Morphological characteristics of flashing jet throughout superheated liquid release

Many release problems involve two-phase releases of hazardous materials of superheated liquids with high energy into the atmosphere. Such accidents are accompanied by violent phase transition and form catastrophic flashing jets. In this work, experimental and theoretical analyses were conducted to investigate dynamic characteristics of flashing jet morphology and their dependence on pressure-decay dynamics under different storage pressures, superheats, and nozzle diameters. Flashing jet morphology and angle throughout two-phase releases were captured by a high-speed camera, and the corresponding source pressure in the superheated liquid tank was measured simultaneously. Results show that three typical phases, expansion, stabilization, and decay, are characterized throughout two-phase release based on the evolution of flashing jet morphology. The jet initially expands gradually due to the enhancement of phase transition intensity, and then keeps stable when the intensity reaches its maximum, and terminally decays rapidly due to the depletion of superheated liquid. Phase transition intensity at the nozzle exit is mainly controlled by the pressure-decay dynamics. Bubbles nucleation inception sites gradually move upstream of the nozzle during the pressure decay process increasing the phase transition intensity. The increase of storage pressure, superheat and nozzle diameter promotes the mechanical and thermodynamic effects on the jet breakup. The significant increase of mechanical and thermodynamic effects effectively accelerates droplets evaporation and further affects flashing jet morphology.     

液氨储罐泄漏扩散模型的改进研究

针对液氨储罐孔洞泄漏的实际工况,综合考虑泄漏及由于泄漏导致液氨闪蒸造成 的罐压变化,以及储罐的许用压力,对其连续泄漏过程进行了分析,以此改进现有的泄漏扩 散后果分析模型,获得较忽略这些因素更为严重的后果.最后针对灾难性事故发生前连续泄 漏最大持续时间的影响因素进行了分析.结果表明,环境温度与初始罐压对其影响较大,是决定...     

Methodology for global sensitivity analysis of consequence models

A methodology is presented for global sensitivity analysis of consequence models used in process safety applications. It involves running a consequence model around a hundred times and using the results to construct a statistical emulator, which is essentially a sophisticated curve fit to the data. The emulator is then used to undertake the sensitivity analysis and identify which input parameters (e.g. operating temperature and pressure, wind speed) have a significant effect on the chosen output (e.g. vapour cloud size). Performing the sensitivity analysis using the emulator rather than the consequence model itself leads to significant savings in computing time.To demonstrate the methodology, a global sensitivity analysis is performed on the Phast consequence model for discharge and dispersion. The scenarios studied consist of above-ground, horizontal, steady-state discharges of dense-phase carbon dioxide (CO₂), with orifices ranging in diameter from ½ to 2 inch and the liquid CO₂ stagnation conditions maintained at between 100 and 150 bar. These scenarios are relevant in scale to leaks from large diameter above-ground pipes or vessels.Seven model input parameters are varied: the vessel temperature and pressure, orifice size, wind speed, humidity, ground surface roughness and height of the release. The input parameters that have a dominant effect on the dispersion distance of the CO₂ cloud are identified, both in terms of their direct effect on the dispersion distance and their indirect effect, through interactions with other varying input parameters.The analysis, including the Phast simulations, runs on a standard office laptop computer in less than 30 min. Tests are performed to confirm that a hundred Phast runs are sufficient to produce an emulator with an acceptable degree of accuracy. Increasing the number of Phast runs is shown to have no effect on the conclusions of the sensitivity analysis.The study demonstrates that Bayesian analysis of model sensitivity can be conducted rapidly and easily on consequence models such as Phast. There is the potential for this to become a routine part of consequence modelling.     

Assessment of the maximum possible liquid superheat during flashing leak flow

During the discharge of flashing liquids through leaks due to abrupt depressurization a transient thermodynamic non-equilibrium in the form of a boiling delay in the superheated liquid flow can occur. As a consequence the actual mass flow quality is smaller than calculated under the assumption of an immediate adjustment of the thermodynamic equilibrium between the phases. For the prediction of the leak mass flow for a given pressure difference the magnitude of this self-adjusting mass flow quality is needed.

