The air shock wave of the explosion causes destruction or damage to the railway track, rolling stock, buildings, communication elements, signaling systems, railway water supply and other elements of the engineering and technical complex (ITC)* of railway transport.
Quality condition destroyed ITC elements in emergency zones are assessed by the corresponding degree of destruction: full, strong, medium And weak.
Complete destruction characterized by the destruction or collapse of all or most of the load-bearing structures, main walls, severe deformation or collapse of interfloor and ceiling ceilings, bridge spans. At the same time, the debris of buildings and structures creates continuous rubble. The main elements of the railway track completely fail. Rolling stock, track machines, station equipment and equipment cannot be restored.
The use of machine elements, rolling stock and destroyed parts of structures is impossible.
Severe destruction are characterized by the destruction of part of the main and most of the remaining walls of buildings, deformation of bridge spans, most supports of the contact network and power lines. Restoring the railway track and structures is possible, but impractical, since it practically comes down to new construction using some surviving elements and structures. Technical and transport vehicles cannot be repaired; some of their parts can be used for repairs in the future.
Medium damage characterized by the destruction of secondary elements (internal partitions, windows, roofs), the appearance of cracks in the walls, the collapse of attic floors and individual sections of the upper floors. No rubble is formed around the buildings, but individual fragments of structures can be thrown over considerable distances. The railway track becomes deformed. Individual elements of bridge spans, individual supports of power lines, contact networks and communication lines are deformed. It is possible to restore buildings, railway tracks, structures, rolling stock, transport and other technical means using major and medium repairs.
Weak destruction buildings are characterized by the destruction of the least durable structures: window and door fillings, light partitions, roofing. The equipment receives minor deformations of minor elements. Restoring the railway track, structures, rolling stock and equipment requires current repairs.
Due to the fact that in case of complete and severe destruction, buildings, structures and technical means are not restored, in reference data and calculations only three degrees of destruction are often used - strong, medium and weak.
When the same parameters of the explosion shock wave are exposed to different elements of the ITC, the degree of their destruction will be different due to their different physical stability.
Under physical stability it is necessary to understand the ability of the structure to withstand the effects of external loads in an emergency situation. This ability is a property of a structure, which depends on its size, design and other parameters and does not depend on any external factors. Such parameters, for example, include: structural rigidity, the presence of a foundation, fastening of elements and other strength properties; material; mass and center of gravity position; dimensions of elements and their configuration; support area; distance between supporting parts, etc.
For example, under the same external loads, multi-storey residential buildings without a frame with load-bearing walls made of bricks, panels and blocks are subject to the greatest destruction. The greatest loads are withstood by massive industrial buildings with a metal frame and internal heavy-duty crane equipment, for which load-bearing columns are installed, which makes the building structure more rigid and durable.
High external loads are withstood by the upper structure of the railway track, which has a rigid structure (connection of the ballast layer, sleepers and rails), a slight elevation above the ground and a low aerodynamic drag coefficient.
Among the various types of railway rolling stock, the most resistant to external loads during explosions are four-axle unloaded platforms (small size with significant weight), loaded tanks (low aerodynamic drag coefficient) and locomotives. The least stable are passenger cars and covered empty freight cars (large in size and relatively light in weight).
A comparative assessment of the stability (by degree of destruction) of ITC elements during explosions is carried out using a single quantitative indicator - the amount of excess pressure in the shock wave front
If the determining factor in the destruction of a structure is not excess pressure in the air shock wave front ΔР f, and the air velocity pressure ΔР ck(in the absence of experimental data on the degree of destruction of structures at the corresponding values ΔР f), then the stability of the structure is calculated based on the action of the velocity pressure ΔР ck. Calculated values ΔР ck are recalculated using formula (3.1) or graph (Fig. 3.3) into ΔР f, which allows you to compare the stability of structures and determine the degree of their destruction using a single indicator ΔР f, (Calculations for the stability of structures are presented in Chapter 8.)
The nature of the dependence of the degree of destruction of a structure on the magnitude of excess pressure in the shock wave front ΔР f can be presented in the form of a graph (Fig. 3.7).
To assess the resistance of structures and devices to the action of a shock wave, it is necessary to know them stability limit - the limiting value of excess pressure in the air shock wave front, above which the operation of structures and devices is impossible.
