Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs by Center for Chemical Process Safety Copyright 1994 American Institute of Chemical Engineers APPENDIX C EFFECTS OF EXPLOSIONS ON HUMANS C-1. INTRODUCTION This appendix is a summary of the work published in the so-called Green Book (1989). Possible effects of explosions on humans include blast-wave overpressure effects, explosion-wind effects, impact from fragments and debris, collapse of buildings, and heat-radiation effects. Heat-radiation effects are not treated here; see Chapter 6, Figure 6.10 and Table 6.6.. Explosion effects are commonly separated into a number of classes. The main division is between direct and indirect effects. Sometimes, direct effects are referred to as primary effects, and indirect effects are then subdivided into secondary and tertiary effects. Direct, Primary Effects The main direct, primary effect to humans from an explosion is the sudden increase in pressure that occurs as a blast wave passes. It can cause injury to pressuresensitive human organs, such as ears and lungs. Indirect Effects Primary fragments originate from the explosion source, for example, a pressure vessel. In general, those fragments have a high velocity. The impact of fragments and debris from sources not originating from the explosion source are secondary effects. Secondary fragments result when the blast tears off parts of structures, for example, bricks, roof tiles, and glass. Such fragments, except for glass, are relatively blunt and have low velocities. Glass window panes and fragments, however, are small, sharp, and sometimes have high velocities. Thus, they are capable of causing injuries at much greater distances from explosion centers than usually result from other secondary fragments. Building collapse can be regarded as a secondary effect, although it is not common to group this effect within any class. 351
352 APPENDIX C The explosion wind following a blast can carry persons away, causing injury as a result of their falling, tumbling over, or colliding with obstacles. This effect is referred to as a tertiary effect. Effects are described, together with criteria to calculate the probable degree of lethality. (2-2. PRIMARY EFFECTS Lethality Due to Lung Injury As the external pressure on the chest wall becomes larger than its internal pressure during the passage of a blast wave, the chest wall moves inward, thus causing injury. Because the inward motion takes time, the duration of the blast wave is important. Results of animal tests indicate that overpressure is only important for long durations, and impulse is important for relatively short durations (White et al. 1971). Most of the criteria found in literature are extracted from Bowen et al. (1968). Diagrams of pressure versus duration are presented for various body positions in relation to the blast wave, from which the chance of survivability can be calculated. Those diagrams were combined in a pressure-impulse diagram, which is depicted in Figure C-1. The scaled overpressure equals P/p,, in which P is the actual - pressure acting on the body, and po is the ambient pressure. The scaled impulse i equals: 5 = i/(p: *m 3) (C-1.1) in which i is impulse and rn is mass of the body. The impulse is the integral of overpressure over the blast-wave duration. For initial calculations, impulse can be approximated by i = 1/2Ptp (C- 1.2) in which tp is the duration of the overpressure in the blast wave. The overpressure P depends on the position of the human body (Figure C-2). If body position is such that no obstruction of the incident wave OCCUTS, P equals the side-on overpressure P, of the blast wave (Figure C-2A). If the body is upright (Figure C-2B), the incident wave is disturbed. Because the human body is small in relation to the length of the blast wave, the reflection phase can be neglected. Then the resulting overpressure on the chest wall equals the side-on overpressure P, plus the pressure Q caused by the explosion wind multiplied with the drag coefficient C, of the body: 5Pf (C-1.3) = 2Pf + 14 X Id
survivability threshold - * 3 flgure G1. Pressure-impulse diagram for lung injury. P: scaled overpressure. scaled impulse. (Bowen et al. 1968). D Flgure C2. Position of human body. (A) No obstruction of incident wave: P = P,. (6) Diffraction of incident wave: P = P, + 0. (C) Body subjected to reflection (standing): P = P,. (0) Body subjected to reflection (prone): P = P, (Bowen et al. 1968). 353
