Explosion and Fire Phenomena and Effects

Transcription

1 Guidelines for Evaluating Process Plant Buildings for External Explosions and Fires by Center for Chemical Process Safety Copyright 1996 American Institute of Chemical Engineers Explosion and Fire Phenomena and Effects Potential explosion phenomena include vapor cloud explosions (VCEs), confined explosions, condensed-phase explosions, exothermic chemical reactions, boiling liquid expanding vapor explosions (BLEVEs), and pressure-volume (PV) ruptures. Potential fire phenomena include flash fires, pool fires, jet fires, and fireballs. Guidelines for evaluating the characteristics of VCEs, BLEVEs, and flash fires are provided in another CCPS publication (Ref. 5). The basic principles from Reference 5 for evaluating characteristics of these phenomena are briefly summarized in this appendix. In addition, the basic principles for evaluating characteristics of the other explosion and fire phenomena listed above are briefly summarized, and references for detailed evaluation of characteristics are provided. A.1. Exploslon and Flre Phenomena A EX/3/05iOn5 An explosion may be defined as a phenomenon where a blast (pressure or shock) wave is generated in air by a rapid release of energy. This energy may have originally been stored in the system in a variety of forms (e.g., nuclear, chemical, electrical, or pressure energy). To be considered explosive, the release of energy must be rapid enough and concentrated enough to produce a pressure wave that can be heard. The resulting blast wave is largely responsible for the damage that was caused (Ref. 84). Buildings may be damaged and people may be injured by the blast wave, with additional indirect effects from missile generation, crater formation, ground shock, and fire. Generally, as the blast wave travels farther away from the center of the explosion it loses energy, so the magnitude of overpressure and other effects experienced as a result of the blast wave decreases as the distance increases from the explosion source. The decay of overpressure is roughly 131

2 132 Appendix A. Explosion and Fire Phenomena and Effects inversely proportional to the separation distance to the thrd power (the Hoplunson-Cram "Cube Root" scaling law). As a blast wave strikes a surface, pressure increases because of a process calledreflection. Similarly, as a blast wave propagates through a congested region, it is possible for the pressures to be focused such that local regions of increased pressure may exist. For process plants, an important distinction to be made for an explosion caused by the release of chemical energy is whether it may be characterized as a deflagration or a detonation. The difference between a deflagration and detonation is the mechanism whereby energy required to activate the explosive reaction is transferred from reacted to unreacted material. In a deflagation, the mechanism for propagation of the explosion reaction into the unburned material is by heat and mass transfer. Material surrounding an initial exploding site is heated above its autoignition temperature, allowing the reaction to propagate. Transfer of energy by these means is a relatively slow process, always at propagation rates that are less than the speed of sound in the unreacted material. In a detonation, the mechanism for propagation of the explosion is by shock compressive heating. Detonation proceeds very rapidly because of the rapid transmission of the mechanical forces involved. Detonation propagation velocities are always greater than the speed of sound. The overpressure characteristics of deflagrations and detonations as a function of time are very different (Ref. 85). Typical overpressure profiles are compared in Figures A. la and A. 1 b. For a given explosion energy, deflagrations are generally characterized by a gradual increase to peak overpressure with long durations, followed by a gradual decrease in overpressure. This wave form is generally referred to as a pressure wave. Detonations are characterized by a very rapid rise to peak overpressure followed by a steady decrease of overpressure to form the more idealized shock front. Even though these blast pressure waves are initially different shapes, they will both eventually develop a shock front as explained below. Development of a shock front is illustrated in Figure A.2. Figure A.2a shows a pressure pulse of arbitrarily chosen initial configuration. Each portion of this pulse moves outward at its own speed. The higher-pressure portions of the pulse correspond to higher temperatures and hence also to greater speeds. The high-pressure portion moves faster than any preceding lowpressure portion of the pulse. As a result, the wave front becomes progressively steeper, as shown in Figure A.2b. In time, a discontinuity known as a shock front is developed, as shown in Figure A.2c. For a detonation, the shock front, as shown in Figures A. 1 b and A.2c, develops very rapidly at distances relatively close to the explosion source. For a deflagration, the shock wave, as shown in Figure A.2c, forms more slowly and may not be developed until it is a significant distance from the center of the explosion or, in some cases for weak deflagrations, not before the blast wave decays to an acoustic (sound wave) magnitude.

