During the period 1997…2001 a total of 120 fires occurred as a result of exoergic processes representing 60% of the total of 201 underground fires. The underground fires are the potential sources of explosions. Problems associated with an accident of explosions include the safety to human life, and the loss of coal reserves.
Explosion tests were carried out at the “Granit” Experimental Mine in
Galleries with cross section of 9,5 m2 . These tests were carried out with
air - methane mixture explosions, which dynamic pressures of 0,1 up to 0,3
MPa were generated. Methane content was 9,5 % . The measuaring technigue in the galleries correspondent to the standard equipment.
The first result was that explosion suppression effectiveness by means of
gob rocks, among other, depends on the rocks characteristics and emergency conditions, such as dynamic pressure of the explosion.
The second result was that rocks guaranteed the effectiveness of
100 % when its length becomes equal to 8 m.
Technique of realization of researches
Usually in engineering practice the dangerous, labour - intensive and expensive industrial tests are preceded by series of laboratory experiments. It applies in full measure to study of such complex process, as localization of explosive waves by gob rocks, as the unsufficient or, on the contrary, the great number of tests can result on the one hand in receipt of erroneous results, and on another hand - in economically unsuitable expenses. For example, if the influence of rock lumpiness is investigated in wide range of its change (0,1... 1,1ì) it will be very expensive. At the same time absence of reliable criteria of similarity allowing to simulate processes of flame extinction and decrease of explosion pressure by gob rocks complicates transfer of results of laboratory researches to the nature. Therefore researches were carried out in two stages.
At the stage of the laboratory researches rightfulnesshe of the accepted hypotheses and assumptions about the adiabatic law of expansion of explosive gases in the explosion epicenter was examinated as well influ-ence of lumpiness of rocks on the process being researched process was studied.
At the stage of mine tests quantitative characteristics of the process were specified, values of deviations of theoretical date from test data were estimated, suitability of theoretical dependences for the description of the mechanism of flame extinction and pressure reduction in front of the blast wave was determined. The technique of realization of researches was based on results of theoretical investigations as well on data of analysis of mate-rials and documents connected with accident elimination by potential threat of explosions of methane-air mixtures.
The experimental researches were carried out on shock pipes under conditions of natural mine roadway- Granite adit, too. The shock pipe of 11,5 m length is closed on the one hand by a flange and consists of separate sections of 0,4 m diameter. The pipe is equipped with pressure sensors, flame sensors of a film type, as well with an electrical inflammator of a methane-air mixture. At a given distance from the closed end of the shock pipe a rock barrier of 1.1…3.0 m length was spread out, rock lumpiness changed from 0,02 up to 0,18m. Thus, rock gob mixed up with an explosive coal dust.
Preparation of an explosive methane-air mixture was carried out by a standart method. The percentage of methan in the explosive chamber made from 8 up to 10 %.
At the stage of realization of the mine experimental researches the quantitative characteristics of process of localization of explosion of the methane-air mixure by gob rocks were specified.
During carring-out the experiments in the adit parameters of the gob rock barrier were estimated and technology of its creation was worked through. In this case a rock barrier material was selected this ways that it corresponded with real lumpiness and structure of rocks in full measure. For these purposes rock from the development heading of the mine being winded up to the surface was used.
Mine tests were carried out in explosion adit wich represented a horizontal roadway of 300 m length. The finished section of the adit was S = 7,5 ì2.
For measurement of pressure in front of the blast along the whole adit the sensors were located. The pressure sensors were calibrated by a static method with the use of compressed air energy.
For registration of a flame of the explosion and the speeds of its front the flame sensors were used.
During preparation of the experimental researches the following parameters were cheked: general length of gob rocks, volume of the explosion chamber, percentage of methan, rocks lumpiness, location of the explosion initiator.
Methane being necessary for the explosion was delivered in tank-lorries and pumped through the methane pipe – line into the dead part of the adit under a positive pressure of 0.01... 0.03 MPa.
Samples of methane-air mixture were taken with the help of the vacuum pump under following control of methane percentage in air.
Results of experimental researches
The analysis of the results of the experiments has shown that the gob rock barrier can effectively suppres the blast wave and flame of the explosion of binary gas mixtures and aerodispersive systems. Thus, explosion pressure of methane-air mixure behind the rock barrier of 3m length has become 20 times as less and the barrier of 2m length has lowered pressure in front of the blast wave 14.5 times. The shock wave in both cases degenerates in the compression wave extending with subsonic speed.
In the presence of an air backlash equal to 0.1... 0.3 of pipe diameters intensity of pressure decrease diminishes sharply and doesn’t exceed a value being 3... 4 times as large. Explosion flame in all experiments went out within the limits of the rock barrier and did not ignite methane-air mixture behind the barrier even in the presence of an air backlash in the upper part of the pipe reaching 0.1... 0.2 of its diameters. By increase of the backlashes break-through of flame the other side of a rock barrier was to be observed. So in the ninth and tenth series of experiments the sensors have settled ignition of methane-air mixture behind the rock barrier which was twice smaller than in the first and second series. The result of this tests was complete flame suppression. Obviously, it is explained by that fact that the power-intensity of the rock barrier has lesser than energy quantity released as a result of the explosion of a methane-air mixture.
