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Fire and Explosion
Abstract
Compliance with new UK and EU gas safety legislation for chemical processing plants is today a major factor influencing its design and operation. The activities of exploration and production of natural gas are associated with gas transportation, distribution and storage. In industrial, commercial and domestic markets, there are innumerable combusting flows as gas is burned as an end product by customers. In all these activities, accurate assessment of what would happen in the event of an operational or accidental release of gas, particularly where gas dispersion, fire or explosion might be involved, is an essential part of ensuring safe operations.
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Following is the continuation of the list of titles and authors: Reliability Engineering - A Rational Technique for Minimizing Loss. By B. A. Buffham, D. C. Freshwater, And F. P. Lees. High Integrity Protective Systems. By R. M. Stewart. Design for Loss Prevention - Plant Layout. By H. G. Simpson. Loss Prevention Aspects in Process Plant Design. By P. L. Klaasen. Fire Detectors for Use on Chemical Plants. By B. G. Steel. Review of Recent Advances in Fire Extinguishing Chemicals. By W. W. Harpur. Dry Chemical Fire Extinguishing Systems and Installations in Chemical Industries. By F. Emmrich. Relief Venting of Dust Explosions in Process Plant. By K. N. Palmer. Ignition and Burning of Dispersions of Flammable Oil in the Form of Pools. By D. G. Wilde and G. E. Curzon. Safe Dispersal of Large Clouds of Flammable Heavy Vapours. By E. M. Cairney and A. L. Cude. Protective Measures and Experience in Acetylene Decomposition in Piping and Equipment. By Herbert Schmidt.
A previous analysis of the turbulent boundary layer which arises from the injection of premixed reactants into a stream of combustion products is extended to the case of variable enthalpy. The variation in enthalpy can be due either to losses or gains in heat in the product stream or to differences in the stoichiometry of reactants and products and leads to a temperature in the external stream different from the adiabatic equilibrium temperature of the injected reactants. The thermochemistry of the system is described by a multivariate probability density function involving two scalars, the progress variable and an enthalpy variable. Three different forms of the probability density function of the distributed enthalpy within products are considered. An extension to incorporate the pdf of the velocity components and those scalars permits description of the aerothermochemistry of the system. Similarity solutions of the complete set of equations for first and second moment quantities permits the effects of enthalpy variations on the structure of the boundary layer to be investigated. In particular it is shown that countergradient diffusion occurs if the enthalpy difference between reactants and products in the external flow is suitably large. The sensitivity of the predictions to the assumed form of the enthalpy distribution and comparison with available experimental results are discussed.
A novel high liquid pressure fine spray swirl atomizer has been developed, which incorporates a spill-return orifice into the rear face of the swirl chamber with the aim of giving a significant reduction in flowrate while maintaining the droplet size. The initial work modified a commercial atomizer to add spill return. However, drop sizes were considered to be too large and a new design was constructed based on an earlier work on efficient high-pressure (up to 120 bar) swirl atomization. The resulting fine sprays can be used for various applications such as humidification, cleaning, coating, cooling, and decontamination. The atomizer has been characterized for different geometries, supply pressures, and spill-return orifice sizes using a Laser Particle Sizer and Phase Doppler Anemometry. For an exit orifice of 0.3 mm diameter and spill orifice 0.5 mm diameter, the drop size (Sauter mean diameter) is less than 20 μm for flowrates as low as 0.1 litre/min and with a mean axial drop velocity of approximately 12 m/s. An average liquid volume flux of 0.014 (cm3/s)/cm2 is obtained in the spray at 150 mm downstream.
A mathematical model of vented gas-phase deflagrations is presented. By introducing several empirical parameters, account is taken of initial turbulence in the gases, flame acceleration due to hydrodynamic instabilities prior to vent opening, and increased burning velocity due to turbulence generated by the venting process. Additionally, a mixture of burned and unburned gases is vented. Essential information needed to compute the pressure development during vented deflagrations (or in large closed vessels) is the rate of increase of flame area due to cell formation in the flame front prior to the vent opening.The model has been tested against methane/air mixtures at initial pressures of 45 psia in vessels up to 3.8 m3 in volume. Good agreement has been obtained.Further work is underway to gather data on vented deflagrations for gases such as propane, ethylene, and hydrogen (which represent a series of increasing burning velocities) and to investigate more fully the effect of initial turbulence and elevated pressures.
