Flammable Gas Cloud Research Articles

Quantitative risk assessment of gas leakage and explosion accident consequences inside residential buildings

In this study, a quantitative risk assessment methodology is developed for gas leakage and explosion accident consequences inside residential buildings. Meanwhile, the effective risk reduction measures are also proposed. The gas leakage and dispersion behavior is firstly analyzed to provide a real initial scenario for the subsequent explosion simulation using CFD code FLACS. And then, the explosion consequences with a real inhomogeneous flammable gas cloud are accurately predicted. Based on the probit model, the probabilities of different adverse impacts (i.e., eardrum rupture, lung damage and head impact) caused by gas explosion accidents are estimated, respectively. Combining the probability with the consequence severity scores, a risk-based concept is adopted to calculate the explosion risk index at any region of residential buildings. Meanwhile, the higher risk index among these adverse impacts is chosen at each grid point. In addition, a grid-based concept and isoline are further used to represent this explosion risk index. Furthermore, the effects of vent area ratio and broken pressure of glasses are investigated to propose the effective risk reduction measures. This methodology is applied to a typical residential building, located in Harbin, China. The results show that the explosion consequence severity at any region of residential buildings could be presented quantitatively. In addition, the risk region distribution of gas explosion inside residential buildings could be also identified. Furthermore, the minimum vent area ratio and the maximum broken pressure of glasses could be recommended to reduce gas explosion risk index. This study provides an effective analysis method and contributes to develop safety measures and improve the emergency response plans for gas leakage and explosion accidents inside residential buildings.

Effect of wind condition on unintended hydrogen release in a hydrogen refueling station

Ambient condition, especially the wind condition, is an important factor to determine the behavior of hydrogen diffusion during hydrogen release. However, only few studies aim at the quantitative study of the hydrogen diffusion in a wind-exist condition. And very little researches aiming at the variable wind condition have been done. In this paper, the hydrogen diffusion in different wind condition which including the constant wind velocity and the variable wind velocity is investigated numerically. When considering the variable wind velocity, the UDF (user defined function) is compiled. Characteristics of the FGC (flammable gas cloud) and the HMF (hydrogen mass fraction) are analyzed in different wind condition and comparisons are made with the no-wind condition. Results indicate that the constant wind velocity and the variable wind velocity have totally different effect for the determination of hydrogen diffusion. Comparisons between the constant wind velocity and the variable wind velocity indicate that the variable wind velocity may cause a more dangerous situation since there has a larger FGC volume. More importantly, the wind condition has a non-negligible effect when considering the HMF along the radial direction. As the wind velocity increases, the distribution of the HMF along the radial direction is not Gaussian anymore when the distance between the release hole and the observation line exceeds to a critical value. This work can be a supplement of the research on the hydrogen release and diffusion and a valuable reference for the researchers.

The nature of flammable cloud volumes in semi-confined environment under the influence of flow of air

In the Explosion Risk Analysis (ERA), ventilation and dispersion calculations using Computational Fluid Dynamics (CFD) are usually used when the level of confinement and congestion cannot be neglected. As far as the dispersion analysis is considered, alternative approaches are sought when a large number of simulations is required. Setting many scenarios and simulate them all is not always suitable within the time-frame of real engineering design. As a result, semi-empirical dispersion models and several procedures based on statistical approaches using CFD have been proposed by other researchers to improve the robustness and the accuracy of prediction of the flammable gas cloud volume. In addition, despite the use of Response Surface Method (RSM) and Frozen Cloud Approach (FCA), it is convenient to address the problem on the basis of the physics underlying the dispersion of a scalar in the chemical process area. In this work we developed a novel mathematical approach for dispersion calculations based on CFD data. We propose a new dimensionless number balancing the transport of the flammable cloud and the accidental leak rate. Numerical experiments have shown that the dimensionless number is related to the angle of the wind direction as the angles in a circle are related to the phase angle in a sine wave Vˆ≈sin(kφπ+β), where Vˆ is the non-dimensional flammable cloud volume and φ the phase angle. The effect of wake wind is investigated and there seems to be no preferable wind direction that leads to the largest cloud volume in the module for the geometry considered in the current research.

