Vented Deflagrations Research Articles

Influence of hydrogen concentration on the pressure oscillation and flame propagation during vented deflagration of hydrogen-air mixtures

Influence of hydrogen concentration on the pressure oscillation and flame propagation during vented deflagration of hydrogen-air mixtures

Fireball and flame venting comparisons: Test data, CFD simulations, and industry standard prediction

Baker Engineering and Risk Consultants, Inc. (BakerRisk®) has performed vented deflagration testing of congested enclosures over a range of configurations, congestion levels, and fuels. This paper provides a comparison of the measured flame jetting distances to predictions made applying standard methods commonly used to calculate the associated hazard zone. These methods include the National Fire Protection Association Standard on Explosion Protection by Deflagration Venting (NFPA 68 (National Fire Protection Association, 2018, )), the British Standard's Gas Explosion Venting Protective Systems (EN 14994 (British Standard EN 14994, 2007)), and a computational fluid dynamics (CFD) analysis.Nine test series were carried out using BakerRisk's Deflagration Load Generator (DLG) test rig. The DLG is a rectangular steel enclosure measuring 48-feet wide × 24-feet deep × 12-feet tall, yielding a total volume of 13,800 ft3 (391 m3), and is enclosed by three solid steel walls, a roof, and floor. The rig vents through one of the long walls (i.e., 48-foot × 12-foot). The venting face was sealed with a 6-mil (0.15 mm) thick plastic vapor barrier for these tests to allow for the formation of the desired fuel air-mixture throughout the rig. Both slightly hyper-stoichiometric propane and lean hydrogen mixtures have been tested in the DLG. Congestion was provided by an array of vertical cylinders. A range of congestion levels and fill fractions were tested. DLG testing was performed with and without vent panels present.Flame jetting distances from the venting face of the DLG were measured using high-speed video. Flame jetting distances were predicted using the Fireball Dimensions calculation from NFPA 68 and the Flame Effects calculation from EN 14994. Blind (i.e., pre-test) simulations were also performed using the FLACS CFD code (Gexcon, 2014). The flame jetting distance in the CFD simulation was determined as the horizontal distance from the DLG vent to the location where the gas temperature dropped below a specified value; the predicted distance for the fuel concentration to drop below half the lower flammability limit (LFL) was also evaluated to assess jetting distance.

Experimental study on external explosion for vented hydrogen deflagration in a rectangular tube with different vent coefficients

Experiments on hydrogen-air vented deflagration in a smooth rectangular tube were conducted with various vent area for different hydrogen concentration. The high-speed schlieren camera was employed to visualize the external flow field. The internal and external overpressures were obtained from five pressure transducers. The effects of vent area for different hydrogen concentration were discussed on the build-up of internal/external overpressures and the evolution of external flow field. Two types of internal overpressure structure, affected by Helmholtz oscillation and external flow field, were observed for different vent coefficient and hydrogen concentration. The increase of hydrogen concentration promotes external mixture reactivity and the external overpressure. The increase of vent coefficient drives the increase of outlet speed of vented gas and flame and generally promotes external overpressure. The higher speed flame provides stronger ignition energy to external combustible gas cloud. For the high hydrogen concentration of 18 vol%, external overpressure increases linearly with vent coefficient. The rise of external overpressure can cause the increase of internal overpressure when it is large enough, particularly in the case of large vent coefficient, and the internal overpressure peak affected by external explosion can even dominate the maximum internal overpressure.

Vapor cloud explosions in various types of confined environments: CFD analysis and model validation

In the current work, three different types of vapor cloud explosion experiments are simulated. The purpose of the simulations is twofold: firstly, to evaluate a recently developed CFD model and secondly to analyze the involved phenomena with the help of the simulation results. The proposed model, which has been implemented in the ADREA-HF CFD code, utilizes the RANS method using the Kato and Launder modification of k-e model. Combustion is modelled by taking into account the main mechanisms that contribute to the phenomenon such as chemistry, turbulence generated from the obstacles in front of the flame front, flame instabilities and turbulence generated by the flame-front itself. The CFD model is evaluated against different types of explosions in different geometries and with various fuels. Uniform premixed fuel-air mixture is considered in all cases. A large scale vented deflagration experiment in a 10 m length enclosure is firstly simulated using methane as fuel. The external explosion effect is apparent in this case. Then, a hydrogen deflagration experiment in a 78.5 m tunnel is simulated. Four mock-up cars are placed in the premixed region. Finally, propane and methane explosions inside a 1.5 m tube with obstacles and intense turbulence are simulated. Two different obstacle configurations are studied. The model predicts the overpressure values satisfactorily in all the examined cases. The factors that contribute to the pressure rise in each stage of each experiment are discussed based on the simulation results.

