Uphole velocities

  • VM = Velocity in metres per minute
  • X(M3) = M3/min of air passed by hammer
  • DM2 = diameter of hole squared in mm
  • dm2 = diameter of drill tube squared in mm
  • VF = Velocity in metres per minute.
  • Y(CFM) = CFM of air passed by hammer
  • DL2 = diameter of hole squared in ins
  • di2 = diameter of drill tube squared in ins

The above formulas should be used to calculate the uphole velocity of the air being used to flush the hole. It is recommended that a minimum velocity of 3000ft/min (915m/min) be used to carry the cuttings away from the bit face, and back up the hole. When drilling with overburden systems or casing driver bits, the internal diameter of the casing you are using becomes the DL, (DM) value in the above formula.

If through using the calculation you find that your velocity is lower than the recommended figure then there are two possible ways to increase the up hole velocity.

If your compressor is of sufficient size as to allow you to pass more air through the hammer, then by drilling a hole through the hammer choke, you will increase the up hole velocity in proportion to the amount of air you use. To check the extra required CFM use the Bulroc technical information sheet showing air consumption of Bulroc hammers.

If the size of your compressor will not allow you to use extra air for flushing then by reducing the area of annulus around the drill string the up hole velocity will also be increased. To achieve this you will need to attach a shroud to the drill rods and hammer. The outer diameter of this shroud should be calculated by transposing the formula above, and using 3000ft/min as the VF value.

You are left with the required diameter of the shroud. Be sure to check that after calculating the required diameter there is sufficient annulus to allow the chipping's to pass back to the surface.

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