3 ~ A.1. Explosion and Fire Phenomena 133 Detonation c time FlgUre A.1. Pressure vs. time curves resulting from a deflagration (Fig. A.la) and a detonation (Fig. A.lb). f I?2 a a - - Direction of Travel Direction (a) E 5 E - of Travel (b) a - Direction of Travel - (c) Flgure A.2. Development of explosive shock.

4 134 Appendix A. Explosion and Fire Phenomena and Effects Vapor Cloud Explosions A vapor cloud explosion (VCE) results from the ignition of a flammable mixture of vapor, gas, aerosol, or mist, in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure (Ref. 21). VCEs are generally associated with the release of a sufficient quantity of flammable gas or vaporizing (flashing) liquid from a storage tank, process or transport vessel, or piping system. In general, five conditions must be met before a VCE with damaging overpressure can occur (Refs. 5 and 85): 1. The released material must be flammable and at suitable conditions to form a vapor cloud. Some portion of the resulting cloud must mix with air such that concentrations are within the flammable range for the material. 2. An ignition source is needed to initiate the explosion. The presence of an ignition source should always be assumed, because explosions and fires have occurred where no obvious ignition source could be identified. Higher-energy ignition sources can lead to a more severe explosion than do lower-energy sources. 3. Ignition of the flammable vapor cloud must be delayed until a cloud of sufficient size has formed. If ignition occurs as the flammable material is escaping, a large fire, jet flame, or fireball might occur, but a VCE is unlikely. The probability of explosion rather than fire increases with the size of the cloud, since the quantity of the mixture within the flammable range increases. Paradoxically, plant safety measures that eliminate sources of ignition can be contributors to the formation of very large flammable vapor clouds with the potential for very severe explosions. 4. Turbulence is required for the flame front to accelerate to the speeds required for a VCE; otherwise, a flash fire will result. This turbulence is typically formed by the interaction between the flame front and obstacles such as process structures or equipment. Turbulence also results from material released explosively or via pressure jets. The blast effects produced by VCEs can vary greatly and are strongly dependent on flame speed. In most cases, the mode of flame propagation is deflagration. Under extraordinary conditions, a detonation with more severe blast effects might occur. In the absence of turbulence, under laminar or near-laminar conditions, flame speeds are too low to produce significant blast overpressure. In such a case, the cloud will merely bum as a flash fire. 5. Confinement of the cloud by obstacles can result in rapid increases in pressure during combustion. Conversely, absence of confining obstacles allows unlimited outward expansion of the cloud during combustion, limiting the pressure increases. Unconfined clouds usually d not generate sufficient flame speeds to result in overpressure effects. The degree of confinement in process plants, with their congested equipment layout and built-up structures, is generally high (Ref. 16).

5 A.1. Explosion and Fire Phenomena 135 Factors affecting the probability, magnitude, and effect of a VCE include (Refs. 16 and 86): Amount of flammable material in the cloud, within an area where there are objects that will induce turbulence and create a degree of confinement 0 Degree of cloud mixing (cloud composition) 0 Reactivity of flammable material (affects explosion severity; highly reactive materials increase the likelihood of a fireball transition to a VCEl 0 Fundamental burning velocity 0 Energy of ignition source 0 Release conditions (high-pressure releases generate greater turbulence than do low-pressure releases) 0 Presence of obstacles, or confinement, or other turbulence-enhancing mechanisms 0 Cloud configuration (some incidents have exhibited directional blast effects) 0 Wind speed and direction The factors that dominate the development of pressure in VCEs are the presence of obstacles enhancing turbulence, the degree of confinement, and the reactivity of the unburned material (Refs. 5 and 16). Bursting Pressure Vessels, Physical Explosions, and BLEVEs When a vessel containing pressurized gas bursts, a shock wave propagates away from the surface of the vessel. This shock wave may create overpressure effects strong enough to cause damage or injury. In addition, the bursting vessel can form a number of fragments moving away from the vessel at relatively high velocity. These fragments may also cause damage or injury. Mechanisms that can result in vessels being overpressurized include deflagration of flammable gases or runaway chemical reactions. Alternatively, vessels may fail because of faulty construction or corrosion, or may become overstressed from mistreatment. Another failure mode can occur if the vessel is subjected to external fire, causing the unwetted surfaces to overheat, weaken, and fail. Vessel ruptures can also occur when a higher-temperature liquid or solid is combined with a cooler low boiling liquid, transferring sufficient heat from the hotter material to the colder material such that the colder material rapidly vaporizes. No chemical reactions are involved; instead, the explosion occurs because the cooler liquid expands as it is converted to vapor, creating high pressures. These are called physical explosions. A common example is a steam explosion, which occurs when liquid water is accidentally introduced into a process vessel operating at an elevated temperature. If the hotter material is above the superheat limit temperature of the evaporating liquid, initial confinement by a vessel is not required to create an explosion pressure wave.

