46 C.F.R. § 64.19 External pressure.
Title 46 - Shipping
(a) A tank without a vacuum breaker must be designed to withstand an external pressure of 7 (b) A tank with a vacuum breaker must be designed to withstand an external pressure of 3 psig or more.
Title 46: Shipping
PART 64—MARINE PORTABLE TANKS AND CARGO HANDLING SYSTEMS
Subpart B—Standards for an MPT
§ 64.19 External pressure.