Most of the models cited in the literature include only the equilibrium mass quality as limiting quantity and ignore further effects as that of the depressurization velocity or the mean nucleus distance. For the assessment of the maximum possible liquid superheat during flashing only the conduction heat transfer from a stagnant liquid to the bubble surface is used to describe the bubble growth.

The sub-model for the bubble growth due to expansion and mass transfer necessary for the global prediction of the transient thermodynamic non-equilibrium in flashing liquids was validated using bubble radii measured by Hooper et al. [Bubble growth and pressure relationship in the flashing of superheated water. Technical publication 6904, Mechanical Engineering Department, University of Toronto, 1969] for the case of a sudden depressurization of initially saturated water. On this basis the calculated time-dependent temperature field, the actual mass quality, the mean liquid temperature and, in comparison to the corresponding values based on the assumption of immediate thermodynamic equilibrium, the maximum possible liquid superheat are predicted.     

Risk assessment methodology for high-pressure CO2 pipelines incorporating topography

This paper presents a risk assessment methodology for high-pressure CO₂ pipelines developed at the Health and Safety Laboratory as part of the EU FP7 project CO2Pipehaz.Traditionally, consequence modelling of dense gas releases from pipelines at major hazard impact levels is performed using integral models with limited or no consideration being given to weather bias or topographical features of the surrounding terrain. Whilst dispersion modelling of CO₂ releases from pipelines using three-dimensional CFD models may provide higher levels of confidence in the predicted behaviour of the cloud, the use of such models is resource-intensive and usually impracticable. An alternative is to use more computationally efficient shallow layer or Lagrangian dispersion models that are able to account for the effects of topography whilst generating results within a reasonably short time frame.In the present work, the proposed risk assessment methodology for CO₂ pipelines is demonstrated using a shallow-layer dispersion model to generate contours from a sequence of release points along the pipeline. The simulations use realistic terrain taken from UK topographical data. Individual and societal risk levels in the vicinity of the pipeline are calculated using the Health and Safety Laboratory's risk assessment tool QuickRisk.Currently, the source term for a CO₂ release is not well understood because of its complex thermodynamic properties and its tendency to form solid particles under specific pressure and temperature conditions. This is a key knowledge gap and any subsequent dispersion modelling, particularly when including topography, may be affected by the accuracy of the source term.     

A risk assessment methodology for high pressure CO2 pipelines using integral consequence modelling

This paper presents a risk assessment methodology for high pressure CO₂ pipelines developed at the Health and Safety Laboratory (HSL) as part of the EU FP7 CO2Pipehaz project. Until recently, risk assessment of dense phase and supercritical CO₂ pipelines has been problematic because of the lack of suitable source term and integral consequence models that handle the complex behaviour of CO₂ appropriately. The risk assessment presented uses Phast, a commercially available source term and dispersion model that has been recently updated to handle the effects of solid CO₂. A test case pipeline was input to Phast and dispersion footprints to different levels of harm (dangerous toxic load and probit values) were obtained for a set of pipeline specific scenarios. HSL's risk assessment tool QuickRisk was then used to calculate the individual and societal risk surrounding the pipeline. Knowledge gaps that were encountered such as: harm criteria, failure rates and release scenarios were identified and are discussed.     

Bund overtopping — the consequence of catastrophic tank failure

The Health and Safety Executive (HSE) has considerable interest in predicting the consequence of releases of hazardous liquids from storage tanks (HSE, 1998), because of its responsibilities as a statutory adviser to local planning authorities on the siting of hazardous installations and on the control of development in their vicinity, and because of its responsibilities alongside the Environment Agencies as the Competent Authority in the assessment of COMAH safety reports. As a result, HSE has commissioned a number of studies, including: experimental studies of the behavior of liquid spills (Lee and Kountouris, 1992, Skitt and Wheeler, 1989, Bentinck and Crow, 1991, Clark and Savery, 1993, Law and Johnskareng, 1994, Ruddle and Widowson, 1985, Ruddle and Elms, 1985, Cleaver, Cronin, Evans and Hirst, 2001), a review of overtopping data and theoretical models used to study liquid flow (Thyer & Jagger, 1997), the characterization of the profile of the leading edge of the spreading liquid (Thyer, MacMillan, & Jagger, 1999), and the development and use of complex mathematical models to predict overtopping fractions and liquid spill behavior (Daish et al., 1998, Ivings and Webber, 2001).This paper summarizes the consequences of catastrophic tank failure, the current position on the availability of liquid spill data for model validation, and indicates how such data may be used to predict the often considerable fraction of liquid that could overtop retaining bunds around real tanks.     