6 136 Appendix A. Explosion and Fire Phenomena and Effects A BLEVE results from catastrophic failure of avessel containing a liquid at a temperature significantly above its boiling point at normal atmospheric pressure. When the vessel fails, the liquid can evaporate very rapidly (explosive evaporation). The rapidly expanding vapor compresses the surrounding air, creating a blast pressure wave. Also, as the vessel fails, fragmenting may occur. These vessel fragments can be propelled a significant distance at high initial velocities. BLEVEs are commonly associated with releases of pressurized flammable liquids from vessels as a consequence of an external fire. Such BLEVEs can produce thermal radiation effects from a fireball, as well as blast and missile effects. Condensed-Phase Explosions Condensed-phase systems (solids and liquids) that have a high heat of decomposition may be capable of detonating. These materials are routinely found in the explosive or munitions industry but can also be found in the chemical process industry. Examples include some organic peroxides, acetylenic compounds, and nitration mixtures. In addition, this hazard can occur in processes if some unwanted and highly sensitive substance is accidentally allowed to concentrate (Ref. 64). Blast and fragment effects from such explosions may be evaluated by estimating the energy released in an explosion of an equivalent mass of TNT. A.1.2. Fires Fires result from cornbustion, an exothermic chemical reaction in which oxidation takes place. Three essential requirements for combustion exist: oxidizing agent (e.g., oxygen), combustible material (i.e., fuel), and ignition source. If any of these three requirements is missing, combustion will not occur. Generally, the activation energy required to initiate the combustion reaction is initially supplied by the ignition source. After initial ignition, the combustion reaction releases sufficient energy to sustain the reaction without an external ignition source. The type of fire that occurs in a particular situation depends on several factors (i.e., the form of the combustible material, the degree of mixing between the material and air, the location of the combustible material, and the nature of the release of the material). Section 2.1 describes flash fires, pool fires, jet fires, and fireballs. Heat is transferred from a fire by three methods: conduction, convection, and radiation. Conduction is the mechanism by which heat is transferred from one body to another by direct contact between the two. Convection involves the transfer of heat by a circulating medium-ither a gas or a liquid. Radiation is a form of energy traveling across a space or through a material as electromagnetic waves. In the case of small fires (e.g., a candle), most of the heat leaves the flame by vertical convection. However, larger fires release about equal amounts of radiative and convective energy. Radiated energy is potentially

7 A.2. Evaluating Characteristics of Explosions and Fires 137 the most damaging to buildings because a stationary surface (such as a wall or roof) near the fire will absorb most of the radiation incident on it, while most of the convected energy flows past the surface as it moves away with the gas stream. Most of the radiant heat from flames originates from soot in the flames, which glows when heated to flame temperature. Clean-buming flames, such as those from natural gas or hydrogen, emit smaller amounts of radiant heat. Radiant energy travels in straight lines and is absorbed by suspended particles such as smoke and by unsymmetrical molecules in the atmosphere (e.g., water vapor and carbon dioxide). This explains why mists or water sprays effectively attenuate thermal radiation. A.2. Evaluating Characterlstlcs of Exploslons and Flres The characteristics of explosions and fires that determine consequences to people and buildings are identified and briefly described in this section. Also, the basic principles used in quantifymg these characteristics are described. References on detailed methods of evaluating explosion and fire characteristics are also provided. A basic reference is CCPS s Guidehes for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs (Ref. 5). A.2. -I. Explosions Buildings in process plants may sustain structural damage from blast overpressure and fragments from an explosion. Cratering and ground shock resulting from explosions can be important considerations for buried targets (e.g., pipelines), but for aboveground targets the effects are normally small compared to those of the blast pressure wave. Evaluating Overpressure and Impulse Characteristics As discussed in Section A.l, explosions generate a pressure wave that propagates outward from the explosion source. The peak value of the overpressure, positive phase duration, and the resulting impulse are the predominant parameters that determine the destructiveness of an explosion. Extensive data are available for evaluating blast wave characteristics as a function of distance from explosion source and of explosion size, expressed as weight of TNT (Refs. 7, 54, 64, and 70). Figure A.3 shows an example of such data taken from Reference 7, Volume 11. Condensed-phase Explosions. Data such as those shown in Figure A.3 can be used for condensed-phase explosions. For many materials, explosive strengths in terms of %TNT are well established (Ref. 54). For these materials, the weight of material, W, is adjusted by the %TNT for the material. Blast wave characteristic data can then be obtained from relationships such as that shown in Figure A.3.

