基于三维图像分析的浮力扩散火焰几何与辐射特性研究

火焰几何特性和辐射特性是刻画火灾规模及其危害的重要参量。利用三维火焰重构技术,获取了丙烷浮力扩散火焰的火焰高度、表面积、体积和火焰面元视角系数的变化规律。结果表明,三维重构的火焰能够表征真实火焰形态的动态变化。平均火焰表面积和体积均可较好地拟合为热释放速率的幂函数,火焰表面积热释放速率随火焰热释放速率的增加趋于常数。平均火焰高度、表面积和体积与火焰外部平均辐射热流之间具有较好的幂函数关系,且拟合指数随着与火源距离的增大而减小。此外,将点源、圆柱辐射模型和火焰面元积分方法得到的辐射计算值与辐射测量值进行比较,发现火焰面元积分方法能够更好地预测火焰外围的瞬时和平均辐射热流分布。     

Blast overpressures from medium scale BLEVE tests

The measured blast overpressures from recent tests involving boiling liquid expanding vapour explosions (BLEVE) has been studied. The blast data came from tests where 0.4 and 2 m³ ASME code propane tanks were exposed to torch and pool fires. In total almost 60 tanks were tested, and of these nearly 20 resulted in catastrophic failures and BLEVEs. Both single and two-step BLEVEs were observed in these tests. This paper presents an analysis of the blast overpressures created by these BLEVEs. In addition, the blast overpressures from a recent full scale fire test of a rail tank car is included in the analysis.The results suggest that the liquid energy content did not contribute to the shock overpressures in the near or far field. The liquid flashing and expansion does produce a local overpressure by dynamic pressure effects but it does not appear to produce a shock wave. The shock overpressures could be estimated from the vapour energy alone for all the tests considered. This was true for liquid temperatures at failure that were below, at and above the atmospheric superheat limit for propane. Data suggests that the two step type BLEVE produces the strongest overpressure. The authors give their ideas for this observation.The results shown here add some limited evidence to support previous researchers claims that the liquid flashing process is too slow to generate a shock. It suggests that liquid temperatures at or above the Tsl do not change this. The expansion of the flashing liquid contributes to other hazards such as projectiles, and close in dynamic pressure effects. Of course BLEVE releases in enclosed spaces such as tunnels or buildings have different hazards.     

Thermochemical stability: A comparison between experimental and predicted data

The first step to be performed during the development of a new industrial process should be the assessment of all hazards associated to the involved compounds. Particularly, the knowledge of all substances thermochemical parameters is a primary feature for such a hazard evaluation. CHETAH (CHEmical Thermodynamic And Hazard evaluation) is a prediction software suitable for calculating potential hazards of chemicals, mixtures or a single reaction that, using only the structure of the involved molecules and Benson's group contribution method, is able to calculate heats of formation, entropies, Gibbs free energies and reaction enthalpies. Because of its ability to predict the potential hazards of a material or mixture, CHETAH is part of the so-called “desktop methods” for early stage chemical safety analysis.In this work, CHETAH software has been used to compile a complete risk database reporting heats of decomposition and Energy Release Potential (ERP) for 342 common use chemicals. These compounds have been gathered into classes depending on their functional groups and similarities in their thermal behavior. Calculated decomposition enthalpies for each of the compounds have also been compared with experimental data obtained with either thermoanalytic or calorimetric techniques (Differential Scanning Calorimeter – DSC – and Accelerating Rate Calorimeter – ARC).     

Evaluation of thermal hazard characteristics of four low temperature reactive azo compounds under isothermal conditions

The polymerization reaction can lower the threshold of the required energy by the initiator to improve the efficiency of the overall process reaction. Emerging polymerization initiators are also a major focus of process improvement and technological progress. Azo compounds (azos), which used in dyeing applications, are subsequently used in polymerization reactions due to their highly exothermic reaction characteristics. Although higher heat release can promote polymerization and modify the product, heat generation may also cause process hazards.These thermal hazard parameters were studied by selecting dimethyl 2,2′-azobis(2,4-dimethylvaleronitrile) (ABVN), 2,2′-azobis(2-methyl propionate) (AIBME), 2,2′-azobis(2-methylpropionamide) dihydrochloride (AIBA), and 2,2′-azobis(isobutyronitrile) (AIBN), which are common azo initiators at present. Thermal hazards are closely related to the reaction kinetics of the substance itself. The form of the reaction, the apparent activation energy and the thermodynamic parameters of the exothermic mode were also obtained.Kinetic analysis of the actual process using the experimental data of the isothermal calorimetry model is rarely used in the evaluation of related thermal hazard characteristics. The simulation results revealed the kinetic azo models and were further applied to calculate the runaway situations of azo under specific boundary conditions.     

