DOUBLE DECK FLOATING ROOF DETAILS 3

[gerrytt]

Meyers only has a cross section and a vague picture. Does anyone have CAD drawings or PDF files for a DDFR showing details for tank of diameter 300 ft (say), eg pontoon layout, location of support legs, etc, any information will be appreciated. The principle of design as per appendix C of 650 is understood, however looking forward to any advise, guidance or experiences.

 

[georginr]

IFR
Thank you, I'll keep you in mind.
You mentioned some reasons that I had not thought about!! From the information we have right now, I think the seal friction is what is making the roof hang; Since the original design was changed, from wiper seals to mechanical shoe seals,making it produce more friction.

We took immediate action, and right now we are in the investigation process, but to answer your questions; most of the oil was in the foam dam area and there were
some pools all around the floating roof. It does not go away and come back, it accumulates. This is the 3rd time, 1997 being the 1st. The tank is very active.

 

[IFRs]

Seal friction is an odd choice, because presumably it worked fine for years, and only now (1997) is trouble? Are you saying that each time this happens, it is cleaned up and does not repeat itself for another few years?

 

[georginr]

Yes, since the mechanical shoe seals were installed, it took 3 years for oil to appear on the roof. We cleaned it and in 3 years we faced the same problem and for the third time 2 years later. It seems that the mechanical shoe does not let the roof find its free point or in other words does not let the roof move freely as with the wiper seals. Does this make any sense?

 

[IFRs]

It almost sounds like there is a tight spot where the tank shell deforms inwards and binds the seal, holding down the floating roof but only once in a blue moon do you fill the tank to that height. If this is the case, I'd worry about filling the tank much beyond that elevation for fear of sinking the roof. Excess friction or seal binding can permenantly deform the outer edge of a pan floater which can (and has more than once that I know of) lead to sinking the roof.

Is it possible that these episodes involve filling the tank so high that the floater is rammed into the tank roof structure or foam deflectors?

Do you pig, blend or use mixers?

 

[georginr]

We use mixers. We do not fill the tanks TOO high to ram the floating roof to the structure.

 

[jay165]

Not sure if this is relevant, but at a refinery, we had an external floating roof tank with double wiper seals. We bought some waxy crude (Cintas?) that left a thick film on the inside of the tank as the roof went down. When the tank was filled back up, the upper seal wiped the oil off the wall and onto the roof.

 

[jcbb47]

Well, It seems I'm not the only one looking for details 'bout DDFR.

I've "figured out" some information on:

http://www.cbiepc.com/cbi/products/floating-rooftanks_design.html

It's just general information, So if someone of you just have more detailed informatión (I mean drawing, basis for design & sepecs) I would really apprciate to send it to me at:

jcbb47@yahoo.com.

I understand that structural design should be tied to tank dimensions, and sould comply with Appedix C of API-650. Anyway, I'm looking for basis theary about bouyancy that let me get a prelimiar dimension of the floating roof (in current stage of the the project I'm ivolved now don't let me wait for final verdor detials).

Thanks a lot everybody colleges.
Best regards.
jcbb47
BS Mechanical Engineer.

 

[IFRs]

A double deck floater will sink into the liquid about 5". Does that help? What specifically are you needing, design-wise?

 

[jcbb47]

Thanks for your attention.

Well, actually what I need is more theory (basis, fundamentals, principies, best practices, tipical geometry, etc.) that let me make a preliminar design (dimensioning).

However, It doesn't matter roof diameter ? If volume equivalent to this 5" under liquid is displaced by roof - then mass of roof divide by this volumne will be density of roof, and this last figure should be equal or less that fluid density on tank (Even when API-650 states 0.7 SG of fluid for design) (Arquimedes).

Whatever, all what I've said is just my opinion. That's why I need more theorical basis to support my design.

Design-wies would be a good for starting.

 

[IFRs]

jcbb47 - The drawing I sent you has the floating weight, you can use that to estimate the displacement. API650 has a section on the design requirements of floating roofs, but it won't help you do the engineering. You will need an engineer for that. Basically, it is a simple structure that needs to support the API design loads when floating or when supported on its legs. Perhaps contacting a manufacturer will help??

 

[IFRs]

jay165 - I would suggest that you switch to a shoe seal with wax scrapers.

 

[jcbb47]

Thanks a lot.

Excellent information. What I did is just make a calculation (spreadsheet) trying to estimate desck separation (roof hieght) in order to verify bouyancy (Total weight / Total volumen <= 0.7 x 62.4 lb/cu.feet). I've compare results with estimated weight on your informatión (300' diameter, 6 pontoons, etc...!) and results were a pretty good approach (just what I needed). I'll be improving (testing & comparing results) the spreadsheet before trying to make it shareware.

Once again, Thank you very much.

 

[tkequipments]

Georginr
Do you use pressurised nitrogen to clean your inlet and outlet pipes. This could also be the cause of the problem if cleaning is done when floating roof is in the lower portion of the tank.

 

[georginr]

tkequipments

Thank you, I am going to look into that!!

 

[carthago]

Hi IFR

could you please send me information/ typical specification of double deck and if possible other type of floating deck.

Thanks

 

[carthago]

Well guys , I am still waiting for some information regarding how to design very roughly roof decks.

I would like to exchange some ideas, spreadshhets, Mathcad
forms about API 650 and 620 Tanks

 

[JoeTank]

carthago,
I doubt anyone will share much about floating roof design. It a skill honed while in the employ of major tank fabricators. There are many sad stories about folks that have tried to design such items for the first time. Usually the problems that arise are due to not understanding the importance of how construction methods factor into the the design process. Anything you find published will be specifically vague on the really important stuff. It's quite an interesting topic and was a pretty cool thing to learn when I worked for a major tank building firm many years ago.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com