The experimental data obtained during the tests in the explosion test adit have shown that in case of absence of the protective rock barrier (the zero series of experiments) a complex of aerodynamic disturbances consisting of the blast wave and flame front moves along the working. Thus, the blast wave forms at a distance of 50... 55 m from the place of ignition of methane-air-mixture, moves along the working at 150…160 m distance with a speed of 370... 470 m/s, and then degenerates in the compression
wave. Pressure in front of the wave changes from 0.13 up to 0.2 ÌPà. The character of pressure change along the working is shown in figure 1, curves 1…4. Thus, explosion flame propagates at a distance of 60... 70 m. Oscillograms have shown that pressure has various character in different points of the adit. So, the sensors located in a dead part of the working have registered the smooth increase of pressure that testifies to absence of the blast wave. At a distance of 50 m from the initiator of explosion the sensor has fixed likeness of the blast wave, thus pressure has made 0,06 ÌPà. Further by propagation of explosion gases front of the wave acquires sharper outlines and at a distance of 150 m becomes practically flat. From this moment the formed blast wave propagates along the working which gradually loses the energy generated by the explosion. Energy of the blast wave is spent for heating and compression of air as well for friction against walls of the adit. Pressure in front of the wave makes 0.12 ÌPà that is 20 times pressure allowed in accordance with «The Regulation for carring-out the mine-rescue operations ».
The presence of the rock barrier in the adit changes the character of formation of explosion process. In the process of expansion explosion gases reach the limits of the barrier and begin intensively influence on it. Thus, energy of the explosion is spent for performance of work on overcoming forces of internal friction and on moving the rock. The mentioned processes are accompanied by heat exchange between explosion gases and rock which heat capacity is depended on both its nature and lumpiness that determines the total area of a surface absorbing heat.
In the second series of experiments the length of the rock barrier was minimal and made 3 m. Lumpiness of rocks made 0,3õ0,4õ0,2 of m. In this case pressure in a dead part of the adit was reduced in comparison with the zero series of experiments and made 0,07 MPà. Speed of the flame front made 40 m/s, the length of flame propagation was 12 m that testifies that the thermal energy of the explosion was absorbed by rosk. The flame sensors didn`t fixe the presence of a thermal pulse, as it took place in the zero series of experiments, when flame was propagated on 60... 70 m from the dead part of the adit. Directly for for the rosk barrier pressure of the explosion wave mounted 0,05... 0,06 MPà. It is evidence that fact that the rosk barrier has not executed function of pressure reduction up to the safe magnitude. Thus, the rosk barrier of 3m length practically does not influence on the shock wave character.
The logical continuation of experiments was the lengthening of the rosk barrier with the purpose of increase of its power- consumption.
In the third series of experiments the length of the rosk barrier made 6 ì, lumpiness was as follows: in 30 % of cases - 0,04õ0,03õ0,02m and in 70 % of cases - 0,03õ0,3õ0,03 m. In this case pressure in a dead part of the adit has increased up to the maximal value (Ð = 0,195 MPa) and then it began to fall.
Thus, the rosk barrier of 6m length completely extinguishes flame, and reduces pressure in front of the wave 6 times in comparison with the zero experiment.
The fourth series of experiments was carried out under analogous conditions and the length of the rock barrier made 8m.
As oscillograms have shown the blocking of explosion gases in the closed volume took place during 1,1 s. Pressure just has mounted to 0,2 MPà, after what explosion gases have penetrated beyond the rock barrier spending energy on overcoming forces of internal friction, heating and rock moving. Owing to energy losses pressure of explosion gases behind the rock barrier made 0,012 MPà, that was almost 3 times lower than in the previous experiment when the rock barrier length was equel to 6 m. The safe pressure level in the experiment ¹ 4 (the curve 4) was reached at a distance of 20 m from the rock barrier.
Thus, the tests carried out in the test adit have confirmed the assumption about the ability of gob rocks to extinguish flame and to reduce pressure up to the safe level. The following series of researches consisting of 17 experiments was carried out under identical conditions but without application of a coal dust, and was directed toward the more detailed study of the character of propagation of the explosion in the shock pipe with the use of the protective rock barrier and without it. In this case, as well as in the previous series of experiments extinction of explosion flame was registered at the entrance in the rock barrier, and sharp pressure reduction in flame front of the explosion wave was observed, too.
So, by explosions without the rock barrier pressure at the end of a pipe has mounted 0,4... 0,6 MPà. The rock barrier of 1.1…1.9 m length have reduced this pressure up to 0,057 and 0,036 MPà accordingly. Increase of the rock barrier length up to 2.3... 3.0 m reduced pressure up to the safe magnitude equal to 0,006 MPà.
On the basis of processing results of the executed experiments the dependence describing process of localization of a blast wave by gob rocks is received. Thus it is established, that the basic characteristics of process are: pressure in front of a wave, a degree of filling of the produced space, width of a gob rocks and them lumpiness. Expression is received as:
P1 / P0 = hp / [1 + (1 + Y) - 1]*k1*k2*H,
where are: P1 – is a pressure in front of a wave, ÌÏà;
P0 –is an atmospheric pressure, ÌÏà;
Y - is a dimensionless parameter of an adiabatic curve; hp – is a distance from working adit up to the center of a fire, m;
k1 – is a factor of filling of the produced space by gob rocks , ì-1 ; k2 – is a average diameter of rocky pieces, m; H – is a length of a methane-air mixture zone of a mine adit, m.
Legitimacy of the given dependence is confirmed during realization of mine experiments.
The realized complex of laboratory and mine of researches has allowed to develop the scientifically grounded recommendations for use of gob rocks properties during carrying out mine-rescue operations.