(a) The total emergency venting capacity (Q) of the relief devices of an uninsulated MPT must be in accordance with Table 1 or the following formula based upon the pressure relief device operating at a pressure not to exceed the test pressure:
where:
Q = Minimum required rate of discharge in cubic feet per minute of free air at standard conditions (60 °F and 14.7 psia).
M = Molecular weight of the product, or 86.7.
T = Temperature, degrees Rankine (460° + temperature in degrees F of gas at relieving temperature), or 710° Rankine.
A = Total external surface area of the tank compartment in square feet.
L = Latent heat of the product being vaporized at relieving conditions in Btu per pound, or 144 Btu per pound.
Z = Compressibility factor of the gas at relieving conditions, or 1.0.
C = Constant based on relation of specific heats, in accordance with appendix J of division 1 of section VIII of the ASME Code, 1974 edition, or 315.
(b) The total emergency venting capacity (Q) of an insulated portable tank may have a reduction if—
(1) It is shown to the Coast Guard that the insulation reduces the heat transmission to the tank;
(2) The present reduction of the emergency venting capacity (Q) is limited to the percent reduction of the heat transmission to the tank or 50 percent, whichever is less; and
(3) The insulation is sheathed.
Table 1—Minimum emergency venting capacity in cubic feet: free air/hour (14.7 lb/in2a and 60 °F)
| Exposed area square feet1 | Cubic feet free air per hour | Exposed area square feet1 | Cubic feet free air per hour |
|---|---|---|---|
| 20 | 27,600 | 275 | 237,000 |
| 30 | 38,500 | 300 | 256,000 |
| 40 | 48,600 | 350 | 289,500 |
| 50 | 58,600 | 400 | 322,100 |
| 60 | 67,700 | 450 | 355,900 |
| 70 | 77,000 | 500 | 391,000 |
| 80 | 85,500 | 550 | 417,500 |
| 90 | 94,800 | 600 | 450,000 |
| 100 | 104,000 | 650 | 479,000 |
| 120 | 121,000 | 700 | 512,000 |
| 140 | 136,200 | 750 | 540,000 |
| 160 | 152,100 | 800 | 569,000 |
| 180 | 168,200 | 850 | 597,000 |
| 200 | 184,000 | 900 | 621,000 |
| 225 | 199,000 | 950 | 656,000 |
| 250 | 219,500 | 1,000 | 685,000 |
1Interpolate for intermediate sizes.
[CGD 73-172, 39 FR 22950, June 25, 1974, as amended by CGD 84-043, 55 FR 37410, Sept. 11, 1990; 55 FR 47477, Nov. 14, 1990]
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