Efficiency and Losses

The wave loses some6-10% of its energy for even, additional coupling, as it travels along the drillstring. This loss is partly due to the difference in cross-sectional area between the rod and the sleeve, and partly due to imperfect contact between the rod faces. The poorer the contact, the greater the energy loss.

When the shock wave reaches the bit, it is forced against the rock, thereby crushing it. The efficiency at the bit never reaches 100%, because some of the energy is reflected as a tensile pulse. The poorer the contact between the bit and the rock, the poorer the efficiency (Figure 6).

To optimize drilling economy, the drilling parameters for percussion pressure, feed force, and rotation must harmonize.

Percussion Pressure

The higher the pressure, the higher will be the speed of the piston, and consequently, the energy. Where the bit is in good contact with hard and competent rock, the shock wave energy can be utilized to its maximum. Conversely, when the bit has poor contact, the energy cannot leave the drillstring, and reverses up the drill-string as a tensile wave.

It is only when drilling in sufficiently hard rock that the maximum energy per blow can be utilized. In soft rock, to reduce the reflected energy, the percussion pressure, and thus the energy, will have to be lowered (Figure 7).

For any given percussion pressure, the amplitude, and hence the stress in the drillsteel, will be higher with reduced cross-section of the drillrods.

To get the longest possible service life from shank adapters and rods, it is important to ensure that the working pressure is matched to the drillstring at all times.

Feed Force

The purpose of the feed is to maintain the drillbit in close contact against the rock. However, the bit must still be able to rotate. The feed force must always be matched to the percussion pressure. Figure 8 illustrates this relationship.

Rotation

The purpose of rotation is to turn the drillbit to a suitable new position for the next blow. Using button bits, the periphery is turned about 10 mm between blows. Consequently, the rotation rate is increased using higher impact frequency and reduced bit diameter. Using insert bits, the recommended rotation rate is 25% higher.

Setting Parameters

In practice, the driller sets the percussion pressure that the rock can cope with, and then sets the rev/min with regard to the percussive frequency and the bit diameter.

When drilling starts, the feed is adjusted to get even and smooth rotation. In case this is not achieved, which will show up in low shank adapter life, the percussion pressure can be progressively reduced, until even and smooth rotation is reached.

The temperature of the adapter sleeve can be checked to ensure that the drilling parameters are correctly set. Immediately after drilling, the temperature should be 60-70 degrees for dry drilling, and approximately 40 degrees for wet drilling.

Drilling problems, mainly related to loose couplings, may arise whatever parameters are used. In order to tighten the couplings during drilling, the friction of the bit against the hole bottom has to be increased. This can be done by increasing the feed, increasing the rotation rate, or changing the bit.

Flushing

Drill cuttings are removed from the hole bottom to the surface by air blowing or water flushing. As the power output from rock drills increases, accompanied by increased penetration rate, efficient flushing becomes gradu­ally more important. The flushing medium is normally air for surface drilling, and water for underground drilling.

The required flushing speed will depend on: specific gravity - material having a density of 2 t/m requires at least 10 m/sec, whereas iron ore, for example, having a density of 4 t/ m, requires an air speed of 25-30 m/sec; particle size – the larger the particles, the higher flushing speed required; particle shape – spherical particles require more speed than flaky, leaf shaped particles.

Date: 2015-12-24; view: 606