Influences of multi vents’ numbers and distributions on vented ethanol-gasoline vapor deflagrations

The safety issues and existing risks of ethanol-gasoline in industrial production have to be well considered and investigated. Especially when there are more than one thin doors, windows/thin walls in containers were broken once deflagrations occurred, but it appears that few researches deal with multi vented deflagrations. Hence we carried out experiments in a small cuboid container to investigate the effect of multiple vents’ numbers and distributions on vented ethanol-gasoline vapor deflagrations characteristics. Multi overpressure peak structures during multi-vented deflagrations were first observed and analyzed to reveal the complex physical process in the venting. Results show that five overpressure peaks’ structures were found (Pb, Pfv, Pcv, Pext and Pac) when multiple vents with the same area are arranged along the axis of the container’s roof. Flame propagation and overpressure buildup are mainly influenced by the 1st vent (i.e., the nearest one to the spark plug). The 1st vent also determines the number and types of overpressure peak structures. Compared with the single vent, the cases of multiple vents with the constant total vent area may cause larger Pmax, dP/dtmax and KG even if all vents are closer to the spark plug. Consequently, the use of multiple vents does not necessarily help to improve the vent effects.

Effect of vent area, vent location and number of vents on vented hydrogen deflagrations in a 27 m3 chamber

Experiments in a 27 m3 chamber are performed to investigate the effect of vent parameters on the pressure development during homogenous or non-homogenous vented deflagrations, and the performance of Molkov’ models for predicting maximum overpressures is also evaluated. The results show that maximum overpressures exhibit monotonic behaviour as hydrogen concentration increases from 16% to 20.2%. Larger vent area leads to a smaller maximum overpressure, but the rate of maximum pressure decrease with the increase of the vent area is lower as the vent area increases. The vent location and number of vents have a less effect on overpressures during homogenous deflagrations, but the effect becomes more pronounced for non-homogenous deflagrations. Furthermore, two pressure peaks caused by the external explosion appear in the case of two vents. Molkov’ best-fit model over-predicts maximum overpressures for vent area of 1.76 m2, and under-predicts maximum overpressures for vent area of 3.14 m2 located on the roof, but the use of conservative model can give conservative predictions for all tests.

Hydrogen deflagrations in stratified flat layers in the large-scale vented combustion test facility

This paper examines the flame dynamics of vented deflagration in stratified hydrogen layers. It also compares the measured combustion pressure transients with 3D GOTHIC simulations to assess GOTHIC's capability to simulate the associated phenomena. The experiments were performed in the Large-Scale Vented Combustion Test Facility at the Canadian Nuclear Laboratories. The stratified layer was formed by injecting hydrogen at a high elevation at a constant flow rate. The dominant parameters for vented deflagrations in stratified layers were investigated. The experimental results show that significant overpressures are generated in stratified hydrogen–air mixtures with local high concentration even though the volume-averaged hydrogen concentration is non-flammable. The GOTHIC predictions capture the overall pressure dynamics of combustion very well, but the peak overpressures are consistently over-predicted, particularly with higher maximum hydrogen concentrations. The measured combustion overpressures are also compared with Molkov's model prediction based on a layer-averaged hydrogen concentration.