On the non-monotonic wind influence on flammable gas cloud from CFD simulations for hazardous area classification

Hazardous areas are defined as a result of a variety of variables as storage temperature, pressure, leak orifice size, physical properties of flammable substance, and wind characteristics. The potential formation of an explosive atmosphere must be accurately assessed to ensure process safety. Therefore, computational fluid dynamics (CFD) arises as an important tool for accurate predictions as recommended by the international standard IEC 60079-10-1 (2015). This study aims to analyze the influence of wind velocity magnitude and direction on the hazardous area classification. The authors evaluated the extent and volume for methane, propane, and hydrogen leakages from a CFD model. For each flammable gas, the wind velocity magnitude and direction were regularly varied. The outcomes show that the behavior of the plume size as the wind varies mainly depends on the gas concentration. Counter-flow wind directions lead to zero relative velocity closer to the release point, which concentrates the gas, and wind in the release direction promotes a higher dilution of the gas cloud increasing the hazardous extent while decreases the volume. As a consequence, the wind also influences the zone type, which was accurately predicted from CFD simulations and significant differences were found when compared to the standard analyses. These differences are, to some extent, related to the consideration of wind velocity effects on the gas jet release.

Laminar Burning Velocity of the Dimethyl Carbonate–Air Mixture Formed by the Li-Ion Electrolyte Solvent

If a Li-ion cell fails and the electrolyte leaks out into air, a flammable premixed gas cloud can be formed. The electrolyte combustion energy is 65–70% of the total energy content of the cell. The main objective of this study is to determine the laminar burning velocity and the Markstein length for dimethyl carbonate and propane in a 20-liter explosion sphere with initial conditions at 100 kPa and 300 K. Five different stretch extrapolation models for the laminar burning velocity give practically the same result. The experimental results agree well with the previously published data and are slightly lower than the theoretical predictions. The laminar burning velocity for dimethyl carbonate is measured close to the saturation point under the initial conditions, which has not been previously reported.

A numerical study of unintended hydrogen release in a hydrogen refueling station

Risk analysis and consequence assessment aimed at the hydrogen refueling station are of great importance for the large-scale commercialization of the hydrogen refueling station. In this paper, the unintended hydrogen release in both momentum-buoyancy dominated regime and momentum dominated regime is investigated numerically. The profile of the flammable gas cloud in release direction, buoyancy/gravity direction and natural convection direction, namely the y direction, z direction and x direction, is plotted quantitatively and comparison between two regimes is presented in detail. Results indicate that in the momentum-buoyancy dominated regime the profile of the flammable gas cloud moves upward during the continuously diffusion process. While in momentum dominated regime, the flammable gas cloud is more likely to be accumulated near the ground. Along the release direction, the marginal profile of the flammable gas cloud is determined by the total volume of the hydrogen in the tank. When the initial pressure increases from 40 MPa to 80 MPa, the marginal profile of the flammable gas cloud is nearly identical at the end of the release. In the natural convection direction, the hydrogen diffusion is driven by the concentration gradient. The hydrogen diffuses symmetrically along the release hole and the profile of the flammable gas cloud is similar in each condition. The results will provide a valuable reference for the later study of the explosion.

A numerical study of hydrogen leakage and diffusion in a hydrogen refueling station

Studies focused on the behavior of the hydrogen leakage and diffusion are of great importance for facilitating the large scale application of the hydrogen energy. In this paper, the hydrogen leakage and diffusion in six scenarios which including comparison of different leakage position and different wind effect are analyzed numerically. The studied geometry is derived from the hydrogen refueling station in China. Due to the high pressure in hydrogen storage take, the hydrogen leakage is momentum dominated. The hydrogen volume concentration with the variation of the leakage time in different scenarios is plotted. More importantly, profiles of the flammable gas cloud at the end of the leakage are quantitatively studied. Results indicate that a more narrow space between the leakage hole and the obstacle and a smaller contact area with the obstacle make the profile of the flammable gas cloud more irregular and unpredictable. In addition, results highlight the wind effect on the hydrogen leakage and diffusion. Comparing with scenario which the wind direction consistent with the leakage direction, the opposite wind direction may result in a larger profile of the flammable gas cloud. With wind velocity increasing, the profile of the flammable gas cloud is confined in a smaller range. However, the presence of the wind facilitates the form of the recirculation zone near the obstacle. With an increase of the wind velocity, the recirculation zone moves downward along the obstacle. Thus, the hydrogen accumulation is more prominent near the obstacle.

