Hole cleaning should be taken seriously! Recommended uphole air velocity is between 4000 and 7000 feet per minute (1220 and 2135 meter per minute) . Uphole velocity (annular velocity) is determined by the compressor output (CPM or 1/sec), bit diameter and drill rod diameter. Annular velocity below 4000 FPM (1200 MPM) can create the following:
- Poor bit life – large diameter cuttings continually fall back to the bottom of the hole and are re-ground to smaller sizes by the bit until the air velocity is sufficient to lift them to the top. This can cause premature wear.
- Reduce drilling speed – exhaust air at the bit face does not have enough velocity to move the cuttings before the piston strikes the bit again. Therefore, the bit is re-drilling the cuttings rather than breaking fresh rock.
- Lost tools – cuttings will sometimes flow part of the way up the hole and form what is called a collar. This will block any cuttings from exhausting from the hole and eventually cause the tools to become stuck below the collar.
Annular velocity above 7000 FPM (2135 MPM) will result in excessive erosion of the down hole hammer, drill bit, and drill rod.
How to calculate uphole velocity:
Liters/sec x 76404.7 bit dia mm2 – rod dia mm2
283 (l/sec) x 76404.7 = 21,622,530.1
155.58 x 155.58 = 24,205.14
114.3 x 114.3 = 13,064.49
24,205.14 – 13,064.49 = 11,141
21,622,530.1 11,141 = 1,941 annular velocity (M/Min)