(A) Vents for normal breathing. Tanks shall have normal venting capacity sufficient to permit the filling and emptying of such tanks, plus their breathing due to temperature changes, without distortion of tank shell or roof. Tanks storing Class I and Class II flammable liquids shall be equipped where practical with either venting devices which shall be normally closed when not under pressure or vacuum, or with approved flame arrestors, except that tanks under 2,500 gallons capacity for Class I liquids and tanks under 3,000 barrels capacity for crude petroleum in producing areas may have open vents.
(B) Vents for emergency relief. In addition to the provisions for normal venting required in division (A) of this section, every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure, caused by exposure fire, that might cause the rupture of the tank shell or bottom. In a vertical tank, this construction may take the form of a weakened seam in the roof. The joint between the roof and the shell of a tank 36 feet or more in diameter, if built in accordance with the provisions of division (B) of § 120.057, shall be deemed to be a weakened seam for this purpose. Where entire dependence for such additional relief is placed upon some device other than a weak roof seam or joint, 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. Such device may be a self-closing manhole cover, or one using long bolts that permits the cover to lift under internal pressure, or an additional or larger relief valve or valves. When emergency relief is provided by other than a weak roof seam or joint, the total capacity of the normal breather and emergency relief shall be sized according to the following table:
Required Total Pressure Relief Capacity of Vents | ||||||
Capacity of Tank (in gallons) | Forty-Two Gallon Barrels | Total Pressure Relief Capacity (cu. ft. of free air per hour) | Approximate Diameter in Inches of Free Circular Opening for Various Pressures | |||
3 in. of water | 1 PSI | 2½ PSI | 5 PSI | |||
Required Total Pressure Relief Capacity of Vents | ||||||
Capacity of Tank (in gallons) | Forty-Two Gallon Barrels | Total Pressure Relief Capacity (cu. ft. of free air per hour) | Approximate Diameter in Inches of Free Circular Opening for Various Pressures | |||
3 in. of water | 1 PSI | 2½ PSI | 5 PSI | |||
1,000 | 23.8 | 25,300 | 4 | 2½ | 2 | 1½ |
4,000 | 95.2 | 69,500 | 6¾ | 3¾ | 3 | 2½ |
18,000 | 428 | 139,000 | 9½ | 5½ | 4¼ | 3¾ |
25,000 | 595 | 166,000 | 10¼ | 6 | 4¾ | 4 |
56,000 | 1,330 | 253,000 | 12¾ | 7¼ | 5¾ | 5 |
100,000 | 2,380 | 363,000 | 15¼ | 8¾ | 7 | 6 |
155,000 | 3,690 | 458,000 | 17¼ | 9¾ | 7¾ | 6½ |
222,000 | 5,290 | 522,000 | 18¼ | 10½ | 8¼ | 7 |
475,000 | 11,300 | 624,000 | 20 | 11¼ | 9 | 7¾ |
735,000 | 17,500 | 648,000 | 20 | 11½ | 9¼ | 7¾ |
Unlimited | 648,000 | 20 | 11½ | 9¼ | 7¾ | |
Note: Venting equipment installed for normal operation may serve as emergency relief; provided, that it has the requisite capacity under the pressure limitation fixed by this table. Responsibility for selecting the limiting pressure is placed on the owner or operator. | ||||||
('81 Code, § 5.40.190) Penalty, see Ch. 13