Boilover in liquid hydrocarbon tank fires
thesisposted on 11.07.2014, 08:12 by Azizul Buang
Boilover is a violent ejection of certain liquid hydrocarbons due to prolonged burning during a storage tank fire. It happens due to vaporization of the water sub-layer that commonly resides at the base of a storage tank, resulting in the ejection of hot fuel from the tank, enormous fire enlargement, formation of a fireball and an extensive ground fire. Boilover is a very dangerous accidental phenomenon, which can lead to serious injuries especially to emergency responders. The boilover can occur several hours after the fuel in a storage tank caught fire. The delayed boilover occurrence is an unknown strong parameter when managing the emergency response operations. Modelling and simulation of the boilover phenomenon will allow the prediction of the important characteristics features of such an event and enable corresponding safety measures to be prepared. Of particular importance is the time from ignition to the occurrence of boilover. In order to establish a tool for the prediction of the boilover events, it is essential to understand what happens within the fuel during a fire. Such understanding is important in order to recognize and determine the mechanisms for the hot zone formation and growth which are essentials, especially for predicting the onset time of boilover. Accordingly, boilover experiments and tests were planned and carried out at field scale by the Large Atmospheric Storage Tank FIRE (LASTFIRE) project with the intentions to evaluate the nature and consequences of a boilover, and to establish a common mechanism that would explain the boilover occurrence. Undertaking field scale experiments, however, is difficult to carry out so often due to high costs and high safety concerns. In order to obtain more detailed measurements and visual records of the behaviour of the liquids in the pool, a novel laboratory scale rig has been designed, built and commissioned at Loughborough University. The vessels used in the field scale tests and the laboratory scale rig were instrumented with a network of thermocouples, in order to monitor the distribution in temperature throughout the liquid and its variation with time. The temperature distribution variation as a function of time enabled the recognition of the phases of the evolution of the hot zone and hence the mechanism of boilover. The rig has allowed well defined and repeatable experiments to be performed and hence enable to study and assess boilover in a reproducible manner. In addition, visualisation of the fuel behaviour during the experiments could be obtained to better understand the formation and growth of hot zone, the boiling of water layer and hence the boilover occurrence. A number of small and larger scale experiments had been completed to obtain a wide spectrum of results, evaluating the effect of tank diameters, fuel depth, and water depth on the rate and extent of the boilover. The analysis of the results had elucidated further the processes of the hot zone formation and its growth, and hence mechanisms involved in the boilover occurrence. The important observation was that there are three stages observed in the mechanism of boilover incidence. At the start of the fire there is a stage when the hot zone is formed. This is followed by a period when the bottom of the hot zone moves downwards at a pseudo constant rate in which the distillation process (vaporisation of the fuel s lighter ends) is taking place. The final stage involved the heating up of the lowest fuel layer consisting of components with very high boiling points and occurrence of boilover. Based on the observations of the mechanisms involved in the hot zone formation and its growth, predictive calculations were developed which focus on the provision of an estimate on the time to boilover upon the establishment of a full surface fire and an estimate of the amount of fuel remaining in the tank prior to the occurrence of the boilover. A predictive tool was developed in order to provide predictions on the important parameters associated with a boilover event i.e. the time to boilover, the amount of fuel remaining in the tank prior to boilover and hence the quantity of fuel that would be ejected during boilover and the consequences of a boilover i.e. fire enlargement, fireball effects and the ground area affected by the expulsion of oil during a boilover event. The predictive tool developed is capable of providing good estimates of onset time to boilover and predicts consequences of the boilover. The tool predicting the time to boilover of the LASTFIRE field scale test and the laboratory scales tests was shown to produce predictions that correlated with the observed time to boilover. Apart from the time to boilover, the predictive calculation is also able to provide an estimate of fuel amount remained in the tank at the instance of boilover occurrence. Consequently, the tool is capable of predicting the quantity of burning fuel being ejected and hence the area affected by the extensive ground fire surrounding the tank. The predictive results are conservatives but yet show good agreement with observed time to boilover in real boilover incidents. Certain considerations in the development of safe and effective fire fighting strategies in handling fire scenario with a potential of boilover occurrence, can be assessed using the predictive tool developed.
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