15-24-190 Enclosed tank vents.
   (a)   Vent pipes for enclosed storage tanks for fuel oil in connection with oil-burning equipment, and diesel oil in connection with emergency diesel-driven equipment shall be located so that the discharge point is outside of the building, and terminate in a weatherproof hood higher than the fill pipe opening. When such tanks contain heaters or hot wells, the opening of the vent pipe shall not be less than 12 feet above the adjacent ground level. Flame arresters shall be required for two and one-half inch and three inch vent pipes. Where a pump is used with a tight seal fill, the vent shall not terminate on or above any roof.
   (b)   Vent openings and vent pipes shall be of ample size to prevent pressure in the tank during filling but not smaller than the pipe size as specified below:
 
Capacity of Tank
U.S.Gallons
Diameter of Vent, Iron Pipe Size (inches)
500 or less
1-1/4
501 to 3,000
1-1/2
3,001 to 10,000
2
10,001 to 20,000
2-1/2
20,001 to 25,000
3
 
   Vents for Aboveground Flammable Liquid Tanks. Every aboveground flammable liquid tank shall have a vent opening for emergency and normal venting capacity not less than specified in Table 15-24-190(b).
      A.   Normal venting capacity requirements shall be obtained without exceeding the operating pressure or vacuum which may be applied regularly to a tank without causing physical damage or permanent deformation to the tank.
      Inbreathing (vacuum relief) venting capacity requirement for maximum movement out of a tank shall be equivalent to 560 cubic feet of free air per hour for each 4,200 gallons per hour of maximum emptying rate, including gravity flow rate to other tanks, for flammable liquids of any flashpoint. Venting capacity requirement for thermal inbreathing for a given tank capacity for flammable liquids of any flashpoint shall be as shown in Column 2 of Table 15-24-190(a) of this section.
      Outbreathing (pressure relief) venting capacity requirement for maximum flammable liquids movement into a tank and resulting evaporation:
         (a)   Class II and III flammable liquids shall be equivalent to 600 cubic feet of free air per hour for each 4,200 gallons per hour of maximum filling rate.
         (b)   Class I flammable liquids shall be equivalent to 1,200 cubic feet of free air per hour for each 4,200 gallons per hour of maximum filling rate.
      Venting capacity requirement for thermal outbreathing, including thermal evaporation, for a given tank capacity:
         (a)   Class II and III flammable liquids shall be as shown in Column 3 of Table 15-24-190(a) and in no case shall be less than one and one-fourth inch nominal inside diameter.
         (b)   Class I flammable liquids shall be as shown in Column 4 of Table 15-24-190(a) and in no case shall be less than one and one-half inch nominal inside diameter. (See Table 15-24-190(a).)
      B.   Emergency Venting Capacity Requirements. Every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure caused by exposure fires, as follows:
      In a vertical tank the construction referred to above may take the form of a floating raft, lifter roof, or a weak roof-to-shell seam. The weak roof-to-shell seam (maximum three-sixteenths-inch single fillet weld) 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 not be enough to prevent rupture of the shell or bottom of the tank if vertical, or of the shell or heads if horizontal. The total capacity of both normal and emergency venting devices shall not be less than that derived from Table 15-24-190(b). Such device may be a self-closing manhole cover, or one using long bolts that may permit the cover to lift under internal pressure, or a self-closing gauge hatch, or an additional or large relief valve or pressure vacuum valve or valves, or an open vent. 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 above grade of the exposed shell area of a vertical tank.
Table 15-24-190(a)
THERMAL VENTING CAPACITY REQUIREMENTS
(Expressed in Cubic Feet of Free Air Per Hour – 14.7 Pounds Per Square Inch Absolute at 60° F)
Tank Capacity*
Inbreathing (Vacuum) All Stocks
Outbreathing (Pressure)
(Barrels)
(Gallons)
Flashpoint 100° For Above
Flashpoint Below100°F
Tank Capacity*
Inbreathing (Vacuum) All Stocks
Outbreathing (Pressure)
(Barrels)
(Gallons)
Flashpoint 100° For Above
Flashpoint Below100°F
1
2
3
4
60
2,500
60
40
60
100
4,200
100
60
100
500
21,000
500
300
500
1,000
42,000
1,000
600
1,000
2,000
84,000
2,000
1,200
2,000
3,000
126,000
3,000
1,800
3,000
4,000
168,000
4,000
2,400
4,000
5,000
210,000
5,000
3,000
5,000
10,000
420,000
10,000
6,000
10,000
15,000
630,000
15,000
9,000
15,000
20,000
840,000
20,000
12,000
20,000
25,000
1,050,000
24,000
15,000
24,000
30,000
28,000
17,000
28,000
35,000
31,000
19,000
31,000
40,000
34,000
21,000
34,000
45,000
37,000
23,000
37,000
50,000
40,000
24,000
40,000
60,000
44,000
27,000
44,000
70,000
48,000
29,000
48,000
80,000
52,000
31,000
52,000
90,000
56,000
34,000
56,000
100,000
60,000
36,000
60,000
120,000
68,000
41,000
68,000
140,000
75,000
45,000
75,000
160,000
82,000
50,000
82,000
180,000
90,000
54,000
90,000
* Interpolate for intermediate sizes.
 
Table 15-24-190(b)
VENTING CAPACITY
Wetted Area Versus Cubic Feet Free Air Per Hour
(14.7 psi and 60° F)
Wetted Surface, Square Feet (a)
Venting Capacity Cubic Feet Per Hour
Minimum Opening, Iron Pipe Size, Inches (b)
Wetted Surface, Square Feet (a)
Venting Capacity Cubic Feet Per Hour
Minimum Opening, Iron Pipe Size, Inches (b)
20
21,000
2
30
31,600
2
40
42,100
3
50
52,700
3
60
63,200
3
70
73,700
4
80
84,200
4
90
94,800
4
100
105,000
4
120
126,000
6
140
147,000
6
160
168,000
6
180
190,000
6
200
211,000
6
250
239,000
6
300
265,000
6
350
288,000
8
400
312,000
8
500
354,000
8
600
392,000
8
700
428,000
8
800
462,000
8
900
493,000
8
1,000
524,000
10
1,200
557,000
10
1,400
587,000
10
1,600
614,000
10
1,800
639,000
10
2,000
662,000
10
2,400
704,000
10
2,800 and over
742,000
10
 
   (a)   Interpolate for intermediate values.
   (b)   These pipe sizes apply only to open vent pipes of the indicated diameter not more than 12 inches long and a pressure in tank of not more than 2.5 pounds per square inch. If tank is to be equipped with venting device or flame arrester, the vent opening must accommodate the venting device or flame arrester sized in accordance with Column 2 of Table 15-24-190(b).
(Prior code §  129.1-16)