The limits to risk aversion: Part 1. The point of indiscriminate decision

The paper uses utility theory to investigate how much should be spent to avert all costs from an industrial accident apart from direct human harm. These “environmental costs” will include those of evacuation, clean-up and business disruption. Assuming the organisation responsible will need to pay such costs, the difference between its expected utility with and without an environmental protection system constitutes a rational decision variable for whether or not the scheme should be installed. The value of utility is dependent on the coefficient of relative risk aversion, “risk-aversion” for short. A model of an organisation's decision-making process has been developed using the ABCD model, linking the organisation's assets, A, the cost of the protection scheme, B, the cost of consequences, C, and the expected utility difference with and without the scheme, D. Increasing the organisation's risk-aversion parameter will tend to make it less reluctant to invest in a protection system, but can bring about such investment only when the scheme is relatively close to financial break-even. For such borderline schemes, the amount the organisation is prepared to spend on the protection system will rise as the risk-aversion increases. The ratio of this sum to the break-even cost is named the “Limiting Risk Multiplier”, the maximum value of which is governed by the maximum feasible value of risk-aversion. However, the mathematical model shows that increasing the risk-aversion will reduce the clarity of decision making generally. Although the reluctance to invest in a protection scheme may change sign and turn into a positive desire to invest as the risk-aversion increases, the absolute value of this parameter is a continuously decreasing function of risk-aversion, tending asymptotically to zero. As a result, discrimination will gradually diminish, being lost altogether at the “point of indiscriminate decision”. Here the decision maker will be able to distinguish neither advantage in installing the scheme nor disadvantage in installing its inverse. There is a close correspondence between this mathematically predicted state and that of panic, where an individual has become so fearful that his actions become random. The point of indiscriminate decision provides a natural upper bound for the value of risk-aversion. This bounds the Limiting Risk Multiplier in turn, and so sets an objective upper limit on the amount that it is rational to spend on an environmental protection system.     

The limits to risk aversion: Part 2: The permission point and worked examples

Part 2 extends the analysis to show that it is possible to find the “permission point”, the value of (the coefficient of relative) risk-aversion, at which decisions to sanction environmental protection are most likely to be made. The mathematical model describes the process by which the decision maker varies his risk-aversion over a range of feasible values to find the risk-aversion that will give him the greatest desire to invest in the protection system under consideration. If he can find such a risk-aversion before losing discrimination (because the system is too expensive, given its performance), he will adopt it as his “permission point” and decide in favour of the expenditure. The permission point is, of course, bounded above by the point of indiscriminate decision. A maximum Risk Multiplier calculated at the point of indiscriminate decision may be applied to the protection expenditure at monetary break-even to give the maximum, rational outlay on protection. Moreover, it is possible to model how the average UK adult should take decisions on protection to maximise his utility. Different situations will call for different values of risk-aversion, which may explain why economists have come up with differing estimates of this parameter in the past. However, a central, average risk-aversion may be calculated for the average UK adult as 0.85, which is within 4% of the value, 0.82, found from the newly reported method based on a trade-off between income and future free time, and is consistent with several recent economic estimates. Worked examples assess how much an organisation should spend on a protection scheme to prevent accidents with very large environmental consequences.     

Air Traffic Management accident risk. Part 1: The limits of realistic modelling

The prime goal of the Air Traffic Management (ATM) system is to control accident risk. Some key questions are posed, including: What do design safety targets really mean and imply for risk modelling? In what circumstances can future accident risk really be modelled with sufficient precision? If risk cannot be estimated with precision, then how is safety to be assured with traffic growth and operational/technical changes? This paper endeavours to answer these questions by an analysis of the nature of accidents, causal factors and practical collision risk modelling. The main theme is how best to combine sound safety evidence and real-world hazard analysis in a coherent and systematic framework.     

Experimental flammability limits and associated theoretical flame temperatures as a tool for predicting the temperature dependence of these limits

The utility and limitations of adiabatic flame temperature calculations and minimum mixture energies in predicting the temperature dependence of flammability limits are explored. The limiting flame temperatures at constant pressure (1 bar) are calculated using a standard widely-used thermodynamic computer program. The computation is based on the calculated limiting flame temperature value at the reference initial temperature and the experimental limit concentration. The values recently determined in large chambers for the lower and upper flammability limits of a variety of simple organic and inorganic gases (methane, ethylene, dimethy lether, and carbon monoxide) are used as the basis for the predictions of the limiting flame temperature concept. Such thermodynamic calculations are compared with more traditional ones based on a limiting mixture energy and a constant average heat capacity of the reactant mixture. The advantages and limitations of the methods are discussed in this paper.     