用伴随方法对毒气泄漏事件进行危害评估

由于自然、人为等因素造成的有毒气体泄漏事件随时威胁着人们的生命安全。对于这类突发事故进行危害评估 ,在应急救援中 ,可以给决策者提供参考 ,使之采取有效措施 ,最大限度地降低事故危害程度 ,减少事故恶果及引起的恐慌。由于毒气泄漏位置事先无法确定 ,如果用常规方法进行逐点评估 ,将面临着巨大的计算量而难以进行。笔者发展了一种伴随方法 ,对风险函数的表达式进行等价变形 ,通过求解伴随方程 ,可以一次求得任意位置的毒气泄漏的风险值 ,大大降低计算量和工作量。此法在化学工厂和仓库设置 ,煤气网铺设和监测点选取 ,城市应急中心设置等多种安全规划中均有重要价值 ,为快速危害评估 ,降低化学事故危害提供了有力的研究工具和实用方法。     

Effect of turbulence spatial distribution on the deflagration index: Comparison between 20 L and 1 m3 vessels

In this work, the effect of spatial distribution and values of the turbulent kinetic energy on the pressure-time history and then on the explosion parameters (deflagration index and maximum pressure) was quantified in both the standard vessels (20 L and 1 m³).The turbulent kinetic energy maps were computed in both 20 L and 1 m³ vessels by means of CFD simulations with validated models. Starting from these maps, the turbulent flame propagation of cornstarch was calculated, by means of the software CHEMKIN. Then, the pressure-time history was evaluated and from this, the explosion parameters.Calculations were performed for three cases: not uniform turbulence level as computed from CFD simulations, uniform turbulence level and equal to the maximum value, uniform profile and equal to the minimum value. It was found that the cornstarch in the 20 L vessel get variable classes (St-1, St-2, St-3) with respect to the 1 m³ (St-1). However, simulations performed on increasing the ignition delay time, shown that the same results can be attained only using 260 ms as ignition delay time in the 20 L vessel.     

Evaluation of thermal reaction for two azo compounds by using 20-L apparatus and calorimetry

Azo compounds are widely involved in the industrial processes of dyes, pigments, initiators, and blowing agents. Unfortunately, these compounds have a bivalent unstable –NN– composition, which can be readily broken when the ambient temperature is elevated. Self-accelerating decomposition might cause a runaway reaction and lead to a fire, explosion, or leakage when the cooling system fails or other events occur. This study investigated the explosion properties, thermal stability parameters, and thermal hazard and mechanism of 2,2′–azobisisobutyronitrile (AIBN) and 2,2′–azobis–2–methylbutyronitrile (AMBN). We used a 20-L apparatus, vent sizing package 2, synchronous thermal analysis, and differential scanning calorimetry under explosive, adiabatic, and dynamic conditions to acquire the explosive curves, thermal curves, and thermodynamic parameters of the substances. Moreover, the differential isoconversional method (Friedman method) and ASTM E698 equation were employed to obtain the apparent activation energy E_a. All the experimental results revealed that AIBN is more dangerous than AMBN. The E_a of AIBN was lower than that of AMBN. The results can be used to construct an azo compound thermal hazard database for use for searches and reference examples by industry and related research areas.     

Kinetic analysis for spontaneous combustion of sulfurized rust in oil tanks

In order to evaluate the spontaneous combustion hazards of sulfurized rust in oil tanks, one kind of rust was obtained from respiratory valve inner cavity of a crude oil tank in a petrochemical company. The rust was sulfurized in sulfuration experimental apparatus. The production was analyzed by X-ray energy dispersive spectrometry (EDS), scanning electron microscopy (SEM) and then thermo-gravimetric analysis (TGA). The EDS result shows that the main substances are FeS and FeS₂ which are liable to spontaneous combustion. The sulfurized rust gives a short length of side and diamond appearance, and a large pore size in structure based on X-ray Diffraction (XRD). The whole oxidation process has three complex stages. The corresponding apparent activation energy values, most probable mechanism functions and pre-exponential factor values were calculated by Madhusudanan–Krishnan–Ninan method and the master plot method. The results indicate that the first and third stages of mass loss are up to the power function mechanism, but the second stage accords with the nucleation and nucleus growth mechanism. The values of apparent activation energy increase successively from the first stage to the third stage. The second stage has the maximum pre-exponential factor value, while the first has the minimum. With the obtained parameters above, the oxidation process of sulfurised rust could be simulated, which would benefit for monitoring and early warning of oil tanks.     

