46 C.F.R. § 38.20-1 – Venting—T/ALL.

(a) Each safety relief valve installed on a cargo tank shall be connected to a branch vent of a venting system which shall be constructed so that the discharge of gas will be directed vertically upward to a point which shall extend to a height above the weather deck equal to at least one-third the beam of the vessel and to a minimum of at least 10 feet, and shall terminate at a comparable distance from any other living or working space, ventilator inlet, or source of vapor ignition. When special conditions will prevent the vent line header outlets being permanently installed at a height above the deck of one-third the beam of the vessel, then an adjustable system shall be provided which, when extended vertically, shall be capable of reaching a height of one-third the beam of the vessel.
(b) The capacity of branch vents or vent headers shall depend upon the number of cargo tanks connected to such branch or header as provided for in the table 38.20-1(b), and upon the total safety relief valve discharge capacity.
Table 38.20-1(b)—Capacity of Branch Vents or Vent Headers
Number of cargo tanks Percent of total valve discharge
1 or 2 100
3 90
4 80
5 70
6 or more 60
(c) In addition to the requirements specified in paragraphs (a) and (b) of this section, the size of the branch vents or vent headers, shall be such that the back pressure in the relief valve discharge lines shall not be more than 10 percent of the safety relief valve setting. In nonpressure vessel vent systems, however, where the maximum back pressure of 10 percent of the relief valve setting is insufficient to move the gases through any but an extremely large diameter vent pipe, the back pressure may exceed 10 percent provided:
1. (1) The pressure in the tank during venting does not exceed 120 percent of the tank maximum allowable pressure; and,
2. (2) The safety relief valve is sized to discharge the required capacity with the tank pressure and vent back pressure actually used.
(d) Return bends and restrictive pipe fittings are not permitted.
(e) Vents and headers shall be so installed as to prevent excessive stresses on safety relief valve mountings.
(f) The vent discharge riser shall be so located as to provide protection against mechanical injury and such discharge pipes shall be fitted with loose raincaps or other suitable means to prevent entrance of rain or snow.
(g) No valve of any type shall be fitted in the vent pipe between the safety relief valve and the vent outlets.
(h) Provisions shall be made to drain condensate from the vent header piping. Special precautions shall insure that condensate does not accumulate at or near the relief valves.
(i) Relief valves discharging liquid cargo shall not be connected to the branch vent or vent header lines from the cargo tanks. They may, however, be connected to an accumulator, the vapor space of which, may in turn, be connected to the vent header system. Relief valves in the cargo piping system may discharge back to the cargo tanks.
(j) Vapor discharged to the atmosphere in accordance with § 38.05-25(b) shall utilize valves separate from the safety relief valves.

Table 38.20-1(b)—Capacity of Branch Vents or Vent Headers
Number of cargo tanks	Percent of total valve discharge
1 or 2	100
3	90
4	80
5	70
6 or more	60