Generalising two-phase homogeneous equilibrium pipeline and jet models to the case of carbon dioxide

The safety of high pressure pipelines is nowadays routinely analysed using simple integral models to predict the nature of a release following a possible breach. Where two-phase flow may result, models making the assumption of homogeneous equilibrium between liquid and gas phases have been used successfully. In the case of a pipeline carrying carbon dioxide, however, solidification of a significant amount of the gas (owing to its high triple point pressure) is a distinct possibility, necessitating consideration of gas, liquid, and solid phases. This paper shows how integral models of two-phase flow can be generalised to accommodate the solid phase, and investigates some of the details of the flow where the triple point pressure occurs and partial solidification begins. It is shown that two-phase homogeneous equilibrium flow models may be generalised straightforwardly to cover the case of carbon dioxide, and some features are seen to emerge independently of more detailed flow modelling considerations.     

n-Compartment mathematical model for transient evaluation of fluid curtains in mitigating chlorine releases

A simple n-compartment mathematical model is developed to study the effectiveness of fluid curtains and transient behaviour in mitigating the effects of an accidental chlorine release. The model is obtained considering chemical and physical absorption effectiveness of the toxic cloud by a reacting liquid curtain. The characteristics of the curtain and the evolution were deeply studied by means of replicated wind tunnel experimental runs, according to different operating parameters. An analytical solution of the model is presented. A fairly good agreement was verified between the model predictions and the original experimental results here presented.     

Analysis of hazard areas associated with high-pressure natural-gas pipelines

The rupture of a high-pressure natural-gas pipeline can lead to outcomes that can pose a significant threat to people and property in the immediate vicinity of the failure location. The dominant hazards are thermal radiation from sustained fire and collapse of buildings from explosion inside or in a partially confined area enclosed by buildings. A simplified equation has been developed that relates the diameter, the operating pressure and the length of pipeline to the size of the affected area in the event of a full-bore rupture. The equation is based on release rate, gas jet and heat flux from fire to estimate the hazard area. Hazard area is directly proportional to the operating pressure raised to a half power, and to the pipeline diameter raised to five-fourths power, but inversely proportional to the pipeline length raised to a quarter power. The simplified equation will be a useful tool for safety management of high-pressur natural-gas pipelines.     

压力容器气体非稳态泄漏模型研究

为计算气体在非稳态泄漏过程中的泄漏率,提高危害后果评估的量化水平,对压力容器失效后气体泄漏过程进行了研究。基于现有的初始泄漏率模型,结合实际泄漏过程中压力容器内各项状态参数的动态变化规律,构建气体非稳态泄漏模型,并通过计算实例进行分析和验证。结果表明,该模型可计算压力容器气体非稳态泄漏过程中(包括音速泄漏阶段和亚音速泄漏阶段)任意时刻容器内的各项状态参数值和孔口处气体的平均泄漏率;同时,对于储存压力较高(大于3.0 MPa)的容器,提出近似计算总平均泄漏率的2种简化方法。     

Real-time risk assessment of explosion on offshore platform using Bayesian network and CFD

Combustion or explosion accident resulting from accidental hydrocarbon release poses a severe threat to the offshore platform's operational safety. Much attention has been paid to the risk of an accident occurring over a long period, while the real-time risk that escalates from a primary accident to a serious one was ignored. In this study, a real-time risk assessment model is presented for risk analysis of release accidents, which may escalate into a combustion or explosion. The proposed model takes advantage of Fault Tree-Event Tree (FT-ET) to describe the accident scenario, and Bayesian network (BN) to obtain the initial probability of each consequence and describe the dependencies among safety barriers. Besides, Computational Fluid Dynamics (CFD) is applied to handle the relationship between gas dispersion and time-dependent risk. Ignition probability model that considering potential ignition sources, gas cloud, and time series are also integrated into this framework to explain the likelihood of accident evolution. A case of release accidents on a production platform is used to test the availability and effectiveness of the proposed methodology, which can be adopted for facilities layout optimization and ignition sources control.     

