API 650 - Annular Ring Width Calculation

[canadianinspector]

Hello there,
Basic question for some, but confusing for me on logic and I am wondering if I am calculating wrong.

I understand as per API 650 Section 5.5.2 the radial width of the ring shall be from the inside edge of the shell radially as follows:

L=2tb Sqrt Fy/2YGH

Our tank: Height 56foot, diameter 134 foot, btm shell course 1.0" G40-21-44W (Product ST=27,900), CA 0.0625, E=1.0

Tb=nominal thickness of ring to be installed 0.500 inches
Fy=Yield strength of annular plate G40-21-44W 44,000
H= Design liquid max 55.5 Feet
G= Specific gravity of our product is 1.1 but max in API is 1.0
Y= Density factor of water 0.43PSI/foot

L=2 x 0.500 (Sqrt (44,000/2 x 0.43 x 1.0 x 55.5))
L= 30.36 inches

My questions:
1. Does my calculation look correct and if so my next part questions are:

2. If you decrease the Specific Gravity the width of the plate increases and I would think less product weight it would decrease the width of the ring instead.
3. Also if you increase the height of the tank the width of the ring decreases and I would think more shell weight would make an increase in the width for more support.

This does not sound logical to me, could someone let me know if I am on the correct path here?

Thank you so much for the help.

 

[HTURKAK]

Dear canadianinspector (Industrial)(OP),

1. Your calcuation correct,

2. If you decrease the Specific Gravity the width of the plate increases..that is true!.

3. Also if you increase the height of the tank the width of the ring decreases..that is also true...

API assumes a portion of the content resists to overturning together with shell . Pls read Appendix E 6.1.5 Overturning Moment and E.6.2 RESISTANCE TO DESIGN LOADS . I will suggest you to look New Zealand NZSEE: Seismic Design of Storage Tanks to understand the behavior of tank and resistance to overturning loads .

 

[IFRs]

In addition, it may be considered curious that when the thickness of the annular ring increases, the minimum width increases. This is also correct so that the strength of the annular plate does not overwhelm the strength of the corner weld.

 

[HTURKAK]

I just see at web , the document at the following link is very useful to see the behavior and resistance to seismic loading of liquid storage tanks

https://www.nzsee.org.nz/db/Bulletin/Archive/19(4)0272.pdf.

 

[IFRs]

Two of those authors, and others, published another document for New Zealand in 2009 entitled "NZSEE: Seismic Design of Storage Tanks: 2009" that is also a good read.

 

[canadianinspector]

Thank you for all the help guys.
Much appreciated.
Cheers.