Cable Tray Fire Research Articles

Experimental study and modelling of real-scale vertical cable tray fires

Cables are one of the most important fire loads in nuclear power plants. It is therefore important to understand their fire behaviour and to predict their heat release rate curve. Electricité de France carried out a series of full-scale vertical cable tray fire experiments. Relevant measurement systems were used. The aim of this article is to give an overview of these experiments and their main results. Important aspects of the fire behaviour are discussed and compared with the literature. In addition, two models, namely the FLASH-CAT model and the ISO 18195 vertical cable tray model, are compared to the experiments and their ability to predict the heat release rate profile is discussed. The models are also compared with other real-scale experiments available in the literature. The ISO 18195 model shows good agreement with the different experiments for the heat release rate profile. The FLASH-CAT model obtains reasonable conservative results for the maximum heat release rate. However, the model significantly overestimates the growth rate in all the tests.

Common OECD/NEA FIRE and PRISME Cable Benchmark Exercise

This paper deals with the common OECD/NEA FIRE and PRISME Cable Benchmark Exercise conducted for a realistic cable fire scenario in an electrical system of a nuclear power plant. The major goal was to simulate a real cable fire event in order to assess the behaviour of fire models for a complex and real fire scenario with the current state of knowledge. Since a numerical Benchmark Exercise on a real fire event is quite challenging, a three-step methodology was adopted: (1) a calibration phase of the cable fire models, (2) a blind simulation of a cable fire test, and (3) the real cable fire event simulation. The selected fire event from the FIRE Database occurred in 2014 in a heater bay of a nuclear power plant. Thirteen institutions from nine NEA member countries participated in this Benchmark Exercise. Simulations were performed using computational fluid dynamics codes, a lumped parameter code, and zone codes. The simulations have provided a wide range of results which reflect the difficulty for fire simulation codes to correctly predict the development of the fire heat release rate over time of a cable tray fire, even under simple, open atmosphere conditions.

Modelling the heat release rate of PRISME experimental cable fires in a confined, ventilation controlled, environment using FLASH-CAT and FDS

Development of a Heat Release Rate (HRR) to represent a fire scenario with unknown parameters is challenging. A key step in any fire modelling approach is to define a HRR curve which accounts for the fuel type, quantities, and combustion behaviour. Without robust evidence to underpin the HRR definition, approaches must be adopted to address data uncertainty. This paper presents a methodology for defining the HRR of cable tray fires in a confined, ventilation controlled, environment. The HRR model has been developed during the PRISME-3 programme which has included both “open” and “blind” prediction exercises, giving the opportunity to develop and validate a fire growth rate model and address uncertainties. The model was developed from an existing, published, approach (FLASH-CAT, as outlined in NUREG CR-7010), which was modified, using sensitivity analysis, to better replicate experimental fire tests.The modified model, used in conjunction with the Computational Fluid Dynamics (CFD) based fire modelling tool NIST Fire Dynamics Simulator (FDS), provides comparison to experimental HRR and other HRR growth approaches, indicating the relative importance of HRR in fire model definition.

Adapted FLASHCAT methodology to model horizontal cable tray fires using computational fluid dynamics

Nuclear power plants (NPPs) have an extensive network of electrical cables and associated trays to support daily operations. As electrical cables have combustible mass, their presence adds to the fire load in such plants. Thermal stress from cable fires can cause cable failures and spread fire to other redundant cables or other equipment, jeopardizing the safe operation of NPPs. Assessing the heat release rates (HRR) of cable fires is critical for the nuclear industry as HRR regulates the thermal stress in the surrounding of a fire and other fire products that can damage the facility. This paper presents a computational fluid dynamics-based method to model the HRR of such cable fire. The method is partly based on the FLASHCAT model (modified and adapted here) and the surface temperature ignition model of the Fire Dynamics Simulator (FDS) software. The simulation set up with validation objective replicates an experiment of OECD PRISME 3 program in which cables arranged over two horizontal trays are burnt. HRR obtained from the simulation had peak HRR underestimated by 4%, and the time to reach peak HRR overestimated by 5%. The result is encouraging as it provides confidence in using the method outlined in the paper.