A simple computer-based algorithm is presented for estimating the overpressures resulting from a vapour cloud explosion in a vented box with obstacles. The algorithm is based on a very simple description of the physical processes involved. It is compared to a large set of 35 m3 experiments previously conducted by Det norske Veritas. The model contains descriptions of all the most important physical processes, and is capable of systematic refinement. If applied carefully, it could be used as a rapid assessment tool for evaluating the hazard from vented obstructed vapour cloud explosions.
All known experimental values of turbulent burning velocity have been scrutinized. These number 1650, a significant proportion of which at the higher turbulent Reynolds numbers we measured in a fan-stirred bomb. Dimensionless correlations which have a theoretical basis are presented. These are in terms of flame straining rates and the effective r.m.s. turbulent velocity, as well as the laminar burning velocity of the mixture. When a flame develops from an ignition source it is not initially exposed to the lower frequencies of the turbulent spectrum. As the kernel grows the flame is affected by ever-lower frequencies and the turbulent burning velocity increases towards a fully developed value. An experimental dimensionless power spectral density function is presented, and used to show how both effective r.m.s. turbulent velocity and flame straining rate develop in an explosion. The results are relevant to a variety of practical devices, including gasoline engines, as well as atmospheric explosions.
Recent accidents in the North Sea have confirmed that gas explosions constitute a major hazard for gas and oil production on offshore facilities.
Research on gas explosions at CMI has for more than 10 years focused on experimental research into flame propagation in complex geometries as well as the development of a gas dynamic code (FLACS) capable of predicting gas explosion propagation in industrial environments.
The FLACS code calculates the turbulence generated by the interaction of the gas explosion with confinement and equipment and the positive feedback of this turbulence into the gas explosion. The code predicts pressure and drag forces due to gas explosions for specified module layouts and for different fuels and ignition locations.
The paper gives a brief introduction to the nature of gas explosions and an overview of previous research at CMI, followed by a presentation of results from the present research on the subject. As an example the effect of water deluge on explosion propagation will be discussed.
Finally an overview is given of lessons that have been learned by using the FLACS code, which is used by operators to design offshore modules. The effect of vent area location, types of vent area covers, module shape, equipment location and platform layout is discussed.
Introduction
Gas explosion safety is a source for concern in areas where large amounts of combustible substances may be accidentally released. Recent, large accidents involving gas explosions and fires demonstrate the need to continuously address gas safety issues. Areas where this is a concern comprise most areas where gas is present, or may be present through an accident, in large amounts: exploration, production, transport, processing, storage and utilization.
Parts of the accident chain that may have to be addressed, include: leaks, dispersion, ignition, explosions, fires and subsequent loads on people and structures. Possible measures to improve safety include looking at process control, design and safety analysis, working procedures and 'the human factor' and also at mitigation techniques. All of these factors are important and must form part of a total safety control strategy. The research presented here is mainly focused on areas related to design and safety analysis and on mitigation techniques.
Previous research
Gas explosion research has been an important activity at CMI since the late 1970's when a series of large-scale explosion experiments in a 50 m3 tube demonstrated the insufficiency of tools like nomograms or simple formulae for overpressure prediction in partly confined, obstructed geometries.
In 1980 a large, seven-year research programme was initiated where the overall objective was to minimize gas explosion hazards both onshore and offshore. This was to be achieved by generating new knowledge concerning questions like
* what is the likely distribution of hydrocarbon-air clouds generated by accidental releases? * which flame acceleration mechanisms, due to confinement and geometry, can be expected? * what will he the expected maximum explosion pressures in the system? * what are the possibilities for successful venting of a confined explosion?
Answers to these questions were required to enable the generation of practical design principles to minimize the effect of accidental gas explosions.
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The SCOPE 3 model (Shell Code for Overpressure Prediction in gas Explosions) has been developed to predict the overpressures which could be generated by gas explosions in vented enclosures, such as offshore modules. SCOPE 3 attempts, wherever possible, to model the underlying physical processes in an explosion. This phenomenological approach gives greater confidence in predictions for full-scale events than methods based simply on correlations of experimental data.
