A risk-based approach to flammable gas detector spacing

Flammable gas detectors allow an operating company to address leaks before they become serious, by automatically alarming and by initiating isolation and safe venting. Without effective gas detection, there is very limited defense against a flammable gas leak developing into a fire or explosion that could cause loss of life or escalate to cascading failures of nearby vessels, piping, and equipment. While it is commonly recognized that some gas detectors are needed in a process plant containing flammable gas or volatile liquids, there is usually a question of how many are needed. The areas that need protection can be determined by dispersion modeling from potential leak sites. Within the areas that must be protected, the spacing of detectors (or alternatively, number of detectors) should be based on risk. Detector design can be characterized by spacing criteria, which is convenient for design - or alternatively by number of detectors, which is convenient for cost reporting. The factors that influence the risk are site-specific, including process conditions, chemical composition, number of potential leak sites, piping design standards, arrangement of plant equipment and structures, design of isolation and depressurization systems, and frequency of detector testing. Site-specific factors such as those just mentioned affect the size of flammable gas cloud that must be detected (within a specified probability) by the gas detection system. A probability of detection must be specified that gives a design with a tolerable risk of fires and explosions. To determine the optimum spacing of detectors, it is important to consider the probability that a detector will fail at some time and be inoperative until replaced or repaired. A cost-effective approach is based on the combined risk from a representative selection of leakage scenarios, rather than a worst-case evaluation. This means that probability and severity of leak consequences must be evaluated together. In marine and offshore facilities, it is conventional to use computational fluid dynamics (CFD) modeling to determine the size of a flammable cloud that would result from a specific leak scenario. Simpler modeling methods can be used, but the results are not very accurate in the region near the release, especially where flow obstructions are present. The results from CFD analyses on several leak scenarios can be plotted to determine the size of a flammable cloud that could result in an explosion that would generate overpressure exceeding the strength of the mechanical design of the plant. A cloud of this size has the potential to produce a blast pressure or flying debris capable of causing a fatality or subsequent damage to vessels or piping containing hazardous material. In cases where the leak results in a fire, rather than explosion, CFD or other modeling methods can estimate the size of a leak that would cause a fire resulting in subsequent damage to the facility, or would prevent the safe escape of personnel. The gas detector system must be capable of detecting a gas release or vapor cloud, and initiating action to prevent the leak from reaching a size that could cause injury or severe damage upon ignition.     

Total flammable mass and volume within a vapor cloud produced by a continuous fuel-gas or volatile liquid-fuel release

The top-hat jet/plume model has recently been employed to obtain simple closed-form expressions for the mass of fuel in the flammable region of a vapor "cloud" produced by an axisymmetric (round) continuous-turbulent jet having positive or negative buoyancy [1]. The fuel release may be a gas or a volatile liquid. In this paper, the top-hat analysis is extended to obtain closed-form approximate expressions for the total mass (fuel+entrained air) and volume of the flammable region of a release cloud produced by either a round or a plane (two-dimensional) buoyant jet. These expressions lead to predicted average fuel concentrations in the flammable regions of the release clouds which, when compared with the stoichiometric concentration, serve as indicators of the potential severity of release cloud explosions. For a fixed release mass, the combustion overpressure following ignition of a hydrogen/air cloud is anticipated to be significantly lower than that due to ignition of a hydrocarbon/air cloud. The predicted average hydrogen concentration within the flammable region of the release cloud is below the lower detonability limit. The facility with which the expressions can be used for predictions of combustion overpressures is illustrated for propane releases and deflagrations in a closed compartment.     

Expansion of clouds from pressurized liquids

This paper deals with the cloud formed by the expansion of a pressurized liquid as a result of tank rupture. Effects of combustion are ignored in this cloud model and the only energy for cloud formation is that of the pressurized liquid before tank rupture. The present cloud model is applicable to the rupture of a tank containing a volatile non-flammable or flammable liquid before ignition. The rupture of a tank of non-volatile flammable liquid can also result in the formation of a cloud if the liquid is heated before tank rupture. Although the latter case can be treated by the methods described here this paper is focussed on the type of cloud formed by liquids that are normally volatile at ambient temperature and are shipped pressurized at that temperature.     

