UPHOLE VELOCITY

The exhausting air from a Down-the-Hole hammer is used as the flushing air to clear the rock cuttings from the bore hole.

To provide an adequate rate for cutting evacuation, an acceptable uphole velocity must be maintained; this is around 900-1800 m/min (3000-6000 ft/min). The actual uphole velocity will be determined by the hole size, drill tube diameter and air consumption of the hammer in use.

The function of a DTH hammer and drill bit is to break the rock and remove the cuttings from the bore hole immediately as they are formed. If a hammer fails to fulfil this function because the uphole velocity is insufficient, there are several actions that can be taken, although operating conditions may only allow some of these. One such action is to increase the drill tube diameter – on very large hole diameters, shrouded drill tubes, near to the drill bit diameter, may be used. Under these conditions, a sleeve or shroud can be fitted around the hammer of the same diameter as the drill tubes. The use of upset drill tubes which cause turbulence and a reduction in uphole velocity should be avoided. Next, we will look into another alternative action.

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DECREASE THE DRILL BIT DIAMETER

Use foam to assist in cutting evacuation from the bore hole, which will be explained in a later post. Alternatively, you can increase the flushing air available.

This can be done in 3 ways:

1. Intermittently - by simply repeatedly lifting the hammer up from the rock face, the percussion action will cease and extra air, up to twice the normal volume, will pass through the hammer to clean the hole, providing that the compressor has a sufficient reserve of volume.
2. By using a choke or vent in the hammer. Dependent on hammer make, this can either be a plugged orifice in the piston or a bleed facility in the non-return valve, bottom bush or bit retaining rings.
3. By using an air bypass sub adaptor above the hammer. Two holes angled upwards are machined in the adaptor and these bleed extra air upwards to increase the uphole velocity. A caveat is that this air bypasses the face of the drill bit where it would be most effective.

It is important to ensure that when using air bypass sub adaptors, chokes, or vents, the compressor in use has a sufficient reserve of air volume available over and above the amount needed to sustain its output pressure with the selected hammer. Otherwise, a pressure reduction will occur, in addition to the effect of any back pressure caused by the use of the vent or choke.

The pressure reduction will lead generally to a decrease in penetration speed.

THE FORMULA FOR CALCULATING UP HOLE VELOCITY IS AS FOLLOWS:

Where:

vm = Velocity in metres per minute.

x (m3) = m3/min. of air volume passed by the hammer at the selected air pressure.

DM2 = Diameter of hole - squared (in millimetres).

dm2 = Diameter of drill tube squared (in millimetres).

vf = Velocity in feet per minute.

y (cfm) = Volume of cfm of air passed by the hammer at the selected air pressure.

DI2 = Diameter of hole - squared (in inches)

di2 = Diameter of drill tube squared (in inches)

AN ALTERNATIVE METHOD OF CALCULATING UPHOLE VELOCITY IS AS FOLLOWS:

1. Establish the volume of air in m3/min. being passed through the hammer.
2. Establish the velocity per m3 of air, from chart across, with relevant tube and hole diameter.
3. Multiply A x B = answer in m/minute.
4. If Feet per minute required - Multiply m/minute x 3.28

WITH HALCO HAMMERS FOR EXAMPLE

DOMINATOR 600 @ 14 bar (200 psi) = 14.10 m3/min AIR CONSUMPTION

114 mm (4.1/2”) TUBE/165 mm (6.1/2”) BIT = 89 m/min VELOCITY FOR 1 m3/min

UPHOLE VELOCITY = 14.10 x 89 = 1255 m/min

HAMMER AIR CONSUMPTION

The chart, below, lists the velocity in metres per minute for each 1 m3 of air passed through the hammer, for each drill tube and drill bit diameter combination.

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