化学品反应危险性分级方法研究与应用

针对化学品反应危险性的评价,介绍利用最大分解热,反应放热功率,绝热反应参数等方法对化学品反应危险性进行分级的几种方法及各自特点。通过分析整合不同分级方法,提出一种基于实测反应放热参数的分级方法,此方法以初始放热温度和反应热组成指标体系,同时配合燃爆曲线实现对化学品反应危险性的分级,得到的结果对于化学品的安全评价有较强的参考价值。考察了6种有机过氧化物的放热性质,诠释了有机过氧化物的反应危险性,应用所提出的反应危险性分级方法对其进行分级,并对分级方法进行了验证。该方法结合了实验结果和多种分级方法的优点,使化学品反应危险性分级方法具有更强的实用性。     

Experimental research of LNG accidental underwater release and combustion behavior

Evaluating potential hazards caused by accidental LNG release from underwater pipelines or vessels is a significant consideration in marine transportation safety. The aim of this study was to capture the dynamic behavior of LNG jet released under water and to analyze its vapor dispersion characteristics and combustion characteristics on the water surface during different release scenarios. Controlled experiments were conducted where LNG was jet released from a cryogenic storage tank. The dynamic process of LNG being jet released from orifices of different sizes and shapes, as well as the rising plume structure, were captured by a high-speed camera. The leakage flow rate and pipeline pressure were recorded by a flow meter and pressure gauge, respectively. The concentration distribution that emanated from the water surface was measured utilizing methane sensors in different positions with various wind speeds. The flame combustion characteristics of LNG vapor clouds, which immediately ignited upon the enclosed water tank, were also recorded. Additionally, the mass burning rate of the flame on the water surface was evaluated, and a new correlation between the ratio of flame length and width was established. The results indicated a large dimensionless heat release rate (Q*) and a continuous release flow rate in a limited burning area. This study could provide greater understanding of the mechanisms of LNG release and combustion behavior under water.     

Determination of heterogeneous reaction mechanisms: A key milestone in dust explosion modelling

Reaction kinetics is fundamental for modelling the thermal oxidation of a solid phase, in processes such as dust explosions, combustion or gasification. The methodology followed in this study consists in i) the experimental identification of the reaction mechanisms involved in the explosion of organic powders, ii) the proposal of simplified mechanisms of pyrolysis and oxidation, iii) the implementation of the model to assess the explosion severity of organic dusts. Flash pyrolysis and combustion experiments were carried out on starch (22 μm) and cellulose (53 μm) at temperatures ranging from 973 K to 1173 K. The gases generated were collected and analyzed by gas chromatography. In this paper, a semi-global pyrolysis model was developed for reactive systems with low Damköhler number. It is in good agreement with the experimental data and shows that both carbon monoxide and hydrogen are mainly generated during the pyrolysis of the solid, the generation of the latter compound being greatly promoted at high temperature. A simplified combustion model was also proposed by adding two oxidation reactions of the pyrolysis products. In parallel, flame propagation tests were performed in a semi open tube in order to assess the burning velocity of such compounds. The laminar burning velocity of cellulose was determined to be 21 cm s⁻¹. Finally, this model will be integrated to a predictive model of dust explosions and its validation will be based on experimental data obtained using the 20 L explosion sphere. The explosion severity of cellulose was determined and will be used to develop and adjust the predictive model.     

Perceptions of personal vulnerability to workplace hazards in the Australian construction industry

Introduction

The importance of risk perception for workplace safety has been highlighted by the inclusion of risk appraisals in contemporary models of precautionary behavior at work. Optimism bias is the tendency to think that negative events are less likely to happen to oneself than to the average person, and is proposed to be related to the reduced use of precautions.

Method

Building on studies of optimism bias for workplace hazards using samples with heterogenous risk profiles, the current study aimed to investigate whether optimism bias is present in a sample of workers exposed to similar workplace hazards. 175 Australian construction workers completed a brief survey that asked them to rate the likelihood of common construction industry hazards occurring to them and to the average worker of the same age doing the same job. Significant levels of optimism bias were found for many hazards (including being electrocuted, being trapped in a confined space, falling from heights, and causing someone else to have an injury).

Results

Optimism bias was not related to perceived controllability, contrary to findings in other domains, yet consistent with findings of optimism bias for workplace hazards. Optimism bias was not found to be related to a reduction in safe work behaviors, though this may be due to difficulties in measuring safe or precautionary behavior, such as social desirability.

Impact on industry

That most workers think that hazards are less likely to happen to them than to the average worker presents a significant problem because it may ameliorate the efficacy of safety programs, yet constitutes a largely unexplored opportunity for improving workplace safety performance.     