Experimental study on the effect of mechanical ventilation on cable tray fires in compartments

This study deals with the effect of ventilation flow rate and the ventilation configuration on the behavior of a cable tray fire in a confined and mechanically ventilated enclosure. Results are based on large-scale fire tests performed within the framework of the OECD Nuclear Energy Agency (NEA) PRISME projects. The fire source, identical for all tests, consisted in five horizontal cable trays measuring 3 m in length, stacked vertically and filled with halogen-free flame retardant cable-type, non-powered cables. The installation consists of two rooms connected by an open doorway. Two ventilation configurations were considered with two different arrangements of the inlet and outlet branches. The analysis demonstrates the effects of ventilation flow rate and air intake position on the fire heat release rate (HRR). Decreasing the ventilation flow rate reduces the maximum HRR and delays combustion on the trays, which burn one after the other rather than simultaneously. This result is the consequence of oxygen vitiation near the cable trays, which reduces the horizontal flame spread on each cable tray. The analysis also discusses the phenomenon of low frequency oscillatory behavior of fire in a ventilated enclosure. The results highlight that the conditions of occurrence of oscillations combine two criteria: a low ventilation flow rate to favor the release of unburnt gases and an appropriate configuration of the ventilation air intake to generate ignition of premixed gases.

Numerical study to reproduce a real cable tray fire event in a nuclear power plant

In this study, a numerical analysis was performed as part of an international joint research project to reproduce a real cable tray fire that occurred in the heater bay area of the turbine building of a nuclear power plant. A sensitivity analysis was performed on various input parameters to derive results consistent with the sprinkler activation time obtained from the fire event analysis. For all sensitive parameters, the normalized sprinkler activation time correlated well with the power function of the normalized sprinkler height. A correlation equation was developed to identify the sprinkler activation time at any location when determining the slope or fire growth rate under the conditions assuming a linear or t-squared heat release rate (HRR) time curve. Various cable fire growth assumptions were used to determine which assumption was better to provide the prediction coincident with the information given from the fire event analysis in terms of the sprinkler activation time and total energy generated from cables damaged by fire. In the comprehensive analysis of all the sensitive parameters, the standard deviation of the input parameters increased as the sprinkler height decreased. Within the range of the sensitivity parameter values given in this study, when considering all sprinkler heights, the standard deviation of the cable model change was the largest and that of the overhang position change was the smallest.

Application of FDS to Predicting Fire Spread in a Horizontal Cable Tray

With regard to cable tray fires, comparing the prediction results obtained through fire simulations with the measurement results from fire experiments conducted under real-life conditions is essential to ensure the predictive reliability of the model. In determining the validity of the calculation process and procedure performed in a specific fire experiment, the result may vary depending on the user’s capability. This is because of the high user dependence due to the characteristics of the fire model. In this study, we examined whether the actual spread of fire could be similarly predicted when the physical quantities of cable tray fires suggested by previous studies were applied. We also applied a fire modeling method considering the arrangement of cables to predict appropriate fire spread in a nuclear power plant and then performed a comparative analysis of the results and the experimental results. The prediction results, in terms of the fire spread time, varied depending on the arrangement type (whether loosely and tightly arranged) of cables. Finally, we devised a process of accurately predicting the heat release rate (HRR) curve as compared to the experimental results by using a sensitivity analysis of the input parameters (density, specific heat, and thermal conductivity) corresponding to the physical properties considered in the fire model.

Experimental investigation of the effects of a sidewall and cable arrangement on a horizontal cable tray fire in an open atmosphere

AbstractThe work deals with the influence of a sidewall and cable arrangement on the behavior of a fire involving horizontal cable trays in the framework of fire safety assessments in nuclear installations. The analysis is based on large‐scale fire tests performed in the open atmosphere in the frame of the OECD Nuclear Energy Agency (NEA) PRISME 3 project and on the corresponding simulations applying the FLASHCAT model for predicting the fire heat release rate. The fire configuration consists of five horizontal trays filled with PVC‐insulated power cables. The parameters investigated are the presence or absence of a sidewall and the cable arrangement (loose or tight bundles). The results show that the presence of a sidewall increases the fire HRR in comparison to a scenario without a sidewall. This effect is due to the increase of the flame spread velocity on the lower trays. Regarding cable arrangement, a tight configuration in bundles reduces the fire HRR in comparison to a scenario with a loose arrangement. This result is due to the reduction of the fire heat release rate per unit of area (HRRPUA) as well as the flame spread velocity. The performance of the FLASHCAT model in predicting the effects of the sidewall and the cable arrangement was also assessed on the basis of the fire tests and satisfactory agreements are reported. The presented analysis demonstrates that the fire scenario with horizontal cable trays against a sidewall and with a loose cable arrangement represents a conservative scenario for fire risk assessment. In addition, on the basis of these experiments, the effect of cable arrangement is more substantial than the sidewall effect.

