Force and Pressure of Explosion

This chapter has so far described the total chemical energy released when a chemical explosion takes place. This energy is released in 

After the shockwave has moved away from the explosive composition the gaseous products begin to expand and act upon the surrounding medium. A crater will be formed if the medium is earth, in water a gas bubble is formed and in air a blast wave develops. The intensity of the gaseous expansion will depend upon the power (Qx V) of the explosive. 

The pressure of explosion is the maximum static pressure which may be achieved when a given weight of explosive is burned in a closed vessel of fixed volume. The pressure attained is so high that the Ideal Gas Laws are not sufficiently accurate and have to be modified by using a co-volume a. At high pressure 

The pressure of explosions are of a much lower order of magnitude than the detonation pressures. 

The power of high explosives can be expressed in terms of their force and has the units of kJ g or MJkg which is the same units as the power of explosion. The force can be calculated from Equation 5.26, where F is the force constant, n is the number of moles of gas produced per gram of explosive, R is the molar gas 

This chapter has so far described the total chemical energy released when a chemical explosion takes place. This energy is released in the form of kinetic energy and heat over a very short time, i.e. microseconds. In a detonating explosive a supersonic wave is established near to the initiation point and travels through the medium of the explosive, sustained by the exothermic decomposition of the explosive material behind it. On reaching the periphery of the explosive material the detonation wave passes into the surrounding medium, and exerts on it a sudden, intense pressure, equivalent to a violent mechanical blow. If the medium is a solid, i.e. rock or stone, the violent mechanical blow will cause multiple cracks to form in the rock. This effect is known as brisance which is directly related to the detonation pressure in the shockwave front. 

---

Therefore, the force exerted on the surrounding medium depends upon the temperature and pressure of explosion and the initial and co-volume of the explosive materials. 

Pressure is defined as force per unit area. When a rubber balloon is inflated with air, it stretches and maintains its larger size because the pressure on the inside is greater than that on the outside. Pressure results from the collisions of gas molecules with the walls of the balloon (see Figure 12.2). When the gas is released, the force or pressure of the air escaping from the small neck propels the balloon in a rapid, irregular flight. If the balloon is inflated until it bursts, the gas escaping all at once causes an explosive noise. 

For any chemical reaction to take place it is known that definite quantities of constituents are necessary. The quantity of oxygen available in the air near flammable material can only oxidize/bum a certain quantity of the flammable material. A commonly known term, stoichiometric ratio, is responsible for determining the quantity of oxygen necessary to react with available flammable materials. Naturally, when the quantity of the flammable material and the available atmospheric oxygen are near to the stoichiometric ratio, the reaction will be near completion and cause an explosion with increase in temperature and pressure. The explosion will be violent. When the quantity of flammable material is too small, combustion carmot spread and may cease. The situation is similar when the quantity of flammable material is too large, because the lack of the required quantity of O2 means that the reaction carmot proceed further. As indicated in an earlier clause (Fig. X/3.1.2-1), all flammable materials have their explosion ranges and limits LEL and UEL. It may be possible to dilute flammable materials in excess air, but it is very difficult to create a situation where there is a dearth of oxygen because of the work force, hence this is only applicable inside equipment. 

Fire or explosion hazards require special motor enclosures. Hazards include combustible gases and vapors such as gasoline dust such as coal, flour, or metals that can explode when suspended in air and fibers such as textile lint. The land of motor enclosure used depends on the type of hazard, the type and size of motor, and the probability of a hazardous condition occurring. Some available enclosures are explosionproof motors, which can withstand an internal explosion force-ventilated motors cooled with air from a safe location and totallv enclosed motors cooled bv air-to-water heat exchangers and pressurized with safe air, instrument air, or inert gas,... 

Table 6.10 presents some damage effects. It may give the impression that damage is related only to a blast wave s peak overpressure, but this is not the case. For certain types of structures, impulse and dynamic pressure (wind force), rather than overpressure, determine the extent of damage. Table 6.10 was prepared for blast waves of nuclear explosions, and generally provides conservative predictions for other types of explosions. More information on the damage caused by blast waves can be found in Appendix B. 

Fig. 4 shows the accumulated standard deviation error, ss, with the measuring time of the pressure fluctuation for a good fluidization case (a) and a locally poor fluidization case (b). Pressure 1 and 2 were measured at the exit and the center air-headers of the FBAC respectively. Pressure 3 and 4 were measured at the entrance air-headers of the FBAC. As shown in Fig. 4 (a) and (b), the accumulated standard deviation error, ss, stayed in a limited range if the bed is in good fluidization state, but ss for the entrance of the FBAC decreased steadily if the bed is in local poor fluidization state. This may be Ifom the decrease of the bubble explosion force and frequency, which have influence on the standard deviation enxir of the pressure fluctuation, at the bed surface due to the bubble break and bypass around the poor fluidization area. Therefore, we can easily detect the local poor fluidization through this simple method. Additionally, as detecting the local poor fluidization, we could also regulate the overall or local airflow rate to clear up the local poor fluidization, as shown in Fig. 5. The accumulated standard deviation error, deviated from a limited range due to poor fluidization, shows to return into a limited value after regulations of local airflow rates.

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

Where enclosed spaces may produce overpressures blow out panels or walls are provided to relieve the pressure forces. The connections of the panel are specified at a lower strength that normal panels so it will fail at the lower level and relieve the pressures. Similarly, combustible or flammable liquid storage tanks are provided with weak roof to shell seams so that in case of an internal explosion, the built-up pressure is relieved by blowing off the roof and the entire tank does not collapse. 

With the tests of percent hydrogen in air around 25% - 30%, reactions were observed which did not make the lid pop up. For instance, with percent hydrogen in air at 25% - 30% and the pressure at 150 torr, the chamber did not explode with enough force to pop up the lid there was only a quick flash in the chamber. Although an explosion did not occur to over-pressurize the vessel, an internal reaction did, the pressures of which are shown in Figure 5. This reaction, though contained and silent, is evidence that a reaction can occur in deeper vacuum, and in richer and leaner hydrogen concentrations than evidenced by observation of over-pressurization. 

Further aluminum pour tests were made in a heavy-wall stainless steel tank fitted with Lucite side windows. The tank was supported on a force transducer and pressure transducers were located on either end. In a test, after the spill, there was a predetermined delay and then the wire was exploded. The aluminum usuaUy had puddled on the tank bottom before the wire explosion and steam bubbles could be seen. The shock from the wire explosion usually collapsed the film and, following this, the aluminum expanded. If the shock were sufficiently energetic, the aluminum soon fragmented and expelled the water from the tank in a thermal explosion. In such cases, the force transducers on the bottom ranged from 5 to 10 N sec. (The exploding wire alone led to impulses around 1 N sec.) Efficiencies of an explosion calculated as indicated above were low. 

---