The Baker-Strehlow methodology was developed to provide an objective approach to prediction of blast pressures from vapor cloud explosions. The complete methodology was first published in 1994 [1]. Since then, it has evolved through ongoing research and use in VCE hazard analyses, facility siting studies and accident investigations. This article gives a brief overview of a paper on recent developments in the Baker-Strehlow methodology presented at the 31st Loss Prevention Symposium in Houston on March 9-13, 1997. Because the entire paper is too lengthy to be presented here, the following discussions may be lacking in some details. A copy of the complete paper can be obtained from the American Institute of Chemical Engineers (AIChE).
Since the Baker-Strehlow method was first published, it has been used extensively in VCE hazard assessments in refineries and chemical plants. As expected, many practical lessons have been learned during the course of the hazard assessments, and the Baker-Strehlow method has evolved as a result. The changes have been evolutionary, not revolutionary. In keeping with the goals of the original study in which the methodology was developed, all changes have been incorporated with the intent of achieving an objective methodology to provide consistent prediction of VCE blast effects.
The revisions to the Baker-Strehlow method resulting from experience gained during plant walk-downs and hazard assessments include:
Systematic identification of “potential explosion sites” or “PESs,”
Selection of the level of confinement for mixed zones of 2D and 3D confinement,
Deciding on flame expansion when confinement is elevated above the vapor cloud,
Selecting the reactivity for a fuel that is a mixture of fuels with differing reactivities,
Predicting blast loads when there are multiple PES's within a vapor cloud considering different ignition source locations.
A multi-energy method that uses an idealized gas explosion for predicting vapor cloud explosions is described.
Barrier technology offers a way forward to control consequences of potential gas explosions. The potential of this technology was discussed in a previous paper. Here we describe the test to explore the potential application of the Micromist device as a soft suppressive barrier. The Micromist device is based on a proprietary hot-water technology, which allows the production and distribution of a very fine mist suitable for damping down the propagating flame. Towards the end of 1999, BP and the UK Health and Safety Executive jointly funded a test programme to investigate the performance of the Micromist device using the medium-scale test rig at the Christian Michelsen Research. Results of this test are described, and ways forward to be identified. A total of 20 tests were carried out and they showed the Micromist device to be able to arrest a developing gas explosion. In cases where the water droplet loading was not sufficient to arrest the flame, the severity of the gas explosions was much reduced. These tests represent an initial assessment of the technology but there are other elements of the system that still need to be addressed, for example, control and detection strategy. These tests specifically exclude the impact of detection and control on the performance of the Micromist device.
A mathematical model for the prediction of overpressures generated in totally confined and vented explosions is described. The model may be applied to explosions of any gaseous fuel/oxidant mixture, and its application to combustion in cubic or cuboid geometries is discussed. Sample solutions of the model are presented, together with comparisons with experimental data and alternative prediction methods. Recommendations as to the correct choice of a turbulent burning velocity for vented explosions are made.
Risks of personal injury from gas explosion, together with fire and smoke ingress, were among the key hazards that the Eastern Trough Area Project (ETAP) team intended to design out as far as possible. This paper describes the process ETAP followed to achieve this. The process involved the early application of the appropriate advance technology and personnel at the concept selection stage and right through different stages during design, and an integrated team including explosion specialists.All major design decisions on explosion optimisation were made at the early stage of front-end engineering design (FEED), resulting in a relatively straightforward detailed design phase. These early design decisions had the effect of not only reducing gas explosion consequences, but simplifying layout, e.g. reducing pipe run and structures. The end result is a design which gives inherently low risk to personnel and Temporary Refuge impairment without the uncertainties of high cost of late remedial work to take account of high explosion loads, and consequent project delay.
Partial confinement is a major cause of blast in vapour cloud deflagrations. Criteria to identify partial confinement in vapour clouds are indicated. A method for blast prediction is proposed, which fully reflects characteristic features of vapour cloud explosions. Its use is demonstrated in a case study and its applicability is discussed.
Reimpresión en el año 1989 Incluye bibliografía e índice
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