CFD Simulations of Fire Propagation in Horizontal Cable Trays Using a Pyrolysis Model with Stochastically Determined Geometry

In this paper, a pyrolysis model for a PVC cable is constructed using results from thermogravimetric analysis, microscale combustion calorimeter and cone calorimeter experiments. The pyrolysis model is used to simulate fire propagation in horizontal cable trays. The simulated arrangement corresponds to a cable tray fire experiment from OECD PRISME 2 project. As laying the cables loosely along the horizontal trays is a random process, a novel stochastic method is developed for making the simplified cable tray geometries for the computational fluid dynamics model. In addition, as the simplified cable tray geometry has significantly smaller surface area than a real tray full of cables, the surface area was parametrically adjusted. In contrast to most of the earlier published numerical approaches for simulating cable tray fires, the presented approach does not use empirical correlations for predicting fire propagation and does not require any results from full-scale experiments for calibrating the model. The simulation results are compared to experimental results in terms of heat release rate, mass loss, tray ignition times and lateral flame spread rates. The maximum heat release rate was overpredicted by 8% on average.

Experimental study on the maximum excess ceiling gas temperature generated by horizontal cable tray fires in urban utility tunnels

Safety concerns on cable tray fires in urban utility tunnels, which may further trigger huge casualties, ceiling structure damages, power failures and other domino effects, attract increasing attention in recent years. Determining the maximum excess ceiling gas temperature (MECT) induced by cable tray fires in urban utility tunnels is crucial to evaluate the fire risks. A series of one-layer horizontal cable tray fire experiments to explore the MECT were carried out in a large-scale utility tunnel without mechanical ventilations. The number of cables on the tray was varied from 8 to 18 in the experiments. The experimental results showed that the cable tray fire burning could be divided into three distinct stages, including ignition, self-sustaining and decaying stages. In the self-sustaining combustion stage, the cable tray was found to burn relatively steady. The mean MECT was also investigated since it represents one of the main characteristics of the cable tray fire. By redefining two parameters (the heat release rate and the effective ceiling height) in three classical MECT models proposed originally based on pool-fire, these three models could be extended to be able to predict the mean MECT generated from the cable tray fire (solid combustible) within 20% deviations. Consequently, two novel models were respectively proposed to predict the mean MECT at the self-sustaining burning period and the instantaneous MECT of one-layer horizontal cable tray fire in utility tunnel, which would be useful in the field of fire protection engineering.

Improved model for estimating sidewall effect on the fire heat release rate of horizontal cable tray

The occurrence rate of fire events involved with cables is high and a fire hazard analysis is needed. A cable tray may be commonly placed on a wall. The fire hazards of multiple cable trays are affected by the supporting wall to give a higher maximum heat release rate. Taking the wall constraint effect on cable burning into account, a new model for the fire heat release rate of a cable tray fire against a sidewall is developed based on small scale (cone calorimeter) measurements. The newly developed model introduces a constant m to reflect the acceleration in cable burning due to the sidewall. With a properly chosen value of m, the results obtained using the newly developed model agree well with the experimental data. The error in the peak fire heat release rate value is considerably smaller than those of other models. This work improves the understanding of the fire risk of a cable tray with a sidewall.

Experimental investigation on maximum ceiling jet temperature generated by a vertically spreading cable fire

Maximum ceiling jet temperature (MCJT) is an important parameter for evaluating fire risk. An important characteristic of vertical cable tray fire is the upward spreading of the cable flame along the cable tray, which leads to a decrease in the distance between the fire source and compartment ceiling during such a fire. Thus, full-scale fire experiments using vertical cable trays were conducted in a confined compartment to investigate the MCJT. The upward spreading of the cable flame leads to transition of the MCJT dominated by the plume region to intermittent region of the flame. In particular, it is observed that the MCJT suddenly increases from ~200 °C to more than 600 °C at this transition point. Taking the upward spreading cable flame along the vertical cable tray as a point fire source moving with constant velocity, a modified model is proposed to estimate the MCJT. Prediction using the proposed modified model yields promising results. The local prediction error for the modified model is less than 12%, and the global prediction error is between 23.6% and 38.6%. These errors are significantly lower than those for other models that assume a fixed fire source.

