To achieve optimum drilling performance, it is essential to supply a sufficient volume of flushing medium to the bit face. Flushing serves two main purposes: (1) to cool the core bit, and (2) to remove cuttings from the borehole.

If flushing requirements are not adequately met there can be serious damage to the bit due to overheating, and if cuttings are not removed efficiently, the core barrel and rods may jam due to the build up of waste material.

To ensure that drill cuttings are lifted up the borehole, an uphole flush velocity must be achieved that exceeds the particle settling velocity. If water is used as the flushing medium, the desired uphole velocity is equal to 40 cm/s. If air is used, the desired uphole velocity is 20 m/s.

Below, we present a two-step method to calculate volume of flush required using diffrent core barrel and drill rod combinations.

Step 1
Using the chart below, find the annular area between the drill rods and borehole. The drill rod in use is chosen from the red horizontal lines and the point where this intersects the relevant hole size (represented by a blue curve) is marked. A line is then dropped vertically from this point of intersection to the bottom horizontal scale and the annular area is noted.

Examples:
Using NW drill rods in combination with a T6-116 core barrel (116 mm borehole diameter) gives an annular area equal to 70.5 cm²

Using NWY drill rods in combination with an HWAF Air Flush core barrel (99 mm borehole diameter) gives an annular area equal to 45 cm²

Step 2
Note the annular area and proceed to step 2, by clicking on the relevant flushing medium below: