Showing papers in "Journal of Loss Prevention in The Process Industries in 1992"

Probabilistic safety analysis in chemical installations

Abstract: A set of procedures and corresponding methodologies for probabilistic safety assessment (PSA) in chemical installations is presented. State-of-the-art methodology for PSA in nuclear power plants is tested for suitability and applicability to chemical installations and applied on a refrigerated ammonia storage facility. The procedural steps comprise hazard identification, accident sequence modelling, data acquisition and parameter estimation, accident sequence quantification, hazardous substance release categories assessment, consequence assessment and integration of results. The results of the application indicate that the methodology applied is adequate from the procedural and completeness point of view and that no major aspect of PSA of a chemical installation is overlooked. A need for integrated computerized tools has, however, been identified.

Computer modelling of gas explosion propagation in offshore modules

Abstract: A computer model is presented for gas explosion propagation in complex densely packed geometries. The governing equations are formulated according to the quasi-continuum principle. The influence of obstacles is allowed for by specifying volume and area porosites and by including distributed resistances in the calculation domain. Distributed resistance functions are given for various types of obstacles, such as flow parallel to, or normal to, rod bundles and single obstacles, either sharp or rounded. Modified versions of the k -e turbulence model and the eddy-dissipation combustion model, including the porosity/distributed resistance formulae, are presented. The method is validated by calculating the pressure build-up in offshore module geometries and comparisons with experimental data is generally good.

Strategies for the quantification of thermally initiated cascade effects

Abstract: Thermally initiated cascade effects can have devastating results for large petroleum refineries or chemical installations on industrial estates. Adjacent plant installations are involved and the overall damage is potentially several times greater than for individual events. In spite of its great practical importance, consequence analysis of cascade effects has received little attention due to the mechanistic complexities involved. This paper highlights the possible theoretical approaches to assess the consequences of cascade effects. The importance of modelling and linking the time variant thermal interchange between installations, failure of vessels under thermal and other stresses and the subsequent fracture fragmentation and impact phenomena are stressed.

Validation of flame drag correlations with data from large pool fires

Abstract: The validity of correlations describing the effect of wind on the flame of a large pool fire (flame tilt and flame drag) has been investigated on the basis of observations made at two tests and one storage tank fire. It is concluded that the flame drag does not depend on the density of the vapour evaporating from the burning pool. The flame drag correlation derived from large LNG pool fires by Moorhouse predicts the measured flame drag values.

Review of dense gas dispersion field experiments

Abstract: This paper considers field scale experimental programmes performed during the last decade to study the atmospheric dispersion of dense gases. The objectives are to summarily review the programmes, consider what has been learned and identify questions that remain to be answered. Considering the test experience, recommendations are made for a course of action to further increase our understanding.

Calculating the energy of explosion using thermodynamic availability

Abstract: Thermodynamic availability represents the maximum mechanical energy extractable from a material as it reversibly moves into equilibrium with the environment. Since blast overpressure is a form of mechanical energy, thermodynamic availability provides a new and completely general technique for determining the energy of explosion. This paper presents the fundamental basis and shows how thermodynamic availability is used to determine the energy of explosion. It is shown that the method is applicable for any process conditions, including systems at elevated temperatures and pressures and systems composed of many materials, and for reactive explosion and explosions of inert compressed gases. The calculations are quite simple, even for the most complex cases. A table of thermodynamic data and a number of worked examples are provided.

Dense gas dispersion modelling

Abstract: The effects of release of hazardous material will originate from the release point, and will diminish with distance from the site. The greatest potential for significant off-site effects exists when a gas or vapour is released which may be either toxic or flammable. This means that an important part of any consequence analysis study must be the calculation of gas dispersion. In many cases, the action of the atmosphere in dispersing a gas will serve to reduce the hazard to nearby personnel. This particularly true for passive or positively buoyant releases from elevated sources and is used to good effect in the design of stacks for the discharge of normal emissions. However, for releases at ground level, or where the gas is denser than air, dispersion is much slower and hazard ranges could be significant. This paper briefly reviews the nature of atmospheric turbulence and the way in which this affects dispersion. Particular features of dense gas dispersion are discussed and compared with the dispersion of neutrally buoyant gases. Methods for incorporation of topographical and obstruction effects are also discussed, together with a range of mathematical models available for the prediction of dense gas dispersion. Case studies from a range of industries are presented in order to demonstrate the use of the methods discussed.