Experimental studies on external pressures during vented lean hydrogen deflagrations in a 27 m3 chamber

A series of experiments were carried out in a 27m3 cubic chamber with different vent areas to study the external pressure during vented lean hydrogen deflagrations. Uniform hydrogen-air mixtures with concentrations ranging from 15 % to 21 % and stratified mixtures with maximum concentrations equalled to average values of uniform mixtures were tested. Different layouts of obstructions were used in the experiments to achieve different volumetric blockage ratios. Experimental data from vented deflagration tests were presented, and the effect of hydrogen concentration, vent area, non-homogeneous mixtures and obstacles on the external pressure during vented deflagrations was investigated. The results show that the external explosion has an effective influence on the internal pressure by several ways. The external pressure increases with the hydrogen concentration inside the chamber, and the maximum external overpressure with back ignition is higher than that with center ignition. Larger vent area leads to a higher external pressure measured at the position far from the vent, but an opposite trend is found at the position close to the vent due to the higher velocity of jet flame corresponded to the smaller vent. As the volumetric blockage ratio increases, the external overpressure increases accordingly, and the difference in magnitude between back ignition and center ignition becomes more pronounced. However, for the non-homogenous hydrogen deflagration, whose external pressure is mainly governed by the maximum hydrogen concentration inside the chamber, only when the volumetric blockage ratio inside the chamber is large enough, the obstruction can have a significant influence on the external pressures, but its effect is much stronger for the homogenous deflagration.

Importance of properly designing dust explosion protection systems: Case study – 2014 plywood manufacturing facility fire and explosion

When designing handling and conveying equipment for combustible dusts, it is crucial to properly implement protective measures (e.g., deflagration venting, suppression, and isolation) capable of mitigating the potential consequences in the event the dust is ignited. Generally speaking, dust processing, storage, and collection equipment, such as cyclones, mills, baghouses, dryers, and silos are typically connected via pipes, pneumatic conveyors, and dust extraction or aspiration lines. If a fire or dust explosion occurs in one vessel, or the interconnecting piping, the flame front or pressure can quickly propagate through these conduits to other parts of the plant and potentially escalate to even worse consequences than the original event. Improper design of isolation devices can be costly, as illustrated by this case study on the fire and explosion in a baghouse that occurred on April 26, 2014 at a plywood manufacturing facility in Texas. More specifically, numerous deficiencies in the system design were identified, which included (1) required air stream flow rates; (2) proper isolation designs; (3) exclusion zones in the anticipated path of a vented deflagration; and (4) impeding deflagration vents. The root cause analysis identified that had a dust hazard analysis been properly performed, these deficiencies would have been identified and the explosion in the baghouse could have been avoided. © 2019 American Institute of Chemical Engineers Process Saf Prog: e12036 2019

Numerical modelling of vented lean hydrogen deflagations in an ISO container

Hydrogen process equipment are often housed in 20-foot or 40-foot container either be at refueling stations or at the portable standalone power generation units. Shipping Container provide an easy to install, cost effective, all weather protective containment. Hydrogen has unique physical properties, it can quickly form an ignitable cloud for any accidental release or leakages in air, due to its wide flammability limits. Identifying the hazards associated with these kind of container applications are very crucial for design and safe operation of the container hydrogen installations. Recently both numerical studies and experiment have been performed to ascertain the level of hazards and its possible mitigation methods for hydrogen applications. This paper presents the numerical modelling and the simulations performed using the HyFOAM CFD solver for vented deflagrations processes. HyFOAM solver is developed in-house using the opensource CFD toolkit OpenFOAM libraries. The turbulent flame deflagrations are modelled using the flame wrinkling combustion model. This combustion model is further improved to account for flame instabilities dominant role in vented lean hydrogen-air mixtures deflagrations. The 20-foot ISO containers of dimensions 20′ × 8′ × 8′.6″ filled with homogeneous mixture of hydrogen-air at different concentration, with and without model obstacles are considered for numerical simulations. The numerical predictions are first validated against the recent experiments carried out by Gexcon as part of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU) under the Horizon 2020 Framework Programme for Research and Innovation. The effects of congestion within the containers on the generated overpressures are investigated. The preliminary CFD predictions indicated that the container walls deflections are having considerable effect on the trends of generated overpressures, especially the peak negative pressure generated within the container is overestimated. Hence to account for the container wall deflections, the fluid structure interactions (FSI) are also included in the numerical modelling. The final numerical predictions are presented with and without the FSI. The FSI modelling considerably improved the numerical prediction and resulted in better match of overpressure trends with the experimental results.

