repair of a double bottom api tank

[lganga]

We have modified 10 years ago, our API tanks,by inserting internally, another bottom as second bottom.
So the tanks has the principal bottom beneath the second one.

We can only make the inspection of this secondary bottom, wich is welded to the wall with a fillet weld between the first shell and the second bottom plates .

So the only inspecction is a visual one and US thickness including a leaking test,by pressurizing the chamber between both bottoms. (mbars.equlizin the bottom weight)

We have one of the 5 courses with problems, and has to be replaced in its half circle.
Can anybody tell me if we can do this repair considering only the inspection for the second bottom only?.
Between both bottoms we have a romboidal expanded metal as a separator,and four 1" connections around it as witness. The gap between both bottoms is about 3 inches.

regards
Luis

 

[Duwe6]

Vacuum-box or dye-penetrant test the upper floor, and take enough thickness readings to be 'comfortable' that you have established minimum remaining thickness. Do NOT pressure up the space between the floors. This will put a permanent bulge in the upper floor, and might bulge the lower one too.

These, with the Visual, will cover the API-653 requirements, and are the only testing that is applicable. MFL or other floor scanning is not needed as the upper floor is protected on the underside. If the top side looks good, the floor is good.

 

[IFRs]

API 650 and 653 do not allow fillet welding the second bottom to the shell without the second bottom penetrating the shell like the original one did. Your tank is not in compliance.

 

[lganga]

IFR

That is correct it is not in compliance with API 650.

By API, the secondary bottom it is bellow the principal.

The bottom tank was settled 25 years ago on a concrete slab with piles . (appendix F tank).

Some one decided (10 years ago) a second bottom inside the tank, fillet welded to the shell

We have inspected all the fillet weld with PT and we did a leak test with air, only some buttweld between the plates has seen leaking an has to be repaired.

The problem with this design is that we can not inspect the principal bottom, That inspection implies a big job that will arrive to a cost that will be very close to a new one tank.

Thanks

 

[Duwe6]

It is understood by 650/653 that the lower bottom of a double-walled floor is uninspectable. Put that in the Inspx report and stop worrying about it. It is impossible.

 

[lganga]

DUWE

Exactly the lower bottom it is not possible to inspect.

The second bottom, according to API, should be located bellow the principal bottom (the resistant bottom ).
In the principal bottom, the shell lies 2" inside the external diameter of the bottom plate and, and as it is allways in the upper part of the seoncdary bottom, then the visual inspection of the resistant bottom is possible.

That is my concern about this design.

By contrary , this design, of a second bottom with a fillet seal weld around the shell.it means that this is not a full penetration weld,but it has to support the total load and the deformation of the shell.

Any way the bottoms are very close each other, with a metal support in between. That is an advantage..
i will try to recover it.

Regards
Luis