Vessel wall thickness (Mandatory Appendix 3-2): • Required Thickness(tr): This represents the minimum thickness of the vessel wall, calculated according to the code, before any corrosion allowance (CA) is added. It is the theoretical minimum thickness needed to withstand the design conditions. • Design Thickness(td): This is the required thickness after incorporating the corrosion allowance. It is calculated by adding the specified corrosion allowance (CA) to the required thickness. This thickness accounts for potential material loss due to corrosion over the vessel's lifespan. • Nominal Thickness(tn): This is the thickness of the vessel wall that will be used in fabrication. It is selected from commercially available plate thicknesses and must be equal to or greater than the design thickness (td).