46 CFR § 54.15-25 - Minimum relief capacities for cargo tanks containing compressed or liquefied gas.

(a) Each tank shall be fitted with one or more safety relief valves designed, constructed, and flow tested in accordance with subpart 162.017 or 162.018 in subchapter Q (Specifications) of this chapter. Valves conforming to specification subpart 162.017 shall be limited to use on tanks whose maximum allowable working pressure is not in excess of 10 pounds per square inch. With specific approval of the Commandant, such valves may be connected to the vessel in lieu of being directly fitted to the tanks.

(b) The discharge pressure and the maximum overpressure permitted shall be in accordance with § 54.15-5.

(c) The rate of discharge for heat input of fire must meet the following formula:

Q = FGA 0.82

where:

Q = minimum required rate of discharge in cubic meters (cubic feet) per minute of air at standard conditions 15 °C and 103 kPa (60 °F and 14.7 psia).

F = fire exposure factor for the following tank types:

F = 1.0 for tanks without insulation located on the open deck.

F = 0.5 for tanks on the open deck having insulation that has approved fire proofing, thermal conductance, and stability under fire exposure.

F = 0.5 for uninsulated independent tasks installed in holds.

F = 0.2 for insulated independent tanks in holds or for uninsulated independent tanks in insulated holds.

F = 0.1 for insulated independent tanks in inerted holds or for uninsulated independent tanks in inerted, insulated holds.

F = 0.1 for membrane and semi-membrane tanks.

G = gas factor of:

$\begin{array}{c}"G=\left[\left(177/\mathrm{LC}\right)\left(\sqrt{\mathrm{ZT}/M}\right)\right]\text{SI units}\\ G=\left[\left(\mathrm{633,000}/\mathrm{LC}\right)\left(\sqrt{\mathrm{ZT}/M}\right)\right]\text{English units"}\end{array}$

where:

L = latent heat of the material being vaporized at the relieving conditions, in Kcal/kg (Btu per pound).

C = constant based on relation of specific heats (k), table § 54.15-25(c) (if k is not known, C = .606(315)).

Z = compressibility factor of the gas at the relieving conditions (if not known, Z = 1.0).

T = temperature in degrees K = (273 + degrees C) (R = (460 + degrees F)) at the relieving conditions (120% of the pressure at which the pressure relief valve is set).

M = molecular weight of the product.

A = external surface area of the tank in m 2 (sq. ft.) for the following tank types:

For a tank of a body of revolution shape:

A = external surface area.

For a tank other than a body of revolution shape:

A = external surface area less the projected bottom surface area.

For a grouping of pressure vessel tanks having insulation on the vessel's structure:

A = external surface area of the hold without the projected bottom area.

For a grouping of pressure tanks having insulation on the tank:

A = external surface area of the pressure tanks excluding insulation, and without the projected bottom area. 1

1 Figure 54.15-25(c) shows a method of determining the side external surface area of a grouping of vertical pressure tanks.

Table 54.15-25(c) - Constant C

k	C
1.00	.606	(315)
1.02	.611	(318)
1.04	.615	(320)
1.06	.620	(322)
1.08	.624	(324)
1.10	.628	(327)
1.12	.633	(329)
1.14	.637	(331)
1.16	.641	(333)
1.18	.645	(335)
1.20	.649	(337)
1.22	.652	(339)
1.24	.658	(341)
1.26	.660	(343)
1.28	.664	(345)
1.30	.667	(347)
1.32	.671	(349)
1.34	.674	(351)
1.36	.677	(352)
1.38	.681	(354)
1.40	.685	(356)
1.42	.688	(358)
1.44	.691	(359)
1.46	.695	(361)
1.48	.698	(363)
1.50	.701	(364)
1.52	.704	(366)
1.54	.707	(368)
1.56	.710	(369)
1.58	.713	(371)
1.60	.716	(372)
1.62	.719	(374)
1.64	.722	(376)
1.66	.725	(377)
1.68	.728	(379)
1.70	.731	(380)
1.72	.734	(382)
1.74	.736	(383)
1.76	.739	(384)
1.78	.742	(386)
1.80	.745	(387)
1.82	.747	(388)
1.84	.750	(390)
1.86	.752	(391)
1.88	.755	(392)
1.90	.758	(394)
1.92	.760	(395)
1.94	.763	(397)
1.96	.765	(398)
1.98	.767	(399)
2.00	.770	(400)
2.02	.772	(401)
2.20	.792	(412)

(c-1) For an independent tank that has a portion of the tank protruding above the open deck, the fire exposure factor must be calculated for the surface area above the deck and the surface area below the deck, and this calculation must be specially approved by the U.S. Coast Guard, Office of Design and Engineering Standards (CG-ENG)..

(d) In determining the total safety valve relieving capacity, the arrangement and location of the valves on the tank will be evaluated. The valves shall be placed so that a number of valves sufficient to provide the required relieving capacity shall always be in communication with the cargo vapor phase. The possible motions which the tank may see in its intended service and attendant changes in cargo liquid level will be considered. Shut off valves shall not be installed between the vessel and the safety relief valves. Manifolds for mounting multiple relief valves may be fitted with acceptable interlocking shut off valves so arranged that the required capacity of discharge will be “lined up” at all times.

(e)

(1) Each safety relief valve shall be tested in the presence of a marine inspector before being placed in service except as noted otherwise in paragraph (e)(2) of this section. The test shall satisfactorily show that the valve will start to discharge at the required minimum pressure.

(2) Each safety relief valve fitted with a breaking pin and rupture disk need not be tested in the presence of a marine inspector before being placed in service. In lieu thereof, a certificate shall be furnished with the valve attested to by the manufacturer that the test requirements of paragraph (e)(1) of this section have been met.

[CGFR 68-82, 33 FR 18828, Dec. 18, 1968, as amended by CGD 74-289, 44 FR 26007, May 3, 1979; CGD 82-063b, 48 FR 4781, Feb. 3, 1983; CGD 95-072, 60 FR 50462, Sept. 29, 1995; CGD 96-041, 61 FR 50728, Sept. 27, 1996; USCG-2004-18884, 69 FR 58346, Sept. 30, 2004; USCG-2007-29018, 72 FR 53965, Sept. 21, 2007; USCG-2009-0702, 74 FR 49228, Sept. 25, 2009; USCG-2012-0832, 77 FR 59777, Oct. 1, 2012]