8 138 Appendix A. Explosion and Fire Phenomena and Effects 100.o0o 50,OW 1,om 500 a lo.m 5.m loo 50.,k.ooo.# i ni % 1 ) ' Scaled Dlstance Z=RJW'D paa Pr is IW" i, MI'" ta/w '' ~IW y3 U W R 113 = Peak Positive Pressure, psi = Peak Positive Normal Reflected Pressure, psi = Scaled Unit Positve Incident Impulse. psi - mdib = Scaled Unit Positve Normal Reflected Impulse. psi - mdib = Scaled Time of Arrival of Blast Wave, msab = Scaled Positive Duration of Positive Phase, mdb = Shock Front Velocity.ftlms = Charge Weight, Ibs = scaled Wave Length of Positive Phase, Wlb = Distance from Explosion, R Flgure A.3. Positive phase shock wave parameten for a hemispherical TNT explosion on the surface at sea level (Ref. 7).

9 A.2. Evaluating Characteristics of Explosions and Fires 139 Vapor Cloud Explosions. Blast wave data such as in Figure A.3 have not been developed for VCEs. However, because of the volume of data available, a TNT equivalence method for predicting VCE overpressures and duration is often used. Establishing TNT equivalency is much more difficult and less reliable for VCEs than for condensed-phase explosions. In the TNT equivalence approach, the blast effects from VCEs are correlated with those from equivalent explosive charges of TNT as a means of quantifying the severity of explosions (Ref. 88). Many organizations use variations of this approach. In TNT equivalence methods, the equivalent weight of TNT representing the VCE, WTNT, is expressed as: WfHf TNT=ae H ~ ~ ~ where Wr = weight of fuel involved [lbm (kg)] Hf = heat of combustion of fuel [ft. lbr (I&)] HTNT = TNT detonation energy [ft. lbf (Jkg)] = 4.52 MJkg ae = TNT equivalency based on energy Differences in the approach by various organizations are in: (1) portion of fuel included, (2) TNT equivalency (also called yield or efficiency), (3) TNT blast data, and (4) TNT blast energy. Normally, only a small proportion of the heat of combustion of the fuel involved in an explosion appears as energy in the shock wave. It is therefore assumed that only a certain proportion (usually 1%-10% based upon energy) of the fuel released contributes to the explosion. This mass of fuel is converted to an equivalent mass of TNT, taking into account the combustion energy of the fuel and the detonation energy of TNT. Overpressures are determined from TNT blast curves, which have been well established from experimental data (Figure A.3). Reference 5 gives detailed information on using the TNT equivalence method. The TNT equivalence method is used because: [ 1 ) it is easy to use, (2) it requires limited assumptions for determining the initial spill size and TNT equivalency, and (3) data on damage generated at given distances by high explosives are well known and widely available. The method does have serious limitations and deficiencies (Ref. 16), including: (1) TNT equivalency is not clearly defined, (2) blast wave characteristics from TNT are very different from those for a VCE (TNT produces a shorter-duration and higher overpressure blast wave than does a VCE for the same energy), and (3) blast attenuation differs between TNT and a VCE (the TNT method overpredicts near-field effects and underpredicts far-field effects, which has caused incident investigators to overestimate the TNT equivalence of VCEs when based on far-field blast damage such as glass breakage). The use of different TNT equivalencies in the near and far fields can overcome the modeling deficiency described in item 3 (Ref. 5).

