Kroon's Method for Bottom Annular Plates on API 650 Tanks

[marcleblanc12]

I've been using the attached spreadsheet to perform an elastic analysis on annular rings. I can manage to get a thickness that works so that: θs = θc and θb = 0 with a reasonable stress, however I find the majority of the time I can't get the inside projection to work out to match code requirements.

From what I understand API 650 states that the inside projection should be > 18 in but need not be more that 0.035*D.

Most of the time I can't get the inside projection to be large enough to fall within this range. If I try to set the inside projection within this range, I need an annular so thick it makes no sense at all.

Is the inside projection in this calculation only theoretical? Meaning, if I calculate the thickness with an inside projection less than 18 in can I just physically make the annular ring between 18in and 0.035*D?

Let me know if any of you have run across this situation.

Thanks,

Marc

 

[HTURKAK]

Dear marcleblanc12 (Mechanical),

The width calculation of annular plates in API 650 is not based on KROON'S theory... API 650 assumes cantilever beam theory and the calculation is based on yield of annular plate.

The annular plates must have width at least 24 in between the inside bottom plates and the shell and they must project outside the shell by at least 2 in.

In your case,if bottom shell course thk is 15.875 mm, the annular plate thk. should be between 6 -9 mm depending on the stress level in first Shell Course ..

I will suggest you to look API 650 ( 5.5 ANNULAR BOTTOM PLATES ) , E.6.2.1.1.2 Annular Ring Requirements , Self anchorage requirements.

The following excerpt from Structural Analysis and Design of Process Equipment (by Maan H. Jawad ,James R. Farr )

If you have further questions , pls provide more info ( seismicity, stress level in First Shell Course...)

 

[marcleblanc12]

Don't forget about Clause 5.5.3 "Table 5.1a and Table 5.1b are applicable for effective product height of H x G <= 75 ft. Beyond this height an elastic analysis must be made to determine the annular plate thickness."

So I do need to use Kroon's method to determine the thickness of the annular plate, but the only trick is determining the internal projection. Clause E.6.2.1.1.3-3 talks about "when the bottom plate under the the shell is thicker than the remainder of the tank bottom, the internal projection needs to me the greater of 0.45m (1.5 ft) or L = 0.216*t*SQRT(Fy/(H*G)) (Equation E.6.2.1.1.2-1b in US UNITS) but need not be greater than 0.035*D."

So in my case the internal projection doesn't need to be more than 21.084 in since my diameter is 50.2 ft (602.4 in) L = 0.035*602.4 = 21.084 in.

I've just noticed a note in "Guide to Storage Tanks and Equipment" by Long and Garner that states about the internal projection: "The API minimum requirement is very conservative compared with all THEORETICAL REQUIREMENTS to H. Kroon's theory of 322mm (12.68 in)." That being said, the internal projection in Kroon's method is strictly theoretical and that's why the calculation doesn't work if you try to use actual API 650 internal projections in this calculation. Therefore I just need to pick a plate thickness and then by iteration determine the theoretical projection of the annular plate. If the projection is over 322mm, and the stress is below allowable then I'm good. After I've confirmed that I can increase the actual internal projection to suit API 650 since the calculation is only giving me a theoretical internal projection.

Looks as though I've answered my own question, but if I wouldn't have asked then I wouldn't have stumbled upon this note.

Marc

 

[HTURKAK]

Will you pls show from any reputable code ( API 650, BS 2654, EC 3 ) that Kroon's method is used for elastic analysis?

the following paragraphs copy and pasted from API 650 for wind and seismic situation;


5.11.2.3
2) When the bottom plate under the shell is thicker due to wind overturning than the remainder of the tank bottom,
the minimum projection of the supplied thicker annular ring inside the tank wall, L, shall be the greater of
450 mm (18 in.) or Lb, however, need not be more than 0.035D.


E.6.2.1.1.3 Annular Ring Welding Requirements

3) When the bottom plate under the shell is thicker than the remainder of the tank bottom, the minimum projection,
L, of the supplied thicker annular ring inside the tank wall shall be the greater of 0.45 m (1.5 ft) or as determined
in equation (E.6.2.1.1.2-1); however, L need not be greater than 0.035 D:


If this is an API 650 tank, you SHALL comply with the requirements of the standard.

API 650 annular plate width calculation is based on cantilever base plate assumption and the length has a FS =2.0
During partial uplift, ( which is necessary to mobilize the wt of liquid content to resist the OT moment ) , the annular plate behavior will be essentially cantilever.


I will suggest you to look the following doc. ( which i think useful to understand the behavior of a tank during a seismic event ) SEISMIC DESIGN OF STORAGE TANKS M. J . N. Priestley1 J . H. Wood2 and B. J . Davidson*

and FIG. 9 EQUILIBRIUM OF UPLIFTING CIRCULAR TANK.