泡沫混凝土和混凝土耐火极限的比较研究

泡沫混凝土用在建筑节能保温工程中,其耐火性能对提高建筑物的抗火灾能力非常重要。在模拟火灾条件下,通过测定不同密度、不同煅烧时间、不同含水量的泡沫混凝土和混凝土的抗压强度值,比较泡沫混凝土和混凝土的耐火极限的变化规律。结果表明,在火灾条件下,泡沫混凝土和混凝土的抗压强度损失率均随密度的增大而降低;密度为300kg/m3和800kg/m3的泡沫混凝土,在800℃下煅烧20min后,其抗压强度损失率分别为66.3%和25.5%;在同样的煅烧条件下,密度为2200kg/m3和2400kg/m3混凝土的抗压强度损失率分别为18.6%和15.8%;泡沫混凝土和混凝土的含水量越高,耐火极限就越长。泡沫混凝土为具有不燃特性A级保温材料,被用于建筑外墙保温隔热材料时,其抗压强度会因火场可燃材料的高温煅烧而有所降低,但其耐火极限能完全满足《建筑防火设计规范》。     

J-value analysis of different regulatory limits for workers and the public

The J-value technique allows an objective determination to be made of the resources that should be applied cost effectively to improve heath and safety. This is essential if capabilities are to be employed optimally and risks reduced in a way that reflects their severity. Although other considerations such as good practice and socio-political influences may affect a final decision on the resources to be sanctioned, the incorporation of these additional factors should be made transparent if the decision is no longer to be based on cost effectiveness. The J-value provides an objective criterion by which to judge when “reasonable practicability” has been achieved in committing resources for safety improvement, which is the legal requirement under health and safety law in the UK.Moreover, the J-value methodology also allows other related issues to be addressed objectively. Regulatory bodies apply different limits for workers and the general public, with higher risks being permitted for workers. Although a factor of about 10 has been used in several contexts, no objective rationale has been developed for this particular figure until now. However, it is shown that application of the J-value analysis can provide a justification for a ratio of workers’ risk to public risk of approximately this size if certain reasonable assumptions are made. Thus the paper provides the first quantitative explanation for the different levels of protection demanded by regulators nationally and internationally for workers and public.     

The fatality consequences of the 65 mph speed limits, 1989

This study examined whether the number of fatalities on rural interstates in 1989 was higher than would be expected based on experience during 1982–1986 and experience on all other roads. Among the 40 states that increased the speed limit to 65 mph on rural interstates, the number of fatalities was 29% higher than expected. Among the eight states retaining a 55 mph maximum speed limit on rural interstates, the observed number of fatalities was 12% lower than expected, although this reduction was not statistically significant. After adjusting the fatality risk on rural interstates for differences in vehicle miles traveled on those roads and for higher passenger vehicle occupancy rates attributable to possible increases in vacation travel, the increased fatality risk was 19%. These data suggest that the majority of the estimated increase in fatalities on rural interstates in 1989 (almost 400 of the approximately 600 extra deaths) can be attributed to the higher speeds resulting from the higher speed limits. Changes in mileage account for the remaining 200 extra deaths.     

Determination of the performance limits of flame arresters at increased oxygen concentrations

This investigation shows how an increased oxygen concentration influences the performance limits of crimped ribbon deflagration flame arresters at elevated pressures. An evaluation of the maximum experimental safe gap (MESG) as reliable criterion for describing the performance limits under non-atmospheric conditions is given. Measurements of MESGs and flame arrester performance tests were performed. Various fuel/oxygen/air mixtures containing ethylene and propane were used as testing gases. Former studies on the pressure dependence and the influence of oxygen on the MESG were initially confirmed. Furthermore, performance tests using a commercial deflagration flame arrester revealed that such a flame arrester may prevent flame transmission also at non-atmospheric conditions within a limited range. For various oxygen concentrations the performance limits were reached at the same MESG. Hence, it can be assumed that a flame arrester possesses a device- and fuel-specific maximum experimental safe gap for a specific gas mixture in different concentrations and at different pressures. This performance-related maximum safe gap can be used as a parameter for estimating and describing the performance limits of a flame arrester. It offers an attempt to simplify the testing and qualification of deflagration flame arresters for non-atmospheric conditions.     

