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API 650 Annex F tank - Roof plate thickness

API 650 Annex F tank - Roof plate thickness

API 650 Annex F tank - Roof plate thickness

Dear all,

im designing a tank under api 650 annex F for academic purpose.

the roof is cone type (self-supporting) with 9.5º slope

Using the equation given in F.6.1 (written below) i got 0.046 mm

t = (P x R)/((cos(alpha)*Sd*E) + CA
t = (0.363*2.865)/(0.165*137*1) = 0.046 mm

here is my data:

Intenal pressure (P) = 0.363 kPa
Nominal Radius (Rt) = 2.865 m
alpha = (180 - 2*9.5)/2 = 80.5º
allowable stress (Sd) = 137 MPa
Joint eff (E) = 1

Im using API 650, 12th Ed.

Thanks for answers !

RE: API 650 Annex F tank - Roof plate thickness

Hi jrafael95 (Mechanical),

I checked ur calculation with (lb, in) units also .. Your calculation is correct but i am not sure the formula is OK. I have API 650 11 th ed. and there is no such a formula ..

Just by looking , i suspect ,this formula derived from buckling load for curved roof so can be used for ( gravity loading (DL+LL )+ VACUUM ) external loading...

However your design shall comply with other requirements ( i.e. for gravity loading , minimum thk. requirements at , APP V etc..)

RE: API 650 Annex F tank - Roof plate thickness

Dear @Hturkak, thanks for your answer!

Well, I'll use the formula given in 5.10.5.

RE: API 650 Annex F tank - Roof plate thickness

I believe that formula is based on tension in the roof plate itself. For most tanks, that formula will not control. When it doesn't, I usually reset joint efficiency lower as well.
For self-supporting cone or umbrella/dome roofs, see the separate formulas for thickness in the main body of the code; or in Annex V if applicable, which are based on buckling and compression in the roof.
For supported-cone roof tanks, see the rafter-spacing requirements which will limit roof thickness, and Annex V if applicable.
Also note the minimum roof thickness requirements.
That formula should be checked, but is not normally the controlling factor.
Similarly, there are separate formulas for top angle area based on tension (in the base code or Annex V) and based on compression (Annex F), and both should be checked. In that case, the Annex F will more likely control.

RE: API 650 Annex F tank - Roof plate thickness


I tried to use the worst joint eff possible, but still got low thicknesses comparated to the method descripted in 5.10.5.

I guess you and Hturkak are both right.

Really aprecciate your answer.

I hope the best for both :)

RE: API 650 Annex F tank - Roof plate thickness

The formula in F.6.1 only determines the required roof plate thickness for internal pressure. This is a simple tension loading, and very small thickness may be sufficient, as you have discovered.

You must also check the required thickness for external loads (self-weight, employees, vacuum during product withdrawal, etc.) using the formula in 5.10.5. This is related to plate buckling and for most tanks would control.

Your required thickness will then be the greater of these two calculations, though given your small tank size it may limited by the 4.8 mm minimum roof plate thickness.

RE: API 650 Annex F tank - Roof plate thickness

Hi Geoff13, thanks for your answer.

Thats exactly what i did, found 6.3mm for roof plate thickness.

Many thanks for your answers :)

RE: API 650 Annex F tank - Roof plate thickness

All professionals should use the current edition:
API 650 13th ed March 2020


RE: API 650 Annex F tank - Roof plate thickness

r6155 - yes, and also understand the differences between editions...

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