Effectiveness of various rock dusts as agents of coal dust inerting

Abstract: An experimental investigation of requirements for coal dust inerting was conducted in a 26 l, spherical chamber. A mine-face sample of coal from the Prince mine of the Cape Breton Development Corporation was tested; rock dusts used were limestone, dolomite (two particle sizes) and magnesite. Limestone, dolomite and magnesite (all having approximately the same particle size) were found to be similar in inerting effectiveness, indicating that the decomposition reaction of the endothermic rock dust did not exert an effect over the short time scales during which the volatiles burn during an explosion. Tests with the two sizes of dolomite showed that a decrease in rock dust particle size could significantly lower the amount required to inert.

Plant level safety analysis

Abstract: The advantages and constraints of safety analysis at the plant level are discussed, based mainly on the results of a hazard and operability study of a pulp process carried out at the plant level and compared with the results of a more detailed analysis. The general level safety analysis, which could be rapidly performed, identified the potential major hazards and the countermeasures required.To discover and analyse the causes of hazardous situation, however, requires that a more detailed hazard analysis is performed.

Development and application of a pilot scale facility for studying runaway exothermic reactions

Abstract: This paper describes a facility for investigating the control and stability of exothermic reactions and the performance of relief systems in a pilot scale installation. A series of experiments is reported in which a simple exothermic reaction is shown to proceed under isothermal conditions, subcritical conditions (with some self-heating) and supercritical conditions leading to exothermic runaway.

Exposure of organophosphorus pesticides to turbulent diffusion flames

Abstract: The development of effective fire-fighting strategies for warehouses storing large quantities of toxic chemicals depends on an understanding of the uncontrolled combustion of such toxics. This paper describes an experimental system for studying fires of this kind. Results are presented from a test programme involving fires with rates of convective heat release in the range 3–30 kW and four organophosphorus pesticides. This work on the combustion chemistry of toxic materials is part of a wider study, involving a number of laboratories, into other inter-related aspects of warehouse fires. The significance of the results for hazard assessment and emergency planning is considered.

Some experiences in hazard identification and risk shortlisting

Abstract: ‘Rapid ranking’ has been used for many years as a method of determining the priority which should be given to formal investigation of the wide range of hazards and risks present on major process industry sites. Such methods tend to evolve to meet the needs of specific applications. Some historical and recent applications are outlined, and the nature of the developments which they led to are discussed. Particular topics discussed include: development of the method such that it is usable by operating managers rather than specialists; use to raise risk consciousness; adaptation for use as an aid to hazard identification; use to define both which risks should be studied and which risks, even if low, should be incorporated in routine monitoring and periodic auditing programmes; use as a basis for comparison of the relative risks presented by a variety of different industrial installations; use for a range of different types of risk; and extension beyond ranking to include ‘shortlisting’ i.e. deciding which risks to include in a study programme and which to leave out. Philosophical difficulties with the approach are discussed, including the danger of omitting from detailed study a risk which is erroneously shown as low, and the uncertainty of the absolute level of the ranked risks. Approaches to minimizing these problems are discussed.

Experimental study on flammability limit of a chlorine trifuloride/dichlorosilane/nitrogen mixture

Abstract: The flammability limit of a chlorine trifluoride (CIF3)/dichlorosilane (SiH2Cl2)/nitrogen (N2) mixture was investigated experimentally. In this study, a SiH2Cl2/N2 mixture was first introduced into an explosion vessel and a CIF3/N2 mixture was then injected into the vessel. Spontaneous ignition was observed during the injection when the final CIF3/SiH2Cl2/N2 mxiture was in a flammable range. Various compositions were tested to determine the flammability diagram of a CIF3/SiH2Cl2/N2 mixture. From the flammability diagram, the lower flammability limits for spontaneous ignition were found to be 0.3 vol% for both CIF3 and SiH2Cl2. Below this limit ignition did not occur even when an electronic spark was applied.

Improved F/N graph presentation and criteria

Abstract: The hazards attending a process from which hazardous materials could be released can be assessed in terms of frequency of casualty-causing incidents and the number of casualties in each such incident. Historical data for accidents can be graphed in the form of frequency versus number of casualties ( F/N ) curves, for use in assessing public exposures to various types of accident, including transportation and industrial incidents. Similar curves can be constructed for industrial exposure of employees. These can be used to determine the need for process improvement and in assigning priorities for recommendations. The resulting ‘two-dimensional’ graphical presentation can be used to show ‘existing’ and ‘upgraded’ process performance and overcomes some statistical problems of ‘one-dimensional’ safety assessments and criteria. Appropriate criteria for public exposures would be the historical F/N curve for acts of God or acts of nature, with a suitable safety margin. The curves can be normalized to account for differing numbers of potentially-exposed members of the public or employees, allowing comparison of the hazards encountered in different industries, environments or countries. The development of F/N curves, particularly in Europe, and their probable application in evaluating the effectiveness of process safety management are presented.

