The possibilities of electropulse destruction of nonconductive materials and boring at various laboratory and field conditions have been studied at Tomsk Polytechnic University during many years. This work is devoted to determining the possibilities of water use to demolish granite and concrete at large distance between the electrodes. It was found that the breakdown voltage during the pulse drop is 1.5-2.0 times less than that for the case when the discharge happens during the pulse rise. Consequently, we have less voltage amplitude and current leakage in water. The possibilities and the efficiency of the electropulse destruction of rock and concrete in water are discussed.Words: breakdown voltage, interelectrode distance, material destruction, granite, pulse efficiency, specific energy. IntroductionElectric pulse technology of rock drilling and demolition is based on the fact that at a very short voltage pulse the breakdown inside the solid dielectric occurs faster than the surface flashover between the electrodes. This fact was discovered in early sixties by Vorobiev [l]. The experimental and theoretical studies on the applications of this effect were summarized in [2] for the time period of over 30 years. It was shown that the most of the solid dielectrics could be broken down in the insulating liquids like transformer oil and diesel &el and even in water.Recently, the main issue in the electric discharge drilling technology has been how to make it ecologically clean. Instead of mineral or organic oils, technical water is used as a liquid dielectric. Diesel &el and transformer oil are good as insulation liquids, but cause ecological problems. The usage of water as an insulation liquid have not shown good results, because water has less breakdown voltage and provides considerable current leakage.The objective of our investigation is to determine effective conditions of material destruction due to the discharge channel in water. The following characteristics are important for the electropulse technology of material destruction and boring: amplitude of applied voltage U,, time of
It has now become possible [1][2][3] to find methods of rock breaking by means of various forms of energyelectrical, thermal, electromagnetic, etc.The most promising method is the use of electrical energy without intermediate conversion to mechanical energy. The operative medium is an electric spark, and the solid body is fragmented by the high pressures in the discharge zone. The creation of fundamentally new rock-breaking equipment and the development of suitable techniques require a wide knowledge of the electrical properties of rocks. There is very little available information on the pulsed dielectric strengths of rocks [4], and we are only beginning to study them in connection with electrical rock breaking.In this article we give the results of experiments on pulsed electrical breakdown of certain types of rock over a wide range of voltage durations in a nonuniform field. We give data on the breakdown voltages of rocks in relation to their moisture contents and to whether they are impregnated with insulating liquids. The results of these experiments may prove useful in the design of rock-breaking equipment or in the design and construction of grounding electrodes in ledge rock.Our source of single high-voltage pulses was a multistage pulsed voltage generator (Russian "GIN") working at 480 kV with a discharge capacitance of 6250 pF. The pulse voltages were recorded by an electronic oscillograph with a capacitative voltage divider. Combined calibration of the oscillograph and divider with respect to the voltage amplitude was effected with the aid of sphere gaps (standard wave 1.5/40 gsec, positive pulses). We used plane-parallel specimens of sandstone, fine-and coarse-grained marble, granite, plagioclase granite, fine siRstone, quartz porphyry, and coal, 50 x 50 x 10 mm in size.To eliminate overlapping of the specimen surfaces with these dimensions, the experiment was performed in transformer oil of strength 35 kV in a standard breakdown tester. To eliminate end effects [5] during pulsed breakdown, a conical 45-degree hole was drilled at one end of the rock specimen and the other end was polished. The thickness of the specimens at the point of breakdown was taken as 10 mm and was measured with an IZV-1 indicator gauge.The moisture contents of the rock specimens were estimated from the weight losses of specimens dried in an oven at t = 150~ (the weights of the wet specimens were measured after immersion in technically pure water with a conductivity of o = 0.15 9 10 -3 mho/cm for 10 days). The effect of soaking in transformer oil on the rock's breakdown voltage was investigated on specimens of sandstone and coarse-grained marble (the specimens were immersed in transformer oil for 24 h). The volt-second characteristics of the dry rocks were recorded for specimens covered with a film of paraffin. This avoided penetration of transformer oil into the pores of the rock, and did not affect its breakdown voltage.The electrodes were a brass flat, 120 mm in diameter, and a bross rod, 8 mm in diameter, on to which was pre...
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