Fire development in multi‐compartment facilities: PRISME 2 project

Fire development in multi‐compartment facilities: PRISME 2 project

Compartment temperature estimation of a multiple-layer cable tray fire with different cable arrangements in a closed compartment

Fire hazard analysis of multiple-layer cable tray is an important part of nuclear safety analysis. Large-scale cable fire experiments with a three-layer horizontal cable tray were conducted in a closed compartment. The vertical temperature profile in the middle of the room was acquired. Distinctive stratification phenomena were found in the vertical temperature distribution. The interface between the upper and lower layer was located at approximately the height of the top cable layer. Heat transfer between the smoke and compartment walls occurred mainly above the smoke interface. A modified non-steady energy balance model in a closed compartment which included the effect of smoke interface height was used to estimate the compartment temperatures. Compared with the experimental results, the modified model for the multiple-layer cable tray fire in a closed compartment provides better estimation than the original model.

Investigation on Vertical Temperature Profile Generated by Fire on Horizontal Multi-Layer Cable Tray in Closed Compartment of Nuclear Power Plant

Focusing on the fire risk of the horizontal multi-layer cable tray in a nuclear power plant, the characteristics of the smoke layer temperature profile generated by the fire on the multi-layer cable tray in a closed compartment and the prediction model for the smoke layer temperature were investigated. Fire experiments of a three-layer cable tray with two typical cable arrangements were conducted in a closed compartment. The experimental results showed that the stratification of vertical smoke layer temperature was evident. Based on the vertical temperature profile in the compartment center, the thermal environment resulted from the fire can be classified as top ceiling jet layer, middle smoke layer, and lower cool air layer. The unsteady-state temperature prediction model in a closed compartment was used to predict the smoke layer temperature resulted from the fire on the horizontal multi-layer cable tray. The comparison of the predicted results with the experimental temperature for the smoke layer showed that this model can accurately predict the maximum temperature of the smoke layer, with relative error less than 1%. the prediction model underestimated the temperature generated by the decay stage of the cable tray fire, and thus the global error is within 16.3% and 27.8%.

Cable tray fire tests with halogenated electric cables in a confined and mechanically ventilated facility

SummaryCable fires are one of the main fire hazards present in nuclear power plants (NPPs). Therefore, as part of the Organisation for Economic Co‐operation and Development (OECD) PRISME‐2 project, cable tray fire tests were performed both in open atmosphere conditions and in a confined and mechanically ventilated facility, called DIVA. These tests aim at showing the effects of a confined and ventilated environment on fire characteristics and consequences. This study deals with five fire tests, which used halogenated (poly [vinyl chloride] or PVC) cable types. Two tests were carried out in open atmosphere and three tests in the DIVA facility. The latter used a ventilation renewal rate (VRR) of either 4 or 15 h−1. The confined and ventilated conditions reduced the mass loss rate and heat release rate than did those obtained in open atmosphere. Furthermore, the three confined tests produced unburnt gases, which ignited in the fire room. Two explosions were highlighted for the tests that used a VRR of 4 h−1. These explosions indeed led to fast flame propagations over the entire upper part of the fire room and steep overpressures of almost 150 hPa. The low‐qualified PVC cables and the ventilation set‐up used in this study strongly contributed to the occurrence of these explosions.

Thermal Characteristics of Vertically Spreading Cable Fires in Confined Compartments

Fire risk analysis for cables in a vertical cable tray is important for fire protection design in nuclear power plants. One important characteristic of vertical cable tray fires is the upward spreading of the cable flame along the cable tray, which should be considered as a time-varying fire source. Experiments involving vertical cable tray fires with a spacing of 0 and 8 mm between cable lines were conducted in confined compartments with dimensions of 3 m × 3 m × 3 m (length × width × height). The cable in a 0.3 m wide × 2 m long vertical cable tray was ignited using a line-source gas burner of a 20-kW premixed flame system. The experiments indicate that a room with a vertical cable tray fire could be divided into three zones with respect to thermal characteristics. Notably, a middle zone having reduced temperature was observed, even though the intensity of burning did not decrease. By considering the effect of a time-varying fire source, an expression for predicting the ceiling jet temperature was proposed. For the prediction of the maximum ceiling jet temperature, the relative errors were less than 18.8%; for the entire cable fire process, the normalized Euclidean distances were less than 20.4%. The available literature data concerning a vertical cable tray fire in a confined room were used to validate this expression. It is further confirmed that the proposed expression predicts well the literature data for vertical cable tray fires in a confined space. This new expression can be used for estimating the hazards from a vertically spreading fire in a compartment in nuclear power plants.

