Question:

I am designing a vertical vessel having one chamber which has 2 sections of same material. The corrosion allowance , radiography etc. for the 2 sections are identical, there is no wind or seismic load. Why do I have slightly different thicknesses for the 2 shells? Also the heads attached to the 2 shells have different thicknesses.

Answer:

There are many reasons why the shells may have different thicknesses. In the above case specifically the reason for the different thickness is different hydrostatic pressure. When the thickness is calculated the applied pressure is design pressure + hydrostatic pressure.

In the above case the hydrostatic pressure for the lower section (31.05) is obviously more than upper section (31.06) and then finally when the thickness round up of 1/16 in is applied, the final thickness turned out to be different. See the report below:

Also see the excel calculation details:

Since the program matches the head thickness with the associated shell thickness, the head thicknesses also vary.