A review of quantitative risk assessment of onshore pipelines

This paper reviews and analyses frequency and consequences of failure of onshore pipelines transporting oil, refined products and natural gas. Generally accepted risk levels are indicated and a desirable risk range is proposed.Pipeline failure statistics from the United States (US), Canada, Europe and Brazil are compared. Failure rates for internal and external corrosion, human action and natural forces are analyzed and the expected failure rate for each failure mechanism is indicated. The effects of relevant construction and environmental factors on the failure frequency are studied and mean trends are obtained. Furthermore, the sizes of the holes indicated at different databases are compared and a typical distribution of failure sizes is proposed for each mode of failure. Finally, the frequency of ignition after a loss of containment is studied for gas and liquid pipelines.Historical data on consequences of the accidental loss of containment of onshore pipelines is reviewed. Property damage and environmental reparation costs are evaluated directly from pipeline failure data. Straightforward regression models are proposed to quantify these types of consequence taking into account the released fluid and the characteristics of the environment. Societal impact is evaluated by combining simple fire models, heat versus mortality correlations and population density.Finally, values for the desired risk level are evaluated by three methods: i) a risk value representing the good engineering practice; ii) the risk associated to the most relevant codes and regulations concerning pipeline risk assessment and/or construction and operation; iii) an analytically derived optimal risk level. The risk values obtained by the three methodologies are similar and a desirable risk range is proposed.     

Analyzing effective factors on leakage-induced hydrogen fires

Currently, novel energy resources are receiving increasing attention as a response to the limitation in fossil fuels as well as their adverse effects on human health. Hydrogen, one of the most abundant elements on the earth, can be regarded as a new energy source to replace fossil fuels. Therefore, safety assessment of the relating processes is very crucial by increasing use of hydrogen as a fuel source. In this regard, consequence analysis for risk assessment and power reduction is very important. The present study aims at modeling hydrogen dispersion along with consequence analyses for such events as jet fire and flash fire. The model was validated by using the data derived from a study on hydrogen leakage in supply pipelines in the laboratory of the University of Pisa. Modeling results reveal that ambient conditions will impose a milder impact on leakage consequences if internal pressure is high in release source. The safe distance was also estimated to be 14 m. Dispersion consequence modeling was performed, followed by the evaluation of the effect of environmental (i.e., stability, ambient temperature, surface roughness, wind speed, and humidity) and process (i.e., vessel temperature and pressure, leakage diameter, and releasing point height) parameters on maximum size flammable vapor cloud and maximum level jet fire radiation on the ground. The size of flammable vapor cloud (consequence dispersion index) and the maximum flux of radiation were affected by process parameters more than ambient parameters. Leakage diameter and the vessel pressure were found to have the highest impact on the operational parameters.     

Case study: Assessment on large scale LPG BLEVEs in the 2011 Tohoku earthquakes

After the 2011 Tohoku earthquakes, several chemical and oil complexes on the Pacific Ocean shoreline of northeast Japan experienced massive losses. In Chiba, a refinery operated by Cosmo Oil lost 17 LPG storage vessels which were either heavily damaged or totally destroyed by fires and explosions in the refinery. These large vessels ranged in size from 1000 to 5000 m³. The estimated volume of LPG at the time of the incident was between 400 and 5000 m³ for each vessel. Five boiling liquid expanding vapor explosions (BLEVEs) of LPG occurred, resulting in huge fire balls measuring about 500 m in diameter.A BLEVE is defined as the explosive release of expanding vapor and boiling liquid when a container holding a pressure-liquefied gas fails catastrophically. It is thus important to estimate the physical properties of superheated liquids: the thermodynamic and transport properties, the intrinsic limits to superheating and depressurization, and the nature of thermodynamic paths. Also it is hoped to provide better understanding of the vessels designed, manufactured, installed, and operated to reduce or eliminate the probability that a sequence of events will result in BLEVE or loss of primary containment. Knowledge of these matters is still incomplete. The objective of this research is to estimate the significant BLEVE phenomenon in very large scale spherical vessels based on published information in Japan. There are some models predicting BLEVEs. However, it is essential to know if this is true for very large scales such as spheres since validation is usually rare to provide confidence in estimating the superheated liquids behaviors. To this end, comparing with the information on this event, the conditions in the five LPG vessels at the time of the BLEVE were determined in terms of: duration of vessel failure (time to BLEVE); mass fraction in the vessel with time; temperature distribution in the liquid and vapor region and pressure within the vessel (e.g. initial pressure and internal high-speed transient pressure during failure), by means of a computer program AFFTAC Analysis of Fire Effects on Tank Cars, which solves heat conduction, stress and a failure model of the tank, a thermodynamic model of its fluid contents, and a flow model for the lading flowing through the safety relief device. Subsequently, the consequences from the sphere BLEVE, such as the expected fireball diameter and duration and the expected blast overpressure produced by the BLEVE failures, are also subjects of active research. Here the blast using the methods of PHAST and SFPE Handbook of Fire Protection Engineering was calculated.Results suggest that methodologies here used gave reasonable estimations for such real and huge BLEVEs in a validated way, which may provide valuable guidance for risk mitigation strategy with regard to LPG facility in design, emergency planning, resiliency, operations, and risk management.     

