Powder discharge characteristics of pneumatic double pipe directional drilling in broken soft coal seams and its application

To study the influence of casing, casing bit size and casing rotational speed on pulverized coal discharge in double pipe directional drilling technology, three kinds of casing and casing bits in different sizes are designed respectively against the background of pneumatic double pipe directional drilling in the broken soft coal seam, the changes of the discharge rate and outlet velocity of pulverized coal under different combinations are analyzed, and the influence law of different casing rotational speeds on the suspension rate and conveying trajectory of pulverized coal are obtained. The numerical simulation results show that the ø120/96 mm casing with different casing bits has a high pulverized coal discharge rate. The conveying speed of pulverized coal decreases with the increase of the total cross-sectional area of the annular channel and increases with the increase of the cross-sectional area of the inner channel. The suspension degree of pulverized coal first increases and then decreases with the increase of casing rotational speed. Based on this, field tests are carried out to compare and analyze the effects of the powder carrying capacity of different casing combinations and changes of the casing rotational speed on pulverized coal discharge. The combination of the ø140 mm casing bit and ø120/96 mm casing achieves the maximum drilling depth at the casing rotational speed of 40 r/min. The research results provide technical reference for the selection of double pipe directional drilling parameters and the selection and development of drilling equipment for broken soft coal seams.