Internal deflagration explosions.

Internal deflagration explosions.

Internal deflagration explosions

Characteristics of an internal deflagration explosion

The most common explosions are deflagration explosions that occur indoors. They are characterized by a variety of manifestations, since the layouts of residential, office and industrial premises are different.

The main physical parameters of an exploding gas-vapor-air mixture are:

• normal combustion rate Un — combustion rate of mixture particles:

• expansion coefficient of a flammable mixture during explosive combustion e — this is the ratio of the density of the initial mixture to the density of the explosion products.

To understand what happens during explosions indoors, let's consider a number of simple cases, combinations of which are real accidents.

If a combustible mixture is ignited, a fireball consisting of water vapor, carbon dioxide, and nitrogen explosion products heated to 1600-1800°C is formed in the center of the ignition. Between the fireball and the walls of the structure is the original combustible mixture, which has not yet had time to burn. Since the flame inside the structure spreads (depending on the type of combustible mixture) at a speed of only 8-20 m/s, and the speed of sound, with which disturbances are transmitted (in this case, an increase in pressure), is 340 m/s or more, then due to multiple runs of the sound wave, the pressure at all points inside the room is equalized almost instantly. This process is called quasi-static. By the end of explosive combustion, the excess pressure in a strong structure becomes approximately equal to 700-800 kPa and is determined by a very simple formula

Dр=р0(e-1)kPa,     (1)

where p0 is the atmospheric pressure, equal to 101.3 kPa.

Of course, no structure, except special bunkers for testing explosives, can withstand such enormous pressure. Usually, during deflagration explosions inside residential and industrial premises (due to the presence of easily destructible glazing of windows and door panels), the pressure is 10-15 kPa. In the case under consideration, an important factor is the quasi-stationarity of the explosive process, when all structures are loaded almost simultaneously and the weakest ones begin to collapse earlier, thereby reducing the pressure and preventing the destruction of load-bearing structures.

Explosion in a room communicating with the atmosphere through an open opening

In this case, simultaneously with the development of the fireball, the process of the outflow of the initial mixture from the room through the opening begins, and then the explosion products. Calculations and experience show that in this case about 85% of the combustible mixture is emitted from the room into the atmosphere and only about 15% burns inside the room, or more precisely 1/e of its part. Thus, if the room were gassed by only 15% of its volume, the effect of the explosion inside the room would be the same as with complete gassing. The process of explosive combustion here is also quasi-static. However, in this case it is much more complicated. This is evident even from the formula by which the excess pressure inside the room is calculated in the simplest case, when the opening is large and the pressure inside the room does not exceed 10-15 kPa:

where: rj is the density of the gas flowing through the open aperture (index 1 refers to the fresh mixture, 2 to the combustion products);
Dр is the excess pressure;
Uн is the normal combustion rate;
e is the coefficient of expansion of the mixture during combustion;
a — coefficient of intensification of explosive combustion during the outflow of the initial mixture and explosion products;
µ — coefficient of consumption during the outflow of the initial mixture and explosion products;
S(t) — area of ​​the flame surface during combustion;
Sотв area of ​​the opening through which the outflow occurs.

From formula (2) it follows that the main parameter determining the value of excess pressure is the ratio of the flame area to the area of ​​the opening. The pressure on the wall opposite the opening is always higher than on other walls due to the addition of the reactive force from the escaping jet. If there are several openings, then the location of the openings is important.

In this case, a so-called two-stage explosion occurs. After the explosion begins, the combustible mixture begins to flow into the second room through the opening. This causes strong turbulence of the mixture, causing a sharp increase in the combustion area in the second room. As a result, even if the second room communicates with the atmosphere, the pressure in it is 2-3 times higher than in the first, and begins to be transmitted back. For this reason, it is recommended to protect technological openings between production rooms with strong doors and provide for airlocks so that both doors are not open at the same time. It is even better to avoid openings between explosive rooms if possible. Two-stage and three-stage explosions (for example, kitchen-hallway-living room) most often occur during accidents.

Glazed openings in industrial explosive premises are used as safety structures (FS) that help reduce pressures during emergency explosions to values ​​at which the supporting enclosing structures remain intact. It is natural to want the FS glazing to be destroyed as early as possible, at lower pressures in the room. Calculations and experience show that this requires installing thin glass and making the spans between the frames large. It is clear that the glazing must withstand loads from strong winds and retain heat in the cold season. These contradictory requirements are reconciled after conducting multi-variant optimization calculations using the theory of load determination, the theory of glazing operation, and the theory of building structure operation during internal deflagration explosions. Unfortunately, building codes lag far behind the requirements of practice and scientific recommendations in the field of ensuring explosion resistance. The standards require that there should be 5 m2 of safety structures (FS) per 100 m3 of room volume, and that the minimum glass area in one cell should be no less than 1 m2. Thus, depending on the room volume, the FS area according to the regulatory requirement may be either sufficient or insufficient in terms of explosion resistance.