Rice. 3.7. The nature of the dependence of the degree of destruction on the magnitude of excess pressure in the shock wave front:
I - zone of weak destruction; II - zone of medium destruction; III - zone of severe destruction; IV - zone of complete destruction; - stability limit of the structure;
Operating radius - distance from the center of the explosion, at which it is numerically equal to the stability limit
Beyond the stability limit ITK element is accepted lower limit of average damage(at a certain distance from the center of the explosion) (Fig. 3.7).
The meaning of this provision is that, falling into zone I - weak destruction (Fig. 3.7), the structure requires ongoing repairs, but its temporary use is possible with certain restrictions.
If the stability limit of a structure is exceeded (it falls into zone II), further use of the structure becomes impossible without carrying out moderate repairs.
Thus, the stability limit and the degree of destruction of ITC elements are quantitatively characterized by the boundary values ΔР f, For the main structures and devices of railway transport, these values are given in table. 3.3.
Indicated in the table. 3.3 intervals with minimum and maximum values of excess pressure, characterizing a certain degree of destruction, take into account possible differences in the design of structures and the position of structures in relation to the direction of propagation of the shock wave front.
For the railway track and rolling stock, the data in Table. 3.3 are given for the case when the shock wave front propagates perpendicular to the axis of the track and the side of the rolling stock (worst case). When a shock wave propagates along the axis of the railway track, the rolling stock withstands excess pressure (velocity pressure) 1.5-2 times greater than the table values, and the railway track suffers severe and complete destruction, mainly within the radius of the crater.
In table 3.3, the pressure values in the shock wave front that cause a certain degree of destruction are given for a nuclear explosion. It is believed that the same degree of destruction by a shock wave from a nuclear explosion and an explosion of VM, GVS or UVG occurs if the pressure in the shock wave front of the explosion of these explosive substances is approximately 1.5 times higher than the pressure in the shock wave front of a nuclear explosion.(For VM, DHW and UVG, the tabular data increases by 1.5 times).
Unlike cities and economic objects, which, as a rule, contain the same type of elements - buildings, railway (transport) objects contain various types of structures and devices,
ensuring the movement of trains and having unequal stability. For this reason, at railway transport facilities in the zone of emergency explosions, it is impossible to distinguish general zones of complete, strong, medium and weak destruction. For each type of structure these zones will have their own dimensions.
An explosion is a fast-paced process of physical and chemical transformations of substances, accompanied by the release of a significant amount of energy in a limited volume, as a result of which a shock wave is formed and spreads, which can and does lead to a man-made emergency.
Characteristic features of the explosion:
- * high speed of chemical transformation;
- * a large amount of gaseous products;
- * strong sound effect (roar, loud sound, noise, loud bang);
- * powerful crushing action.
Explosions are classified according to the origin of the released energy into:
- · Chemical.
- · Explosions of pressure containers (gas cylinders, steam boilers):
- · Explosion of expanding vapors of a boiling liquid (BLEVE).
- · Explosions when releasing pressure in overheated liquids.
- · Explosions when mixing two liquids, the temperature of one of which is much higher than the boiling point of the other.
- · Nuclear.
- · Electrical (for example, during a thunderstorm).
- Supernova explosions
Depending on the environment in which explosions occur, they can be underground, ground, airborne, underwater and surface.
The extent of the consequences of explosions depends on their power and the environment in which they occur. The radius of affected areas during explosions can reach several kilometers.
There are three explosion zones.
Zone I is the zone of action of the detonation wave. It is characterized by an intense crushing action, as a result of which structures are destroyed into separate fragments that fly away at high speeds from the center of the explosion.
Zone II is the zone of action of explosion products. It involves complete destruction of buildings and structures under the influence of expanding explosion products. At the outer boundary of this zone, the resulting shock wave breaks away from the explosion products and moves independently from the center of the explosion. Having exhausted their energy, the products of the explosion, having expanded to a density corresponding to atmospheric pressure, no longer produce a destructive effect.
Zone III - the zone of action of the air shock wave - includes three subzones: III a - severe destruction, III b - moderate destruction, III c - weak destruction. At the outer boundary of zone III, the shock wave degenerates into a sound wave, which can still be heard at considerable distances.