Flexible alarming mechanism of a general GDS deployment for explosive accidents caused by gas leakage

Gas detection system (GDS) is critical for identifying and suppressing the flammable or hazardous gas leaked by incidents. The spread of a flammable gas cloud is susceptible to external factors and highly uncertain, but the GDSs deployed today often raise alarms based on certainty. It means that the threshold value set in a GDS for triggering alarms is a constant. Such an approach sometimes cannot accurately determine the leakage location, and so it may result in a great delay of gas detection. To reduce the risk of gas leakage, this paper proposes a flexible triggering mechanism for GDSs, where an alarm is raised or not depending on the number of sensors whose reading are reaching or closing to the preset threshold. An equivalent gas cloud (EGC) model is introduced here to provide design inputs for such a flexible triggering GDS. The volume and location distribution of an EGC are estimated with the Gaussian dispersion model, based on the changes of wind speed and direction. A case study in a terminal station of LNG is conducted, to illustrate the effectiveness of the new approach. The findings show that the flexible triggering mechanism is able to make alarms of a GDS more accurate, and meanwhile avoiding unnecessary measures, which can effectively optimize alarming thresholds.

Study of Gas Fuel Leakage and Explosion in the Engine Room of a Small LNG-Fuelled Ship

The engine fuel piping in LNG-fuelled ships’ engine room presents potential gas explosion risks due to possible gas fuel leakage and dispersion. A 3D CFD model with chemical reaction was described, validated and then used to simulate the possible gas dispersion and the consequent explosions in an engine room with regulations commanded ventilations. The results show that, with the given minor leaking of a fuel pipe, no more than 1kg of methane would accumulate in the engine room. The flammable gas clouds only exit in limited region and could lead to explosions with an overpressure about 12 mbar, presenting no injury risk to personnel. With the given major leaking, large region in the engine room would be filled with flammable gas cloud within tens of seconds. The gas cloud might lead to an explosion pressure of about 1 bar or higher, which might result in serious casualties in the engine room.

Simulation and assessment of gas dispersion above sea from a subsea release: A CFD-based approach

This paper presents a comprehensive simulation and assessment of gas dispersion above sea from a subsea release using a Computational Fluid Dynamics (CFD) approach. A 3D CFD model is established to evaluate the behavior of flammable gas above sea, and a jack-up drilling platform is included to illustrate the effect of flammable gas cloud on surface vessels. The simulations include a matrix of scenarios for different surface release rates, distances between surface gas pool and offshore platform, and wind speeds. Based on the established model, the development process of flammable gas cloud above sea is predicted, and the dangerous area generated on offshore platform is assessed. Additionally, the effect of some critical factors on flammable gas dispersion behavior is analyzed. The simulations produce some useful outputs including the detailed parameters of flammable gas cloud and the dangerous area on offshore platform, which are expected to give an educational reference for conducting a prior risk assessment and contingency planning.

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.

Gas dispersion and deflagration above sea from subsea release and its impact on offshore platform

Flammable gas dispersion and deflagration above sea resulting from subsea release have the possibility to cause an adverse impact on the operation safety of surface vessels. This paper presents a systematic simulation of gas dispersion and deflagration above sea from a subsea release using CFD-based approach, aiming to study the evolution process of accident scenario and assess its impact on surface vessels. A jack-up drilling platform is used as a case to discuss the impact of gas dispersion and deflagration. The uniqueness of this study is the integration of release, dispersion, and deflagration scenarios. The simulation includes the modelling of wind flow, gas dispersion and subsequent deflagration due to accidental ignition. The wind flow above sea surface is given using wind speed profile equation to generate steady wind field around offshore platform. Simulation of gas dispersion is conducted based on the generated wind field. The development of flammable gas cloud is predicted, and the dangerous area on offshore platform is assessed. Considering the appearance of ignition source, a deflagration simulation based on dispersion results is performed to predict the consequences including overpressure, high temperature, and heat radiation. Eventually, the damage criterion is utilized to evaluate their impact on offshore platform.