Appraising the flammability hazards of hydrocarbon refrigerants using quantitative risk assessment model. Part II. Model evaluation and analysis

Hydrocarbon refrigerants present are fire and explosion hazards due to their flammability. This paper describes a quantitative risk assessment (QRA) model to evaluate the potential for ignition when hydrocarbons are employed in stationary refrigeration and air-conditioning equipment. QRA enables examination of the effects that design, installation of equipment and external conditions on the frequency of ignition of the refrigerant and its consequences. Part I of this study presents the modelling approach for ignition frequencies, sub-models for refrigerant leakage and development of flammable concentration, and the associated consequences, being overpressures and thermal radiation. Part II provides recommended empirical input data and example results generated from the model.     

Application of CFD (Fluent) to LNG spills into geometrically complex environments

Recent discussions on the fate of LNG spills into impoundments have suggested that the commonly used combination of SOURCE5 and DEGADIS to predict the flammable vapor dispersion distances is not accurate, as it does not account for vapor entrainment by wind. SOURCE5 assumes the vapor layer to grow upward uniformly in the form of a quiescent saturated gas cloud that ultimately spills over impoundment walls. The rate of spillage is then used as the source term for DEGADIS. A more rigorous approach to predict the flammable vapor dispersion distance is to use a computational fluid dynamics (CFD) model. CFD codes can take into account the physical phenomena that govern the fate of LNG spills into impoundments, such as the mixing between air and the evaporated gas. Before a CFD code can be proposed as an alternate method for the prediction of flammable vapor cloud distances, it has to be validated with proper experimental data. This paper describes the use of Fluent, a widely-used commercial CFD code, to simulate one of the tests in the "Falcon" series of LNG spill tests. The "Falcon" test series was the only series that specifically addressed the effects of impoundment walls and construction obstructions on the behavior and dispersion of the vapor cloud. Most other tests, such as the Coyote and the Burro series, involved spills onto water and relatively flat ground. The paper discusses the critical parameters necessary for a CFD model to accurately predict the behavior of a cryogenic spill in a geometrically complex domain, and presents comparisons between the gas concentrations measured during the Falcon-1 test and those predicted using Fluent. Finally, the paper discusses the effect vapor barriers have in containing part of the spill thereby shortening the ignitable vapor cloud and therefore the required hazard area. This issue was addressed by comparing the Falcon-1 simulation (spill into the impoundment) with the simulation of an identical spill without any impoundment walls, or obstacles within the impoundment area.     

可燃气云抑爆技术初探

试验表明,在可燃气云爆炸引发过程或爆炸初始阶段,通过喷洒抑爆材料可抑制爆炸燃烧反应进程,继而中断爆炸反应或显著削弱爆炸强度.无机粉末、惰气和水雾是性价比较高的抑爆材料,具有大面积推广的价值.探讨了可燃气云的抑爆机理,并指出可燃气云抑爆技术后续研究中亟待解决的问题,为可燃气云抑爆技术的实际应用提供了技术参考.     

Using transportation accident databases to investigate ignition and explosion probabilities of flammable spills

Risk assessment of hazardous material spill scenarios, and quantitative risk assessment in particular, make use of event trees to account for the possible outcomes of hazardous releases. Using event trees entails the definition of probabilities of occurrence for events such as spill ignition and blast formation. This study comprises an extensive analysis of ignition and explosion probability data proposed in previous work. Subsequently, the results of the survey of two vast US federal spill databases (HMIRS, by the Department of Transportation, and MINMOD, by the US Coast Guard) are reported and commented on. Some tens of thousands of records of hydrocarbon spills were analysed. The general pattern of statistical ignition and explosion probabilities as a function of the amount and the substance spilled is discussed. Equations are proposed based on statistical data that predict the ignition probability of hydrocarbon spills as a function of the amount and the substance spilled. Explosion probabilities are put forth as well. Two sets of probability data are proposed: it is suggested that figures deduced from HMIRS be used in land transportation risk assessment, and MINMOD results with maritime scenarios assessment. Results are discussed and compared with previous technical literature.     

