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API 620 thickness calculation - understanding of load consideration

API 620 thickness calculation - understanding of load consideration

API 620 thickness calculation - understanding of load consideration

(OP)
Hi,

For learning purposes I'm looking over a calculation done for one of our tanks designed to API 620 with the following information:

Max design pressure = 1 psi
Min design pressure = 2" WC
Tank Radius = 1044 inches

Pressure due to liquid = 27 psi
Pressure due to gas = 1 psi

P = 28 psi

Weight of tank shell = 1,346,000 lbs
Weight of roof = 621,400 lbs

Live Loads:
Pressure: 1 psi = 253,150 lbs
2" WC = -3,425,150 lbs
Roof Load = 25 psf = 596,600

Now for the thickness calculation for shell course 1, we have to find W which is the total weight of the tank

W = weight of shell + weight of roof + weight of live loads.


Next they have max W(live) = 253,150 + 596,000 = +849,150 lbs (which is the 1 psi load + roof live load)

and then min W (live) = -3,425,150 lbs (which is the 2" WC load).

then

max W(comp) = 1,346,000 + 621,400 + 849,150 = 2,816,550 lbs
min W(tension) = 1,346,000 + 621,400 -3,425,150 = -1,457,750 lbs.

My confusion comes into why is the 2" WC load negative as this should be acting opposite to the weight whilst the 1 psi load should be in the same direction.

Also confusing is the calculation of the compressive load and tensile load. Shouldn't the 2: WC be causing compression of the tank as it is making a vacuum within the tank and the 1 psi does the opposite?

Yet to calculate the unit forces T1 and T2, in the equation for T1, then used W = + 1,457,750. Shouldn't they have used the larger load of 2,816,550 ?

I suspect my confusion is possibly to do with the direction of the forces based on the free body diagram. Can anyone shed some light to help me understand better?

RE: API 620 thickness calculation - understanding of load consideration

The pressure and vacuum numbers appear to be swapped?
Also, pressure/vacuum don't enter into the W term, they are separate entries in the T1 and T2 equations. W is just the weights in the freebody.
The product weight is missing from the W term?

RE: API 620 thickness calculation - understanding of load consideration

(OP)
Any further assistance with this?

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