49 CFR §179.220-13

1. (a)Inner container manway nozzle must be of approved design with access opening at least 18 inches inside diameter, or at least 14 inches by 18 inches obround or oval.
2. (b)Manway covers must be of approved type. Design must provide a secure closure of the manway and must make it impossible to remove the cover while the tank interior is under pressure.
3. (c)All joints between manway covers and their seats must be made tight against leakage of vapor and liquid by use of suitable gaskets.
4. (d)Manway covers must be cast, forged, or fabricated metal complying with subsection § 179.220-7(g) of this section.
5. (e)A seal must be provided between the inner container manway nozzle and the opening in the outer shell.