Quest an Improved Model for the Prediction of Radiant Heat from Fireballs

This paper presents an integrated model for predicting the radiant heat effects of fireballs created by near-instantaneous releases of superheated flammable liquids. Films and videos of fireballs show that a fireball grows quickly in size, rises into the air after some time delay, then dissipates when the available fuel is consumed. Most of the published fireball radiation models ignore this behavior and simply assume the diameter, location, and surface emissive power of the fireball are constant over the full duration of the event. In contrast, the model described in this paper provides a more realistic representation of the true behavior of fireballs by employing equations that account for fireball growth, lift-off, and changing radiative characteristics. Thermodynamic changes that occur during the release of superheated liquids are incorporated into the model, making it suitable for predicting the radiant heat effects of fireballs formed as a result of cold catastrophic failures of pressure vessels, as well as fireballs created by BLEVE incidents. Predictions of the time-varying radiant heat flux incident upon targets located outside the fireball are shown to agree well with results from moderate-scale experiments. INTRODUCTION The sudden release of superheated flammable liquid from a storage tank or process vessel is the beginning of a complex event that often ends in the formation of a short-lived fireball. The event starts with a major failure of the container. Because the pressure in the container is greater than atmospheric pressure, much of the liquid is quickly expelled into the atmosphere. In response to this rapid drop in pressure, a portion of the liquid flashes to vapor nearly instantaneously. This vapor expands rapidly, shattering some of the remaining liquid into small drops, thereby creating a turbulent aerosol cloud consisting of vapor, liquid drops, and air. The aerosol cloud quickly increases in size, entraining more air as it grows. Ignition of this aerosol cloud results in a fireball that exists until the vapor and liquid fuel within the cloud are consumed. The fireball can emit a large amount of radiant energy during its brief life, and is capable of causing injuries and damage over an area several times greater than the size of the fireball. Therefore, when conducting a hazards or risk analysis of process vessels or storage tanks that contain superheated flammable liquids, it is important to be able to accurately model the radiant heat effects of fireballs. Most fireball radiation models ignore the dynamic nature of fireballs and simply treat them as static events. This simplification often causes such models to overpredict the extent of potentially damaging or Copyright 1999, Quest Consultants Inc., 908 26th Avenue NW, Norman, Oklahoma 73069, USA. All rights reserved. Copyright is owned by Quest Consultants Inc. Any person is hereby authorized to view, copy, print, and distribute documents subject to the following conditions. 1. Document may be used for informational purposes only. 2. Document may only be used for non-commercial purposes. 3. Any document copy or portion thereof must include this copyright notice.