云计算在核安全分析中的应用研究

在进行核安全分析时,需要考虑核反应堆设计、运行、管理及计算评估等诸多方面的复杂问题。而人员进行风险评估会在很大程度上影响核电站的正常运行时间,并带来人因失误的不确定性增大。利用云计算,可以有效地解决核安全分析面临的资料繁杂,模型构造麻烦和信息不共通等缺点。提出核安全云的概念,将云计算与核安全分析结合起来,借助计算机平台对核电厂的选址定点、建造、调试、运行及退役过程中可能产生的风险进行分析计算,更全面的分析各种不利条件,及时地做出决策,可以有效的降低风险发生的概率以及降低计算成本,可更准确地实现核电站安全分析的目标。     

Safety is the antonym of risk for some perspectives of risk

Safety and safe are terms closely related to risk, but do they extend beyond the realm of risk? Is safety/safe just the antonym of risk, or the same as acceptable risk? This issue has been given due attention in the literature and in this paper we explore how different perspectives of risk affect the relationship between safety/safe and risk. If risk is defined as an expected value or as a measure of the probability and severity of adverse effects, the conclusions would be different than if the essential component of risk is uncertainty. For risk understood as uncertainty about and severity of the consequences of an activity we argue that safe means acceptable risk. Three dimensions of safety are discussed; events and consequences (harm), probability and uncertainty.     

低压下近熄灭极限区域纸火蔓延实验研究

对低压下的近熄灭极限区域水平纸火蔓延进行了实验研究。通过降低环境压力和氧气浓度,确定了水平纸火蔓延的着火极限,并得出了在极限氧气浓度条件下的火蔓延速度变化规律。在相同氧气浓度下(43%)进行了不同压力的水平纸火蔓延实验。结果表明,火蔓延速度在近熄灭极限区域内非线性增加,通过比较分析前人火蔓延速度实验结果,确定了火蔓延近熄灭极限区域和线性增长区域的分界压力值。此外,得出了压力分界处的火焰变化特征,并根据火焰图像与理论分析,得出了不同区域内的火蔓延传热机制。     

油页岩含氧干馏工艺流程爆炸极限分析

根据干馏工艺流程配入适量氧气,可以降低载热气体需要预热的温度,以实现低能耗、易于工业生产的特点,设计了一套新型的有氧干馏工艺流程。有氧干馏工艺因其过程中存在可燃性混合物,有发生爆炸事故的可能性,通过实验对所收集的不同温度下的干馏气体的成分与含量进行了分析,结合爆炸极限理论,对该有氧干馏工艺流程的不同温度、不同惰性气体含量条件下可燃气体爆炸极限进行了分析计算。结果表明,可燃气体的浓度在整个反应升温过程中始终没有进入爆炸危险区域,说明该实验装置不具备爆炸危险性;对干馏工艺流程中氧气的输入量的控制,可以防止该工艺流程的火灾爆炸的发生。     

Evaluation of limits for effective flame acceleration in hydrogen mixtures

Results of experiments and data analysis on turbulent flame propagation in obstructed channels are presented. The data cover a wide range of mixtures: H₂/air, H₂/air/steam (from lean to rich) at normal and elevated initial temperatures (from 298 to 650 K) and pressures (from 1 to 3 bar); and stoichiometric H₂/O₂ mixtures diluted with N₂, Ar, He and CO₂ at normal initial conditions. The dataset chosen also covers a wide range of scales exceeding two orders of magnitude. It is shown that basic flame parameters, such as mixture expansion ratio σ, Zeldovich number β and Lewis number Le, can be used to estimate a priori a potential for effective flame acceleration for a given mixture. Critical conditions for effective flame acceleration are suggested in the form of correlations of critical expansion ratio σ^* versus dimensionless effective activation energy. On this basis, limits for effective flame acceleration for hydrogen combustibles can be estimated. Uncertainties in determination of critical σ^* values are discussed.     

基于风险的报警优先级量化设定方法研究

当前主流报警优先级设定方法操作性不佳,在进行装置报警系统优化时由于考虑维度少而无法较好优化报警系统优先级,针对该问题,建立了基于风险的报警优先级设定新方法。将标定的风险图法应用于报警优先级设定,假定某报警不存在,对出现危险情况的频率、后果严重度、可用时间、避免事故后果的可能性等进行标定,并对危险状况出现频率与可用时间进行矩阵分析,将结果运用于风险图中;针对某柴油加氢装置报警系统,应用该方法进行了报警优先级优化。结果表明:基于风险的报警优先级量化设定方法能够有效应用于优化装置报警优先级;经该方法优化的各级报警,数量分布更接近EEMUA的推荐值;对于“在所考虑的报警不存在时每年发生事故的次数”因素,研究给出3个不同的档次供企业选择,实践中,企业可结合自身可接受标准,作进一步完善。     

Relationship of traffic fatality rates to maximum state speed limits

Objectives: The objective of this study was to examine the safety effects of increases in U.S. state maximum speed limits during the period 1993–2013.

Methods: Poisson regression was used to model state-by-state annual traffic fatality rates per mile of travel as a function of time, the unemployment rate, the percentage of the driving age population that was younger than 25, per capita alcohol consumption, and the maximum posted speed limit on any road in the state. Separate analyses were conducted for all roads, interstates and freeways, and all other roads.

