49 CFR §179.103-1

1. (a)Tanks built under this section may be of any approved cross section.
2. (b)Any portion of the tank shell not circular in cross section shall have walls of such thickness and be so reinforced that the stresses in the walls caused by a given internal pressure are no greater than the circumferential stresses which would exist under the same internal pressure in the wall of a tank of circular cross section designed in accordance with paragraphs § 179.100-6 (a) and (b), but in no case shall the wall thickness be less than that specified in § 179.101.
3. (c)[Reserved]
4. (d)Valves and fittings need not be mounted on the manway cover.
5. (e)One opening may be provided in each head for use in purging the tank interior.