Floor Plate Projection distance problem

[KiwiTanks]

Can anyone tell me why API650 asks for a minimum 50mm (2 inches) floor plate projection from the outside of the shell?
I have a contractor who has ended up with 45mm (just less than 1-13/16") and hence the tank does not strictly comply with API650.

For tanks already in service, API653 would reign, and the minimum could then drop to 9.5mm (3/8 inch).

So what is the main purpose for the 50mm. Is it something to do with the HAZ? to allow for proper sealing? or just good amount?

Ideas anyone?

 

[MJCronin]

Kiwi..

I believe that I read that the minimum floor plate distance involves the resistance to rotation that is desired at the bottom of the shell. This slight roation occurs when the tank is full and varies with the tank design as well as the liquid SG.

If the distance is too short, then the tank is (slightly)more prone to rotation at the shell to floor joint.

Try posting on this forum

http://www.ast-forum.com

Regards and please tell us what you find out

-MJC

 

[JoeTank]

The chime provides distribution of the shell weight onto the earthen subgrade so may have to be greater than 2" sometimes. If it's on a concrete ringwall it is less critical. Either way the Standard now requires a minimum of 2".

Joe Tank

 

[gdubyuh]

If this is a shop fabricated tank, designed per App. J, J.3.2.3 still requires just the 1" + weld thickness outside the shell.
As a matter of consistency, I design all tanks (including App. J tanks) with the 2" chime.

 

[KiwiTanks]

This is a 19.5m dia tank constructed on an existing foundation. The spec and drawings all show "50mm min" but it's not what we've ended up with.

I think that originally the annulars were cut to the correct diameter (with no allowance) then the tank has been fabricated using the top down method (jacking the tank roof and shell strake by strake from the ground). The bottom strake of the shell may now be approx 10mm larger diameter than originally proposed, (hence the 5mm each side too small)

 

[JoeTank]

Kiwi,
Are you the tank fabricator or the poor inspector that is stuck with this problem?

Joe Tank

 

[KiwiTanks]

JoeTank,

I'm the client's designer / engineer (and also API guy), and noticed this when I went for a site visit after the tank has been constructed and was being painted.
The Client has their own staff monitoring the job, but obviously weren't checking these details. They are more involved in safety.
So I guess I'm kinda stuck with it. Not sure what (if anything) can be done about it. All comments welcome

 

[IFRs]

What type of foundation is there under the shell?

 

[KiwiTanks]

The foundation is an Earth mound with asphalt over-lay

 

[IFRs]

How extensive is the problem?

 

[KiwiTanks]

The floor plate projection (chime) is the same the whole way around the tank - 5mm short

 

[JoeTank]

Kiwi,
The 2" projection is just a nominal dimension. It's not based upon a stress analysis, it's just a practical figure. The recent API actions to make it 2" for all tanks with either sketch plates or annular plates was based on practical considerations and provide one set of rules, not two. I'd figure out a way to have all parties accept it. Trying to rework it would do more harm than good.

Joe Tank