An improved CFD model for vented deflagration simulations – Analysis of a medium-scale hydrogen experiment

Hydrogen is a promising alternative fuel which is expected to gain a significant portion of the energy market in the near future. However, it is a flammable gas with significant safety concerns as it can easily cause deflagrations or even detonations. The aim of this work is twofold: firstly, to derive a new CFD model for vented deflagration simulations improving an existing one and secondly, to better understand the physics of the phenomenon. A recent experiment conducted in Karlsruhe Institute of Technology is used for that purpose. The new model improves substantially the predictions representing successfully the experiment. The analysis indicates that turbulence generated outside the vent is responsible for the violence of the external explosion which leads to the sudden pressure increase. Rayleigh-Taylor instability on the other hand does not seem to contribute to burning velocity increase. From the analysis that is made combustion models are formulated highlighting the important components for a successful CFD deflagration model.

Vented hydrogen deflagrations in containers: Effect of congestion for homogeneous and inhomogeneous mixtures

This paper summarises the results from 66 vented hydrogen deflagration experiments performed in 20-foot shipping containers: 42 tests with initially homogeneous and quiescent mixtures, and 24 tests with inhomogeneous mixtures. Other parameters investigated include hydrogen concentration, vent area, type of venting device, ignition position, and the level and type of congestion inside the container. The results confirm that internal congestion can increase the maximum reduced explosion pressure in vented deflagrations significantly, compared to vented deflagrations in empty enclosures. As such, it is important to incorporate the effect of congestion in the theoretical and/or empirical correlations recommended in standards and guidelines for explosion protection. The work reported here is a deliverable from work package 2 (WP2) in the project “Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations” (HySEA). The project received funding from the Fuel Cells and Hydrogen Joint Undertaking (FCH JU).

Small scale experiments and Fe model validation of structural response during hydrogen vented deflagrations

University of Pisa (UNIPI) conducted a series of vented deflagration tests at B. Guerrini Laboratory. The tests were part of the experimental campaign performed by UNIPI for the European HySEA project (Hydrogen Safety for Energy Applications). Experiments included homogeneous hydrogen-air mixture contained in an about 1 m3 enclosure, called SSE (Small Scale Enclosure). The mixture concentration was variable between 10% and 18% vol. During the deflagrations, structural response was investigated by measuring the displacement of a test plate. The collected data were used to validate the FE model developed by IMPETUS Afea. In this paper experimental facility, displacement measurement system and FE model are briefly described, then comparison between experimental data and simulation results is discussed.

Comparison of large-scale vented deflagration tests to CFD simulations for partially congested enclosures

This paper presents a comparison between the results from a test program carried out to characterize the blast load environment within BakerRisk's Deflagration Load Generator (DLG) test rig, and predictions made using the FLACS computational fluid dynamics (CFD) code. The test data was also compared to internal peak pressure predictions made using the newest version (2018) of the National Fire Protection Association's Standard on Explosion Protection by Deflagration Venting (NFPA 68) (National Fire Protection Association, 2018). The purpose of these tests was to provide data for comparison with standard methods used to predict internal blast loads in a vented deflagration. The tests also provided a characterization of the internal DLG blast load environment for equipment qualification testing.The DLG test rig is 48 feet wide × 24 feet deep × 12 feet tall and is enclosed by three solid walls, a roof, and floor, with venting through one of the long walls (i.e., 48-foot × 12-foot). During testing, the venting face of the rig was sealed with a 6 mil (0.15 mm) thick plastic vapor barrier to allow for the formation of a near-stoichiometric propane-air mixture throughout the rig. The flammable gas cloud was ignited near the center of the rear wall. Congestion inside the rig was provided by a regular array of vertical cylinders (2-inch outer diameter) that occupied the rear half of the rig; the front half of the rig was uncongested (i.e., as would be the case for equipment qualification testing). Forty-three pressure transducers were deployed internal and external to the rig to measure blast pressure histories.Three series of tests were conducted with congestion levels corresponding to area blockage ratios (ABR) of 11%, 7.6%, and 4.2% in test series A, B and C, respectively. The obstacle-to-enclosure surface area ratio (Ar), a parameter used within the NFPA 68 correlations to quantify congestion within the vented enclosure, was equal to 0.39, 0.27, and 0.15 for test series A, B, and C, respectively. The peak pressures and impulses for each test are provided, along with pressure histories internal and external to the rig for selected tests. Comparisons of the test data to predictions made using the FLACS CFD code and NFPA 68 (2018) venting correlations are also provided.