10 140 Appendix A. Explosion and Fire Phenomena and Effects Because of the deficiencies of the TNT equivalence model for representing a VCE, additional methods for evaluating VCEs have been proposed. These methods address a major feature of gas explosions not considered by TNT equivalence methods; namely, the blast source is not a point, and blast strength varies over the extent of the blast source. Such methods, grouped as those based on fuel-air charge blast, are covered in Reference 5. Basic principles of the Multienergy method (Ref. 88) and the Baker-Strehlow method (Ref. 64) are briefly discussed below. For VCEs, observations indicate that little correlation exists between the quantity of fuel and the equivalent weight of TNT required to simulate its blast effects. For VCEs, blast effects are determined primarily by the size and nature of partially confined and obstructed regions within the cloud. The Muftienergy method recognizes these principles for VCE blast modeling, emphasizing the influence of obstacles and partial confinement of the cloud. The incident is modeled by dividing the vapor cloud into volumes according to their degree of obstructiodconfinement. An explosion strength index, varying from 1 to 10, is assigned to each volume. The partly confined and/or obstructed volumes of a cloud will be major blast sources. The unconfined and unobstructed volume of the gas cloud will bum without contributing significantly to the main blast (Ref. 16). An explosion strength value of 1 represents a very weak explosion due to a virtually unobstructed or unconfined cloud, whereas a value of 10 represents a detonation. An explosion strength of 7 would correspond to a very strong explosion caused by a highly congested area such as within a dense process block or a bank of pipes. Potential sources of strong blast include: (1) extended spatial configuration of objects such as process equipment in chemical plants or refineries and stacks of crates or pallets, (2) spaces between extended parallel planes (e.g., those beneath closely parked cars in parking lots and open buildings such as multistory parlung garages), (3) spaces within tube-like structures (e.g., tunnels, bridges, corridors, sewage systems, and culverts), and (4) an intensely turbulent fuel-air mixture in a jet resulting from release at high pressure. Each volume of the subdivided cloud is modeled as a hemispherical, stoichiometric, fuel-air charge with an appropriately assigned explosion strength. The major determining factor for the initial blast strength is the degree of confinement and obstruction. Plots of scaled overpressure and explosion duration as a function of scaled distance and explosion strength (Figures A.4 and A.5) enable blast effects to be predicted. Again, refer to Reference 5 for more information. See Figure A.5 for definition of the scaled axis variables. The Baker-Strehlow method (Refs. 5, 64, and 89) for VCE blast modeling uses numerical and experimental data relating the structure of blast waves generated by constant velocity and accelerating flames propagating in a spherical geometry. These data are expressed by plots of dimensionless overpressure and positive impulse as a function of energyscaled distance from the cloud center. Application of the Baker-Strehlow

11 A.2. Evaluating Characteristics of Explosions and Fires I combustion energy-scaled distance (R) Flgure A.4. Scaled overpressure versus energy-scaled distance by Multienergy method (Ref. 5). method requires estimation of maximum flame speed attained and equivalent energy of the explosion. Maximum flame speed attained is a function of confinement, obstacle density, fuel reactivity, and ignition intensity. The maximum flame speed that will be achieved with a particular combination of confinement, obstacles, fuel reactivity, and ignition source is estimated (Ref. 5). Energy represents the heat that is released by that portion of the cloud contributing to the blast wave. Any of the following methods of calculating vapor cloud

12 142 Appendix A. Explosion and Fire Phenomena and Effects I-+ 10 Y v) v) - al.- C 0 v) C E?j RO combustion energy-scaled distance (R) ke P - APs, - t+co - R APs--, t+=-. R= PO (wpo)1/3 (m0)1/3 *,.-... Po - atmospheric pressure co = atmospheric sound speed E I amount of combustion energy Ro I charge radius Flgure A.5. Scaled duration versus energy-scaled distance by Multienergy method (Ref. 5). explosive energy are applicable to the Baker-Strehlow method: ( 1) estimating thevolume within each congested region, multiplying fuel mass by heat of combustion in that region, and treating each congested region within the flammable portion of the cloud as a separate blast source, (2) estimating total release of flammable material within a reasonable amount of time and multiplying by the heat of combustion and an efficiency factor, and (3 J estimating the amount of material within flammable limits (usually by dispersion modeling) and multiplying by the heat of combustion. BLEVEs and Bursting Pressure Vessels. In Reference 5, three methods for calculating values of the blast wave parameters of pressure vessel bursts and BLEVEs are presented. The choice of method depends on the phase of vessel contents (i.e., liquid, vapor or nonideal gas, and ideal gas) and the distance to the blast wave target. For PV ruptures, the first method includes the following steps:

13 A.2. Evaluating Characteristics of Explosions and Fires Collect data on vessel internal pressure, ambient pressure, volume of gas-filled space, ratio of specific heats of the gas, distance from vessel to target, and shape of vessel. 2. Calculate the energy of compressed gas. 3. Evaluate scaled distance from actual distance, energy, and ambient pressure. 4. Check scaled distance region, as different methods apply in nearand far-field regions. 5. Determine scaled side-on pressure as a function of scaled range from empirical curves. 6. Determine scaled positive impulse as a function of scaled distance from empirical curves. 7. Adjust scaled pressure and impulse for geometry effects. 8. Evaluate actual side-on pressure and positive impulse. 9. Check resulting pressure against initial vessel pressure and make adjustments, if necessary. The second and third methods include enhancements to the basic method described above for different parameters. The second method represents refinements for blast wave characteristics in the near field of the vessel. The third method specifically addresses explosively flashing liquids and pressure vessel bursts with vapor or nonideal gas. Fragment Characteristics In addition to the blast pressure wave, explosions can also create fragments. Characteristics of fragments to be used in consequence evaluation include their number, shape, velocity, and trajectory. A BLEVE or bursting pressure vessel can produce fragments that fly away from the explosion source. These primary fragments, which are part of the original vessel, are hazardous and may result in injuries to people and damage to structures. Also, a blast wave from a condensed-phase explosion, VCE, or BLEVE may pick up and hurl objects (projectiles) because of the blast wind by the associated blast wave propagation. The effects of fragments resulting from a BLEVE or bursting pressure vessel are considered in detail in References 5 and 64. Specifically, methods for calculating the number, range, and velocity of fragments are discussed in depth. A good approximation for the velocity attained by blast wind projectiles is available by using the impulse-momenturn exchange theory (Ref. 84). For explosions inside vessels, the initial velocity of the projectiles can be estimated, for example, by predicting what fraction of the available energy is transferred from the expanding fluid to the fragments (Ref. 84). Energy available from an explosion within a vessel will be divided between work to propagate cracks to cause rupture, lunetic energy of fragments, energy in the shock wave (some but not all of which can do work; i.e., cause

14 144 Appendix A. Explosion and Fire Phenomena and Effects damage), heat in products, and plastic strain energy in fragments. Another method for evaluating the initial velocity of projectiles involves a step-bystep analysis of the momentum transfer from the fluid escaping through cracks in between the projectiles to the projectiles themselves. For brittle vessels, or in the case of detonations, severe fragmentation is possible, while for ductile vessels, a limited number of fragments will be generated (generally less than 10). The velocity of fragments from a pipeline will be greater than that from an isolated vessel at the same conditions because of the replacement of gas loss by flow from the intact pipe (Ref. 84). The influence of drag and lift on the fragment may significantly affect its range and velocity such that the velocity with which it strikes an object can be markedly different from its initial velocity. Neglecting such aerodynamic effects is llkely to lead to a serious underestimation of the velocity with which fragments impact a particular object. A.2.2. Jet Fires: Direct Contact and Conduction Released heat is transmitted to the surroundings by convection and thermal radiation. For large fires, thermal radiation is the main hazard and can cause severe bums to people and secondary fires to nearby structures. Two methods or models used to describe the radiation from a fire are the point source model and the surface-emitter model, or solid flame model, which are discussed in detail in Reference 5. Reference 5 also provides discussion of a flash fire thermal radiation model and means of estimating fireball size and duration along with a fireball radiation model. Additional material on fireball modeling may be found in References 63 and 90. Reference 33 provides information on modeling pool fires. In particular, the flame height above the pool surface and the angle of tilt of the flame from the vertical as a function of wind velocity and direction may be estimated from Reference 33. References 33 and 91 provide methods of calculating the flame length and diameter of a turbulent gas jet buming in still air. A.3. Effects of Exploslon on Bulldlngs Following an explosion, a blast propagates through the air outward from the explosion source (Figure A.6). Depending on the nature of the explosion, the blast wave can be a shock wave with instantaneous rise to the peak overpressure or a pressure wave with a more gradual rise time. The blast wave produces diffraction and drag load on structures in its path. The diffraction loading process on buildings can be illustrated by considering a rectangular structure with one side (called the front of the structure in subsequent discussion) facing the explosion, as shown in Figure A.7. The blast wave is generally assumed to be a plane wave (the

