The difference between 3 rock drilling methods &#; Rotary drilling, DTH drilling, and Top Hammer drill

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Bonmach Q.D

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4 min read

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Feb 28,

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Original from Stella-Blog of BONMACH

Drilling and blasting is an important section during mining production. Let&#;s talk about the three methods of rock drilling &#; Rotary drilling, DTH (down the hole) drilling and Top hammer drilling. These three ways are suitable for different mining operations, and the wrong choice will cause huge loss.

First of all, we have to explain the working principles of them.

Rotary drilling

In rotary drilling, the rig provides enough shaft pressure and rotary torque. The bit drills and rotates on the rock at the same time, which exerts both static and dynamic impact pressure on the rock. The bits rotate and grind continuously in the bottom of the hole to make the rock to fracture. Compressed air under a certain pressure and flow rate are sprayed from the nozzle through the inner of the drill pipe, to make the slag continuously blown from the bottom of the hole along the annular space between the drill pipe and the whole wall to the outside.

Down the hole (DTH) drilling

Down-the-hole drilling is to drive the hammer which behind the drill bit by compressed air via the drill pipe. The piston strikes the bit directly, while the hammer external cylinder gives straight and stable guidance of the drill bit. This makes the impact of energy is not lost in joints and allowing for much deeper percussion drilling.

Furthermore, the impact force acts on the rock at the bottom of the hole, which is more efficient, and straighter than other methods of the drilling operation.

And DTH is more suitable for the big hole of hard rock drilling, special for rock hardness over 200Mpa. However, for the rock below 200 MPa, it will not only energy waste, but also in low drilling efficiency, and serious wear to the drill bit. It&#;s because while the piston of the hammer strike, the soft rock cannot absorb the impact completely, which seriously lower the efficiency of drilling and slagging.

Top Hammer Drilling

The percussive force of the top hammer drilling produced by the piston of the pump in the hydraulic drilling rig, it is transmitted to the drill bit via shank adaptor and drill pipe, This is the difference between DTH drilling. Meanwhile, the percussion system drives the drilling system rotation. When the stress wave reaches the drill bit, the energy is transmitted to the rock in the form of bit penetration. The combination of these functions enables drilling holes into the hard rock, and the air compressor only performs dust removal and slagging in top hammer drilling.

The combination of these functions enables drilling holes into the hard rock, and the air compressor only performs dust removal and slagging in top hammer drilling.

Impact energy multiplied by impact frequency together creates the percussive output of the drifter. However, usually, top hammer drilling used for hole diameter maximum 127mm, and hole depth less than 20M, which in high efficiency.

Conclusion

Characteristics of three rock drilling methods and comparison for adapted to hole size, depth, rock adaptation, slag flushing media, efficiency, and straightness.

Rotary Drilling

&#;Mainly suitable for large holes (coal, limestone) usually diameter larger than 200 mm.
&#;Drilling depths up to several thousand meters or even tens kilometers.
&#;Adapted to soft and medium hard rock.
&#;The slag flushing medium can be compressed air, air foam, mud, etc.
&#;Inefficient for hard rock drilling.
&#;Drilling straightness is lower than the down the hole (DTH) drilling type.

Down the hole (DTH) drilling

&#;The most suitable hole diameter is 100&#;254 mm, in special applications can be up to 2M And the minimum can be around 50 mm as well.
&#;Theoretically drilling depth is limited by back pressure only (can be up to several kilometers).
&#;Adapt to hard rock and rock formation conditions (compared to top hammer).
&#;Mainly use pressurized air for slag flushing, and sometimes air foam for special application.
&#;Highly efficient in hard rock drilling (compared to the rotary drilling).
&#;Drilling straightness is best in these three methods.

Top hammer

&#;The most suitable for hole diameter between 25&#;127 mm.
&#;Generally, the hole depth is less than 25 meters.
&#;Adapt to a stable and uniform hard rock.
&#;The slag flushing medium is compressed air or water.
&#;High efficiency in shallow holes drilling, the deeper the lower efficiency.
&#;Most easily to skew in these three drilling methods.

Read more

The reason customer want to drill the hole is that drill and blast is the most efficient and economic way to break rock instead of excavating it.

