API-620 Free Body Analysis

[gdubyuh]

I am designing an F&D bottom head for an 11'-0" I.D x 15"-0" tall, 304L storage tank via API-620. Assuming the bottom head approximates a spherical segment, I am using equations (4) and (5) from section 5.10.2.5. to calculate T1 and T2.

My question regards the correct value for 'W' along with its sign convention in the equation.

5.9.2 discusses how to perform FBA from the top of the tank working downward in successive levels. With that in mind, the bottom head would be a seperate level, and thus, the area of the tank under consideration for the FBA would just be the bottom head.

Moving along to 5.10.1, the definition of 'W' is given as "total weight of that portion of the tank (the head)and its contents (either above or below the level under consideration) that is treated as a free body...Strictly speaking, the total weight would include the weight of all metal, gas, and liquid in that portion of the tank".

So here is my first question. For the value of 'W' for the free body analysis of the dished bottom head, does that just include the weight of the bottom head plus the liquid in the head, or does it include all the weight above the dished bottom (weight from the liquid in the shell, the weight of the shell, plus the weight of the roof)? Just from reading the code, it seems like its just the head and it contents, but I cannot come to terms with the idea of not including the weight above it.

Second question. The tank has 4" W.C. internal pressure. Since internal pressure acts outwardly from the inside of the tank, the pressure would be pushing 'downward' on the bottom head, and thus be in the same direction as the weight, so the equation would be [P+(W/At)]? On a side note, what direction is P considered for equation (10)? Internal pressure acts radially against the cylindrical sidewall. Is it considerd that P is positive and W is negative in this case?

Thanks in advance for your commentary.

 

[doct9960]

Before you design to API 620, a few questions:

1.) Have you considered designing and building your storage tank to ASME VIII Div. 1 (without the stamp)?

2.) Is your tank above ground? How is it supported? by legs? skirt?

 

[JStephen]

If you take the bottom head, or portions thereof, as a free body, then the W term would be all the weight of the head and contents that are below the point being analyzed. The pressure would be the vapor pressure over the liquid plus the hydrostatic pressure of the liquid at that depth. The pressure is acting vertically downward at the plane where you are analyzing the head. P and W should be of the same sign.

Be aware that API-620 can give unreasonable results for small-radius knuckles. If so, consider using ASME-VIII as mentioned above.