15 A.3. Effects of Explosion on Buildings 145 I= FlgUre A.6. R surface Blast wave propagation along ground surface to location of structures. B Back face Front face I,' Flgure A.7. Blast wave striking a closed rectangular structure. blast wave front is sufficiently large compared to dimensions of the structure) with the blast front perpendicular to the surface of the ground (explosion initiated near the ground surface), as shown in Figure A.6. Figures A.8 and A.9 illustrate the behavior of a blast wave striking this structure. When the blast wave with a shock front strlkes the front of the structure (Figures A.8a and A.9a), the overpressure rises to a value in excess of the peak overpressure in the incident blast wave. This increased overpressure is called the reflected overpressure and is a function of the peak side-on overpressure. The blast wave then bends (or diffracts) around the structure, exerting pressures on the sides and roof and finally on the back face (Figures A.8b

16 146 Appendix A. Explosion and Fire Phenomena and Effects and A.9b). The structure is thus engulfed in the high pressure of the blast wave. Because the reflected pressure on the front face is greater than the pressure above and to the sides, the reflected pressure cannot be maintained, and it soon decays to the incident overpressure. The decay time of the reflected pressure is roughly that required for a rarefaction wave to sweep from the edges of the front face to the center of this face and back to the edges. The decay time may be approximated by 3S/U, where S is the stagnation distance (Figure A.7) and U is the shock front velocity. The reflected pressure wave amplitude and impulse for shock waves associated with detonations are well documented, as shown in Figure A.3 (Ref. 7, Volume 11). Less information is available on reflected overpressure and impulse resulting from deflagration pressure waves. Reference 67 documents approaches for evaluating reflected overpressure from weaker blast pressure waves. Forbes [Ref. 71) suggests the following approximate relation to model the more complex relations in Reference 64: PJPS = (Ps) for 0.c P, < 20 psi (1.4 bar) where Pr is peak reflected pressure and Ps is peak incident side-on pressure. Reference 64 recommends assuming that the ratio of reflected impulse to incident impulse is the same as the ratio of reflected pressure to incident pressure. Note that the above relation is a conservative estimate of reflected pressure for VCEs, since in most cases the VCE blast waves may not be characterized by an ideal shock front. The reflection process for a nonideal blast wave or pressure wave is not well understood, but it is unlikely that such blast waves are fully reflected by the front wall of a building. front 2 c LL Flgure A.8. Blast wave and structur-levation view.

17 - Incident shock front d A.3. Effects of Explosion on Buildings 147 Vortex-- :I- Rarefaction Reflected shock front wave -. I--- shock front Shock front Diffracted shock front Figure A.9. Blast wave and structure-plan view. However, it is normal practice in blast-resistant design to assume conservatively that the VCE blast wave is fully reflected from the front wall as an ideal shock. The pressures on the sides and roof of the structure build up to the incident overpressure as the blast wave traverses the structure. Traveling behind the blast wave front there is a short period of low pressure caused by a vortex formed at the front edge during the diffraction process (Figures A.8c and A.9c). After thevortex has passed, the pressure returns essentially to that in the incident blast wave. The air flow causes some reduction in the loading to the sides and roof, because the drag pressure has a negative value for these surfaces. When the blast wave reaches the rear of the structure, it diffracts around the edges and travels across the back surface (Figures A.8d and A.9d). The pressure takes a certain rise time to reach roughly a steady-state value equal to the sum of the overpressure and the drag pressure, the latter again having a negative value for the back surface.

18 148 Appendix A. Explosion and Fire Phenomena and Effects Also important to consider is the net horizontal dlffraction loading on the entire structure, corresponding to the loading on the front face minus that on the back face. The net horizontal loading during the diffraction process is high because the pressure on the front face is initially the reflected pressure, and no loading has reached the back face. When the diffraction process is complete, the overpressure loading on the front and back faces is essentially equal, and the net horizontal loading is then relatively small. Because the time required to complete the diffraction process depends on the size of the structure, rather than on the positive-phase duration of the incident blast wave, the net horizontal loading impulse is greater for large structures than for small ones. In summary, an explosion causes overpressure and drag pressures on buildings and other structures. The overpressure produces the largest loads on the side of buildings facing the explosion because of reflection and lesser loads on the roof and other sides. Drag pressures produce loads on slender structures such as stacks and towers. These loads cause buildings and other structures to deform, and if deformations are sufficiently large, damage and failure can result.