Annular sketch plate lap 1

[hamidun]

Hi all,

I am working on a Storage Tank design calculation as per API 650. I am currently stuck with Annular Bottom Plate width.

In API, it is stated that 5.5.2 "Annular bottom plates shall have a radial width that provides at least 600 mm (24 in.) between the inside of the
shell and any lap-welded joint in the remainder of the bottom. Annular bottom plate projection outside the shell shall
meet the requirements of 5.4.2
In 5.4.2 "Bottom plates of sufficient size shall be ordered so that, when trimmed, at least a 50 mm (2 in.) width will
project outside the shell".

And I got the drawing reference attached in this thread. What becomes my concerns are,

1. How the drawing finds the length of L which is Annular sketch plate lap as in API 650 I have so far not found any statements that mention about Annular sketch plate lap length.

2. Does the drawing, draw/translate the statement 5.5.2, 5.4.2 correctly?

3. Or can I just ignore the annular sketch plate lap in my calculation? And just go with formula given by the standard?

Thanks for any help!

 

[JStephen]

The lap onto the annular plate is just another lap, see 5.1.3.5.
I usually figure the 24" dimension to the toe of the weld, not to the edge of the plate.
I usually round dimensions up some, and/or add some extra on, for field tolerances.
And I usually adjust width of annular plates to match the plate I'm cutting from if convenient.

 

[hamidun]

JStephen,

So, in general this sketch plate lap is designed basically based on the need of the tank designer? Or we can follow 5.1.3.5. Do you translate "any lap-welded joint" as "the toe of the weld"?

IFRs,

You mean the foundation? Yea, I am using cone down foundation. What? "The annular plate is on top of the bottom plate"? Isn't the annular sketch plate lap is on top of annular bottom plate?

The attached file is my current drawing of the annular bottom plate. I really appreciate any comment on my drawing.

Thanks!!

 

[IFRs]

Sir -
I'm used to the term "cone-down" meaning that the middle of the tank bottom is lower than the outer edge. Lap joints should be arranged to drain like roof shingles. The lap as shown in your image is for a "cone-up" bottom - high in the middle relative to the shell. It otherwise accurately represents a proper bottom lap and annular plate size. For more guidance on if the 24" is to the toe of the weld, please see section 5.5.2 where L is defined.

 

[JStephen]

The API section referenced gives minimum lap dimension as installed. This is usually detailed larger as IFRs indicated, to allow for tolerances in size and placement of plates, etc.

 

[hamidun]

IFRs,

Oh yea, I need to bend the annular sketch plate lap in the drawing as in the drawing of the standard, it is also bent.

Well, in section 5.5.2, this "in the remainder of the bottom" line makes me a bit confused, what is the remainder, as if we translate any lap-welded joint as toe of the weld, then there is no more remainder of the bottom, am i right? So, the L is still not defined, only if as you said in section 5.1.3.5, we can define the L = 2 in or 1 in Correct Me If I am Wrong!

I just want to make sure that I don't get the translation wrong.

 

[IFRs]

5.5.2 defines L as "the minimum width of annular plate as measured from inside edge of the shell to the edge of the plate in the remainder of the bottom, mm (inch);" ( my emphasis )

To me this is pretty clear - the weld is not part of this length.

L is the greater of 24" and the result of the equation in this section.

The Lap is defined in 5.1.3.5 and is 1 inch maximum for single sided lap joints which this is.

API 650 does not use "L" to represent "Lap" and this may be a source of confusion for you.

I don't usually see annular plates "bent" to make the lap drain properly, for a cone down to the center slope I'd simply lay the annular plate on top of the bottom plate, or shove the bottom plate under the annular plate, etc. Bending the annular plate introduces stresses you don't want. In addition, according to API 650 "the foundation must provide uniform support under the full width of the annular plate" ( see the footnote in table 5.1 ) and bending annular plates adds complexity to this detail.

 

[JStephen]

IFRs-
I hadn't noticed the wording in 5.5.2, which clarifies that.
Section 5.1.5.5 gives the 12" minimum between 3-way laps and per Figure 5.3d, this is measured toe-to-toe of weld, so I was assuming the weld clearance at the annular ring would be measured similarly.
In looking at this just now, I see a different definition of L in E.6.2.1.1.3 which requires 1.5 ft minimum, etc., which seems to be at variance with the section above.

 

[hamidun]

IFRs,

As I mentioned in my first post, actually my concern is about how the author of the attached file (my first post) find the L? And I think you have answered that with 5.1.3.5.

And what is "in the remainder of the bottom" bottom plate or annular bottom plate? As if the L is "measured from inside edge of the shell to the edge of the plate in the remainder of the bottom" the bottom should be for bottom plate, if that so the annular plate is extended either 1 in or 2 in depending type of lap joint used, therefore the annular plate is 24 + extended (1 in or 2 in) Correct Me If I am Wrong...

I am just still confused about that.

 

[IFRs]

Sir -

Please accept my apologies, I would like to correct my first post in this thread.

The annular plate width is always increased by the lap.

5.5.2 "Annular bottom plates shall have a radial width that provides at least 600 mm (24 in.) between the inside of the shell and any lap-welded joint in the remainder of the bottom."

This is clear - measure to the lap-welded joint.
Not clear - what portion of the lap welded joint do we measure to?
Keep reading...

5.5.2 continues with an equation for L and the following definition of L: "the minimum width of annular plate as measured from inside edge of the shell to the edge of the plate in the remainder of the bottom, mm (inch)"

This is clear - measure to the edge of the bottom plate. No distinction is made for where the bottom plate is relative to the top surface of the annular plate nor is the weld itself mentioned. This clarifies the uncertainty of where to measure the lap weld. We measure to the nearest edge of the bottom plate at the lap and ignore where the weld line occurs.

The annular plate width is always increased by the lap.

The width of the annular plate is the same for any bottom slope or lap configuration.

Again, my apologies for my first post which says to ignore the lap if the lap is for a cone down bottom.

Comments are welcome!

 

[hamidun]

IFRs,

Well, now it's clear that the annular is always increased which means the result of equation L will be added between 1" or 2" depending of the type of weld.

Again, just to make sure...

 

[IFRs]

That's how I now read those paragraphs. Others may not. I try to take the words literally and not inject my bias. In some respects, it does make sense - some of the original work and subsequent research papers define the inside of the annular plate at an inflection point or plastic hinge location and it would make sense that this occurs where the bottom thickness abruptly increases, or where the "remainder of the bottom" starts. I believe I have talked myself into this interpretation.

Remember: this is the internet and I am one voice of many. Caveat emptor !

 

[hamidun]

Alright IFRs and JStephen, I think I'm pretty much done with this and thank you for you time and energy wasted on this thread!!!