The effect of an explosion on buildings, structures, equipment.
Large buildings and structures with light load-bearing structures that rise significantly above the ground are subject to the greatest destruction by explosion products and shock waves. Underground and buried structures with rigid structures have significant resistance to destruction.
Destructions are divided into complete, strong, medium and weak.
Complete destruction. The floors of buildings and structures collapsed and all the main supporting structures were destroyed. Restoration is not possible. Equipment, mechanization and other equipment cannot be restored. In utility and energy networks, there are cable breaks, destruction of pipeline sections, overhead power line supports, etc.
Severe destruction. There are significant deformations of load-bearing structures in buildings and structures, and most of the ceilings and walls have been destroyed. Restoration is possible, but impractical, since it practically boils down to new construction using some surviving structures. The equipment and mechanisms are mostly destroyed and deformed. In utility and energy networks, there are breaks and deformations in certain sections of underground networks, deformations of overhead power and communication lines, and breaks in process pipelines.
Medium damage. In buildings and structures, it was mainly not load-bearing structures that were destroyed, but secondary structures (light walls, partitions, roofs, windows, doors). There may be cracks in the outer walls and collapses in some places. The ceilings and basements are not destroyed, some of the structures are suitable for use. In utility and energy networks, there is significant damage and deformation of elements that can be eliminated by major repairs.
Light damage. In buildings and structures, some of the internal partitions and filling of door and window openings were destroyed. The equipment has significant deformations. There are minor damages and breakdowns of structural elements in utility and energy networks.
According to the origin of the released energy.
Chemical explosions.
There is no consensus on which chemical processes should be considered an explosion. This is due to the fact that high-speed processes can occur in the form of detonation or deflagration (combustion). Detonation differs from combustion in that chemical reactions and the process of energy release occur with the formation of a shock wave in the reacting substance, and the involvement of new portions of the explosive in the chemical reaction occurs at the front of the shock wave, and not through thermal conductivity and diffusion, as in combustion. As a rule, the detonation speed is higher than the combustion speed, but this is not an absolute rule. Differences in the mechanisms of energy and matter transfer affect the speed of processes and the results of their action on the environment, however, in practice, very different combinations of these processes and transitions from detonation to combustion and vice versa are observed. In this regard, various fast processes are usually classified as chemical explosions without specifying their nature.
There is a more stringent approach to defining a chemical explosion as exclusively detonation. From this condition it necessarily follows that during a chemical explosion accompanied by a redox reaction (combustion), the combustion substance and the oxidizer must be mixed, otherwise the reaction rate will be limited by the speed of the oxidizer delivery process, and this process, as a rule, has a diffusion nature. For example, natural gas burns slowly in the burners of home cookstoves because oxygen slowly enters the combustion area through diffusion. However, if you mix the gas with air, it will explode from a small spark - a volumetric explosion.
Individual explosives, as a rule, contain oxygen as part of their own molecules, moreover, their molecules are essentially metastable formations. When such a molecule is given sufficient energy (activation energy), it spontaneously dissociates into its component atoms, from which explosion products are formed, releasing energy exceeding the activation energy. Molecules of nitroglycerin, trinitrotoluene, etc. have similar properties. Cellulose nitrates (smokeless gunpowder), black powder, which consists of a mechanical mixture of a combustible substance (charcoal) and an oxidizing agent (various nitrates), are not prone to detonation under normal conditions, but they are traditionally classified as explosives.
Explosions of pressure vessels
Pressure vessels are hermetically sealed containers designed for conducting chemical and thermal processes, as well as for storing and transporting compressed, liquefied and dissolved gases and liquids under pressure. The main danger when operating such vessels is the possibility of their destruction due to sudden adiabatic expansion of gases and vapors (i.e., a physical explosion). The causes of explosions of pressure vessels may be errors made in the design and manufacture of the vessel, material defects, loss of strength as a result of local overheating, shocks, excess operating pressure as a result of the absence or malfunction of instrumentation, the absence or malfunction of safety valves, membranes, shut-off and shut-off valves. Explosions of vessels containing flammable media are especially dangerous, because fragments of tanks of even large mass (up to several tons) scatter over a distance of several hundred meters and when falling on buildings, technological equipment, tanks cause destruction, new fires, and loss of life.
