Mitigation of compartment jet fires using water sprays
The safe design and operation of Process plants requires an ability to predict hazard consequences reliably. One particular hazard is a jet fire that might arise from the ignition of an accidental release of pressurised gas or liquid. On offshore gas and oil production platforms and also on land-based gas facilities, accidental releases might occur of high pressure natural gas sometimes containing higher molecular weight components. Industries continue to seek efficient and cost-effective means of protecting their plants and personnel from the hazards of fires. Following disasters which occurred in the past, the need for effective mitigation systems has, once again, been highlighted. Mitigation systems involving agents such as halons, which are perceived to be environmentally damaging, are currently out of favour and interest has revived in the use of water sprays. The research work presented here addresses the problem of the suppression of a compartment jet fire by water sprays. This involved studying the interaction between water spray and a turbulent jet flame inside a compartment of dimension 6x2.4x2.4 m3. The fuel used for the jet fire was propane emerging from a 2.0 cm diameter vertical nozzle and at a mass flow rate of 0.1 kg/s. The objectives of the research are to investigate the mitigation of compartment jet fires by using water sprays by the application of a computational fluid dynamics (CFD) methodology incorporating iv Summary combustion and a radiation model to study the jet fire behaviour and the temperature distribution in a compartment. In order to achieve the above objectives, it is necessary to produce a workable CFD model of an offshore module. The radiative heat exchange is considered in the modelling by using the Discrete Transfer Radiation Method (DTRM). The study of the sprays requires details of the individual drops' sizes. The Malvern Particle Sizer was used to measure the drop size of water sprays from the different spray nozzles which have been investigated in this study. The obtained drop sizes of the spray nozzles investigated are used to model the spray in FLUENT, which is a well developed CFD package used in industry and university research. The research started with the CFD modelling of the compartment fire, followed by experimental work done at the university laboratory at Buxton to validate the result of the modelling. In contrast to previous studies in which the combustion reaction was treated as a simple heat source this CFD has included a model of the combustion reaction. Comparisons are made between the experimental data and the predictions of different scenarios (i. e. steady state, different water spray arrangement and time dependent). The predicted temperature distributions from FLUENT, which includes radiation and surface heat transfer, are found to be in close agreement with the experimental data. Modelling results showed that the current version of the CFD code is able to provide a satisfactory and practical means of modelling jet fire and extinguishment processes.