exLOPA for explosion risks assessment

The European Union regulations require safety and health protection of workers who are potentially at risk from explosive atmosphere areas. According to the requirements, the operators of installations where potentially explosive atmosphere can occur are obliged to produce an explosion protection document. The key objective of this document is the assessment of explosion risks. This paper is concerned with the so-called explosion layer of protection analysis (exLOPA), which allows for semi-quantitative explosion risk assessment for process plants where explosive atmospheres occur. The exLOPA is based on the original work of CCPS for LOPA but takes into account some typical factors appropriate for explosion, like the probability that an explosive atmosphere will occur, probability that sources of ignition will be present and become effective as well as the probability of failure on demand for appropriate explosion prevention and mitigation means.     

Heavy gas dispersion: integral models and shallow layer models

Integral models for heavy gas dispersion approximate a dispersing cloud in terms of a small number of variables; each of these is ultimately a function of an independent variable which is usually time (instantaneous releases) or downwind distance (continuous releases). This type of model is used almost exclusively in risk assessment [HSE's risk assessment tool, RISKAT, in: Major Hazards: Onshore and Offshore, October 1992, pp. 607-638; Ann. Rev. Fluid Mech. 21 (1989) 317], but many distinct integral models exist. The code comparison exercise of Mercer et al. [CEA/AEA exchange agreement on external event. Comparison of heavy gas dispersion models for instantaneous releases: final report, Technical Report IR/L/HA/91/6, Health and Safety Laboratory, Sheffield, June 1991; J. Hazard. Mater. 36 (1994) 193] presented the results from a number of integral models in a common format; Mercer found that the range of predictions for some scenarios exceeded three orders of magnitude. Here, the TWODEE shallow layer model [J. Hazard. Mater. 66 (3) (1999) 211; J. Hazard. Mater. 66 (3) (1999) 227; J. Hazard. Mater. 66 (3) (1999) 239] is added to Mercer's code comparison exercise. The physical assumptions used in shallow layer models differ profoundly from those used in integral models and the implications of these differences for risk assessment are discussed. TWODEE was used to simulate four representative cases considered by Mercer. In terms of cloud averaged concentration (CAC) vs. centroid position, the present model gave predictions that were consistent with the integral models used by Mercer. As the model neglects horizontal diffusion for passive clouds, overprediction at large downwind distances was expected, but not generally observed.     

Modelling of catastrophic flashing releases

Several low boiling point materials are stored in closed vessels at ambient temperature, using their own vapour pressure to maintain a liquid state. These materials are often toxic, flammable, or both, and thus any uncontrolled release can have potentially disastrous consequences. There are many ways in which an accidental release can occur, the most severe being due to catastrophic vessel failure. Although not the most common, this mode of failure has the potential to result in an instantaneous loss of the entire vessel inventory in the form of a rapidly expanding, two-phase, vaporising cloud. This paper provides a comprehensive review of the physical processes of existing models and of available experimental and incident data to model such scenarios. Subsequently, this has enabled the development of an improved methodology for the characterisation of the source conditions following catastrophic vessel failures.     

Study of the spread of a cold instantaneous heavy gas release with surface heat transfer and variable entrainment