Nuclear explosion
A nuclear explosion is an uncontrolled process of releasing large amounts of thermal and radiant energy as a result of a chain nuclear fission reaction or thermonuclear fusion reaction in a very short period of time. By their origin, nuclear explosions are either a product of human activity on Earth and in near-Earth space, or natural processes on certain types of stars. Artificial nuclear explosions are powerful weapons designed to destroy large ground and protected underground military facilities, concentrations of enemy troops and equipment (mainly tactical nuclear weapons), as well as the complete suppression and destruction of the opposing side: the destruction of large and small settlements with civilian populations and strategic industry (Strategic nuclear weapons).
Fission chain reaction
The atomic nuclei of some isotopes of chemical elements with a large atomic mass (for example, uranium or plutonium), when irradiated with neutrons of a certain energy, lose their stability and disintegrate with the release of energy into two smaller and approximately equal in mass fragments - the fission reaction of the atomic nucleus occurs. In this case, along with fragments with high kinetic energy, several more neutrons are released, which are capable of causing a similar process in neighboring similar atoms. In turn, the neutrons produced during their fission can lead to the fission of new portions of atoms - the reaction becomes a chain reaction, acquiring a cascade character. Depending on external conditions, the amount and purity of the fissile material, its flow can occur in different ways. The emission of neutrons from the fission zone or their absorption without subsequent fission reduces the number of fissions in new stages of the chain reaction, which leads to its attenuation. If the number of split nuclei in both stages is equal, the chain reaction becomes self-sustaining, and if the number of split nuclei in each subsequent stage exceeds the number of split nuclei, more and more atoms of the fissionable substance are involved in the reaction.
Thermonuclear fusion
Thermonuclear fusion reactions with the release of energy are possible only among elements with a small atomic mass, not exceeding approximately the atomic mass of iron. They are not of a chain nature and are possible only at high pressures and temperatures, when the kinetic energy of colliding atomic nuclei is sufficient to overcome the Coulomb barrier of repulsion between them, or for a noticeable probability of their merging due to the tunneling effect of quantum mechanics. To make this process possible, work must be done to accelerate the original atomic nuclei to high speeds, but if they merge into a new nucleus, the energy released will be greater than the energy expended. The appearance of a new nucleus as a result of thermonuclear fusion is usually accompanied by the formation of various kinds of elementary particles and high-energy quanta of electromagnetic radiation.
Phenomena during a nuclear explosion
The phenomena accompanying a nuclear explosion vary depending on the location of its center. Below we consider the case of an atmospheric nuclear explosion in the surface layer, which was the most common before the ban on nuclear tests on the ground, under water, in the atmosphere and in space. After the initiation of a fission or fusion reaction, a huge amount of radiant and thermal energy is released in a very short time of the order of fractions of microseconds in a limited volume. The reaction usually ends after evaporation and disintegration of the explosive device structure due to the enormous temperature (up to 10 7 K) and pressure (up to 10 9 atm.) at the point of explosion. Visually, from a great distance, this phase is perceived as a very bright luminous point.
During the reaction, the light pressure from electromagnetic radiation heats and displaces the surrounding air from the point of explosion - a fireball is formed and a pressure jump begins to form between the air, compressed by the radiation, and the undisturbed one, since the speed of movement of the heating front initially exceeds the speed of sound in the medium many times over. After the nuclear reaction decays, the energy release stops and further expansion occurs due to the difference in temperatures and pressures in the area of the fireball and the surrounding air.
The nuclear reactions occurring in the charge serve as a source of various radiations: electromagnetic in a wide spectrum from radio waves to high-energy gamma rays, fast electrons, neutrons, and atomic nuclei. This radiation, called penetrating radiation, gives rise to a number of consequences characteristic only of a nuclear explosion. Neutrons and high-energy gamma quanta, interacting with atoms of the surrounding matter, transform their stable forms into unstable radioactive isotopes with different paths and half-lives - creating the so-called induced radiation. Along with fragments of atomic nuclei of fissile matter or thermonuclear fusion products left over from an explosive device, the newly produced radioactive substances rise high into the atmosphere and are capable of dispersing over a large area, forming radioactive contamination of the area after a nuclear explosion. The spectrum of unstable isotopes formed during a nuclear explosion is such that radioactive contamination of an area can last for millennia, although the intensity of the radiation decreases over time.
