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TL on What is the difference between vessel and tank
By: TL  Date: Monday, May 05, 2008 Time: 7:31:25 AM
The simpliest of answers is that ASME (Sec. VIII) refers to pressure vessels that can come in may forms that are operating under pressures greater than that which API 650 (generally just atmospheric and contents pressure) and API 620 (generally pressure under 15psi). 650 and 620 are considered to be "vertical, cylindrical, aboveground, closed- and open-top, welded steel storage tanks in various sizes and capacities", as per the opening paragraph in 650; with 620 covering spherical shaped, as well. I'm sure someone else can give a better and more detailed answer than this.