§ 14a.8. Vents.
(a) Normal venting. The normal venting requirements for construction and use of tanks shall conform to the following:
(1) Atmospheric tanks shall be adequately vented to prevent, as a result of filling or emptying and atmospheric temperature changes, development of pressure or vacuum sufficient to distort the roof of a cone roof tank or pressure exceeding the design pressure in the case of other atmospheric tanks.
(2) Low pressure tanks shall be adequately vented to prevent development of pressure or vacuum, as a result of filling or emptying and atmospheric temperature changes, from exceeding the design pressure of the tank. Protection shall also be provided to prevent overpressure from any pump discharging into the tank where the pump discharge pressure may exceed the design pressure of the tank.
(3) If a tank has more than one fill or withdrawal connection and simultaneous filling or withdrawal can be made, the vent size shall be based on the maximum anticipated simultaneous flow.
(4) Unless the vent is designed to limit the internal pressure to 2 1/2 psig or less, the outlet of vents and vent drains shall be arranged to discharge in such a manner as to prevent localized overheating of any part of the tank in the event vapors from such vents are ignited.
(5) Tanks storing class IA liquids shall be equipped with venting devices which shall be normally closed except when venting under pressure or vacuum conditions. Tanks storing class IB and class IC liquids shall be equipped with venting devices which shall be normally closed except when venting under pressure or vacuum conditions, or with approved flame arresters. This paragraph shall not apply to tanks with capacities of 3,000 barrels or less containing crude petroleum in crude producing areas and outside aboveground atmospheric tanks with capacities under 1,000 gallons containing other than class IA liquids which may have open vents.
(6) Flame arresters or venting devices required in paragraph (5) may be omitted for class IB and class IC liquids if conditions are such that their use may, in case of obstruction, result in tank damage. Liquid properties justifying the omission of such devices include, but are not limited to, condensation, corrosiveness, crystallization, polymerization, freezing, or plugging. When any of these conditions exist, consideration may be given to heating, use of devices employing special materials of construction, the use of liquid seals, or inerting; for further information, see Standard No. 69—1978, Explosion Prevention Systems.
(7) Normal vents shall be sized in accordance with either the American Petroleum Institute, Standard No. 2000—1982, Venting Atmospheric and Low-Pressure Storage Tanks, or other accepted standard, or shall be at least as large as the filling or withdrawal connection, whichever is larger, but shall not in any case be less than the requirements set forth in the following table:
Capacity of Tank
(in gallons)Diameter of Vent Fitting Iron Pipe
Size (in inches)500 or less 1¼ 501 to 3,000 1½ 3,001 to 10,000 2 10,001 to 20,000 2½ 20,001 to 35,000 3 (8) Skid tanks and similar portable equipment shall be provided with a free opening vent of the following nominal pipe size, to relieve vacuum or pressure which may develop in normal operation or from fire exposure:
Tank Size Vent Size
(in inches)0- 275 1½ 276- 660 2 661- 900 2½ 901-1,000 3 (i) Vents shall be arranged to discharge in such a way as to prevent localized or overheating of, or flame impingement on, any part of the tank in the event vapors from such vents are ignited.
(ii) Drums equipped with dispensing devices must be provided with pressure and vacuum relief vents with flame arresters. These vents are to be in addition to any vent which may be an integral part of the dispensing device being used in connection with the drum.
(9) For tanks in excess of 1,100 gallons capacity, emergency relief venting for fire exposure shall be as provided under subsection (b).
(b) Emergency relief venting for fire exposure. Every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure caused by exposure fires, and such construction or device shall conform to the following:
(1) In a vertical tank, the construction may take the form of a floating roof, lifter roof, a weak roof-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 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 permit the cover to lift under internal pressure.
(2) If emergency venting is not provided in accordance with paragraph (1), the total capacity of both normal and emergency venting devices shall be not less than that derived from the following table, except as provided in paragraphs (4) and (5):
Wetted Area Versus Cubic Feet Free Air per Hour
at 14.7 p.s.i.a. and 60°F.