A ground nuclear explosion, unlike a conventional one, also has its own characteristics. During a chemical explosion, the temperature of the soil adjacent to the charge and involved in the movement is relatively low. During a nuclear explosion, the temperature of the soil increases to tens of millions of degrees and most of the heating energy in the very first moments is radiated into the air and additionally goes into the formation of thermal radiation and a shock wave, which does not happen during a conventional explosion. Hence the sharp difference in the impact on the surface and the soil mass: a ground explosion of a chemical explosive transfers up to half of its energy into the ground, and a nuclear explosion transfers only a few percent. Accordingly, the size of the crater and the energy of seismic vibrations from a nuclear explosion are several times less than those from an explosive explosion of the same power. However, when the charges are buried, this ratio is smoothed out, since the energy of the superheated plasma goes less into the air and is used to do work on the ground.
8.2. Explosion classification
At explosive sites the following are possible: types of explosions:
1. Explosions of condensed explosives (CEC). In this case, an uncontrolled sudden release of energy occurs in a short period of time in a limited space. Such explosives include TNT, dynamite, plastid, nitroglycerin, etc.
2. Explosions of fuel-air mixtures or other gaseous, dust-air substances (PLAS). These explosions are also called volumetric explosions.
3. Explosions of vessels operating under excess pressure (cylinders with compressed and liquefied gases, boiler plants, gas pipelines, etc.). These are so-called physical explosions.
Main damaging factors of the explosion are: air shock wave, fragments.
Primary consequences of the explosion: destruction of buildings, structures, equipment, communications (pipelines, cables, railways), injury and death.
Secondary consequences of the explosion: collapse of structures of buildings and structures, injury and burial of people in the building under their rubble, poisoning of people with toxic substances contained in destroyed containers, equipment, and pipelines.
In explosions, people will suffer thermal, mechanical, chemical or radiation injuries.
To prevent explosions at enterprises, a set of measures is taken, depending on the nature of production. Many measures are specific, characteristic only of one or several types of production. However, there are measures that must be observed in any production. These include:
1) placement of explosive production facilities, storage facilities, explosive warehouses in uninhabited or sparsely populated areas;
2) if the first condition cannot be met, then such facilities may be built at safe distances from populated areas;
3) to reliably supply explosive industries with electricity (in this case, the technological regime is disrupted), it is necessary to have autonomous power supply sources (generators, batteries);
4) on long oil and gas pipelines it is recommended to have emergency teams every 100 km.
8.3. Characteristics and classification of condensed explosives
By KVV we mean chemical compounds located in solid or liquid state, which, under the influence of external conditions, are capable of rapid self-propagating chemical transformation with the formation of highly heated and high-pressure gases, which, when expanding, produce mechanical work. This chemical transformation of explosives is called explosive transformation.
Explosive transformation, depending on the properties of the explosive and the type of impact on it, can occur in the form of an explosion or combustion. The explosion propagates through the explosive at a high variable speed, measured in hundreds or thousands of meters per second. The process of explosive transformation, caused by the passage of a shock wave through an explosive substance and occurring at a constant (for a given substance in a given state) supersonic speed, is called detonation. If the quality of the explosive decreases (humidification, caking) or the initial impulse is insufficient, detonation may turn into combustion or die out completely.
The combustion process of high explosives proceeds relatively slowly at a speed of several meters per second. The burning rate depends on the pressure in the surrounding space: with increasing pressure, the burning speed increases and sometimes the burning can lead to an explosion.
Excitation of explosive transformation of explosives is called initiation. It occurs if the explosive is given the required amount of energy (initial impulse). It can be transmitted in one of the following ways:
Mechanical (impact, puncture, friction);
Thermal (spark, flame, heating);
Electrical (heating, spark discharge);
Chemical (reactions with intense heat release);
Explosion of another explosive charge (explosion of a detonator capsule or a neighboring charge).
