Enter the design length of the section, typically the length of the vessel plus one-third the depth of the heads or, alternately, the distance between stiffening rings. For a vessel with 2 elliptical heads and no intermediate stiffeners, the design length is the tangent length plus the diameter/6. For a vessel with 2 spherical heads and no intermediate stiffeners, the design length is the tangent length plus the diameter/3. For a vessel with 2 flanged and dished heads and no intermediate stiffeners, the design length is the tangent length plus the diameter/9. When analyzing a head, enter zero for the length.