Square Feet CFH Square Feet CFH Square Feet CFH 20 21,100 160 168,000 900 493,000 30 31,600 180 190,000 1,000 524,000 40 42,100 200 211,000 1,200 557,000 50 52,700 250 239,000 1,400 587,000 60 63,200 300 265,000 1,600 614,000 70 73,700 350 288,000 1,800 639,000 80 84,200 400 312,000 2,000 662,000 90 94,800 500 354,000 2,400 704,000 100 105,000 600 392,000 2,800 742,000 120 126,000 700 428,000 and over 140 147,000 800 462,000
(3) The wetted area of the tank, when such term is used in paragraph (2), shall be calculated on the basis of 55% of the total exposed area of a sphere or spheroid, 75% of the total exposed area of a horizontal tank, and the first 30 feet abovegrade of the exposed shell area of a vertical tank. Approximate wetted areas for horizontal tanks may be found in the following table:
Tank Approximate Wetted Areas for Horizontal Tanks Diameter (Wetted Area Equals 75% Total Area) (feet) 3 4 5 6 7 8 9 10 11 12 Tank
LengthApproximate Wetted Area Of Tanks With Flat Heads (feet) 3 32 4 39 55 5 46 65 88 6 53 74 100 128 7 60 84 112 142 173 8 67 93 124 156 190 226 9 74 102 136 170 206 245 286 10 82 112 147 184 223 264 308 353 11 88 121 159 198 239 283 329 377 428 12 95 131 171 213 256 301 350 400 454 509 13 102 140 183 227 272 320 371 424 480 537 14 109 150 194 241 289 339 393 447 506 565 15 116 159 206 255 305 358 414 471 532 594 16 123 169 218 269 322 377 435 495 558 622 17 130 178 230 283 338 395 456 518 584 650 18 137 188 242 298 355 414 477 542 610 678 19 197 253 312 371 433 499 565 636 707 20 206 265 326 388 452 520 589 662 735 21 216 277 340 404 471 541 612 688 763 22 225 289 354 421 490 562 636 714 792 23 235 300 368 437 508 584 659 740 820 24 244 312 383 454 527 605 683 765 848 25 324 397 470 546 626 706 791 876 26 336 411 487 565 647 730 817 905 27 347 425 503 584 668 754 843 933 28 359 440 520 603 690 777 869 961 29 371 454 536 621 711 801 895 989 30 383 468 553 640 732 824 921 1018 31 395 482 569 659 753 848 947 1046 32 496 586 678 775 871 973 1074 33 510 602 697 796 895 999 1103 34 524 619 715 817 918 1025 1131 35 539 635 734 838 942 1051 1159 36 553 652 753 860 966 1077 1187 37 567 668 772 881 989 1103 1216 38 685 791 902 1013 1129 1244 39 701 810 923 1036 1155 1272 40 718 828 944 1060 1181 1301 41 734 847 966 1083 1207 1329 42 751 866 987 1107 1233 1357 43 767 885 1008 1130 1259 1385 44 904 1029 1154 1284 1414 45 923 1051 1178 1310 1442 46 941 1072 1201 1336 1470 47 960 1093 1225 1362 1498 48 979 1114 1248 1388 1527 49 998 1135 1272 1414 1555 50 1157 1295 1440 1583 51 1178 1319 1466 1612 52 1199 1342 1492 1640 53 1220 1366 1518 1668 54 1246 1389 1544 1696 55 1263 1413 1570 1725 56 1437 1593 1753 57 1460 1622 1781 58 1484 1648 1809 59 1507 1674 1839 60 1531 1700 1866 61 1726 1894 62 1752 1923 63 1778 1951 64 1803 1979 65 1829 2007 66 1855 2036 67 2064 68 2092 69 2120 70 2149 71 2177 72 2205 (4) The total emergency relief venting capacity for any specific liquid may be determined by the following formula:
Cubic feet of free air per hour =
V = cubic feet of free air per hour from the table of paragraph (2).
L = latent heat of vaporization of specific liquid in Btu per pound.
M = molecular weight of specific liquid.
(5) The required air flow rate, as set forth in paragraphs (2) and (3), may be multiplied by the following appropriate factor set forth when protection is provided as indicated; only one factor may be used for any one tank:
(i) Factor .5 for drainage in accordance with the drainage provisions of § 14a.6 (relating to dikes, walls and drainage) for tanks over 200 square feet of wetted area.
(ii) Factor .3 for approved water spray.
(iii) Factor .3 for approved insulation.
(iv) Factor .15 for approved water spray with approved insulation.
(6) Venting devices shall be certified by the manufacturer as to actual rated flow capacity in cubic feet of free air per hour, specifying the pressure in psig at which the flow capacity was determined. The stamping of such information on the devices by the manufacturer shall be acceptable as certification for this purpose.
(7) The outlet of all vents and vent drains on tanks equipped with emergency venting to permit pressures exceeding 2½ psig 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.
(c) Vent piping for aboveground tanks. Vent piping for aboveground tanks shall conform to the following:
(1) Vent piping shall be constructed under § § 14a.51—14a.58 (relating to piping, valves and fittings). Where vent piping for tanks storing class I liquids is adjacent to buildings or public ways, the vent piping shall be located so that the vapors are released at a safe point away from buildings and not less than 12 feet above the adjacent ground level. In order to aid their dispersion, vapors shall be discharged upward so that flammable vapors will not be trapped by eaves or other obstructions and shall be at least 5 feet from building openings. Vent outlets for all classes of flammable and combustible liquids shall be above normal snow accumulation on top of tanks.
(2) When tank vent piping is manifolded, pipe sizes shall be able to discharge, within the pressure limitations of the system, the vapors they may be required to handle when manifolded tanks are subject to the same fire exposure. The manifolding of tank vent piping shall be avoided except if required for special purposes such as vapor recovery, vapor conservation or air pollution control.
(3) Vent piping for tanks storing class I liquids shall not be manifolded with vent piping for tanks storing class II or class III liquids unless positive means are provided to prevent the vapors from class I liquids from entering tanks storing class II or class III liquids to prevent contamination and possible change in classification of the less volatile liquid.
Authority The provisions of this § 13.8 issued under act of April 27, 1927 (P. L. 450, No. 291) (35 P. S. § § 1181—1194); transferred from 37 Pa. Code and renumbered as 34 Pa. Code § 14a.8 under section 4(b) of the Combustible and Flammable Liquids Act (35 P. S. § 1244(b)).
Source The provisions of this § 13.8 amended through June 22, 1984, effective June 18, 1984, 14 Pa.B. 2131; transferred from 37 Pa. Code and renumbered as 34 Pa. Code § 14a.8, February 28, 2014, effective March 1, 2014, 44 Pa.B. 1233. Immediately preceding text appears at serial pages (259913) to (259919)
Cross References This section cited in 34 Pa. Code § 14a.25 (relating to vents for underground tanks); 34 Pa. Code § 14a.111 (relating to storage and handling); 34 Pa. Code § 14a.141 (relating to scope and application); 34 Pa. Code § 14a.151 (relating to scope and application); and 34 Pa. Code § 14a.161 (relating to general provisions).
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