The effect of venting process on the progress of a vented deflagration

Vented deflagrations are one of the most challenging phenomenon to be replicated numerically in order to predict its resulting pressure time history. As a matter of fact a number of different phenomena can contribute to modify the burning velocity of a gas mixture undergoing a deflagration, especially when the flame velocity is considerably lower than the speed of sound. In these conditions acceleration generated by both the flow field induced by the expanding flame and from discontinuities, as the vent opening and the venting of the combustion products, affect the burning velocity and the burning behavior of the flame. In particular the phenomena affecting the pressure time history of a deflagration after the flame front reaches the vent area, such as flame acoustic interaction and local pressure peaks, seem to be closely related to a change in the burning behavior induced by the venting process. Flame acoustic interaction and local pressure peaks arise as a consequence of the change in the burning behavior of the flame. This paper discuss the analysis of the video recording of the flame front produced during the TP experimental campaign, performed by UNIPI in the project HySEA, to describe qualitatively the contribution of the generated flow field in a vented deflagration and its influence in the peaks of the pressure-time history.

Deflagrations of localised homogeneous and inhomogeneous hydrogen-air mixtures in enclosures

Two original models for use as novel tools for the design of hydrogen-air deflagration mitigation systems for equipment and enclosures are presented. The first model describes deflagrations of localised hydrogen-air mixtures in a closed space such as a pressure vessel or a well-sealed building while the second model defines safety requirements for vented deflagrations of localised mixtures in an enclosure. Examples of localised mixtures include ‘pockets’ of gas within an enclosure as well as stratified gas distributions which are especially relevant to hydrogen releases. The thermodynamic model for closed spaces is validated against experiments available from the literature. This model is used to estimate the maximum hydrogen inventory in a closed space assuming the closed space can withstand a maximum overpressure of 10 kPa without damage (this is typical of many civil structures). The upper limit for hydrogen inventory in a confined space to prevent damage is found to be equivalent to 7.9% of the closed space being filled with 4% hydrogen. If the hydrogen inventory in a closed space is above this upper limit then the explosion has to be mitigated by the venting technique. For the first time an engineering correlation is presented that accounts for the phenomena affecting the overpressure from localised vented deflagrations, i.e. the turbulence generated by the flame front itself, the preferential diffusion in stretched flames, the fractal behaviour of the turbulent flame front surface, the initial flow turbulence in unburnt mixture, and the increase of the flame surface area due to the shape of an enclosure. Validation of the new vented deflagration model developed at Ulster has been carried out against 25 experiments with lean stratified hydrogen-air mixtures performed by the Health and Safety Executive (UK) and Karlsruhe Institute of Technology (Germany).

The use of a safety construction for explosion relief venting

The emergency venting of gaseous deflagration is widely used to prevent severe consequences of a confined explosion. Processes of vented deflagrations have been studied extensively and experimental data has been analysed. It is determined that venting starts after a safety construction is destroyed and the area of expiration of gases is opened. In the protecting structures of buildings of explosive production it is necessary to establish protective designs. In the work, triplex panels are considered as protective designs. Process of opening and driving of the panel with release of the area for a depressurization is studied during explosion development. In the work, questions of influence of pressure of opening and speed of explosive combustion at pressure peak size are investigated. The criterion for determination of efficiency of opening of panels is injected.