Experimental study of gasoline vapor deflagration in a duct with an open end

An experimental investigation was conducted on gasoline vapor deflagration in a duct (100 × 100 mm, L/D = 4, 6, 10) with the ignition end closed and the other end open. The tests were focused on the flame behavior and the pressure generation inside and outside the duct. High-speed flame images, schlieren images, and the instantaneous pressure at different test points were recorded. The experimental results verified Bychkov's model [Combust. Flame 150 (2007) 263] in predicting the flame propagation at the early stages. Moreover, a new model was proposed to predict the axial flame moving distance when the flame front moved out of the tube. This model predicts the experimental data precisely when L/D = 4–10, and the deviation increases with larger L/D ratio. Effusive gas mixture gave rise to a vortex ring at the edge of the opening and leads to the formation of a mushroom-shaped gas cloud. A distortion of the flame skirt was observed several milliseconds after the flame ejection at about 10–40 mm away from the opening. The entire external flame was enveloped by the flammable gas cloud. Three peak pressures were obtained for each test point inside the vessel. The first peak was observed when the flame shirt touched the side walls. The second peak appeared when the flame front reached the opening. The third peak pressure was a result of the external explosion. For external test points, the pressure-time curves reached their peaks when the combustion reactions ended, and then dropped below zero due to the dissipation of energy and the low density of the burned gas.

Probabilistic explosion risk analysis for offshore topside process area. Part II: Development of gas cloud multivariate frequency distribution (MVFD)

Flammable gas cloud frequency distribution forms a key part of the CFD-based probabilistic explosion risk analysis (ERA). As part of this series work, a new type of gas cloud frequency distribution, namely, ignited gas cloud frequency distribution has been proposed in the previous study (Part I). On that basis, an advanced gas cloud frequency distribution is to be introduced continuously, which is the ultimate goal of this series work. The current study contributes to proposing a multivariate frequency distribution (MVFD), which can provide both gas cloud size and position to investigate explosion scenarios. As a matter of fact, the consequences of explosion accidents may vary greatly, depending on variables such as ignition position, gas cloud size, position and shape etc. So far, except for the gas cloud size that can be provided by the gas cloud frequency distribution, the remaining variables are more likely to be determined by engineering judgement and experience. Though this process follows standard guidelines or recommended practices, the determined variables can vary widely depending on engineers. In the present study, however, the gas cloud position is determined probabilistically as well. In order to input the gas cloud position into the proposed MVFD, the volumetric center of the equivalent gas cloud (ESC) is used to quantify the cloud position. To this end, the methodology introduced in Part I is consistently adopted in this paper, but the volumetric center is additionally monitored in the whole transient process of cloud propagation. Using the proposed distribution, the explosion scenarios can be investigated without the subjective engineering judgment of the gas cloud position, and therefore one can expect that the investigated explosion scenarios become more accurate than before. This paper provides a detailed procedure on how to obtain the proposed distribution and then presents case studies to demonstrate how the ERA results can change with the choice of the gas cloud position. Through the case studies, it is proven that the proposed MVFD is more reasonable for evaluating correct explosion design accidental loads.

