off in a fire, that's probably not a big concern.

RE: frangible roof tank as per api 650

Dear Sir,

Thank you for your support, i have few more questions that are given below. Please advice/suggest the answers since i m new in this, and off course
my design will be checked.
1. Design pressure is given 20mbar(g) and design vaccum is 10mbar(g), it means internal pressure (excluding static head) is 2KPa and external pressure is 1KPa. please advice.
2.in data sheet it is given that
Type of Head = Fixed cone roof
Frangible roof joint = yes
as per above answer i understood that frangible roof comes in clause. but my question is that how to know that roof is self supported or supported. please suggest.

3. in data sheet it is given that
type of bottom = single bottom
type of foundation = concrete foundation slab
slope from center(mm/m) = >1:100

my question is how to know annular plate is required or not because clause 5.5.1 gives information for only butt or lap joint annular plate.
since in data sheet it is given single bottom then it means no annular plate is required ????

Thank You

RE: frangible roof tank as per api 650


I suppose that's a fair assessment. I was coming at the anchorage from a perspective of an Annex F tank. It may also be an indirect way to direct the designer to certain features (e.g. a cone roof instead of a dome roof). I will definitely ask around about it at tne next meeting. Especially if it is something that others are encountering on a regular basis.

RE: frangible roof tank as per api 650

Using API-650 design, that pressure would fall under Appendix F and the external pressure would require the use of Appendix V.
The choice of supported or self-supported may be dictated by the purchaser or made by the designer depending on the situation. Usually, smaller tanks will have self-supported roofs, larger tanks will have supported roofs. For that diameter, self-supporting would be most common, but that would depend on the material, the allowable roof slope, and the erection equipment available.
For a standard (carbon steel) tank, an annular plate is required if mandated by 5.5.1, or by Appendix M for a heated tank, or may be used for seismic uplift per Appendix E or may be specified by the purchaser. If there is no requirement for it, it would not be used.
For a titanium tank, there are no requirements as it simply isn't covered by the standard.

I submitted a request for interpretation on the point, so we'll see what it comes up with. I've had some of these get answered fairly fast, some take forever, some just disappear and you never hear about it again, so no telling.

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