In rooms cluttered with equipment installed in them, a fireball of a regular shape does not arise, the profile of the movement of the explosion products is more similar to the profile of a hydraulic flow spilling over a dam; the role of the dam here is played by obstacles in the path of the flame. The combustible mixture is strongly turbulent, which leads to an increase in the intensity of explosive combustion and, as a consequence, to an increase in the loads on building structures. In order to estimate such explosive loads, it is necessary to resort to modeling, since analytical calculation in this case gives very approximate results.

In addition to rooms, all kinds of channels are often gassed, including pipes, gallery walls, galleries, tunnels, corridors, adits, mine shafts, elevator shafts, chimneys, flues, etc. Experimental explosions conducted in pipes have shown a sharp difference in explosive combustion in them and in rooms. In such a channel, explosive combustion accelerates and, if the channel is long enough, deflagration can turn into detonation. Experiments have also shown a sharp difference in the mechanism of explosive combustion propagation in pipes during ignition of a combustible mixture at closed and open ends. During ignition at a closed end, the explosion flame continuously accelerated and at a distance equal to 60-70 pipe diameters, the deflagration explosion turned into a detonation explosion. Due to the roughness of the walls in front of the flame, the original combustible mixture was strongly turbulent and this led to an acceleration of the flame.

Such a high flame speed has never been achieved during ignition from the open end of a pipe. This is explained by the fact that the visible flame speed is made up of the flow speed of the combustible mixture and the speed of flame propagation along the particles. During ignition from the open end of a pipe, the combustible mixture is practically motionless in front of the flame front, since this mixture has nowhere to move. From the facts given above, it follows that in explosive premises, measures must be taken to prevent or reduce the possibility of the formation of a high degree of turbulence and the flow of fresh mixture, using various types of fairings for this purpose, and not allowing the installation of equipment and structures with sharp edges. During explosions inside channels with open or easily opened openings on the side walls, the outflow of explosion products through these openings was observed, as a result of which an increase in pressure and significant acceleration of the flame did not occur.

In 1980, in the Smolensk region, it was decided to use the basement of a building under construction to store archives. The transverse load-bearing walls in the basement were arranged in the form of labyrinthine partitions. The walls were painted with paints diluted with a solvent that evaporated quickly. A portable electric lamp connected to the power grid with a cable was used for lighting. When the painting was finished, the painter tripped at the exit from the basement, the lamp broke, and this ignited a mixture of solvent and air. As a result of the deflagration explosion, the ceiling above the basement was bulged, and the partially built walls of the first floor were destroyed. The configuration of the basement plan corresponded to a structure called by specialists a «detonation box». Each turn led to additional turbulence of the mixture, and therefore to an increase in pressure during explosive combustion. If the ignition had occurred in the blind end of the basement, detonation and complete destruction of the building would have been inevitable.

Features of the operation of building structures and glazing during internal deflagration explosions

In 1974, an explosion of wood dust accumulated in the ventilation system occurred in the radio receiver body grinding shop at the Minsk Radio Plant. It happened right after the lunch break. One of the shop workers was late from lunch. As he approached the shop, he saw the following scene: the building suddenly began to swell, then the walls moved apart and the roof rose, from under which the flame of the explosion burst forth, then the roof collapsed down. The man lost consciousness from what he saw.

Usually, buildings and structures are designed to withstand external vertical and horizontal (wind) forces directed inside the structure. In the case of an internal explosion, significant loads act in the opposite direction. Experts in explosion resistance of buildings and structures recommend providing direct and reverse connections between the structural elements of explosive buildings and structures. In explosive premises, glazing of window openings is considered the main safety structure that reduces explosive loads to an acceptable level safe for load-bearing structures, although in addition to this, various easily resettable structures (LSC) are also offered that quickly release openings at the very beginning of the occurrence of explosive loads.

Studying the operation of glazing shows:

• glazing never breaks instantly. This happens gradually as the load increases. First of all, glass with various types of defects breaks. At low loads, only the part of the glazing closest to the center of the explosion breaks;

• glass with large spans and areas should be used for effective protection of load-bearing structures;

• the best protection is provided by single glazing and then double glazing. The destruction of glazing has a probabilistic nature and is subject to the Weibull distribution. A thorough experimental and theoretical study of the operation of glazing was conducted by Russian scientists L.P. Pilyugin A.N. Litvin (MGSU).

We provide a description of a typical explosive accident, the consequences of which could have been less severe if the architects had consulted with experts in ensuring the explosion resistance of buildings and structures during the design process. We are talking about the explosion in house No. 5 on Akademika Volgina Street on March 9, 1996.

