API 650 Tanks - bottom plates deformations 3

[Allcciinneeii]

Hello!
We have faced some issues during site erected tanks when it comes to bottom plates welding. Our tanks which are erected at site have their bottom plates welded by butt joints with the usage of permanent backings strips. Even taking good care during welding, considering proper welding method and sequence, it´s almost impossible to avoid some deformation of the bottom plates. We normally map those deformations after finish welding, measuring peaks and valleys in different regions of the bottom plates. My question is how much we can accept in terms of deformation of the bottom plates. I couldn´t find any specific tolerance for that in API 650.

 

[IFRs]

The bottoms for API 650 tanks are supposed to be fully supported by the foundation. As such there is very little stress in them during tank operation. Most bottoms have wrinkles in them which are totally inconsequential and the bottoms flatten out when uniformly loaded by the product. API 650 does not have tolerances for the flatness of the bottom because it does not matter.

 

[JStephen]

From AWWA D100-05, "Out-of-plane distortion equal to 1 percent of the tank radius is considered acceptable."
While that's not an API requirement, that should be a measure of industry standard.

 

[HTURKAK]

I have copied and pasted relevant API 650 sections;

7.2.2 Bottoms
7.2.2.1 After the bottom plates are laid out and tacked, they shall be joined by welding the joints in a sequence that the Manufacturer
has found to result in the least distortion from shrinkage and thus to provide as nearly as possible a plane surface.


7.3.6.8 Internal bottom elevation measurements shall be made before and after hydrostatic testing. Measurements shall be
made at maximum intervals of 3 m (10 ft) measured on diametrical lines across the tank. The diametrical lines shall be spaced at
equal angles, with a maximum separation measured at the tank circumference of 10 m (32 ft). A minimum of four diametrical
lines shall be used.

Apparently , API 650 specifies the procedure how to record buckling of the bottom plate however, it does not specify any tolerance for the same.
Make measurements after hydrotest , and you may see ,significant amount of the distortions will disappear. What is the type of foundatiom? ring type?

 

[JStephen]

I assume that is intended to measure bottom settlement, not weld distortion in the bottom.

 

[HTURKAK]

I assume that is intended to measure bottom settlement, not weld distortion in the bottom.

Yes , what you said is true from a certain point of view.

I just shared my two penny experience (based on 40+ years experience ) and knowledge.

According to the best of my knowledge ,
The settlement of the tank during hydrostatic testing is measured from the exterior of the tank. For this purpose ,the initial level of anmular plate projecting outside the tank shell is recorded . Tank settlement measurements carried out in general at seven steps (empty, 1/4H,1/2H,3/4H,H,1/2H, empty). The difference between max. settlement (at test water H level) and after emptying will be the elastic rebound of found. and rhe difference between initial reading and final reading will be plastic , permanent settlement. (Total consolidation settlement is a different story..).

IMO, the initial measurements of bottom from the inside will include also buckled level of bottom plate (if exists). The measurements after completion of test in general will include plastic soil settlement and fixed level.

 

[Allcciinneeii]

Hello again! Thanks for the comments!
The foundation is normally ring type.
I´m talking about weld distortion indeed, not bottom or foundation settlement.
As I could not find any applicable tolerance, except for this new information coming from JStephen "From AWWA D100-05, "Out-of-plane distortion equal to 1 percent of the tank radius is considered acceptable.", sometimes we had some difficulties to convince the customer that the tank bottom plates after welding were approved or rejected. There was one case where the customer decided to inspect the bottom plates using a small hammer, and while hammering, if they found a "hollow" sound, they said that there was a space between the foundation and the bottom plates, and decided to reject the tank. I mean, when we don´t have a specific tolerance for something, it creates opportunity to approve or disapprove anything based on any inspection method.

Thanks once again! Feel free to add new comments!

Best regards,
Alcinei Nery

 

[IFRs]

I would ask them what standard are they using to evaluate the construction, if they could share that document with you and if they could show you where in your contract this is mentioned. If there is no technical or contractual basis for their decision and your work meets the requirements of the codes specified in your contract you should be good. I would be very hesitant to bring another standard into play unless you want to change the contract and if you are 100% sure it won't open a can of worms no one needs, now or for future projects. .

 

[HTURKAK]

The foundation is normally ring type.
I´m talking about weld distortion indeed, not bottom or foundation settlement....

API 650 at dimensional tolerance section, '7.5.5.2 Where foundations true to a horizontal plane are specified, tolerances shall be as follows:
a. Where a concrete ringwall is provided under the shell, the top of the ringwall shall be level within ±3 mm (1/8 in.) in any 9 m
(30 ft) of the circumference and within ±6 mm (1/4 in.) in the total circumference measured from the average elevation.'

This statement implies tanks constructed on concrete ring wall foundation should have a buckling tolerance above this value to accommodate distortion of the bottom plate due to welding. The practical welding distortion tolerance based on experience for tanks with concrete ring foundation is (1/2”) in.