What is difference between pressure vessel and tank ?

As the term indicates, "Pressure vessels" refers to a equipment/ component segment that is designed over an atmospheric pressure (above 15 psig) and mandates design and boiler safety code compliance to ASME Sec VIII Div. 1 or 2 or 3, as applicable. Equivalent International standard compliance in European region is BS5500 and/ or other nationally recognized code.

While tanks are meant to be for static storage of fluids at or below atmospheric pressure (of 15 psig). Accordingly, Storage tanks generally applicable for service and respective maximum pressures are as below, complying to respective industry codes:

• API 650 Welded Steel Tanks for Oil Storage - often specified for service other than oil- Max pressure 2.5 psig / max temperature 90°C / non-refrigerated
• API 620 Design and Construction of Large, Welded, Low-Pressure Storage Tanks - Max pressure 15 psig / Max temperature 250°F
• AWWA D100 Welded Steel Tanks for Water Storage

Lastly, pressure vessels are closed units, while storage tanks can be either closed or open top type (especially for vertical cylindrical construction).