Liquid discharge from process and storage vessels

Abstract: A generalized model to represent the orifice discharge of liquid from any point on the surface of a storage or process vessel is presented. The model is useful for determining the vessel leakage characteristics for specified geometries or using numerical volume versus height data acquired from the tank. Specific results are presented for vented tank geometries represented by tanks of constant cross-section (vertical cylinders and rectangular tanks), horizontal cylinders and spheres. Both algebraic and numeric results are presented. An interesting result is that the maximum leakage time for the sphere occurs when the leak is 25% of the diameter above the bottom of the tank. For the horizontal cylinder, the maximum leakage time occurs for leak 17% of the diameter above the bottom.

Testing a new device for suppressing electrostatic charges on polymer granules

Abstract: Large discharges along the surface of the silo heap of polyethylene granules were reproducibly generated during filling of a 3 m diameter earthed metallic silo. The specific charge acquired by the granules during pneumatic transport was of the order of 2 μC kg−1 and the mass flow rate of the granules was approximately 1 kg s−1 (3.6 metric tons h−1). The rate of discharges depends on the height of the heap and varied from 4 to 8 discharges min−1. By operating a three nozzle injector of bipolar ions in a carefully optimized arrangement at a short distance from the granules' entry port, all discharges were immediately suppressed.

Flow Visualizations during Top Venting from a Small Vessel

Abstract: To study the venting characteristics of foams and high-viscosity fluids, a 13 litre glass facility was built. The fluids used were pure water, foamy water and fluds with viscosities of 0.013 and 0.19 Pa s (giving values 73 and 1061 times that of water at 150°C). The resulting blowdowns show that differences in venting characteristics (pressure history and vented mass) are due to differences in the flow regimes. The low viscosity fluids have a churn-turbulent flow pattern, while the high viscosity fluid forms a ‘two-phase emulsion’ through which large bubbles rise. The experimental results are compared with existing data. Good agreement between the two sets of water and foam data is demonstrated but there is a discrepancy between the two sets of high viscosity data.

Reactivity limits of aluminium and halohydrocarbon mixtures: determination by the ASTM CHETAH program

Abstract: The CHETAH program has been successfully used to assess potential hazards of binary mixtures of aluminium with several halogenated compounds. From this thermochemical evaluation, conducted on more than 2000 mixtures, it was possible to distinguish mixtures with very low reactivity from mixtures with high reactivity delimited by two well defined concentrations of Al. These two concentration limits,analogous to flammability limits, can be considered as upper and lower reactivity limits. The CHETAH evaluation,carried out on mixtures with the highest reactivity values, in the presence of different amounts of an ‘inert’ compound (heptane), gives a triangular reactivity diagram similar to that frequently used in flammability characterization of combustible gases and vapours.

Benchmark exercises on the emergency venting of vessels

Abstract: The benchmark exercises concentrated on the hydrodynamic aspects of venting of vessels containing fluids under high pressure as well as the problems associated with the theoretical modelling of such processes with four types of code. The key features of these codes are illustrated and the parameters which characterize the emergency relief of the reactor vessel are identified. The experiments analysed in the benchmark were carried out at different plants in order to provide information about the dependence of the depressurization process on the size of the vessel, on the type of vent installed and on the initial conditions. Six of these venting examples are analysed together with the data calculated by the relevant codes.

Practical uncertainty assessment of reasoning paths (fault trees) under total uncertainty ignorance

Abstract: An engineer needs to know probabilities of different events. He cannot evaluate them using conventional formal calculations (e.g. statistical analysis) because of an information shortage. This paper presents two algorithms which calculate probabilities of a set of terminal hypotheses of a reasoning path (e.g. fault tree). This calculation is based exclusively on common sense metaheuristics and structure (topology) of a reasoning path. Therefore no uncertainty information is required (an assumption of total uncertainty ignorance). The first algorithm introduces a symbolic distance between two knowledge items. If a distance is small then items of uncertainty are similar. The second algorithm is based on a ‘water’ flow through a reasoning path (a long subpath has ‘large resistance’ and therefore low fluid probability). The main advantages of this methodology are its information non-intensity, simplicity and ability to absorb common sense heuristics. Therefore it can easily be put to practical use. A case study (fault tree with 38 terminal hypotheses)is given.