All VVVs used in production are classified into three groups:
- initiating(primary), they have a very high sensitivity to shock and thermal effects and are mainly used in detonator capsules to detonate the main explosive charge (mercury fulminate, nitroglycerin);
- secondary explosives. Their explosion occurs when they are exposed to a strong shock wave, which can be created during their combustion or using an external detonator. Explosives of this group are relatively safe to handle and can be stored for a long time (TNT, dynamite, hexogen, plastid);
- gunpowder. Impact sensitivity is very low and burns slowly. They ignite from a flame, spark or heat, burn faster in the open air. They explode in a closed container. The composition of gunpowder includes: charcoal, sulfur, potassium nitrate.
In the national economy, KVVs are used for laying roads, tunnels in the mountains, breaking up ice jams during the period of ice drift on rivers, in quarries for mining, demolishing old buildings, etc.
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Explosions that are most often encountered in practice can be divided into two main groups: physical And chemical(see Fig. 7.2).
Physical explosions include processes that lead to an explosion and are not accompanied by a chemical transformation of the substance.
Chemical explosions include processes of chemical transformation of a substance, manifested by combustion and characterized by the release of thermal energy in a short period of time and in such a volume that pressure waves are formed, propagating from the source of the explosion.
Accidental explosions are most often caused by combustion processes. Explosions of this kind most often occur during the storage, transportation and manufacture of explosives. They occur when handling explosives and explosive substances in the chemical and petrochemical industries; in case of natural gas leaks in residential buildings; during the production, transportation and storage of highly volatile or liquefied flammable substances; when washing liquid fuel storage tanks; in the manufacture, storage and use of flammable dust systems and some spontaneously combustible solid and liquid substances.
Rice. 7.2. Classification of explosions most often encountered in practice
At physical explosion the released energy is the internal energy of the compressed or liquefied gas (more strictly, liquefied steam). The strength of such explosions depends on internal pressure, and destruction can be caused by a shock wave from expanding gas or fragments of a ruptured tank. In a number of accidents, physical explosions resulting from the complete destruction of tank trucks were noted. Depending on the circumstances, parts of such a tank were scattered hundreds of meters.
The same thing can happen (on a smaller scale) with portable gas cylinders if such a cylinder falls and the pressure-reducing valve breaks. There are numerous cases of such purely physical explosions of vessels with liquefied gases under pressure not exceeding 4 MPa.
Physical explosions also include the phenomenon of so-called physical (or thermal) detonation, which occurs when hot and cold liquids are mixed, when the temperature of one of them significantly exceeds the boiling point of the other (for example, when molten iron is poured into water). In the resulting vapor-liquid mixture, evaporation can proceed explosively due to the developing processes of fine fragmentation of melt droplets, rapid removal from them and overheating of the cold liquid. Physical detonation is accompanied by the formation of a shock wave with excess pressure in the liquid phase, reaching in some cases hundreds of megapascals. This phenomenon can cause major accidents in nuclear reactors and at industrial enterprises in the metallurgical, chemical and paper industries.
Sources of energy for compressed gases (vapors) in closed volumes of equipment can be either external or internal. External is electrical energy used to compress gases and pump liquids; coolants, including electric ones, providing heating of liquids and gases in closed equipment volumes. Internal sources include the energy of exothermic physicochemical and heat and mass transfer processes in a closed volume of equipment, leading to intense evaporation of liquid media or gas formation, an increase in temperature and pressure without internal explosive phenomena.
Chemical explosions divided into volumetric (see Fig. 7.3) and explosions of condensed explosives. The source of a chemical explosion is rapidly occurring self-accelerating exothermic reactions of interaction of flammable substances with oxidizing agents or thermal decomposition of unstable compounds. Under some circumstances, uncontrolled reactions are possible, accompanied by an increase in pressure in the reaction vessel, which can completely collapse if there is no safety valve. This can create a shock wave and a fragmentation field.
