Plasma Drilling

a method of drilling based on the use of a plasma drill, or specially designed plasmatron. Plasma drills with air-eddy stabilization, or “swirling,” of the electric arc discharge, which is the plasma source, have received considerable acceptance. The temperature of the plasma jet in plasma drilling may be as high as 5000°K, which is sufficient for the destruction of the rocks at the bottom of the drill hole. The plasma-forming materials in plasma drills are air, inert gases, water vapor, and mixtures thereof. The axial position of the arc in the plasma drill permits high power outputs to be obtained with a small outer diameter.

Figure 1. Air-cooled plasma drill: (1) output electrode, (2) internal electrode, (3) swirl cone, (4) stem, (5) drill rod, (6) casing, (7) arc

The principle of operation of a simple air plasma drill (Figure 1) is as follows. Compressed air is supplied through a hollow drill rod to the plasma drill, where it is separated into two streams. One stream proceeds to the internal electrode through a spiral swirling channel, feeds the discharge, and by blowing on the arc forces it to rotate. The rotation displaces the electrode spots of the arc over the surface in the interior of the electrode and thereby prevents the premature burning out of the electrode. The second stream cools both electrodes by flowing around their cooling fins. A part of the second stream proceeds through tangential openings in the insulating sleeve into the discharge chamber. The plasma that has been formed flows out through one or more nozzles toward the bottom of the drill hole. After the cooling of the electrodes, a large part of the second stream is ejected to the outside through openings in the plasma drill cover and carries the drilling debris out of the drill hole.

Other plasma drill designs are also widely used, for example, the coaxial whirl design with water-cooled electrodes. Air-water mixtures or steam may be used in plasma drills as the working medium. This lowers or practically eliminates the toxicity of the waste gases—a result of particular importance in subterranean conditions—and increases the specific heat flux of the plasma drill.

Plasma drilling is most efficient in hard rocks, such as granites, quartzites, and porphyries. The rate of drilling is directly proportional to the power rating of the plasma drill. Plasma drills with air-swirl arc stabilization and air cooling are capable of producing drilling rates of up to 4.5 m/hour (hr) in granodiorites for drill-hole diameters of up to 130 mm and power inputs of up to 100 kilowatts (kW). Coaxial whirl plasma drills with the introduction of hydrocarbon fuel into the plasma have attained drilling rates in the ferruginous quartzites of the Krivoi Rog Basin of up to 10–25 m/hr (based on a 50 mm hole) for a plasma drill power input of 81–150 kW.

Plasma drilling is used in drilling and widening holes and wells, in crushing oversized rocks, in extracting and processing block rubble and in cutting and processing concrete.