As numbers, considering absolute pressure in and out appears to be OK, but when you use a software like PV elite, it is understood that you have two scenarios- one with external pressure alone, one with internal pressure alone.
In fact a pressure vessel is essentially designed to the relative internal pressure. In some cases, you must consider the external pressure which can be created in three ways:
-From a vacuum inside a vessel and atmospheric pressure outside (two simple examples of sources of vacuum – unloading a vessel or a tank that is not adequately vented – or – cooling down a vessel filled with steam that condenses to water);
-From a pressure outside a vessel greater than atmospheric (typically from some types of jacket or a surrounding vessel)
-From a combination of the first two – vacuum inside + pressure greater than atmospheric outside.
To answer specifically to your question, you may design your tank according to EN 12285-2:2005 (Workshop fabricated steel tanks - Part 2: Horizontal cylindrical single skin and double skin tanks for the aboveground storage of flammable and non-flammable water polluting liquids) to the limits of this standard, or any other specification having this subject.
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