This paper asserts that the API-521 proposed default average and peak values of ϵ and Tf for use with Annex A equation A.1 are most suitable for hydrocarbons. They are not suitable for large pool fires of hydrogen (LH2), LNG, and low carbon number alcohols such as methanol and ethanol.
Upon ignition, a spilled combustible liquid pool will burn in the form of a large turbulent diffusion flame. Calculating the incident flux to an observer or to plant equipment involves four steps: (a) geometric characterization of the flame, (b) estimation of flame thermal radiation properties, (c) estimation of the atmospheric attenuation coefficients, and (d) computation of the geometric view factors between the observer or plant equipment and flame.
The size of the flame will depend upon the spill surface and thermochemical properties of the spilled liquid. In particular, the diameter of the fire (if not confined by a dike), the visible height of the flame, the tilt and drag of the flame caused by wind, and the burning velocity of the liquid.
The radiative output of the flame will depend on the fire size, the extent of mixing with air, and the flame surface temperature. Some fraction of the thermal radiation is absorbed by carbon dioxide and water vapor in the intervening atmosphere. In addition, large combustible liquids pool fires produce thick smoke which can significantly obscure flame radiation. Finally, the incident flux at an observer location will depend on the thermal radiation view factor, which is a function of the distance from the flame surface, the observer’s orientation, and the flame geometry.
In recent editions of API-521 [4, 6], a fundamental equation (Annex A, A.1) for incident fire flux is provided. Equation A.1 enables a better assessment of both relief requirement and vessel integrity depending on fire type and duration. However, reasonable values of the flame emissivity, ϵ, and the average and peak flame surface temperatures, Tf, are required to calculate thermal radiation heat transfer rates to equipment and vessel surfaces that are either engulfed by the fire or exposed to thermal radiation from the fire. The vessel outer wall temperature, inner wall temperature, and heat transfer rates to the vessel vapor and liquid contents, all depend on the temperature difference between the flame surface and the vessel outer wall surface. Credible vessel wall temperatures are required to assess the estimated time to failure or estimated time to yield. Vessel wall temperatures cannot be calculated properly without reasonable values of the flame average and peak surface temperatures.
The performance of API-521 [4, 6] equation A.1 is evaluated using the proposed default values of emissivity ϵ and flame temperature (Tf ) for a variety of fuel types. The recommended peak values for LNG are ϵ ≃ 1, Tf ≃ 1, 500 K and the recommended values for ethanol are ϵ ≃ 0.18, Tf ≃ 1, 273 K.
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