Results: A 5 mph increase in the maximum state speed limit was associated with an 8% increase in fatality rates on interstates and freeways and a 4% increase on other roads. In total, there were an estimated 33,000 more traffic fatalities during the years 1995–2013 than would have been expected if maximum speed limits had not increased. In 2013 alone, there were approximately 1,900 additional deaths—500 on interstates/freeways and 1,400 on other roads.

Conclusions: There is a definite trend of increased fatality risk when speed limits are raised. As roadway sections with higher speed limits have become more ubiquitous, the increase in fatality risk has extended beyond these roadways. The increase in risk has been so great that it has now largely offset the beneficial effects of some other traffic safety strategies. State policy makers should keep this trade-off in mind when considering proposals to raise speed limits.     

Explosion limits of mixtures relevant to the production of 1,2-dichloroethane (ethylene dichloride)

A simple method exists to estimate the limiting oxygen concentration (LOC) based upon the lower explosion limit (LEL) by assuming (1) that the LOC lies at the apex of the explosion area, (2) that the LEL is unaffected by nitrogen addition and (3) that the apex of the explosion area lies on the stoichiometric line. This estimation method is assessed for mixtures relevant to the production of 1,2-dichloroethane. To this end, the explosion areas of ethylene/hydrogen/nitrogen/air, ethylene/nitrogen/air and ethylene/1,2-dichloroethane/hydrogen chloride/nitrogen/air mixtures are determined at typical process conditions. The experiments are performed in a closed spherical 8 l vessel. The mixtures are ignited by fusing a coiled tungsten wire, placed at the centre of the vessel. A 5% pressure rise criterion is used to determine the explosion limits. The experimental procedure is based upon EN 14756. It is found that a safe estimate of the LOC of ethylene/hydrogen/nitrogen/air mixtures can be found based upon the LEL of these mixtures.     

An investigation of flammability limits of gaseous mixtures at reduced pressures

An experimental investigation of flammability limits of hydrogen, methane and propane in air and oxygen at reduced pressures was carried out. A slow influence of sizes of an experimental vessel of a diameter higher than 125 mm on the flammability limits was revealed, but an influence of a type of an oxidizer (air or oxygen) and an ignition energy is significant. Critical values of an initial pressure for a possibility of a flame propagation were determined. The limiting values of the ignition energy were determined, for which an elevation of this parameter does not influence the critical pressure and the flammability region. A qualitative interpretation of obtained experimental results is given, which is based on a peculiarities of a flame initiation.     

基于动态风险平衡的海洋平台事故连锁风险研究

针对海洋平台事故风险特点,提出动态风险平衡概念,以此建立事故动力模型,并将该模型运用到墨西哥湾"深水地平线"井喷事故。动态风险平衡表征事故动力与事故阻力之间的动态平衡状态,具有动态性和暂时稳定性。事故动力模型以海洋平台可能发生的重大事故为研究对象,从工艺、技术和管理等角度分析事故可能致因和事故发展可能影响因素。该模型首先分析对象的初始事故动力,建立事故连锁风险图,然后计算初始动力发生情况下,传递动力和传递阻力的概率分布,最后提出相应风险控制措施。实例分析表明,基于动态风险平衡建立的事故动力模型能有效分析海洋平台事故连锁风险。     

Experimental investigation on the lower flammability limits of diethyl ether/ n-pentane/epoxypropane-air mixtures

Diethyl ether (DEE), epoxypropane (PO) and n-pentane have excellent ignition and combustion performance; hence, they have a wide variety of applications in industry and advanced aviation propulsion systems. As these fuels are flammable at normal temperature and pressure, their explosive characteristics need to be explored. In this study, the lower flammability limits (LFLs) of vapor mixtures of DEE/PO/n-pentane in air were measured in 20 L, closed, stainless steel spherical vessels. Experimental results were obtained at ambient atmospheric pressure and an initial temperature of 40 °C. The experimental results show that the LFLs of DEE-air, n-pentane -air, and PO-air are 1.81 vol%, 1.41 vol% and 2.44 vol%, respectively. The LFLs of binary/ternary fuel mixtures under different compositions were tested, and the experimental results are compared with the classical Le Chatelier's formula. The results show that, for the binary fuels (i.e., DEE/PO, DEE/n-pentane, PO/n-pentane)-air mixtures, the maximum difference of the LFLs between Le Chatelier's formula and the experimental results is 6.10%. For the ternary fuels (i.e., DEE/PO/n-pentane)-air mixtures, the maximum difference of the LFLs between the two results is 6.33%. Due to the adiabatic flame temperature of each single fuel mixture being close, the Le Chatelier's formula is applicable for an estimation of the LFL for DEE/PO/n-pentane-air mixtures.