Font Size: a A A

Coal Breakage Impact By High Pressure Rotary Water Jet And Induced Pressure Relief And Permeability Enhancement By Hydraulic Flushing Cavity: Mechanism And Application

Posted on:2017-05-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:W WangFull Text:PDF
GTID:1221330509954818Subject:Safety science and engineering
Abstract/Summary:PDF Full Text Request
On account of the high coal and gas outburst danger and gas drainage difficulty in the high gas content and low permeability soft coal seam, gas drainage method of the gas pressure relief and permeability increasing caused by the hydraulic flushing cavity was proposed. By means of bedding and cross boreholes enter into the internal of coal seam. Large amount of coal and gas was dashed out by water jet, and a big cavity was generated. The cavity could provide enough space for the coal expansion deformation and the gas desorption, and make the fracture network, which result in the releasing of the coal elastic energy and the gas dilatation energy. Then the coal and gas outburst danger was eliminated. In the paper, the soft coal in Wolonghu mine was taken as the research object, the method of combining theoretical analysis, laboratory experiment, numerical simulation and field engineering practice were adopted. The pressure relief mechanism of the hydraulic flushing cavity was revealed according to the range of the plastic zone and the stress distribution. The physical and mechanical property and the permeability were tested under the stress changed induced by cavity. Furthermore, the effects of the submerging condition, pump pressure, nozzle parameters and rotational speed on the structure and impact force of water jet. Based on the above, the coal breakage process impact by water jet was studied, and the effects of the jet velocity, size, impact time, drift angle and translational velocity. Moreover, integration equipment used for drilling and hydraulic flushing have developed tested in the Xinjing mine. The main research results are as follows:(1) After hydraulic flushing cavity, the horizontal stress decreases sharply while the vertical stress increases first and induced coal damage then decreased and transferred to the distant coal. The plastic zone induced by horizontal stress decrease and vertical stress increase during hydraulic flushing, its radius is13 times larger than the borehole. The larger the pressure-relief zone, the higher the stress concentration degree. The stress concentration factor can be 1.3 while the radius of the pressure-relief zone is larger than 3.0m. The pressure-relief effect is much more significant and the pressure-relief radius can be more than 6.0m. Under the superposition effect of cavity, the horizontal stress relief is more effective. The pressure-relief effect is much more obvious of in the bedding seam while the hydraulic flushing cavity by the cross measure boreholes is much safe.(2) Soft coal has the characteristics of strain softening and plastic flow, peak intensity is low and the phenomenon of stress suddenly drop after peak is not obvious. The peak strength and elastic modulus increase with confining pressure increasing. The phenomenon of plastic flow and expansion and damage are obvious under low confining pressure stage. Pore gas has attenuation on the strength, meanwhile, absorbed gas have un-mechanical effect. The permeability decreased first then increased in conventional triaxial loading. Permeability reach the lowest before peak strength and increase with the damage degree. In the process of constant different stresses and unloading confining pressure, the permeability get the minimum at the unloading point. Permeability increase with damage degree increase, after the confining unloading.(3) The impact distance and the impact pressure of the high pressure water jet are greatly effected by the pumping station pressure, environmental media, impact distance, nozzle structure and rotation speed. The impact speed of the high pressure water jet increases with the increase of the pumping station pressure and the decrease of the reduction angle. While the increasing of the pumping station pressure, the impact distance of the high pressure water jet can be decreased due to the fact that the reduction angle may be decreased and the water jet can even be pulverizated, thus the reduction angle can be chosen about 24° when taking the cumulative effect and the length condition into consideration. Due to the fact that the nozzle rotation can induce the rotation of the water jet and then weaken its continuity and incentive function to decrease its effective impact distance, the nozzle rotation speed can be better controlled under 90 r/min during the process of hydraulic flushing cavity. Besides, it’s better to keep the condition of coal breakage by non-submerged water jet, control the flow rate and guarantee the water can be drained away easily.(4) Based on the development of pore fracture and physico-mechanical properties of soft coal, high pressure water jet structure and impact force distribution, coupling model of coal breakage by water jet has constructed. Coal breakage process has derived. The impact force of water jet in the bottom of coal body exhibits the horseshoe distribution, the largest stress locates in below contact surface 20~30m. Coal breakage by water jet needs a certain jet velocity, size and impact speed. With the increases of jet velocity and impact time, the failure degree gradually increases, the low- velocity jet(20m/s) only results in deformation. The depth and radius of the pit increase as the jet size increase, when the jet size increases to a certain extent about 32 mm, the cracking volume is closely related to the sectional area of the pit. The failure area of coal body increases first and then decreases with the jet angle. Considering the maximum diameter of the cavity the nozzle should perpendicular to the coal body. However, from the efficiency of breaking coal, the optimum angle is about 45°. The translator velocity increases the surface area of breakage and weakens the strength of water jet.(5) Integration equipment of drilling and hydraulic flushing used in underground coal mine have developed, which can crawler walking linkage. Technology system of hydraulic flushing cavity, pressure relief gas drainage and effectiveness investigate were proposed. Field engineering test has implement in Xinjing mine, the effect of coal breakage and prevention of coal and gas outburst are remarkable. Coal quantity flushed by one cavity is about 0.8-2.5 t, average of 1.2 t. The radius of the hole is about 0.46-0.75 m, the amount of gas drainage increased 5.6-6.9 times.
Keywords/Search Tags:soft coal, high pressure water jet, hydraulic flushing cavity, pressure relief and permeability enhancement, numerical simulation
PDF Full Text Request
Related items