Large-scale vented deflagration tests

This paper presents results from a test program carried out to determine the peak deflagration pressure achieved within a congested enclosure vented through one wall of the enclosure. The industry standard in the United States for predicting the peak pressure developed in a vented deflagration is the National Fire Protection Association's Standard on Explosion Protection by Deflagration Venting (NFPA 68). The NFPA Explosion Protection Committee has compiled a database of published and unpublished explosion venting test data. This data was summarized in a 2008 report (Zalosh) that served as the foundation of the development for the vented deflagration correlations in the latest (2013) edition of NFPA 68. In this latest edition, NFPA 68 (2013), the vent area correlation accounts for varying degrees of congestion if the ratio of the obstacle surface area (Aobs) to that of the enclosure internal surface area (As) is greater than 0.4 (i.e., Ar = Aobs/As > 0.4). Congestion is accounted for within the correlation at all values of Ar, however when Ar is < 0.4, variations in the level of congestion are not accounted for. The tests described in this paper were performed using an obstacle array with an Ar ratio of less than 0.4.These tests were conducted in a rig with a 48-foot width, 24-foot depth, and 12-foot height. The rig is enclosed with solid walls, roof, and floor, allowing for venting through one of the long walls (i.e., 48-foot by 12-foot). The venting face of the rig was sealed with a 6 mil (0.15 mm) thick plastic vapor barrier to allow for the formation of a near-stoichiometric propane-air mixture. The flammable gas cloud was ignited near the center of the rear wall. Steel vent panels (20-gauge, 2 lbm/ft2) were installed over the plastic vapor barrier using explosion relief fasteners. The vent panels were configured to release at 0.3 psig; vent panel restraint devices were not utilized. The congestion inside the rig was provided by a regular array of vertical cylinders (2-inch schedule 40 pipe and 2-inch outer diameter cylinders) giving area and volume blockage ratios (ABR and VBR) within the congestion array of 4.9% and 0.5%, respectively. The obstacle-to-enclosure surface area ratio (Ar) is 0.3 with the array extended throughout the rig and vent panels installed, which is less than the critical value to account for congestion in the NFPA 68 correlation.Four series of tests were conducted with varying vent parameters, flammable gas cloud sizes, and congestion levels. Baseline tests were performed with the congestion array and flammable gas cloud extending throughout the entire rig without vent panels present (i.e., vapor barrier only). The second test series included the addition of vent panels for the same congestion pattern as that employed for the baseline tests. The third test series utilized a flammable gas cloud that filled only the back half of the rig. For the fourth test series, the congestion array occupied only ¼ of the rig. The peak pressures and impulses for each test series are provided, along with pressure histories internal and external to the rig for selected tests. The steel vent panel throw distance is also provided as a function of internal peak pressure.The test data were compared with the predictions of the vent area correlations provided in NFPA 68. For all but the fourth test series (i.e., congestion array occupying ¼ of the rig), the average internal peak pressures were approximately a factor of 2 larger than those predicted by NFPA 68. Adjustments to the NFPA 68 correlation were investigated to improve the agreement with the current test data.

Scaling effects of vented deflagrations for near lean flammability limit hydrogen-air mixtures in large scale rectangular volumes

Combustion-generated overpressures in nuclear containment buildings during a severe accident may be relieved by venting gases to adjacent compartments through relief panels or existing openings to avoid compromising a containment breach. Experimental studies on the dynamics of vented hydrogen-air combustion were extensively performed using vessels varied in shape and size at the Canadian Nuclear Laboratories. In this paper, the scaling effects are examined for near lean flammability hydrogen-air mixtures (6–12 vol.% H2) with tests performed in rectangular volumes (25, 57 and 120 m3) with a scaled vent area (Av/V2/3) of 0.02–0.05 under both initially quiescent and fan-induced turbulent conditions. This study has found that the maximum peak overpressure of all quiescent tests are dominated by the acoustic coupled effect for the hydrogen concentration greater than 8 vol.%, and the acoustic effect becomes insignificant under turbulent conditions. The measured peak over-pressures are generally over-predicted for the quiescent tests and better predicted for the turbulent tests by the well-known Bradley–Mitcheson and Molkov correlations.

Analysis of acoustic pressure oscillation during vented deflagration and proposed model for the interaction with the flame front

Explosion relief panels or doors are often used in industrial buildings to reduce damages caused by a gas explosion. Decades of research produced a significant contribution to the understanding of the phenomena involved. Among the aspects that need further research, interactions between acoustic oscillations and the flame front is one of the most important. Interactions between the flame front and acoustic oscillations has raised technical problems in lots of combustion applications as well and has been studied theoretically and experimentally in such cases. Pressure oscillations have been observed in vented deflagrations and in certain cases they are responsible for the highest pressure peak generated during the event. At Scalbatraio laboratory of Pisa University a test facility (CVE) was built in order to investigate vented hydrogen deflagrations. This paper presents an overview of the results obtained during several experimental campaigns. The tests are analysed with the focus on the investigation of flame acoustic interaction phenomena. Qualitative and quantitative analysis is presented and the generated possible physical phenomena are investigated.