Air quality models help in developing relationships between the amount of pollutant released into the ambient atmosphere by a source and the corresponding incremental contribution in the atmospheric concentration. Of the various dispersion models, the heavy gas models help in predicting the concentrations due to release of gases heavier than air and the risks associated with the increased concentrations. Several differences exist among the various models developed to study the heavy-gas dispersion phenomena. These differences mainly arise because of the varied treatment given to physical processes involved in the dispersion mechanism. One of these processes, which have not been fully considered in many of the existing models, is the effect of ground heating on the movement of a cloud under windy conditions. In this study, the box model developed by Kunsch and Fannelop (J. Hazard. Mater. 43 (1995) 169) is extended to incorporate variable air entrainment on the dispersion of a heavy gas cloud spreading in a channel under windy conditions. The air entrainment was assumed to be proportional to the cloud frontal velocity. The semi-analytical equations developed were then solved by numerical methods to provide a heavy gas cloud dispersion profile. The popular Runge-Kutta fourth order technique was adopted to solve the differential equations numerically. The model was applied on a cold cloud released instantaneously and the results indicated that the model behavior follows closely the expected dispersion trends and observed cloud characteristics reported in a laboratory study. The trial run carried out in order to model the scenario of no heat transfer, by equating the source temperature to the ambient temperature, followed variations observed in the field. The analysis of cloud behavior indicated that the cloud length is strongly influenced by source density and initial cloud temperature.     

Experimental study of flammability limits of natural gas-air mixture

Flammability limits data are essential for a quantitative risk assessment of explosion hazard associated with the use of combustible gas. The present work is to obtain the fundamental flammability data for prevention of the hazards in the practical applications. Experiments have been conducted in a constant volume combustion bomb, and the fuel considered here is natural gas (NG). The pressure histories in the combustion bomb are recorded and a criterion of 7% pressure rise has been used to judge a flammable mixture. The effects of ethane on NG-air flammability limits have been investigated. By adding diluent (carbon dioxide, nitrogen or their mixture) into NG-air mixture, the dilution effects on the flammability limits have been explored as well, and the results are plotted as functions of diluent ratio.     

Equipment design and installation features to disperse refrigerant releases in rooms—part II: determination of procedures

There is always a risk of leakage of refrigerant into a room that refrigeration and air conditioning equipment occupies. Mitigation of build-up of flammable concentrations from leakage through appropriate equipment construction and installation criteria minimises the potential for ignition. This paper is the first part of an investigation into design and installation measures to disperse leaked flammable refrigerant. It mainly describes the experiments and provides an analysis of the data. The paper describes a purpose built test facility, which was used to carry out experiments to study the dispersion of carbon dioxide to simulate leaked refrigerant. By measuring carbon dioxide concentrations and making flow visualisation, the effects of parameters such as equipment airflow and installation height were observed. The observed trends provide guidance for designing refrigeration and air conditioning equipment, which helps to ensure rapid dispersion of flammable concentrations in the event of a leak of flammable refrigerant. A second paper (Part II) discussed the development of numerical correlations, which are used in the resulting design procedure.     

Determination of flash point in air and pure oxygen using an equilibrium closed bomb apparatus

The standard closed testers for flash point measurements may not be feasible for measuring flash point in special atmospheres like oxygen because the test atmosphere cannot be maintained due to leakage and the laboratory safety can be compromised. To address these limitations we developed a new "equilibrium closed bomb" (ECB). The ECB generally gives lower flash point values than standard closed cup testers as shown by the results of six flammable liquids. The present results are generally in good agreement with the values calculated from the reported lower flammability limits and the vapor pressures. Our measurements show that increased oxygen concentration had little effect on the flash points of the tested flammable liquids. While generally regarded as non-flammable because of the lack of observed flash point in standard closed cup flash point testers, dichloromethane is known to form flammable mixtures. The flash point of dichloromethane in oxygen measured in the ECB is -7.1 degrees C. The flash point of dichloromethane in air is dependent on the type and energy of the ignition source. Further research is being carried out to establish the relationship between the flash point of dichloromethane and the energy of the ignition source.     

