Jet fire models

The most common application of jet fire models is the specification of exclusion zones around flares. 

Jet fire models based on point source radiation approximations will give poor thermal flux estimates close to the jet, and more mechanistic models should be used. The radiant energy fraction is also a source of error. The models presented here do not apply if wind is present, see Mudan and Croce (1988). 

The physical models described in Chapter 2 generate a variety of incident outcomes that arc caused by release of hazardous material or energy. Dispersion models (Section 2.3) estimate concentrations and/or doses of dispersed vapor vapor cloud explosions (VCE) (Section 3.1), physical c q)losion models (Section 3.3), fireball models (Section 3.4), and confined explosion models (Section 3.5) estimate shock wave overpressures and fragment velocities. Pool fire models (Section 3.6), jet fire models (Section 3.7), BLEVE models (Section 3.4) and flash fire models (Section 3.2) predict radiant flux. These models rely on the general principle that severity of outcome is a function of distance from the source of release. 

When a jet fire impinges on an object, its shape may be very distorted compared to the free-field shapes modeled. If the jet fire impinges perpendicularly on a flat object such as a fire-wall or deck, it will produce a thin circular flame over the object s surface. 

To estimate the impact of the jet fire on process equipment located 20 m from the source, the point source model can be used to determine the incident heat flux from the jet flame to the equipment. The incident heat flux per unit surface area of target, q" is calculated as follows. 

KAMELEON FIRE E-3D— This model is a program specifically designed to deal with hydrocarbon fires in the form of both liquid pool fires and gas jet fires. 

Calculate the thermal radiation of jet fire by simplified model proposed by Croce and Mudan (Croce Mudan 1986, Mudan 1987). 

ABSTRACT The determination of loads from accidental fires with realistic accuracy in the oil gas industry offshore and petrochemical industry onshore is important for the prediction of exposure of persoimel, equipment and structures to the fires. Standards, Codes of Practice and other similar publications refer to thermal loading from jet fires from 100 to 400kW/m and from 50 to 250kW/m for pool fires. The application of inappropriate fire loads may lead to incorrect predictions of fatalities, explosion of pressure vessels and collapse of structures. Further uncertainties are associated with heat transfer from the flame to pressure equipment and strucmres, and their behaviour when affected by accidental fires. The Paper presents results of a review of fire models from various Standards and Codes of Practice, and data obtained from full scale tests. A parametric study of the various methods used in the industry is presented. A simulation-based reliability assessment (SBRA) method has been applied to quantify potential accuracy range and its consequences to fire effects on structures. 

The review of fire models focuses on the hydrocarbon jet fires, but some pool fire data are also mentioned. 

The sensitivity of random input parameters (radiative and convective heat fluxes, flame emissivity and convection heat transfer coefficient) to the output variable of the model (time-to-loss-of-strength) have been performed in three case studies for confined jet fires and the ranges of variables in Table 2. Case four has been carried out to cover the full range of flame emissivity in both Table 1 and 2. The data used in these studies are summarised in Table 4. 

The uncertainty intervals of these four input random parameters are for the confined jet fire in Table 2. No additional information about the probabUity distributions of input random variables and their mutual relationships is assumed for modelling. For this reason, the randomness of input parameters was modelled by the independent uniform distributions between the lower and upper values in the range. Due to the robustness ofthe simulation approach, it is possible to extend the presented approach and to include more complicated behaviour of the input parameters, when this information becomes available (Ref. 9). The simulation model was implemented in MS Excel and run 100 times by the Monte Carlo method directly in the MS Excel environment. 

Thermal modeling and rate of fire progression, including jet fires and plume fires. 

The API (1996) method was originally developed for flare analysis, but is now applied to jet fires arising from accidental releases. Flare models apply to gas releases from nozzles with vertical flames. For accidental releases, the release hole is typically not a nozzle, and the resulting flame is not always vertical. For the modeling approaches presented here, the assumption will be made that the release hole can be approximated as a nozzle. The asstomption of a vertical flame will provide a conservative result, since the vertical flame will provide the largest radiant heat flux at any receptor point. 

Desensitization of heterogeneous explosives by shocks too weak to initiate propagating detonation is an important feature that must be included in numerical modeling of energetic materials under projectile or to jet impact if the system being modeled includes layers of inert materials in contact with the energetic material. The multiple shock Forest Fire model will model both desensitization and failure to desensitize effects that occur when explosives or propellants are multiply shocked. 

A simplified Chamberlain model is used for modelling the jet fire. Contrary to the fire ball modelling, the meteorological conditions are considered in this case and the computation of flame shape and heat flux is done for different directions and speeds of wind. Determination of the thermal dose takes into account the exposure time which limited by duration of gas leak (upper hmit) and the time necessary to escape from the area affected by the jet fire (lower limit). The individual risk is computed for weighted combination of different meteorological conditions. 

Various models are used for computing the heat flux of individual types of fire. In general, every model contains calculation of following quantities mass flow, duration of the gas release, shape of the gas cloud, heat generated due to combustion, heat flux. Meteorological conditions are taken into account in limited extent in models of jet fire and flash fire. Uncertainty in calculation of heat flux, selection of particular meteorological conditions and their influence on distribution of heat flux in time and space are main uncertainties of this part of the QRA. 

Consequence assessment is the main part of risk based assessment and models for different outcome types-dispersion, flash fire, jet fire, vapor cloud explosion are presented in the later sections. As the Figure 5 shows, consequence areas, component damage areas and personal injury areas concluded, of each identified hazard with a range of hole size is determined based on LNG gas properties and ambient condition. 

Walton, W. D. and K. A. Notarianni, 1993, Comparison of Ceiling Jet Temperatures Measured in an Aircraft Hanger Test Fire with Temperatures Predicted by the DETACT-QS and LA VENT Computer Models, NIST, NISTIR 4947. 

Yao, X. and Marshall, A. W., Characterizing Turbulent Ceiling Jet Dynamics with Salt-Water Modeling, Fire Safety Science-Proceedings of the Eighth International Symposium, Eds. Gottuk, G. T. and Lattimer, B. Y., pp. 927-938. 

The point source model assumes that the fire can be represented as a point that is radiating to a target at a distance, R, from the point. The model is most appropriate for calculating incident heat fluxes to targets where fluxes are in the range from 0 to 5 kW/m (SFPE, 1999). The point source model has been shown to be accurate for calculating the incident heat flux from a jet flame to a target outside the flame (Beyler, 2002). The literature contains more refined line or cylinder models (Beyler, 2002 SINTEF, 1997). 

Figure 1 Sketch of the fixed-bed biomass furnace showing the different bed modeling sections, under-fire and overfire air jet levels, and measurement ports M-2, M-3 and M-9. 

Early explosion tests with gas clouds were used to derive simple empirical correlations concerning flame extension or lifetime as a function of fuel mass released, e.g., WHAZAN, a software package of the World Bank [115]. Other approaches are subjected to a turbulent jet flame [74] or the dynamics of a rising and expanding fire ball or the impact by the heat radiation flux [31]. Common to all models is the fact that they are based on a more or less empirical approach with a contentious capability of predicting the consequences of an explosion. 

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