Pressure drop characteristics of reverse circulation pneumatic cuttings removal during coal seam drilling.

To solve the problems that the borehole depth is shallow and the drilling efficiency is low during the gas drainage drilling in soft coal seam with current cuttings removal method, a new technology of reverse circulation pneumatic cuttings removal is proposed. The working principle of reverse circulation pneumatic cuttings removal is analyzed, and the kinetic equation of cuttings in the inner hole of the drill pipe is established. Through experiments, the pressure drop in the drill pipe is measured to reveal the effects of air velocity, cuttings mass flow rate, and cuttings particle size on the pressure drop in inner hole of the drill pipe. When the cuttings mass flow rate is constant, the pressure drop increases with the increase in air velocity. When the air velocity is constant, the pressure drop increases with the increase in cuttings mass flow rate. At low air velocity, the pressure drop of cuttings is primary. As the air velocity increases, the pressure drop ratio of cuttings decreases. Under the same conditions, the order of pressure drop with different particle size cuttings is coarse cuttings > medium cuttings > fine cuttings. Empirical equation of pressure drop coefficient of cuttings is established, which is in good agreement with the actual data.