Horizontal cable tray fire in a well‐confined and mechanically ventilated enclosure using a two‐zone model

SummaryElectrical cable trays are used in large quantities in nuclear power plants (NPPs) and are one of the main potential sources of fire. A malfunction of electrical equipment due to thermal stress for instance may lead to the loss of important safety functions of the NPPs. The investigation of such fires in a confined and mechanically ventilated enclosure has been scarce up to now and limited to nuclear industry. In the scope of the OECD PRISME‐2 project, the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) conducted more than a dozen fire tests involving horizontal electrical cable trays burning either in open atmosphere or inside mechanically ventilated compartments to investigate this topic. A semi‐empirical model of horizontal cable tray fires in a well‐confined and mechanically ventilated enclosure was developed. This model is partly based on the approach used in FLASH‐CAT and on experimental findings from IRSN cables fire tests. It was implemented in the two‐zone model SYLVIA. The major features of the compartment fire experiments could then be reproduced with acceptable error, except for combustion of unburned gases. The development of such a semi‐empirical model is a common practice in fire safety engineering concerned with complex solid fuels.

Numerical simulations of a full‐scale cable tray fire using small‐scale test data

SummaryThis paper presents a computational fluid dynamics (CFD)‐based modeling strategy for the prediction of cable tray fire development. The methodology is applied to a set of five horizontal trays (each 2.4‐m long and 0.45‐m wide) that are positioned with a 0.3‐m vertical spacing and set up against an insulated wall. Each tray contains 49 power PVC cables. Ignition is performed with an 80‐kW propane burner centrally positioned at 0.2 m below the lowest tray. A collection of four groups of cables per tray (made of one homogeneous material) is considered. These groups are separated by longitudinal slots of air to “mimic” their relatively “loose arrangement.” The thermal properties and surface ignition temperature are estimated from cone calorimetry (CC). When the ignition temperature is reached, the cables burn according to a prescribed heat release rate per unit area (HRRPUA) profile obtained from CC, as is or in a modified shape. A realistic flame pattern is predicted. Furthermore, using only data from CC, the peak HRR is underpredicted, and the time to reach the peak is overpredicted. The proposed “design” for the modified HRRPUA CC‐profile significantly improves the results.

Simulation of the pyrolysis of charring polymers: influence of the porous media properties

Recent large-scale experiments of cable tray fires have shown that the composition of the cable-sheath materials have a strong influence on the fire growth and spread. Thus, the correct prediction of the fire scenario requires the characterization of the sheath materials thermal and thermokinetic parameters. Yet, this task may be tricky for the materials whose pyrolysis generally leads to the formation of a porous residual. Whereas the physical constants related to the material degradation kinetics and to the virgin material properties can usually be determined using thermogravimetric analysis and differential scanning calorimetry, the porous residue properties are difficult to obtain and rarely available for pyrolysis models validation.The present paper deals with the simulation of reference cone calorimeter experiments carried out on PVC samples. An extension to porous media of the pyrolysis model of the CALIF3S-ISIS fire simulation software, developed at IRSN and validated on non-charring polymers, is used for the simulations. Special attention is paid to the modelling of the thermal diffusion effects in the porous residuals. On the one hand, based on the intrinsic conductivity of the different media (virgin material, intumescent intermediate state, char and gases), several effective conductivity models are tested to evaluate the relevant range for the global material conductivity. On the other hand, it is observed that the porous residuals of PVC generally contain large pores due to the material intumescence. That is why a radiative conductivity is included in the model to account for the radiative transfer in the pores. Without precise knowledge of the PVC residuals morphology, a Penetrable Sphere Model (PSM) is chosen to represent the porous medium geometry. This model is parametrized by a constant porosity deduced from the experimental data, and a variable pore size. It allows deducing a porous medium radiative conductivity which is added to the usual effective conductivity.It is shown that, in the particular case of PVC pyrolysis, the uncertainty on the purely conductive effects induces a very limited dispersion on the simulation of the Heat Release Rate (HRR), since all the considered models yield very similar results. Conversely, the sensitivity of the radiative conductivity and therefore of the pyrolysis kinetics to the PSM pore size is much larger. The optimal pore size with respect to the HRR temporal evolution is of the same order as the pore size observed in some PVC residuals. Finally, pyrolysis simulations are performed with this optimized parameter for different cone heater induced heat fluxes and sample sizes. A good overall agreement is found between the experimental and the numerical HRR evolution, except for the thinnest sample for which the numerical HRR is overestimated. More generally, this work shows that further effort on the morphological description of the porous residuals and on the modelling of the radiative transfer is required if one aims at performing predictive pyrolysis simulations.