有毒有害蒸气云扩散的危害估算

因事故排放形成的有毒有害蒸气云,有可能造成爆炸、火灾、中毒等危害后果,但危害后果的大小(危害程度),则与事故造成危害的时刻(延迟时间)有关。笔者在以往研究成果基础上,考虑该因素,并通过引入正态分布假设,时、空变换及误差分析,给出了估算危害程度(特别是延迟爆炸危害)的有效而简便方法,补充了以往危害估算方法(不考虑延迟时间)的不足,提高了危害后果估算精度,可广泛应用于生产安全评价、环境风险评价、风险工程设计、事故应急预案、事故应急救援等工作领域,对于保障环境安全具有显著的科学价值与现实意义。     

绿色技术评价磺化反应过程热危险性

反应量热仪RC1研究磺化反应过程中热危险性具有评价路线简单、易于操作、过程绿色环保等优势,近年来逐渐成为研究的热点.磺化反应过程中由于工艺的不同,不同磺化反应过程的热危险性也具有很大的差别.通过反应量热仪RC1、差示扫描量热DSC、绝热加速量热仪ARC对10种不同工艺的磺化反应过程的热危险进行了深入的研究,对企业实践生...     

危险源理论研究及在事故预防中的应用

为提高事故预防和控制水平,以现代安全科学的观点及事故致因理论为基础,运用事故案例分析研究方法,从危险源概念辨析入手,认为,危险源是指一切可能导致事故发生的能量、能量载体或危险物质,而不具有能量的隐患不是危险源。进一步探讨危险源的分类、层次结构及基于危险源理论的事故预防控制模型;同时,分析各类危险源在事故预防控制模型中的地位,指出事故的原因及防范对策。结果表明,事故是由直接危险源、第一触发危险源和初始触发危险源等不同种类的危险源的相互作用的结果,加强对不同危险源的研究,采取有针对性措施可有效提高生产安全管理水平及事故控制水平。     

A framework for evaluating OSH program effectiveness using leading and trailing metrics

Introduction

Many employers and regulators today rely primarily on a few past injury/ illness metrics as criteria for rating the effectiveness of occupational safety and health (OSH) programs. Although such trailing data are necessary to assess program success, they may not be sufficient for developing proactive safety, ergonomic, and medical management plans.

Methods

The goals of this pilot study were to create leading metrics (company self-assessment ratings) and trailing metrics (past loss data) that could be used to evaluate the effectiveness of OSH program elements that range from primary to tertiary prevention. The main hypothesis was that the new metrics would be explanatory variables for three standard future workers compensation (WC) outcomes in 2003 (rates of total cases, lost time cases, and costs) and that the framework for evaluating OSH programs could be justifiably expanded. For leading metrics, surveys were developed to allow respondents to assess OSH exposures and program prevention elements (management leadership/ commitment, employee participation, hazard identification, hazard control, medical management, training, and program evaluation). After pre-testing, surveys were sent to companies covered by the same WC insurer in early 2003. A total of 33 completed surveys were used for final analysis. A series of trailing metrics were developed from 1999-2001 WC data for the surveyed companies. Data were analyzed using a method where each main 2003 WC outcome was dichotomized into high and low loss groups based on the median value of the variable. The mean and standard deviations of survey questions and 1999-2001 WC variables were compared between the dichotomized groups. Hypothesis testing was performed using F-test with a significance level 0.10.

Results/Discussion

Companies that exhibited higher musculoskeletal disorder (MSD) WC case rates from 1999-2001 had higher total WC case rates in 2003. Higher levels of several self-reported OSH program elements (tracking progress in controlling workplace safety hazards, identifying ergonomic hazards, using health promotion programs) were associated with lower rates of WC lost time cases in 2003. Higher reported exposures to noise and projectiles were also associated with higher rates of WC cases and costs in 2003.

Impact on Industry

This research adds to a growing body of preliminary evidence that valid leading and trailing metrics can be developed to evaluate OSH effectiveness. Both the rating of OSH efforts and the regular trending of past loss outcomes are likely useful in developing data-driven improvement plans that are reactive to past exposures and proactive in identifying system deficiencies that drive future losses.     

Confidence-based quantitative risk analysis for offshore accidental hydrocarbon release events

Quantitative risk analysis (QRA) has been widely used to conduct the assessment of offshore accidental risks. However, the accuracy and validity of QRA is significantly affected by uncertainties when subjective judgments are involved. Therefore, it is unrealistic to determine the probability of a hazardous event by using one single explicit value when safety experts have a relatively low confidence level in their judgments. This paper proposes a new methodology for incorporating uncertainties into conventional QRA using the concept of confidence level. Offshore hydrocarbon release hazards are focused on and a barrier and operational risk analysis (BORA-Release) method is selected as the basic model to illustrate the proposed methodology. A left–right (L–R) bell-shaped fuzzy number is employed and its membership curve is able to control its shape to represent different confidence levels. As to the complex geometry of the bell-shaped fuzzy number, an α-cut operation is introduced to conduct the arithmetic operations of the fuzzy number, and a defuzzification method with total integral value is chosen to match the α-cut operations and acquire complete information for the fuzzy numbers. In the meantime, an optimism index is used to describe the attitude of the decision-maker. One case study is provided in this paper to demonstrate the implementation of this method.