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Earl   AST Forum  

Center pole bouyancy

Tuesday, January 03, 2012   By: Tankee [235] 4 Stars
Someone recently told me that if a 24 diameter center pole didnt have mouse hole cutouts in the bottom, there would be a buoyancy issue when the tank was filled. I understand the dynamics and physics of this statement but is it an actual relevant and valid concern when the pole is weighted down by large I-beam rafters and the cone roof material on top of it? Is it more of an issue of applied stresses to the roof and rafter connections, and not an actually worry about the center pole floating out of position?

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Wednesday, January 04, 2012   By: DavidR@Fast.net [39] 5 Stars
Hard to believe it will be a problem, but easily calculated.

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Thursday, January 05, 2012   By: CNT [492] 4 Stars
Buoyancy can throw my head for a loop sometimes, but this is what it looks like to me:

2 dia x 52 tall tank neglecting wall thickness -- Volume1,222 gallons.
1 gal. water @ 8.34 lbs -- Displaced weight Buoyant force 10,200 lbs. H2O -- 7,344 lbs .72 Sp.Gr.
Assume the center column is actually 55 ft long.
Std. wall 24 pipe weighs 94lbs/ft and Sch. 40 weighs 171lbs/ft not sure what schedule would be used for a center column. So the pipe is going to weigh at least 5,190 lbs and maybe 9,400 lbs or more.

Even with the thinner std. wall pipe, I would think your downforce from the rafters and such would be more than 5,000 lbs. Using a heavier wall pipe, nearly negates the buoyant force all together.

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Thursday, January 05, 2012   By: CNT [492] 4 Stars
Regardless of the buoyancy question, I think mouse holes are a good idea. Nice big ones. The issue is with the potential for pin-holes in the welds. If there is a leak, the center column could be full of fuel and it cant be tested. There is also potential for fumes to get into the center column from the top. Now it seems to me that there would be a hazard in your tank and it would be potentially dangerous to weld attachments to the column as is common to do during construction/repair work.

With the mouse holes nice big ones and at least two please the column can be washed out, sniffed, and proven to be free of liquids and vapors.

You want at least two holes and they need to be big enough that they wont clog with gunk and that they can be washed out. Two holes lets the cleaners wash from one side and have the gunk and liquid flow out the other. 1 to 1.5 mouse holes should be good. I have found anything smaller than 1 leads to clogging problems, in which case you know you have liquids and vapors trapped, but cant get them out and purge holes or other mouse holes have to be air drilled.

I will acknowledge that there are arguments against mouse holes that are probably just as sound as my arguments for mouse holes. Arguments against typically assume the base plate weld is good, and keeping the inside of the column dry and clean is better than getting it wet and gunked up. Ive dealt with refined products. In crude and asphalt service, mouse holes might be a bad idea.

Ive run into enough columns where the base plate weld has had a pin-hole in it and product leaked out during cleaning, that I have gone to insisting on mouse-holes to avoid the unexpected cleaning/gas-free delays.

Then there is the discussion of Pipe versus Structural Columns. Structural eliminates the whole mouse-hole debate, but causes floating roof seal issues that you may or may not have anyway with a pipe column.

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Thursday, January 05, 2012   By: DavidR@Fast.net [39] 5 Stars
Some folks put couplings and plugs in the column to have it dry inside but have a way to check it also.
Columns with holes have emission issues when there is a floating roof.

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Friday, January 06, 2012   By: Tankee [235] 4 Stars
We are going with mouse holes and there is not going to be a floating roof. We just have a big corrosion issue and I was concerned about inside the center pole since we cannot properly apply the lining inside. We will be lining the tank internally and have a nitrogen purge blanket. I got them to go to a thicker schedule pipe, but I suggested no mouse holes because of the corrosion issues and that was when the foreman mentioned the bouyancy issue. I figure we could get the inside coated, but it would probably be costly and tough to verify. I have seen the problems of leakage inside a supposedly seal welded center pole I got a mouthful of crude when I drilled a hole in one to check. Appreciate the comments!

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Friday, January 06, 2012   By: BABRTs 973 [53] 5 Stars
Sch 80 Center column, welded to a 1/2 inch thick base plate, with the rafters and the roof holding it down.
I have not done the calculations but somehow I do not think bouyancy will be an issue.

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Saturday, January 07, 2012   By: LL [391] 1 Stars
Ive installed centre column 12 pipe on 26m height x 12m dia tank since 1999. So far its been working well and no bouyancy issue.

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Published on ast-forum.com


Atmospheric Design Pressure
By: Fathi [46]

The whole story is to avoid brittle fracture=

Test temperature shall be equal or warmer than Minimum Metal Design temperature at Test pressure.

I would limit my minimum testing temperature for a tank (or even for a vessel or a piping system) by the =

Temp = maximum of [ Min(lower shell) , Min(shell# 2) ....Min (Upper shell) ]

For a Shell# i the minimum is function ( of shell tickness , Material group ) and this can be found in API 650 , Fig 2-1

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