Replace sectional of annular plate. 1

Pls have a look (Figure 9.13—Typical Welded-on Patch Plates on Tank Bottom Plates). What is the reason for replacing one or afew annular plates?




Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

The main problem as I see it is the heat affected zone just past the weld. You 'can' remove the corner weld and replace the plate under the shell, but the question is 'should' you do that, what are the risks and whose risk is it to take? The risks are mostly brittle fracture of the tank shell above the new corner weld. You can mitigte that with NDE and hardness testing but probably not eliminate the risk. I'm sure you understand that the tank shell, bottom plate under the shell and corner weld area are the highest stressed areas of the tank and also experience deformation under load and the design failure mode includes plastic deformation. You might be able to limit the problem to the transition areas at the beginning and end of the replacement plates by bending the plate up to meet the shell and fully suppoorting the annular plate with grout. Also I'm sure you know that when used per API 650 annular plates are required because of specific conditions that increase the stresses in the corner weld area, ie higher strength shell plate, thicker tank shells and tanks in more severe service.



All in all I would try hard to do as API653 suggests. The tank is a large, expensive piece of capital equipment that deserves a 'do the right thing' mentality and whose failure puts the public at risk.

To HTURKAK: in case you cannot use patch-plate due to maximum 24in in length for tombstone plate at critical zone.
To IFRs: I agree we shall follow the code API653 cutting 1/2in above corner weld, however if we do it when sectional replace, the tank shell will short a distance because the cut off area, then how can we re-weld the big gap between the tank shell and the new annular plate? A head up to you, we cannot do follow the attached figure in your reply.

Creed - the figure illustrates the deformations that occur when the tank is filled. The deflections are exaggerated but give you an idea of what is happening and why the corner joint is complicated and the area is called the critical zone.

If you are not replacing the entire annular plate and the owner accepts the risk, I think the best you can do is to taper the shell from 1/2" above the corner weld to the existing bottom where you remove the old corner weld. You have to remove 12" of the corner weld on both sides anyway, you can use another 12" to taper the shell up to the 1/2" above the corner weld elevation.

The annular plate will bend as needed, use wedges under the annular plate to force it up into the shell at the transition. You might consider hardness testing and UT of the shell where you remove the old corner weld. You have to evaluate the risk of doing it this way or removing the corner weld and not cutting 1/2" above the corner weld for this repair.

Where you have cut 1/2" above the corner weld, remember to fully support the new annular plate for its full radial width (as the original whould have been).

Evaluate the risk and choose a repair technique considering the tank dia and height, liquid stored, shell material, shell thickness, bottom thickness, foundation type and condition, length of service before a complete replacement, brittle fracture, stress due to settlement, stress due to shell bottom discontinuity, metal temperature, fracture mechanics, the extent and quality of NDE, etc.

I was expecting some data for the reason of replacement of some of the annular plates. If some of them need to be replaced, probably bottom plates are also in not good position. Considering replacement of entire basement could be an option.






Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM