Depends on the quantity you're going to store.
For smaller quantities, it's more convenient a cylindrical, horizontal vessel with semielliptical heads. For larger quantities it's more convenient a spherical vessel.
What's the difference? The spherical vessel has a thinner wall thickness than the cylindrical one. Less weight, lower cost.
BUT, the cylindrical vessel is easier and cheaper to fabricate. The shell is rolled and the heads are either pressed or spun. The cylindrical vessel is also easier to erect. Less manhours, lower cost.
So, you should make a cost estimate for both cases and choose the more convenient one from an economical point of view.
Just for your information, here in Brazil, where both vessels would be designed and fabricated to ASME VIII Div 1 (Div 1 is OK for 500 psig), the point of equilibrium is about 140 cubic meters. For smaller volumes the cylindrical vessel is more convient, for larger volumes it's more convenient the spherical vessel.
Now, this is in Brazil. You're probably using the Australian pressure vessel standard and in your country (Singapore?) the price of steel, wages, government taxes etc. are different. So you should make a cost estimate before taking a decision.
Giovanni S. Crisi
Sao Paulo - Brazil
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