Residential building No. 5 has 16 floors (Fig. 1). Each entrance has 4 apartments on the floor. The explosion occurred in the first entrance on the third floor. The entrance has a stairwell 20. The entrance is located on the side of Akademika Volgina Street. There are passenger 15 and freight 16 elevators. The doors of all four apartments No. 66, 67, 68.69 located on the floor open into elevator corridor 9. The elevator corridor communicates through vestibule 17 with two doors with aeration platform 18. The platform is ventilated through an aeration opening leading outside, covered with grating 23. The apartments have loggias. The kitchens are equipped with electric stoves.

2. Development of the accident

In the hallway of apartment 7 #66, two propane cylinders with a capacity of 27 and 5 liters were stored, prepared for the dacha: the 27-liter cylinder turned out to be faulty and started leaking. After some time, a deafening bang was heard in the hallway (the first explosion of the propane-air mixture), audible in all the apartments on all floors of the entrance. A fire immediately started in the hallway. The injured and burned owner of the apartment could not get into room 5, where his wife and child remained, because of the fire. In a panic, he ran out through the elevator corridor 9 and two vestibules 17, 19 to the landing 20. The bang, having created excess pressure in hallway 7, squeezed out most of the propane-air mixture into adjacent rooms 5, 6, 8, etc. apparently, into the hallway of apartment 10 No. 67, and also through leaks near the water pipes in bathroom 8 into seventeen voids of ceiling slabs 27 (three slabs). Approximately 2-5 minutes after the start of the fire, a 5-liter cylinder exploded due to heating. The explosion (the strongest of all) shattered the parquet floor and broke through the reinforced concrete floor slab above the apartment on the second floor. This was the so-called «physical explosion of a vessel operating under pressure». For this, it is not necessary for the cylinder to contain flammable gas: any gas under high pressure will give the same effect. However, the propane released after the explosion soon mixed with air and the resulting mixture exploded in rooms 7.5 and 6.

Thus, three explosions occurred in succession in the rooms and seventeen — in the voids of the roof slabs 27, where the combustible mixture was pressed through the leaky ends. Concealed electrical wiring passed through the voids of the ceiling slabs.

Apartments No. 66 and 67 suffered the most damage. All partitions between rooms, kitchens and other premises were destroyed. The diagram shows the direction of the blast wave with arrows. Not only the glass was knocked out, but also the frames. The blast wave tore off the reinforced concrete fences of the loggias measuring 580×100 cm. There are extensive soot stains from the explosions and fire above the loggias on the 3rd, 4th and 5th floors.

The total length of the corridor is about 15 m on each floor, and there are two 90° turns. Such long corridors with turns create a labyrinth effect, i.e. they contribute to the transformation of the compression wave into a shock wave, and reflections in the corners of the turns and in the dead-end ends cause an increase in the explosive load. At the end of the corridor adjacent to apartments No. 66 and 67, a deflagration explosion also occurred, which led to the formation of a compression wave. In this section, the corridor is heavily sooted, and in the rest of the soot is absent.

After the explosion in corridor 9, the compression wave destroyed the elevator doors and burst into the elevator shafts, from where it then spread to all floors of the entrance. On all these floors, the elevator doors are pressed through from the inside.

Spreading horizontally, the compression wave reached vestibule 17, which became an obstacle to the wave's exit into the atmosphere through the aeration opening. The torn off vestibule door knocked out metal grating 23. This allows us to estimate the intensity of the reflected compression wave at 10-15 Pa.

The apartment was not directly exposed to the flames of the explosion, but its front door was shattered into pieces by the compression wave that burst from the elevator corridor.

At that time, there was a woman in the apartment. The increase in pressure did not harm her and did not cause any pain in her ears. This allows us to conclude that the wave that burst into the apartment was a compression wave, in which the pressure increases smoothly, and not a shock wave. The eardrums are capable of withstanding high pressure when it increases gradually (pearl divers experience pressure up to 300 kPa), but when exposed to a shock wave, the eardrums begin to rupture at 10-15 kPa.

From the Akademika Volgina Street side, the glazing of the stairwell of entrance #1 was destroyed on all floors by the compression wave that penetrated stairwell #20 through the destroyed doors of vestibules #17 and #19. The blast wave blew away the loggias of apartments #66 and #67 facing the courtyard. The glazing of apartments on all floors of the first entrance facing the courtyard was destroyed. This can be explained by the fact that the flammable mixture squeezed out of apartments #66 and #67 ignited on the outside of the building from the products of the explosions in the premises, resulting in an external deflagration explosion. Glass fragments thrown out of the windows and doors of apartments #66 and #67 were scattered across the courtyard for a distance of up to 80 m.

The emergency explosion in building No. 5, which started in one apartment, resulted in the entire entrance being damaged. The increased volume of destruction is explained by the unfortunate placement of the elevator shafts and the isolation of the elevator corridor from the stairwell by two vestibules. In those buildings where the elevator entrances are located on the landings of the stairwell, the elevators remain operational during such explosions and do not transfer the destructive compression wave to all floors. The volume of destruction could have been significantly less if the elevator shafts had at least openings (protected by nets) leading to the stairwell.