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Subchapter 7. General Industry Safety Orders
Group 20. Flammable Liquids, Gases and Vapors
Article 145. Tank Storage

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§5593. Emergency Relief Venting for Fire Exposure for Aboveground Tanks.

(a) Except as provided in (b), every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure caused by exposure fires.

(b) Tanks larger than 12,000 gallons capacity storing Class IIIB liquids and not within the diked area or the drainage path of Class I or Class II liquids do not require emergency relief venting.

(c) In a vertical tank the construction referred to in (a) 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.

(d) 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 FL-16 except as provided in (f) or (g). 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 above grade of the exposed shell area of a vertical tank.

(e) For tanks and storage vessels designed for pressures over 1 psig, the total rate of venting shall be determined in accordance with Table FL-16, except that when the exposed wetted area of the surface is greater than 2,800 sq. ft., the total rate of venting shall be in accordance with Table FL-17 or calculated by the following formula:

CFH = 1,107 A0.8 2

CFH = venting requirement, in cubic feet of free air per hour.
A = exposed wetted surface, in square feet.

The foregoing formula is based on Q = 21,000 A0.8 2

(f) The total emergency relief venting capacity for any specific stable liquid may be determined by the following formula:

Cubic feet of free air per hour = V         1337
                                     L x square root of M. 

V = cubic feet of free air per hour from Table FL-16.
L = latent heat of vaporization of specific liquid in BTU per pound.
M = molecular weight of specific liquids.

                    Table FL-16
             [14.7 psia and 60 deg. F.]
        |         |        |         |        |

 Square |         | Square |         | Square |
  feet  |   CFH   |  feet  |  CFH    |  feet  |   CFH
        |         |        |         |        |
     20 |  21,100 |    200 | 211,000 |  1,000 | 524,000
     30 |  31,600 |    250 | 239,000 |  1,200 | 557,000
     40 |  42,100 |    300 | 265,000 |  1,400 | 587,000
     50 |  52,700 |    350 | 288,000 |  1,600 | 614,000
     60 |  63,200 |    400 | 312,000 |  1,800 | 639,000
     70 |  73,700 |    500 | 354,000 |  2,000 | 662,000
     80 |  84,200 |    600 | 392,000 |  2,400 | 704,000
     90 |  94,800 |    700 | 428,000 |  2,800 | 742,000
    100 | 105,000 |    800 | 462,000 | see (e)|
    120 | 126,000 |    900 | 493,000 |        |
    140 | 147,000 |  1,000 | 524,000 |        |
    160 | 168,000 |        |         |        |
    180 | 190,000 |        |         |        |
    200 | 211,000 |        |         |        |
*Interpole for intermediate values.

Table FL-17
Sq. Ft. CHF Sq. Ft CFH
2,800 724,000 9,000 1,930,000
3,000 786,000 10,000 2,110,000
3,500 892,000 15,000 2,940,000
4,000 995,000 20,000 3,720,000
4,500 1,100,000 25,000 4,470,000
5,000 1,250,000 30,000 5,190,000
6,000 1,390,000 35,000 5,900,000
7,000 1,570,000 40,000 6,570,000
8,000 1,760,000    

(g) For tanks containing stable liquids, the required air-flow rate of (d) or (f) 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.

.5 For drainage in accordance with Section 5595(b) for tanks over 200 square feet of wetted area.

.3 For approved water spray in accordance with Standard for Water Spray Fixed Systems for Fire Protection, NFPA No. 15, and drainage in accordance with Section 5595(b).

.3 For approved insulation in accordance with (g)(1).

.15 For approved water spray with approved insulation in accordance with (g)(1) and drainage in accordance with Section 5595(b).

(1) Insulation systems for which credit is taken shall meet the following performance criteria and shall be subject to approval of the authority having jurisdiction:

(A) Remain in place under fire exposure conditions.

(B) Withstand dislodgment when subjected to hose stream impingement during fire exposure. This requirement may be waived where use of solid hose streams is not contemplated or would not be practical.

(C) Maintain a maximum conductance value of 4.0 Btu's per hour per square foot per degree oF (Btu/hr./sq.ft./ oF) when the outer insulation jacket or cover is at a temperature of 1,660o F and when the mean temperature of the insulation is 1,000o F.

(h) The outlet of all vents and vent drains on tanks equipped with emergency venting to permit pressures exceeding 2.5 psig shall be arranged to discharge in such a way as to prevent localized overheating of or flame impingement on any part of the tank, in the event vapors from such vents are ignited.

(i) 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. If the start to open pressure is less than 2.5 psig and the pressure at full open position is greater than 2.5 psig, the flow capacity at 2.5 psig shall also be stamped on the venting device. The flow capacity shall be expressed in cubic feet per hour of air at 60 F. and 14.7 psia.

(1) The flow capacity of tank venting devices under 8 inches 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 a qualified impartial outside agency. The flow capacity of tank venting devices 8 inches nominal pipe size and larger, 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.

(2) A suitable formula for this calculation is:

CFH = 1,667 CfA x square root of (Pt - Pa)

where CFH = venting requirement in cubic feet of free air per hour
Cf = 0.5 (the flow coefficient)
A = the orifice are in sq. in.
Pt = the absolute pressure inside the tank in inches of water
Pa = the absolute atmospheric pressure outside the tank in inches of water

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