CFR > Title 46 - Shipping > CHAPTER I—COAST GUARD, DEPARTMENT OF HOMELAND SECURITY > SUBCHAPTER D—TANK VESSELS (parts 30 to 39) > PART 39—VAPOR CONTROL SYSTEMS > SUBPART 39.20—Design and Equipment (§39.20-1 to §39.20-13) > 39.20-11—Vapor overpressure and vacuum protection—TB/ALL.

39.20-11—Vapor overpressure and vacuum protection—TB/ALL.

(a) The cargo tank venting system required by § 32.55 of this chapter must:
(1) Be capable of discharging cargo vapor at 1.25 times the maximum transfer rate such that the pressure in the vapor space of each tank connected to the vapor collection system 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) Not relieve at a pressure corresponding to a pressure in the cargo tank vapor space of less than 1.0 psig;
(3) Prevent a vacuum in the cargo tank vapor space, whether generated by withdrawal of cargo or vapor at maximum rates, that exceeds the maximum design vacuum for any tank connected to the vapor collection system; and
(4) Not relieve at a vacuum corresponding to a vacuum in the cargo tank vapor space of less than 0.5 psi below atmospheric pressure.
(b) Each pressure-vacuum relief valve must:
(1) Be tested for venting capacity in accordance with paragraph 1.5.1.3 of API 2000; and
(2) Have a means to check that the device operates freely and does not remain in the open position, if installed after July 23, 1991.
(c) The relieving capacity test required by paragraph (b)(1) of this section 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 ft.) per second.