46 CFR § 154.801 - Pressure relief systems.
(a) Each cargo tank that has a volume of 20m 3 (706 ft. 3) or less must have at least one pressure relief valve.
(b) Each cargo tank that has a volume of more than 20m 3 (706 ft. 3) must have at least two pressure relief valves of the same nominal relieving capacity.
(c) Each pressure relief valve must:
(1) Meet Subpart 162.018 of this chapter or, if the valve is also capable of vacuum relief and the MARVS is 69 kPa gauge (10 psig) or less, Subpart 162.017 of this chapter, and have at least the capacity required under § 154.806;
(2) Not be set for a higher pressure than the MARVS;
(3) Have a fitting for sealing wire that prevents the set pressure from being changed without breaking the sealing wire;
(4) Be fitted on the cargo tank to remain in the vapor phase under conditions of 15° list and of 0.015 L trim by both the bow and stern;
(5) Vent to a vent mast under § 154.805, except a relief valve may vent to a common tank relief valve header if the back pressure is included in determining the required capacity under § 154.806;
(6) Not vent to a common header or common vent mast if the relief valves are connected to cargo tanks carrying chemically incompatible cargoes;
(7) Not have any stop valves or other means of isolating the cargo tank from its relief valve unless:
(i) The stop valves are interlocked or arranged so that only one pressure relief valve is out of service at any one time;
(ii) The interlock arrangement automatically shows the relief valve that is out of service; and
(iii) The other valves have the relieving capacity required under § 154.806, or all relief valves on the cargo tank are the same size and there is a spare of the same size, or there is a spare for each relief valve on a cargo tank.
(d) The pressure relief system must:
(1) If the design temperature is below 0 °C (32 °F), be designed to prevent the relief valve from becoming inoperative due to ice formation; and
(2) Be designed to prevent chattering of the relief valve.