354 APPENDIX c In general the drag coefficient depends on the shape of the structure. In the case of a human body a value of 1 is considered to be accurate enough. If the body is near a surface against which the blast wave can reflect (Figures C-2C and C-2D), the pressure P acting on the body equals the reflected pressure P,: (C- 1.4) Ear Damage The ear is a very sensitive and complex organ that responds to very small variations in pressure. It was argued in Hirsch (1968) that ear drum rupture is decisive as to ear damage from blast waves. Figure C-3 shows the percentage of eardrum ruptures as a function of side-on overpressure P,. Overpressure duration probably has some influence on ear damage, but no literature on this subject was found. Because the ear can respond to high frequencies, blast wave loading normally lies in the pressure region rather than in the impulse region. 98 I I I I l l l 1 I I I I 95 - - 90- h 2 5 80 E g 70 REIDER 1968-40 HENRY 1945 20 10 2- I I I I I I l l I 1 I 1 1 2 4 6 105 2 4 6 p, (Pa) Figure C-3. Eardrum rupture as a function of overpressure (Hirsch 1968). -
APPENDIX c 355 TABLE C1. Criteria for Skull Fracture Due to Impact of a Mass of 4.5 kg Impact Velocity (mls) Level of Injury 3.1 Mostly safe 4.6 Threshold 7.0 Near 100% lethal C-3. SECONDARY EFFECTS For purposes of determining fragment effects on humans, cutting and noncutting fragments should be distinguished from each other. Cutting fragments penetrate the skin, whereas injuries from noncutting fragments result from contact pressure at impact. The open literature contains only scarce and incomplete data. However, criteria were found to describe the impact of a mass of 4.5 kg to the head (Table C-1). A fragment is generally considered to be dangerous if it has a kinetic energy of at least 79 J. But values of 40 to 60 J were found to cause serious wounds. Kinetic energy Ek equals: Ek = 1/2m,$ where m, is the fragment mass in kilograms and vf is the impact velocity in meters per second. The kinetic energy criterion can be applied for fragment masses between 4.5 and 0.1 kg. For smaller masses, the following equation can be used: in which v, = 1247P3mF3 + 22.03 v, = penetration velocity at which 50% of fragments penetrate the skin k = a shape factor which equals 4740 kg/m3n for the most damaging fragments mf = fragment mass The equation was derived empirically from experiments on animals, isolated skin, and materials resembling skin. Collapse of Buildings Humans inside collapsing buildings are subjected to the impact of very heavy structural parts. Pictures taken after earthquakes or bomb attacks reveal that vertical members usually fail, leaving a stack of floors on top of another. Although a
356 APPENDIX C TABLE C2. Injury Criteria for Whole Body Impact Impact Velocity (mls) Injury 3.0 6.4 16.5 42.0 Mostly safe Lethality threshold Lethality near 50% Lethality near 100% building collapse may appear total, it is not unusual for some people to survive within the spaces formed by the collapsed structure. Earthquake statistics reveal that about 50% of those inside a collapsing building will be killed. either immediately or as a result of injuries sustained. Other data are lacking, but one could assume a similar percentage for people inside buildings that collapse as a result of blast. This assumption is supported by the fact that, in both cases, the event is sudden and unexpected, so there is neither a place nor the time to find other shelter. C-4. TERTIARY EFFECTS Air particles in a blast wave have a certain velocity which, in general, flow in the same direction as the propagation of the blast wave. This explosion wind can sweep,031 102 I I I I 1 1 I I I 103 104 105 106 107 Is (P0.s) Figure C4. Impact velocity and injury criteria as a function of side-on overpressure and impulse (Bowen et al. 1968).
APPENDIX C 357 people away, carry them for some distance, and throw them against obstacles. Upright people are most vulnerable (Figure C-2B). No lethal injuries are likely to be incurred as a person tumbles and slides along the surface, but upon collision with an obstacle, consequences may be deadly. Such consequences depend upon velocity at impact, the hardness and shape of the obstacle, and the portion of the body involved in the collision. Table C-2 gives injury criteria. Based on the pressure and impulse of the incident blast wave, the maximum velocity can be calculated of a human body during transportation by the explosion wind. Figure C-4 shows the impact velocity V,,, for the lethality criterion for whole body impact as a function of side-on overpressure P, and impulse is. REFERENCES Bowen, J. G., E. R. Fletcher, and D. R. Richmond. 1968. Estimate of man s tolerance to the direct effects of air blast. Lovelace Foundation for Medical Education and Research. Albuquerque, NM. Green Book 1989. Methods for the determination of possible damage to people and objects resulting from releases of hazardous materials. Published by the Dutch Ministry of Housing, Physical Planning and Environment. Voorburg, The Netherlands. Code: CPR.6E Hirsch, F. G. 1968. Effects of overpressure on the ear, a review. Ann. NY Acud. Sci. White, C. S., R. K. Jones, and G. E. Damon. 1971. The biodynamics of air blast. Lovelace Foundation for Medical Education and Research. Albuquerque, NM.