46 CFR § 153.500 - Inert gas systems.
When Table 1 refers to this section, a cargo containment system must have a permanent inert gas system that:
(a) Maintains the vapor space of the containment system in an inert state by filling the vapor space with a gas that is neither reactive with the cargo nor flammable;
(b) Has a pressure control system that:
(2) Maintains at least a 3.5 kPa gauge (approx. 0.5 psig) pressure within the containment system at all times, including cargo discharge;
(c) Has storage for enough inerting gas to replace that normally lost while the tank's atmosphere is maintained in an inert condition (e.g. through tank breathing and relief valve leakage), but in no case an amount less than 5 percent of the tank's capacity when measured with the gas at −18 °C (approx. 0 °F) and a pressure equal to the cargo tank's relief valve setting; and
(d) Has connections for any supplemental gas supply necessary to maintain the inert gas pressure described in paragraph (b) of this section during cargo discharge.