API 653 Repairs within the critical zone 1

[Andres Romero]

Hello, I have a question about the interpretation of paragraph 9.11.1.2.2 of API 653 on repairs in the critical zone.

What exactly does at the toe of the internal shell-to-bottom refer to?

I attach the paragraph:

9.11.1.2.2 A welded-on patch plate shall not be used if the covered bottom plate minimum remaining thickness at the toe of the internal shell-to-bottom weld will be less than the minimum thickness required by 4.4.5 and 4.4. 6 at the next internal inspection.

Best regards

 

[JStephen]

 

[Andres Romero]

Hello, but what does the entire paragraph refer to?
to the area of the lower sheet that is covered by the patch? or only to the area of the lower plate where the inner bottom-shell weld is located?

Best regards

 

[HTURKAK]

I have 5th edition and the same clause has different number.

9.10.1.2.2 A welded-on patch plate shall not be used if the covered bottom plate minimum remaining thickness at
the toe of the internal shell-to-bottom weld will be less than the minimum thickness required by 4.4.5 and 4.4.6 at the
next internal inspection.

So , the minimum remaining thickness is measured at the toe of the internal shell-to-bottom weld.



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

NEW ENGLAND MAXIM

 

[Creed2023]

So if the thickness at that area is not satisfactory then what can we do to repair to bring back the tank in service? replace the annular?

 

[Louisiana Inspector]

9.11.1.3 "The use of welded-on patch plates that do not meet the requirements of 9.11.1.1 or 9.11.1.2 is permitted if the repair method has been reviewed and approved by an engineer experienced in storage tank design in accordance with API 650". This can be done if your thickness is less than .100". Without an engineering evaluation your only option is to replace the plate.

Now, if you remaining thickness is .100" or greater you can put a welded patch plate.

 

[Dhurjati Sen]

The portion of the plate where the 'toe' of the shell to annular / bottom plate weld exists carries the entire load of the shell and so it is the critical zone. If its thickness at the next internal inspection (could be after 10 years) goes down below the minimum, patching is not allowed.
Then?
Replace the tank bottom.
Or?
Put patches / weld build ups and ensure bottom replacement is done before the scheduled next internal inspection.
But I would never approve the second option.

DHURJATI SEN
Kolkata, India