I've been designing this vessel which runs in the atmospheric pressure conditions. But I faced some issues after taking atmospheric pressure as the design pressure. I've been getting very low thickness values for the shell. How can I solve this?
I'm not sure what you're trying to solve. If a vessel is designed to hold a gas, but that gas will only be at atmospheric pressure, then the walls can be very thin indeed. No? If the pressure is the same on both sides of the walls of the vessel, the pressure difference, inside and out, is zero. A paper bag can be sealed shut, and it will easily hold air inside, at atmospheric pressure.