Rice. 7.3. Classification of volumetric explosions
The energy carriers of chemical explosions can be solid, liquid, gaseous substances, as well as air suspensions of flammable substances (liquid and solid) in an oxidizing environment (often in air). Explosions of gas mixtures and air suspensions of flammable substances are sometimes called volumetric explosions. Solid and liquid energy carriers in most cases belong to the class condensed explosives. These substances or mixtures thereof include reducing agents and oxidizing agents or other chemically unstable compounds. When an explosion is initiated in these substances, exothermic redox reactions or thermal decomposition reactions with the release of thermal energy occur at enormous speed (during explosions of condensed explosives, carbon and hydrogen atoms in the molecules of the substance are replaced by nitrogen atoms).
Gaseous energy carriers They are homogeneous mixtures of flammable gases (vapors) with gaseous oxidizers, such as air, oxygen, chlorine, etc., or unstable gaseous compounds, such as acetylene, ethylene (prone to thermal decomposition in the absence of oxidizers). The source of explosions of gas mixtures are exothermic oxidation reactions of flammable substances or decomposition reactions of unstable compounds.
Two-phase explosive air suspensions consist of finely dispersed flammable liquids (“mists”) or solids (dust) in an oxidizing environment, mainly in the air. The source of energy for their explosions is also the heat of combustion of these substances.
A technological system is explosive if it has a potential energy reserve that is released at such a high speed that it can generate an air shock wave (ASW) capable of causing crashes or injury to people. The amount of potential energy is determined by the corresponding physicochemical laws of energy release.
The explosion energy of vapor-gas environments is determined by the heat of combustion of flammable substances mixed with air (oxidizing agent); condensed explosives - by the heat released during their detonation (decomposition reaction); during physical explosions of systems with compressed gases and superheated liquids - according to the energy of adiabatic expansion of vapor-gas media and liquid overheating.
The rate of energy release is generally expressed as specific power, i.e. the amount of energy released per unit time per unit volume. In chemical explosions, the rate of energy release can be determined by the speed of detonation or flame propagation in a gaseous environment. The speed of detonation propagation in a solid or liquid explosive approximately corresponds to the speed of sound in the substance and is in the range of 2. 10 3 -9. 10 3 m/s; During gas physical and chemical explosions, compression waves move at a speed close to the speed of sound in air.
Chemical explosions caused by exothermic decomposition reactions in condensed explosives or unstable compounds in the gas phase are accompanied by the formation (increase) of the number of moles of gases. For example, the explosion of 1 kg of trinitrotoluene (TNT), which is a substance with a negative oxygen balance, produces approximately 20 moles of gases (vapors) (0.6 - CO; 10.0 - CO 2; 0.8 - H 2 O; 6. 0 – N2; 0.4 – NH3; 4.7 –CH3OH; 1.0 – HCN) and 15 mol of carbon. Most other high explosives (with the exception of nitroglycerin) are also substances with a negative oxygen balance, that is, the number of oxygen atoms in their molecules is not enough to completely convert the existing carbon atoms into CO 2 and hydrogen into H 2 O. The ability of a substance to undergo an explosive process is subject to the laws of thermochemistry, according to which, if in a given reaction the sum of the heats of formation of the products is less than the heat of formation of the initial compound, then this substance is potentially explosive. For example, if substance A, decomposing by the reaction A → B + C + D, is explosive, then the following condition must be met:
q(A) ≥ q(B) + q(C) + q(D),
where q is the enthalpy (heat) of formation; q has positive values for compounds formed with the absorption of heat (endothermic processes) and negative for compounds formed with the release of heat (exothermic processes).
In this way, only the ability of a substance to undergo an explosive process can be assessed, and the energy and power of the explosion are determined by the reaction rate.
Sources of explosion energy can be redox chemical reactions, in which
air or oxygen reacts with the reducing agent.
Along with flammable gases, reducing agents can be
fine flammable solids (dusts) or
dispersed liquids. Redox reactions under these conditions can occur in both closed and open volumes at sufficiently high speeds at which shock waves are generated that can cause significant destruction.
General information about the explosion
An explosion is a fast-flowing process of physical and chemical transformations of substances, accompanied by the release of a significant amount of energy in a limited volume, as a result of which a shock wave is formed and spreads, exerting a shock mechanical effect on surrounding objects.
CHARACTERISTIC FEATURES OF THE EXPLOSION:
High speed of chemical transformation of explosives;
a large amount of gaseous explosion products;
strong sound effect (rumble, loud sound, noise, loud bang);
powerful crushing action.
