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Subchapter 1. Unfired Pressure Vessel Safety Orders
Article 10. Safe Practices

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Appendix A

Minimum Capacity Requirement Of Relief Valves for Fire Exposure For Liquefied Natural Gas Tanks and Vaporizers

A1. The minimum total heat influx during a possible fire exposure of an insulated container shall be computed by the applicable formula:

NOTE: Equation (1) shall be used for vaporizers with external insulation and Equation (2) for uninsulated vaporizers. The value of A is obtained by adding the area of the shell in contact with LN-Gas and the area of heat exchange surface in contact with LN-Gas.

(1) H = 1560C(1)A0.82 + Hn


H = Total heat influx, Btu per hour.

C(1) = Conductance of the insulation, Btu/sq. ft.-hr.

-deg.F. (The value of C increases with temperature

and a mean value for the range from -260 F to +

1660 F should be used.)

A = Total exposed wetted surface area in sq. ft.

Hn = Total normal heat gain to the stored liquid without

fire exposures and at maximum ambient temperatures,

Btu per hour.

A2. If the insulation system, including any jacketing material, is such that it will disappear, deteriorate or dislodge in an exposure fire, a higher heat gain will occur. This requires special consideration depending upon the extent of loss of the insulating properties. If only a part of the insulation is lost, the heat gain may be estimated by the formula:

(2) H = (34,500 - 360C(2)) A0.82 + Hn

In this case, the value of C(2) should be the mean value for the range from -260 F to +100 F.

A3. The required relief valve capacity shall be computed by the formula:


(3) Qa = 3.09 (H/L) \/ (T/M)


Qa = Required flow capacity of air, cu. ft. per hour at

60 F and 14.7 psia.

H = Total heat influx, Btu per hour from the formula

(1) or (2)

L = Latent heat of vaporization of the stored liquid,

Btu per lb. (219 may be used for LN-Gas).

T = Absolute temperature of the gas at the relief valve

inlet, deg. R.

M = Molecular weight of the gas (16.0 may be used for



1. Relocation of Appendix A filed 12-14-76; effective thirtieth day thereafter (Register 76, No. 51).

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