1
Introduction

1.1 What Is an Explosion? Types of Explosions Covered in this Book

To introduce the concept of explosion protection, one must first understand what is an "explosion?" The dictionary definition of an explosion is "the action of going off with a loud noise or of bursting under the influence of a suddenly developed internal energy." A more relevant definition related to the scientific study of the problem is the release of energy to generate a pressure wave of finite amplitude traveling away from the source. This energy may have been stored in various forms such as nuclear, chemical, electrical, or pressure energy [1]. The release of energy is not considered explosive unless it is rapid and concentrated enough to produce a pressure wave that one can hear. Even though many explosions damage their surroundings, an explosion doesn't need to create external damage.

Explosions can occur in any media, such as air or condensed phases like liquid or solids. In all cases, the critical aspect is the generation of energy and pressure, which is released in a short time. The magnitude of energy release and its rate of release thus constitute the basis of the classification of different types of explosions. Zalosh [2] describes this using a peak pressure generated vs. a time scale for energy release, as shown in Figure 1.1. The peak pressure is directly related to the total amount of energy1, and the time scale is a result of the spatial scale and reaction rate or the speed with which the energy is released during the explosion. For example, when dynamite is ignited, the chemical reaction front proceeds through the solid at a speed of 4900 m/s. Thus, a 50 cm (0.5 m) stick would release all of its energy in 0.5/4900 = 102 µs. For a gas detonation explosion, typical detonation velocities are in the range of 1500-4000 m/s. For example, stoichiometric acetylene (C2H2)-air mixture's detonation velocity can be calculated from a chemical equilibrium code [3] and equal to 1868 m/s. Thus, in this case, the energy release in a 0.5 m radius would occur in 0.5/1868 = 268 µs. The corresponding energy released would be the heat of combustion of acetylene in air (48.22 kJ/g) times its density (1.2 kg/m3) times the volume () given by 30.3 MJ! The corresponding pressure is equal to 18.2 atm (267.5 psig).

Figure 1.1 Classification of different types of explosions based on peak pressure and time to different kinds of energy release.

Source: Zalosh [2].

Explosions can be either deflagrations or detonations, depending on whether the speed of the chemical reaction front propagating through the combustible mixture is less than or greater than sound speed in the unburned fuel-air mixture. (Sound speed is approximately equal to 347 m/s if the fuel concentration is small compared to the air concentration.2) As shown in Figure 1.1, the peak pressures generated in detonations are at least twice as large as those in deflagrations, and the time scale is often at least an order of magnitude smaller. To begin, let us briefly describe the different types of explosions shown in Figure 1.1 to understand the significance of peak pressure and time for energy release.

1.1.1 Nuclear Explosions

As shown in Figure 1.1, nuclear explosions release the most amount of energy per unit volume. Therefore, they generate the highest pressure on the top right-hand corner of Figure 1.1. Also, the reaction speed is exceptionally high for nuclear explosions, with a tremendous amount of energy released in a microsecond. Both the exceptionally high magnitude of pressures and the extremely short time scales make nuclear explosions extremely damaging.

1.1.2 Pressure Vessel Bursts

Progressing further in a direction of increasing time scale in Figure 1.1, a pressure vessel burst is the release of energy of compression in high-pressure vessels. The release of pressure takes place in a time for a crack3 to propagate sufficiently far to allow the vessel shell to split open. This is typically on the order of 10 µs. The peak pressure is approximately equal to the vessel pressure at the time of bursting, Pb. The isentropic expansion energy, Eburst, for an ideal gas released during the vessel burst is [1]:

where Pb = vessel pressure at the time of bursting, Pa = pressure of ambient air (1 atm = 14.7 psia = 101 kPa at sea level), V = vessel volume, and ? = ratio of specific heats for the gas in the vessel (equals 1.4 for air).

1.1.3 Explosives

Explosions caused by explosives, usually condensed phase have time scales of the order of 100 µs. Figure 1.2 shows an aerial view of the aftermath of an explosion incident involving a special black powder composed of ascorbic acid (combustible powder), potassium nitrate (strong oxidizer), and potassium perchlorate (highly reactive oxidizer). The latter two ingredients were in the form of granular solids requiring milling prior to being mixed in two combination milling/blending machines located where indicated by the arrow in Figure 1.2. There were about 34 kg (75 lb) of black powder in each machine, and the first explosion triggered a second explosion, with the combined effects causing two fatalities in addition to the destruction shown in the photograph. As shown, the relatively small amount of explosive created significant damage to property in a radius of 30.5 m (100 ft). This radius is also called as a "blast debris radius," associated with a blast wave, i.e. a pressure disturbance propagating into the atmosphere away from the source of energy release. We will discuss the damage potential of blast waves based on the initial energy release and distance from the release point in Chapter 7. The knowledge is useful for safe citing of industrial facilities.

Figure 1.2 Aerial view of damage and debris caused by a black powder explosion, with arrow indicating origin.

Source: U.S. Department of Justice.

Energies released by condensed-phase explosives are often quoted in terms of the trinitrotoluene (TNT) equivalent weight. One kilogram of TNT has an explosive energy of 4.2 × 106 J. Most condensed-phase high explosives have an explosive energy per unit mass that is similar to that of TNT. For example, the explosive energy of pentolite (50/50) is 5.1 × 106 J/kg, and that of royal demolition explosive (RDX) is 5.4 × 106 J/kg. The corresponding TNT equivalent of pentolite is 5.1/4.2 = 1.2 kg-pentolite/kg-TNT, and that of RDX is 5.4/4.2 = 1.3 kg-RDX/kg-TNT.

1.1.4 Closed Vessel Detonation

As discussed earlier, a detonation propagates at a speed greater than the speed of sound. A closed vessel detonation is usually the detonation of a flammable gas that is enclosed in a vessel, for example, a pipeline. In this case, ignition leads to a deflagration, which starts slowly, but rapidly accelerates to a detonation after propagating through the pipe for a distance called a run-up distance. These distances are usually large (60-100 tube diameters) and the transition occurs in piping but is very improbable in vessels and equipment unless there is a combination of a fast-burning gas mixture and a highly turbulent flame accelerating situation. The transition from deflagration to detonation is also highly complex. A flammable gas can also be made to detonate without a "run up" by providing a sufficiently large ignition energy. For example, Carlson [4] determined the minimum energy for initiation of detonation in stoichiometric gas-oxygen mixtures, using exploding wires to initiate detonation. The ignition energy to cause direct detonation of a stoichiometric propane-oxygen mixture is 2.5 J [4]. On the other hand, the minimum ignition energy (MIE) to ignite (sustain a propagating flame) in the same mixture is four orders of magnitude lower at 0.26 mJ as shown in Table 1.1. Thus, a combustible gas-air mixture likely will form a sustained flame, which may accelerate to a detonation rather than detonate directly since ignition with such a large energy source is usually unlikely.

Table 1.1 Flammability properties of some common gas air mixtures in air.