Explosion protection for vehicles intended for the transport of flammable gases and liquids--an investigation into technical and operational basics

In Europe, the transport of flammable gases and liquids in tanks has been impacted by new developments: for example, the introduction of the vapour-balancing technique on a broad scale and the steady increase in the application of electronic components with their own power sources; furthermore, new regulatory policies like the ATEX Directives are being enforced in the European Union. With this background in mind, the present investigation aims to provide a basis for future developments of the relevant explosion protection regulations in the safety codes for the transport of dangerous goods (RID/ADR). Specifically, the concentration of gas in the air was measured under various practical conditions while tank vehicles were being loaded with flammable gases or liquids. These spot-test data were supplemented by systematic investigations at a road tanker placed in our test field. With respect to non-electrical ignition sources, a closer investigation of the effect of hot surfaces was carried out. With regard to improving the current regulations, the results of our investigation show that it would be reasonable to implement a stronger differentiation of the characteristics of the dangerous goods (gaseous/liquid, flashpoint) on the one hand and of the techniques applied (loading with and without vapour-balancing system) on the other hand. Conclusions for the further development of the current international regulations are proposed.     

玉米淀粉电火花点火数值模拟

在粉尘云电火花点火实验研究的基础上,结合粉尘云点火机理,建立完整的粉尘云点火模型。通过模型计算玉米淀粉的最小点火能,并模拟点火过程中颗粒温度随时间的变化过程,同时分析在无粉尘粒子情况下电火花温度的变化情况。通过模拟计算,得到玉米淀粉在敏感条件下的点火能为2.9 mJ,计算结果与实验数据基本一致,由此可以进一步理解粉尘云的点火过程及电火花放电过程。     

Mitigation of dense gas releases within buildings: validation of CFD modelling

When an accidental release of a hazardous material is considered within a safety case or risk assessment, its off-site effects are generally assessed by calculating the dispersion of vapour from the site. Although most installations handling flammable materials will be in the open air, many types of plant, particularly those handling toxics, are enclosed, partly to provide some form of containment and hence, to mitigate the effects of any release. When such a release occurs within a building, the gas or vapour will undergo some mixing before emerging from any opening. The degree of mixing will depend upon the building geometry and the nature of the ventilation, which in turn may be modified by the leak. This situation is considered in this paper, with specific application to calculating the rate of release of a dense vapour from a building. The paper describes the application of computational fluid dynamics (CFD) techniques to modelling the release and mixing processes within buildings. Examples of validation calculations for simple geometric arrangements, as well as more complex geometries representative of an industrial site, are described. The results demonstrate the capabilities of CFD for this application but highlight the need for careful modelling of the near-wall flows and heat transfer, and need for an accurate fluid dynamics and thermodynamic representation of the release source.     

Research on the flammability hazards of an air conditioner using refrigerant R-290

Currently, hydrochlorofluorocarbons and hydrofluorocarbons are the most common refrigerants used for air conditioners. Due to ozone depletion and high global warming potential, environmentally benign options such as hydrocarbons are under consideration. Whilst R-290 (propane) has favourable system performance, environmental characteristics and cost, it is a flammable substance, thereby posing additional risks. This study addresses the associated flammability concerns through a number of risk-related sub-studies. These include evaluating the distribution of R-290 following a leak in room, overpressure arising from ignition of a flammable mixture, severity of a secondary fire and total heat release rate in the event of an external fire imposed upon an R-290 system. It is found that the possibility of refrigerant existing within the flammable range is limited only to the region very close to the indoor unit. Besides, low overpressures in the event of ignition and limited additional heat flux in the event of external fire were registered.     

Experimental research on the explosion characteristics in the indoor and outdoor units of a split air conditioner using the R290 refrigerant

Currently, the most common refrigerants used in household split air conditioners are R22 and R410A. Due to environmental concerns, benign options such as R290 (propane) are under consideration as an alternative. However, R290 is flammable, which poses additional fire and explosion risks. The ignition source and location of the leak that may appear within the indoor and outdoor units were analysed. A series of experiments were carried out to better understand the ignition hazard. The explosion characteristics associated with the indoor and outdoor units were studied whereby the overpressure arising from ignition of R290 was measured at different locations. According to the internal volume of indoor and outdoor units, the amount of R290 that should be deposited inside the indoor and the outdoor units is 7 g and 16 g respectively, to form a stoichiometric concentration. The explosion overpressure in the indoor and outdoor units is sufficiently low so as to not damage the air conditioner system. However, if R290 is ignited during the leak, the indoor or outdoor unit will be burned.