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API shell thickness using corrected height

API shell thickness using corrected height

API shell thickness using corrected height

I make excel sheets to help me remember things I figured out before.

On my spreadsheet for tank wall minimum thicknesses, I use a corrected height H'=H+(2.31*P/G). H=actual liquid height, P=pressure, G=specific gravity. In reviewing my spreadsheet, I did not reference where I got H' from and I cannot find it in API 620, 650, or 653. It makes sense to me to use H', but do any of you happen to know where I may have found that and if you agree it should be used? Without it, it doesn't seem to matter what pressure the tank is designed for.

I use it in the shell thickness equation (1-foot method) t=(2.6*(H'-1)*D*G)/SE. D=diameter, S=design stress, E=Joint efficiency.

In summary, my question is: Where would I have found H' and should I use it in the shell thickness equation?

RE: API shell thickness using corrected height

H is the design liquid depth in feet.
H' is the adjusted depth to allow for internal pressure in the vapor space per Annex F.
I don't think API-650 uses the H' terminology, it just says to include the adjustment when calculating shell thickness. In the more recent edition, they've specified to do so if pressure is over 4" water column. In older editions, I think it was only if pressure uplift exceeded the shell and roof weight.
The design pressure of a tank will normally not have a whole lot of influence on shell thickness, but does have a major effect on roof, top angle, and anchorage design.

RE: API shell thickness using corrected height

JStephen nailed it.

RE: API shell thickness using corrected height

Thank you for the response.

RE: API shell thickness using corrected height

In calculating shell thickness for Annex F tanks, H shall be increased by the quantity P/(9.8G) for SI units, or P/(12G) for USC units—where H is the design liquid height, in m (ft), P is the design pressure kPa (in. of water), and G is the design specific gravity.
Design pressures less than 1 kPa (4 in. of water) do not need to be included.

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