There is no accurate short answer to your thickness versus pressure question. Thickness depends on factors other than pressure such as material strength, diameter, corrosion allowance and the best bet if you are concerned with a pressure vessel/pipe whatever is to use the correct national design code such as ASME VIII Div 1. However for a quick and simple check on vessel with a thin wall relative to diameter, t = Pressure*Diameter/(2*material strength). Be aware that the material strength used in the aforementioned code is only about 1/4 of the actual ultimate tensile strenght of the material. Yes, pressure design is conservative for many good reasons!