Outflow from fractured pipelines transporting supercritical ethylene

This paper describes a methodology for predicting outflow from a rupture in a pipeline transporting supercritical ethylene. Ethylene outflow is of particular interest and is a challenging scenario to model as typical operating temperatures are a few degrees above the critical temperature, 283 K. Thus when the pipe fractures the initial rapid depressurisation induces a number of propagation waves to travel up the pipeline initiating phase changes and fundamentally changing the nature of the outflow problem.The methodology presented is primarily based on an outflow model for compressed volatile liquid outflow as this is demonstrated to be the flow regime upstream of the pipe fracture after a small time interval following pipe fracture. The model is validated using a more complex commercially available pipeline model, PROFES for propane outflow as experimental data at relatively low pressure and short pipes exist in the open literature and has been used to validate the models considered here.Finally the ethylene outflow methodology has been applied to a number of different ethylene pipeline scenarios with a range of operating temperatures, pressures and pipeline diameters of interest to confirm that predicted outflow rates trend as expected.     

On the mechanism of a BLEVE occurrence due to fire engulfment of tanks with overheated liquids

The BLEVE (boiling liquid expanding vapor explosion) effect that involves the formation of a fireball occurs at the engulfment by fire of a tank with a highly flammable liquid or a liquid gas. Heating of the tank causes elevation of the liquid phase temperature and pressure inside the tank. A partial rupture of the dry tank walls is possible, with the formation of a rarefaction wave propagating into the liquid phase. An evaporation wave moves after the rarefaction wave and cause a rapid increase of pressure, exceeding the initial pressure before depressurization. Rapid violent destruction of the tank occurs. The mechanism of a BLEVE initiation is considered using Van der Waals isotherms. The following criterion for the possibility of a BLEVE was formulated. If the final state is located on an unstable part of the Van der Waals isotherm, a BLEVE takes place. Limiting values of the temperatures for overheating of certain highly flammable liquids and liquid gases (propane, n-butane, n-pentane, isopentane) were calculated using the proposed method, and were found to be in good agreement with experimental data available in the literature.     

Experimental investigation of the flow characteristics of jettisoning in a CO2 carrier

This experimental study was performed to investigate the flow characteristics in the jettisoning flow line of a liquid CO₂ carrier. When a pressurized liquid CO₂ container loses mechanical integrity, possibly by material or mechanical defects, the liquid inventory should be drained out rapidly for safety reasons using the so-called jettisoning process. In the course of jettisoning, the liquid CO₂ undergoes two phase change stages, from liquid to liquid + vapor and from liquid + vapor to solid + vapor. Consequently, the jettisoning release rate is affected by the characteristics of these phase changes. In this study, liquid CO₂ was discharged through a small tube, representing a jettisoning flow line. The temperature and pressure were measured along the tube, and the locations of the phase changes were inferred from the measured data. The experimental results showed that active nucleation occurred near the tube tip and that the phase change into solid and vapor occurred just after leaving the pipe, irrespective of the tube length in this study.