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Every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure caused by exposure fires.
In a vertical tank the construction referred to in paragraph (i)(2)(v)(A) of this section may take the form of a floating roof, lifter roof, a weak room-to-shell seam, or other approved pressure relieving construction. The weak roof-to-shell seam shall be constructed to fail preferential to any other seam.
Where entire dependence for emergency relief is placed upon pressure relieving devices, the total venting capacity of both normal and emergency vents shall be enough to prevent rupture of the shell or bottom of the tank if vertical, or of the shell or heads if horizontal. If unstable liquids are stored, the effects of heat or gas resulting from polymerization, decomposition, condensation, or self-reactivity shall be taken into account. The total capacity of both normal and emergency venting devices shall be not less than that derived from Table F-10 except as provided in paragraph (i)(2)(v)(E) or (F) of this section. Such device may be a self-closing manhole cover, or one using long bolts that permit the cover to lift under internal pressure, or an additional or larger relief valve or valves. The wetted area of the tank shall be calculated on the basis of 55 percent of the total exposed area of a sphere or spheroid, 75 percent of the total exposed area of a horizontal tank and the first 30 feet (9.12 m) above grade of the exposed shell are of a vertical tank.


[14.7 psia and 60 deg F. (15.55 deg. C) ]
Square feet (m(2))
CFH (m(3)H)
20 (1.84)
21,100 (590.8)
30 (2.76)
31,600 (884.8)
40 (3.68)
42,100 (1,178.8)
50 (4.6)
52,700 (1,475.6)
60 (5.52)
63,200 (1,769.6)
70 (6.44)
73,700 (2,063.6)
80 (7.36)
84,200 (2,357.6)
90 (8.28)
94,800 (2,654.4)
100 (9.2)
105,000 (2,940)
120 (11.04)
126,000 (3,528)
140 (12.88)
147,000 (4,116)
160 (14.72)
168,000 (4,704)
180 (16.56)
190,000 (5,320)
200 (18.4)
211,000 (5,908)
250 (23)
239,000 (6,692)
300 (27.6)
265,000 (7,420)
350 (32.2)
288,000 (8,064)
400 (36.8)
312,000 (8,736)
500 (46)
354,000 (9,912)
600 (55.2)
392,000 (10,976)
700 (64.4)
428,000 (11,984)
800 (73.6)
462,000 (12,936)
900 (82.8)
493,000 (13,804)
1,000 (90.2)
524,000 (14,672)
1,200 (110.4)
557,000 (15,596)
1,400 (128.8)
587,000 (16,436)
1,600 (147.2)
614,000 (17,192)
1,800 (165.6)
639,000 (17,892)
2,000 (180.4)
662,000 (18,536)
2,400 (220.8)
704,000 (19,712)
2,800 (257.6)
742,000 (20,776)
and over
For tanks and storage vessels designed for pressure over 1 p.s.i.g., the total rate of venting shall be determined in accordance with Table F-10, except that when the exposed wetted area of the surface is greater than 2,800 square feet (257.6 m(2)), the total rate of venting shall be calculated by the following formula:


CFH=Venting requirement, in cubic feet (meters) of free air per hour.
A=Exposed wetted surface, in square feet (m(2)).

NOTE: The foregoing formula is based on Q=21,000A0.82.
The total emergency relief venting capacity for any specific stable liquid may be determined by the following formula:

V=1337 divided by L square root M

V= Cubic feet (meters) of free air per hour from Table F-10.

L= Latent heat of vaporization of specific liquid in B.t.u. per pound.

M= Molecular weight of specific liquids.
The required airflow rate of paragraph (i)(2)(v)(C) or (E) of this section may be multiplied by the appropriate factor listed in the following schedule when protection is provided as indicated. Only one factor may be used for any one tank.

0.5 for drainage in accordance with paragraph (i)(2)(vii)(B) of this section for tanks over 200 square feet (18.4 m(2)) of wetted area.

0.3 for approved water spray.

0.3 for approved insulation.

0.15 for approved water spray with approved insulation.
The outlet of all vents and vent drains on tanks equipped with emergency venting to permit pressures exceeding 2.5 p.s.i.g. shall be arranged to discharge in such a way as to prevent localized overheating of any part of the tank, in the event vapors from such vents are ignited.
Each commercial tank venting device shall have stamped on it the opening pressure, the pressure at which the valve reaches the full open position, and the flow capacity at the latter pressure, expressed in cubic feet (meters) per hour of air at 60 deg. F. (15.55 deg C) and at a pressure of 14.7 p.s.i.a.
The flow capacity of tank venting devices 12 inches (30.48 cm) and smaller in nominal pipe size shall be determined by actual test of each type and size of vent. These flow tests may be conducted by the manufacturer if certified by a qualified impartial observer, or may be conducted by an outside agency. The flow capacity of tank venting devices larger than 12 inches (30.48 cm) nominal pipe size, including manhole covers with long bolts or equivalent, may be calculated provided that the opening pressure is actually measured, the rating pressure and corresponding free orifice area are stated, the word "calculated" appears on the nameplate, and the computation is based on a flow coefficient of 0.5 applied to the rated orifice area.

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