Risk assessment and risk contour mapping

Abstract: This paper presents a brief overview of risk assessment methodologies, focusing on safety in design and the different methods which can be combined for a full quantitative risk analysis (QRA). The results of a QRA are normally presented as individual risk and group or societal risk. The calculated individual risks are often presented in the form of an individual risk graph (IRG) or individual risk contours (IRC). The IRGs and IRCs are calculated for an example accident scenario: a release of ammonia at 2 kg s −1 for 1800 s, during different types of meteorological conditions. These examples clearly illustrate the development of the graphs and curves. Finally, it is concluded that a QRA (inclusive risk contour mapping) can be a useful tool, for example in land use planning. However, for the safety of a chemical plant, other methods for risk assessment are also available, and are indispensable.

Prediction of dust cloud minimum ignition energy for organic dusts from modified hartmann tube data

Abstract: The modified Hartmann tube apparatus is recognized to give realistic or safe indications of dust explosion violence. Evidence is presented that results from this apparatus can also be used to indicate minimum ignition energy. Published data show a correlation between minimum ignition energy and minimum explosible concentration, which is refined by also taking into account explosive violence. An empirical model has been derived, relating minimum ignition energy to minimum explosible concentration and explosive violence. The model allows estimation of probabilities of minimum ignition energy falling below specified values, such as those recognized as of practical importance by the relevant British Standard. A limited test of the model suggests that it can be applied to modified Hartmann tube data. It is anticipated that the model will be valuable as a screening tool, especially in the early stages of development of a project.

Performance of a safety relief valve under back pressure conditions

Abstract: By the nature of its design, the performance of a spring-loaded safety relief valve, especially a conventional valve, is influenced by back pressure. A bellows safety relief valve, since its disc is subjected to a much smaller downward force resulting from back pressure, is able to remain stable under much higher back pressure conditions. Due to the wide range of bellows manufacturing tolerances, the bellows safety relief valves can substantially reduce, but cannot totally eliminate, the back pressure effects on its set point and relieving capacity. Set point change in a conventional safety relief valve, due to constant superimposed back pressure, can be easily negated by cold differential set. For a bellows safety relief valve, however, set point change must be corrected by the set point correction factor which is a linear function of the back pressure to the set pressure ratio, R. There exists an allowable back pressure to set pressure ratio, Ra, and a critical back pressure to set pressure ratio, Rc, for each conventional and each bellows safety relief valve. When total back pressure exceeds Ra, the relieving capacity will be reduced, mainly resulting from the valve lift being reduced, and the capacity reduction factor should be applied in valve sizing. Once the Rc is exceeded, the safety relief valve becomes unstable and will totally lose its overpressure protection capability. For a given back pressure to set pressure ratio, R, the capacity reduction factor is a function of system overpressure, but its relationship is non-linear in nature.

Temperature runaway in fixed bed reactors: online and offline checks for intrinsic safety

Abstract: Recent trends in the online detection of thermal runaway precursors in exothermic reaction systems and its applicability to fixed bed tubular reactors are briefly discussed. While online techniques are favoured for restablization purposes, offline predictive methods are suggested for locating the parametrically sensitive zones. A computer-aided method for a priori assessment of parametric sensitivity based on the two-dimensional heterogeneous transport model for packed beds is formulated for a series—parallel reaction scheme. Its utility is demonstrated for two commercially important partial oxidation processes with different levels of reaction vigour and exothermicity.

Sizing pressure relief valves in flashing and two-phase service: an alternative procedure

Abstract: In view of the greater accuracy of the ω HEM correlation and the resulting conservative PR valve sizing (relative to the traditional PR valve sizing methods), the ω HEM-based flow calculations are recommended for all situations where flashing or two-phase flow occurs within the PR valve. However, as with any correlation, the accuracy of the ω HEM correlation results drops as the system to which it is applied diverges from the data used to develop it. Analysis of 15 different systems representative of actual refinery streams indicate that for fluids with a wide boiling range and for very non-ideal systems such as those containing hydrogen, the HEM correlation underpredicts the mass flux significantly (i.e. overpredicts the PR valve size). Since the large deviations are on the conservative side, the procedure will result in excessively large PR valves that may cause problems because of chattering of the PR valves, but would not present potential for vessel failure. The majority of the deviations are a result of the fact that the simplifying assumptions built into the correlation for single component do not truly characterize the actual flashing behaviour of many multicomponent fluids. The alternative approach presented, in which the correlation parameter ω is based on the actual flashing behaviour, eliminates nearly all the deviations and significantly improves the results of the correlation regardless of the system analysed. For trouble systems (those containing more than 0.1 wt% hydrogen and for some multicomponent fluids with a nominal boiling range greater than 80°C (150°F), the alternative approach should be used to define the correlation parameter ω. Use of this alternative approach is valid for any system (and will improve the accuracy of the correlation) but does require an additional flash calculation. However, for other systems the original formulation of ω is adequate and can be used.