Probabilistic explosion risk analysis for offshore topside process area. Part I: A new type of gas cloud frequency distribution for time-varying leak rates

Explosion risk analysis (ERA) is known as one of the dedicated safety studies for offshore installations and its purpose is to evaluate the explosion design accidental loads (DALs) on offshore topside structures and facilities. In general, ERA is more likely to be implemented in a probabilistic manner because it has a problem that needs to deal with a large number of explosion scenarios. In the probabilistic ERA, flammable gas cloud frequency distribution is a kind of intermediate result, which can be obtained by integrating the results of frequency analysis and gas dispersion modeling. In general, this distribution is applied to investigate a certain number of appropriate explosion scenarios to evaluate exceedance curves, which are commonly used to determine the DALs. The use of Computational Fluid Dynamics (CFD) to perform the gas dispersion and associated ignition probability modeling has become a trend in recent offshore projects. In most cases, however, the gas cloud frequency distribution has not yet fully benefited from the CFD models due to the high computational cost. Generally, the distribution is derived only using some particular values rather than reflecting the overall results of the CFD simulations. These particular values are likely to be steady-state results calculated using constant leak rates, which reduces the benefits of the CFD simulations and time dependent ignition probability model (TDIIM). In addition, when using these values, the effect of time-varying leak rates has only been considered with some simplified methods and the accuracy of these methods is still controversial. In the current study, the main effort is focused on proposing a new type of gas cloud frequency distribution that can reflect the overall results of the CFD simulations performed with the time-varying leak rates. One aspect to keep in mind is that every instantaneous gas cloud can pose a hazard of explosion, regardless of whether the leak rate varies with time or not. Corresponding to that, a specific method is further proposed which takes into account the entire transient process of gas cloud propagation, i.e. the time history of gas cloud size and ignition probability. This paper also presents case studies in which the gas clouds are both partially and fully selected in CFD simulations, and the final results are investigated in terms of overpressure exceedance curves. The case studies have shown that the transient process of gas cloud propagation is important for obtaining an accurate ERA, and that the proposed distribution dose not results in a significant increase in total computational cost.

Elongated VCE blast waves and structural response

Elongated congested volumes are common at chemical processing and petroleum refining facilities due to the arrangement of processing units, but there have been relatively few evaluations reported for the blast loads produced by elongated vapor cloud explosions (VCEs). The accidental VCE that occurred at the Buncefield, UK facility in 2005 involved an elongated congested volume formed by the trees and undergrowth along a portion of the site boundary. Some of near‐field damage indicators present at the Buncefield site could not be reasonably explained using existing standard VCE blast load prediction techniques that are based on an assumption that the congested volume filled with flammable gas cloud is hemispherical and located at grade level.This paper summarizes recent work to define the characteristics associated with elongated congested volume VCEs and identify differences relative to standard VCEs involving compact congested volume geometries. The main conclusions from this work with regard to the blast wave shape for an elongated congested volume deflagration are: (1) the blast wave behaves as an acoustic wave along the long axis, (2) the blast wave has a very quick transition from the positive phase peak pressure to the negative phase peak pressure (i.e., quick relative to the positive phase duration), and (3) the magnitude of the pressure drop between the peak positive and negative pressures diminishes quickly with distance outside the congested volume. These observations are not consistent with the behavior of a compact congested volume geometry VCE blast wave.Deflagration and deflagration‐to‐detonation transition (DDT) regimes were also identified for unconfined elongated congested volume VCEs as a function of the normalized flame travel distance and flame speed. These regimes were verified with existing test data, including data from the on‐going RPSEA test program. These observations and the DDT regime identification provide a frame of reference to develop a better understanding of elongated congested volume VCEs.Generic structures, with properties typical of conventional construction, were analyzed to illustrate the effect of the elongated congested volume VCE blast loads on structural response. The results show that, for an elongated congested volume VCE, a high flame speed deflagration may result in more severe structural response in the near‐field than for a detonation (i.e., due to a DDT). These results provide an alternative near‐field damage indicator analysis approach for the investigation of elongated congested volume VCE incidents. © 2017 American Institute of Chemical Engineers Process Saf Prog 37: 283–293, 2018

A probabilistic visual-flowcharting-based model for consequence assessment of fire and explosion events involving leaks of flammable gases

