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Summary of discussion on explosive clouds
Acta Astronautfca Vol.5, pp. 1231-1232
0094-5765/7811101-1231/$02.00/0
© PergamonPress Ltd., 1978. Printedin Great Britain
COMMENT
Summary of discussion on explosive clouds' (Received 24
January 1978)
Abstract--Under the co-chairmanship of Dr. H. J. Pasman (TNO, Holland) and Dr. H. Phillips (HSE, U.K.) a special evening session was organised for those colloquium participants with a special interest in the subject "Explosive Clouds".
A NUMBERof disastrous accidents have involved the release of flammable vapour and its subsequent explosion, with loss of life and property. Consequently in recent years various theoretical and experimental studies of the behaviour of explosive clouds have been carried out with the aim of reducing the probability and the consequences of such accidents and predicting safe distances. Some of the studies were presented as papers during the colloquium, but the special evening session was useful in that it encouraged discussion of the phenomenon. The meeting started with films showing some of the field experiments that have been carried out. The films showed balloon tests with acetylene-air mixtures explosively initiated and leading to detonation (J. Duco, CEA, France), and with fuel-air mixtures of different reactivities (R. T. Luckritz, U.S. Coast Guard). Despite an initiating charge of 2.5 kg composition B explosive, methane-air only produced a deflagration in a 5 meter radius balloon. The flame decoupled from the shock at some distance from the initiating charge. Further balloon tests were shown in which a low initiating energy in a reactive mixture (ethylene oxide-air) lead to deflagration with maximum flame speeds a few times larger than that of the laminar flame (R. T. Luckritz, R. A. Strehlow, Univ. of Illinois, U.S.A.). Finally field spill tests with LNG (China Lake) were shown in which the cloud was ignited with a torch and produced no blast. Brief introductory talks on several selected topics then formed the basis for the discussion. Dr. Phillips (HSE, U.K.) raised the following point: One measure that has been used to define the yield of an unconfined vapour cloud explosion has been the TNT equivalence. It was agreed that this approach could lead to results that were seriously in error when applied to gaseous or vapour explosions. If the TNT yield is to be estimated £rom observations of window and roof damage in the far field, the discrepancy between different estimates of TNT equivalence may be as much as a factor of 10. Part of the variation lies in differences in the plots of overpressure versus scaled radius for solid explosives due to different laboratories (see R. A. Strehlow and W. E. Baker; Evaluation and characterisation of accidental explosions. Prog. Energy Comb. Sci. 2, 27-60, 1976) tThis discussion took place at the 6th International Colloquium Gasdynamics of Explosions and Reactive Systems, Stockholm, Sweden, August, 1977. 1231
1232
Summary o f discussion on explosive clouds
but a more serious cause of error arises because the vapour cloud explosion releases energy slowly and the destructive effects of its blast wave cannot be compared with the effects of the rapid explosion of TNT. This was illustrated by describing the damage resulting from a recent propylene vapour cloud explosion (Mr. Bruning, DSM, NL) and by reporting about a window pane fracture investigation (Mr. Trense, TNO, NL). It was Prof. Strehlow's view, that because of the variations in the rate of energy release there could be no unique plot of overpressure versus scaled radius (see R. T. Luckritz and R. A. Strehlow's paper to the colloquium). The maximum velocity that the flame will reach is the most important variable in determining the overpressure. Flame speed needs to be about 100 times the normal burning velocity to produce serious damage. Studies show that the effect of turbulence on flame speed is less than a factor of 100, so R. A. Strehlow concluded that high flame speeds and damage are not the result of flame acceleration due to turbulence. He suggested that partial confinement and the effect of obstructions are key factors. The effect of partial confinement was elaborated by Dr. Slagg (ARRADCOM, U.S.A.). He suggested that this important parameter should be better defined and investigated. Also aerosol and dust explosions deserve our attention. Nevertheless there are differences between different flammable gases. Prof. J. H. Lee (McGill Univ., Canada) explained that there is a need for a measure of the ability of a fuel to produce damage when involved in an unconfined vapour cloud explosion. He suggested that a suitable parameter of reactivity, which could be determined in the laboratory, might be the minimum energy for direct initiation of a spherical detonation. In Prof. Lee's tracks Dr. Guirao (McGill Univ., Canada) presented results of a summary plot of TNT equivalents as found at a number of incidents and at various pressure levels. She also showed the results of relevant calculations. In particular the calculation of the blast of a detonating ellipsoidal cloud according to the Brinkley-Kirkwood method was of interest. Prof. Wagner (Univ. of G/Sttingen, FRG) concluded that exaggeration of the effects should be avoided and further research could produce valuable results to describe better the potential blast waves due to vapour clouds. The general feeling of the meeting was that the study of unconfined explosions was still in its infancy; research undertaken so far had produced more questions than answers. This would be a fruitful topic for research for some years to come.