Depending on the environment in which explosions occur, they can be underground, ground, air, underwater and surface.
The extent of the consequences of explosions depends on their power and the environment in which they occur. The radius of affected areas during explosions can reach several kilometers.
There are three explosion zones.
3she I- zone of action of the detonation wave. It is characterized by an intense crushing action, as a result of which structures are destroyed into separate fragments that fly away at high speeds from the center of the explosion.
Zone II- area of effect of explosion products. It involves complete destruction of buildings and structures under the influence of expanding explosion products. At the outer boundary of this zone, the resulting shock wave breaks away from the explosion products and moves independently from the center of the explosion. Having exhausted their energy, the products of the explosion, having expanded to a density corresponding to atmospheric pressure, no longer produce a destructive effect.
Zone III- zone of action of the air shock wave - includes three subzones: III a - severe destruction, III b - medium destruction, III c - weak destruction. At the outer boundary of zone 111, the shock wave degenerates into a sound wave, which can still be heard at considerable distances.
EFFECT OF EXPLOSION ON BUILDINGS, STRUCTURES, EQUIPMENT .
Large buildings and structures with light load-bearing structures that rise significantly above the ground are subject to the greatest destruction by explosion products and shock waves. Underground and buried structures with rigid structures have significant resistance to destruction.
Destructions are divided into full, strong, medium and weak.
Complete destruction. The floors of buildings and structures collapsed and all the main supporting structures were destroyed. Restoration is not possible. Equipment, mechanization and other equipment cannot be restored. In utility and energy networks, there are cable breaks, destruction of sections of pipelines, supports of overhead power lines, etc.
Severe destruction. There are significant deformations of load-bearing structures in buildings and structures, and most of the ceilings and walls have been destroyed. Restoration is possible, but impractical, since it practically boils down to new construction using some surviving structures. The equipment and mechanisms are mostly destroyed and deformed.
In utility and energy networks, there are breaks and deformations in certain sections of underground networks, deformations of overhead power and communication lines, and breaks in process pipelines.
Medium damage. In buildings and structures, it was mainly not load-bearing structures that were destroyed, but secondary structures (light walls, partitions, roofs, windows, doors). There may be cracks in the outer walls and collapses in some places. The ceilings and basements are not destroyed, some of the structures are suitable for use. In utility and energy networks, there is significant damage and deformation of elements that can be eliminated by major repairs.
Weak destruction. Some of the internal partitions, windows and doors in buildings and structures were destroyed. The equipment has significant deformations. There are minor damages and breakdowns of structural elements in utility and energy networks.
General information about fire
FIRE AND ITS OCCURRENCE .
A fire is an uncontrolled combustion that causes material damage, harm to the life and health of citizens, and the interests of society and the state.
Essence of Combustion was discovered in 1756 by the great Russian scientist M.V. Lomonosov. Through his experiments, he proved that combustion is a chemical reaction of a combustible substance combining with oxygen in the air. Therefore, for the combustion process to proceed, the following are necessary: conditions:
The presence of flammable substances (except for flammable substances used in production processes and flammable materials used in the interior of residential and public buildings, a significant amount of flammable substances and combustible materials is contained in building structures);
the presence of an oxidizing agent (usually air oxygen is the oxidizing agent when burning substances; in addition, oxidizing agents can be chemical compounds containing oxygen in the composition of molecules: nitrate, perchlorate, nitric acid, nitrogen oxides and chemical elements: fluorine, bromine, chlorine);
presence of an ignition source (open flame of a candle, match, lighter, campfire or spark).
It follows that the fire can be stopped if one of the first two conditions is excluded from the combustion zone.
The possibility of fires in buildings and structures and, in particular, the spread of fire in them depends on what parts, structures and materials they are made of, what their size and layout are. As can be seen from Diagram 2, substances and materials are divided into flammability groups:
For non-flammable substances that cannot burn;
for low-flammability substances that can burn under the influence of an ignition source, but are unable to burn independently after its removal;
for flammable substances capable of burning after removal of the ignition source:
a) difficult to ignite, capable of igniting only under the influence of a powerful ignition source;
b) flammable, capable of igniting from short-term exposure to low-energy ignition sources (flame, spark).