thickness of tank courses confusion

[NovaStark]

I have the calculations of a storage tank designed to API 620 however I am bit confused as to how the shell thickness of the courses were calculated. I have attached piece of the calculation.

From what I can see, they calculated the weight of the entire shell using basically density*volume and then converted that to lbs. However, the summed up the thicknesses, there is no explanation as to how these were initially found. In addition to this, the first course thickness is calculated as 1.346 inches, but I eventually realized that in the formula for the total weight, they used the thicknesses they found.

The confusing part is that it looks like they magically selected the correct thicknesses initially to calculate the weight W, and then used that same W to calculate thicknesses (all of which) correspond to what was being added together in finding W.


Can anyone familiar with tank design please shed some light on this? (Sorry if it sounds a bit confusing but I am a bit lost to it, I may be even overlooking something elementary)

 

[fegenbush]

Page 4 appears to be a summary sheet only. The order of the pages does not necessarily indicate the order of calculation.

You will notice on page 6 that the thickness required for the unit stress T2 is approximately double that required by T1. The unit stress T1 is the one which requires the weight of the shell. In this case and almost all cylindrical ground supported tanks designed to API 620, the shell thickness is controlled by T2. As you move farther up the shell, the ratio of T1 to T2 increases. However, with such a low pressure rating, it will not exceed 1, so T2 will control the shell thickness.

 

[NovaStark]

Page 4 appears to be a summary sheet only. The order of the pages does not necessarily indicate the order of calculation.

You will notice on page 6 that the thickness required for the unit stress T2 is approximately double that required by T1. The unit stress T1 is the one which requires the weight of the shell. In this case and almost all cylindrical ground supported tanks designed to API 620, the shell thickness is controlled by T2. As you move farther up the shell, the ratio of T1 to T2 increases. However, with such a low pressure rating, it will not exceed 1, so T2 will control the shell thickness.

Then basically what was done was T2 was used to calculate the minimum thicknesses. The thicknesses were then used to calculate W to verify that T1 was in fact less than T2?

 

[fegenbush]

While I cannot comment on their order of design operations, since I only see the final result, the order of your calculation would work as well.

 

[NovaStark]

Ah, I see then. I was a bit confused as that is the order in which the calculations were given. So it looked like they magically used the correct thicknesses in the beginning and got the correct ones in the end.

Thanks for that then, I thought I was going a bit mad. Now I know

 

[NovaStark]

Query about the method in calculating T2 and checking T1.

If I used T2 to get all the thicknesses and then check the value of T1; and I find that T1>T2, I will need to re-calculate all of the thicknesses using T1 only right?

I will not have to recheck T1 and T2 for the rest of courses, correct?

 

[fegenbush]

Correct. However, in vertical, cylindrical storage tanks, it is very uncommon for T1 to control.

 

[NovaStark]

ah much thanks then!