46 CFR § 39.2011 - Vapor overpressure and vacuum protection - TB/ALL.
(1) Be capable of discharging cargo vapor at the maximum transfer rate plus the vapor growth for the cargo such that the pressure in the vapor space of each tank connected to the vapor control system (VCS) does not exceed -
(i) The maximum design working pressure for the tank; or
(ii) If a spill valve or rupture disk is fitted, the pressure at which the device operates;
(2) Relieve at a pressure corresponding to a pressure in the cargo tank vapor space not less than 1.0 pounds per square inch gauge (psig);
(3) Prevent a vacuum, which generates in any tank connected to the vapor collection system during the withdrawal of cargo or vapor at maximum rates, in a cargo tank vapor space from exceeding the maximum design vacuum; and
(4) Not relieve at a vacuum corresponding to a vacuum in the cargo tank vapor space between 14.7 pounds per square inch absolute (psia) (0 psig) and 14.2 psia (−0.5 psig).
(b) Each pressure-vacuum relief valve must -
(2) Be tested for venting capacity in accordance with paragraph 126.96.36.199 of API 2000 (incorporated by reference, see 46 CFR 39.1005). The test must be carried out with a flame screen fitted at the vacuum relief opening and at the discharge opening if the pressure-vacuum relief valve is not designed to ensure a minimum vapor discharge velocity of 30 meters (98.4 feet) per second; and
(3) If installed after July 23, 1991, have a mechanism to check that it operates freely and does not remain in the open position.
(c) A liquid filled pressure-vacuum breaker may be used for vapor overpressure and vacuum protection if the vessel owner or operator obtains the prior written approval of the Commandant.
(d) Vapor growth must be calculated following the Marine Safety Center guidelines available in Coast Guard VCS guidance at http://homeport.uscg.mil, or as specifically approved in writing by the Commandant after consultation with the Marine Safety Center.
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