0094-5765/7811101-1231/$02.00/0
© PergamonPress Ltd., 1978. Printedin Great Britain
COMMENT
Summary of discussion on explosive clouds' (Received 24
January 1978)
Abstract--Under the co-chairmanship of Dr. H. J. Pasman (TNO, Holland) and Dr. H. Phillips (HSE, U.K.) a special evening session was organised for those colloquium participants with a special interest in the subject "Explosive Clouds".
A NUMBERof disastrous accidents have involved the release of flammable vapour and its subsequent explosion, with loss of life and property. Consequently in recent years various theoretical and experimental studies of the behaviour of explosive clouds have been carried out with the aim of reducing the probability and the consequences of such accidents and predicting safe distances. Some of the studies were presented as papers during the colloquium, but the special evening session was useful in that it encouraged discussion of the phenomenon. The meeting started with films showing some of the field experiments that have been carried out. The films showed balloon tests with acetylene-air mixtures explosively initiated and leading to detonation (J. Duco, CEA, France), and with fuel-air mixtures of different reactivities (R. T. Luckritz, U.S. Coast Guard). Despite an initiating charge of 2.5 kg composition B explosive, methane-air only produced a deflagration in a 5 meter radius balloon. The flame decoupled from the shock at some distance from the initiating charge. Further balloon tests were shown in which a low initiating energy in a reactive mixture (ethylene oxide-air) lead to deflagration with maximum flame speeds a few times larger than that of the laminar flame (R. T. Luckritz, R. A. Strehlow, Univ. of Illinois, U.S.A.). Finally field spill tests with LNG (China Lake) were shown in which the cloud was ignited with a torch and produced no blast. Brief introductory talks on several selected topics then formed the basis for the discussion. Dr. Phillips (HSE, U.K.) raised the following point: One measure that has been used to define the yield of an unconfined vapour cloud explosion has been the TNT equivalence. It was agreed that this approach could lead to results that were seriously in error when applied to gaseous or vapour explosions. If the TNT yield is to be estimated £rom observations of window and roof damage in the far field, the discrepancy between different estimates of TNT equivalence may be as much as a factor of 10. Part of the variation lies in differences in the plots of overpressure versus scaled radius for solid explosives due to different laboratories (see R. A. Strehlow and W. E. Baker; Evaluation and characterisation of accidental explosions. Prog. Energy Comb. Sci. 2, 27-60, 1976) tThis discussion took place at the 6th International Colloquium Gasdynamics of Explosions and Reactive Systems, Stockholm, Sweden, August, 1977. 1231
1232
Summary o f discussion on explosive clouds
but a more serious cause of error arises because the vapour cloud explosion releases energy slowly and the destructive effects of its blast wave cannot be compared with the effects of the rapid explosion of TNT. This was illustrated by describing the damage resulting from a recent propylene vapour cloud explosion (Mr. Bruning, DSM, NL) and by reporting about a window pane fracture investigation (Mr. Trense, TNO, NL). It was Prof. Strehlow's view, that because of the variations in the rate of energy release there could be no unique plot of overpressure versus scaled radius (see R. T. Luckritz and R. A. Strehlow's paper to the colloquium). The maximum velocity that the flame will reach is the most important variable in determining the overpressure. Flame speed needs to be about 100 times the normal burning velocity to produce serious damage. Studies show that the effect of turbulence on flame speed is less than a factor of 100, so R. A. Strehlow concluded that high flame speeds and damage are not the result of flame acceleration due to turbulence. He suggested that partial confinement and the effect of obstructions are key factors. The effect of partial confinement was elaborated by Dr. Slagg (ARRADCOM, U.S.A.). He suggested that this important parameter should be better defined and investigated. Also aerosol and dust explosions deserve our attention. Nevertheless there are differences between different flammable gases. Prof. J. H. Lee (McGill Univ., Canada) explained that there is a need for a measure of the ability of a fuel to produce damage when involved in an unconfined vapour cloud explosion. He suggested that a suitable parameter of reactivity, which could be determined in the laboratory, might be the minimum energy for direct initiation of a spherical detonation. In Prof. Lee's tracks Dr. Guirao (McGill Univ., Canada) presented results of a summary plot of TNT equivalents as found at a number of incidents and at various pressure levels. She also showed the results of relevant calculations. In particular the calculation of the blast of a detonating ellipsoidal cloud according to the Brinkley-Kirkwood method was of interest. Prof. Wagner (Univ. of G/Sttingen, FRG) concluded that exaggeration of the effects should be avoided and further research could produce valuable results to describe better the potential blast waves due to vapour clouds. The general feeling of the meeting was that the study of unconfined explosions was still in its infancy; research undertaken so far had produced more questions than answers. This would be a fruitful topic for research for some years to come.