Leaks of flammable gases from containing systems pose safety concerns in many industrial settings. In this research, state-of-the-art visual flowcharting methodology is employed to develop a probabilistic model to quantify occupational risks of fire and explosion events initiated by leaks that ignite within enclosed spaces. In this model, leak initiation time and leak type (small, medium, or large) are selected based on user-specified probability distribution function and leak probability ranges, respectively. Other inputs to the model include probability distribution of time to failure of mechanical ventilation in the enclosed space, likelihood of presence of an ignition source with energy ≥ minimum ignition energy (MIE) of formed flammable gas cloud, probability of leak detection prior to ignition, and conditional probabilities of fires and explosions, given ignition. The model checks whether randomly-selected times of leak initiation and ventilation failure are within user-specified mission time. Number of personnel present near leak source is determined by a user-selected probability distribution. Uncertainties of input probabilities are propagated through the model using Monte Carlo sampling technique. Given occurrence of an undetected gaseous leak in conjunction with presence of an ignition source, ventilation failure, and presence of personnel close to the hazard source, the model calculates frequencies of risks of fire or explosion injuries, averaged over 106 Monte Carlo trials per simulation run. Functionality of proposed model is demonstrated by a hydrogen refueling station (HRS) case study in which gaseous hydrogen is postulated to leak from its compressor system. Base case and worst case scenarios as well as sensitivity cases are considered and their simulation results show that, for these postulated scenarios, compressor's small H2 leaks (unlike medium and large leaks) pose intolerable occupational risk frequencies that exceed the acceptable risk level of 1.0E-4/year as well as NFPA's selected risk guideline of 2.0E-5/year which is driven by the comparative risk to gasoline stations. To mitigate predicted occupational risks to acceptable levels, safety control measures and best practices are recommended. The proposed model can be used as a training tool for first responders to fire and explosion events initiated by leaks of flammable gases. The model allows user-specified ‘what-if’ scenarios with or without risk mitigation measures. In addition to HRS, the model can be applied to a broad range of industrial applications such as natural gas refueling stations, indoor chiller systems which employ flammable refrigerants, and warehouses equipped with hydrogen-powered forklifts. Risk insights from this model's simulations can also support safety codes & standards and root cause investigations of industrial fire and explosion events.

Louvered jet fire deflector – concept and concept validation for flame length reduction, flame deflection and explosion overpressure reduction

In hydrocarbon processing industries, the standard approach to prevent jet fire escalation to adjacent facilities is to provide a solid fire wall. However, a solid firewall impedes the air flow through the plant, decreasing ventilation and increasing the potential flammable gas cloud size resulting in the generation of higher explosion overpressures if the gas cloud finds an ignition source. The conflicting impact of solid fire walls in preventing jet fire escalates but conversely increasing the potential explosion overpressures is a perennial design issue faced by safety engineers. This paper presents an alternate louvered jet fire deflector design concept; along with the validation work undertaken to demonstrate the explosion overpressure reduction benefits of the louvered wall application. This louvered jet fire deflector concept was subsequently engineered, fabricated, tested and has been incorporated into the plant design. This paper is written to increase the knowledge and awareness of safety engineers of an alternate option to optimise and balance the fire and explosion risk management.

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.

Optimal blast wall layout design to mitigate gas dispersion and explosion on a cylindrical FLNG platform

This paper presents a comprehensive safety design of blast wall layout on a cylindrical Floating Liquefied Natural Gas (FLNG) platform. Comparing to the mitigation measure – safety gap investigated previously (Li et al., 2016a, 2016b), blast wall is a better option to be utilized in the congested areas with space limitation. Both of deterministic and probabilistic studies on the cylindrical FLNG platform with and without blast walls subjected to gas dispersion and explosion are conducted. The Computational Fluid Dynamics (CFD) simulation results of more than 120 gas cloud sizes and 16,000 gas explosion overpressures indicate that blast walls are exclusively beneficial for mitigating of flammable gas cloud and explosion overpressure, if the initial gas leak rates are high momentous. A series of different blast wall layouts are designed for the cylindrical FLNG. By taking all uncertainties including different leak rates, leak locations, and gas cloud conditions etc. into account, the optimal blast wall layout with lowest explosion overpressure exceedance frequencies is determined.