DTH Drilling Method
No particular water well drilling methods are applicable under all various drilling conditions. Due to wide geological conditions range such as hard rock, like granite and dolomite to completely unconsolidated sediments such as alluvial sand and gravels, various well drilling techniques have been developed. Specific drilling techniques have become popular in certain areas because these techniques are more effective in drilling the local aquifer at lowest cost. In many cases, however the usual drilling procedure depends on the depth and diameter of the well, type of formation, principal use of the well and sanitation requirement. Hence it is very clear that no single drilling technique is suitable for all geological conditions and well installation.
The Down the Hole (DTH) hammer or Hole Hammer drill a pneumatic drill at the end of drill pipes rapidly stakes the rock while drill pipe rotated slowly. The percussive effect at the bottom of the end of the hole is similar to the blows delivered by the cable tool bit. The hammer is constructed from alloy steel with tungsten carbide inserts that provide the cutting or chipping hole powered by compressed air flowing down the center of the drill string into the hammer. Tungsten carbide is extremely resistant to abrasion.
Rotation of the bit with percussive action helps to assures even penetration and therefore straighter holes even in extremely abrasive or resistant rock types. It is used for fast and economical penetration results from the air piston blows are transmitted directly to the bit without losing energy through drill pipes of medium to extremely hard formation higher than those obtained by other drilling methods or other type of tools.
As the cuttings is removed out continuously of the formation by the air used to drive the hammer. Bit that is constantly striking previously broken rock fragments, the air coming out of the bottom of the bit lifts these chips and carries them back to the surface on the outside of the hammer and drill pipe. The bit on the air hammer always strikes a clean surface. So, the huge quantity of air is allowed to flush out from a small hole hence flushing of the cutting material is also very high. The lifting capacity of the air can be enhanced by adding a small amount of surfactant and water solution to the air. Foam also reduces loss of air to the formation. Thus the air hammer is highly efficient.
The Down the Hole hammer method is widely used to drill long holes, because the deviation is less, hammering cost is less, so it is not only used for blasting, but also used for water well drilling and shallow well drilling.
The strike frequency of DTH hammer is usually kept between 600 to 1600 strikes per minutes, the air pressure is used between 10 to 24 bar and rotation speed of the drill stem is about 25 to 100 rpm is kept to drill a smooth wall circular bore.
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