Blast hole drilling equipment

• Handheld rock drills

• Top hammer drills

  •  pneumatic

  • hydraulic

• Down-The-Hole drills
• Rotary drills

  • Rotary cutting

  • Rotary crushing

  • Rotary wearing

 

 

 

 

 

 

 

 

 

 

 

 

 

FOUR ACTIONS IN A DRILLING PROCESS&#;

• ENERGY TRANSFER &#; PERCUSSION / CRUSHING

• FEEDING

• ROTATION

• FLUSHING

 

 

 

 

 

 

 

 

 

Drilling methods        

• Top Hammer
•  Down the Hole
•  Rotary

 Percussion

• Does the real job = breaking the rock.
• Produced by rock drill or hammer&#;s impact energy and frequency.
• Typically between to hits per minute.
• Compared to pneumatic drills, hydraulic drills are capable of higher percussion power and faster penetration rates.
• Percussive drill rig is built around the hammer or rock drill
• in percussive drilling rock is broken with impact energy
• Impact Force per Blow : 20 to 60 Tons
• Blow Frequency:   30 &#; 60 Blows/Sec (Top Hammer), 25 &#; 35 Blows/Sec (DTH)
• Rod/Tube diameters: 23 &#; 140 mm

 

 

 

 

 

 

 

 

 

 

Top hammer drilling principle

• When the piston hits the shank, kinetic energy in the piston creates a compressive wave in the string of drilling tools, that will cause the bit to penetrate the rock
• Piston velocity at impact &#; 10m/s but the compressive wave in the drill steel travels at a velocity of m/s (velocity of sound in steel)
• The bit is forced forward and penetrates the rock around 1.0 mm
• Part of the energy not used to penetrate the rock goes back as a tensile wave and part of it as a compressive wave

 

 

 

 

 

 

 

 

Top hammer drilling energy and efficiency

• Example: Rock drill for TH drilling delivers 18 kW
• Energy is lost in places where the drill string diameter changes, like in joints and threads
• MF rods lost energy about 4% and rods and couplings lost energy around 6%

With competitive price and timely delivery, KSQ Technology sincerely hope to be your supplier and partner.

Efficiency 70% 60% 50% Drilling 12.6 kW 10.8 kW 9 kW Wear / Damage 5.4 kW 7.2 kW 9 kW

 

 

 

 

 

 

 

 

 

 

 

 

 

How rock breaks

• The cemented carbide buttons on the drill bit are put under load each time the piston in the hammer or rock drill strikes
• Rock penetration by percussive drilling can be divided into four phases:

  • Elastic deformation

  • Rock pulverization

  • Crack formation

  • Chipping

• All this happens of course very fast as the hammers strike at a rather high frequency, about 40 to 60 Hz (blows per second)
• How much the button penetrates at each blow depends on the type of rock and shape of the button, but typically it is in the range of 1 mm.

 Rock drill(Top hammer)and  Air hammer(DTH) configuration

 

 

 

 

 

 

 

 

 

 

Percussion pressure or power

• Too high: short steel life
• Too low: poor penetration
• Lower pressure needed when
  • Collaring
  • Hitting fractured and layered rock
  • In soft rock

Feeding

• The purpose of the feed is to maintain the drill bit in close contact against with virgin rock across the vertical plane
• To ensure maximum impact energy transfer.
• The feed force must alway be matched to the percussion pressure
• Feed pressure must be always on right level

 

 

 

 

 

 

 

Feeding pressure

• Too high:
  • Bending of steels, 

    hole deviation

  • Increased rotation resistance,

    tight couplings

  • Risk of jamming

  • Difficult collaring

• Too low:
  • Reduced penetration due to poor rock contact
  • Loose couplings, thread burning
  • Poor shank and steel life
  • Higher drifter maintenance cost
• Drilling with a very worn bit or no rotation will cause a strong compressive wave travelling back in the string of tools
• By way of distinction from a tensile wave, the compressive wave will pass through joints which in this case means, it will &#;jump&#; into rock drill and rig
• Tensile waves are more harmfull to the drill steel than compressive waves A stabilizer takes care of the returning compressive wave that otherwise will reflect back in the drill stell as a tensile wave

Rotation

• Purpose of rotation is to turn the drill bit a suitable new position for the next blow. It means the bit must rotate so the buttons on the bit are moved between each blow
• Using the button bits, the periphery is turned about 10mm between blows.
• The optimum distance generates the biggest chips or cuttings and gives the most efficient drilling.
• Rotation pressure must be kept in reasonable for avoiding overtighted threads.

 

 

 

 

 

 

 

 

 

Drill bit rotation speed

• A good indicator of optimal rotation speed is the size of drill cuttings &#; The bigger the better
• Too high:
  •  Too high: Short bit life
• Too low:
  • Low penetration rate
  • Risk of jamming steels
• Rotation pressure (=torque) is a function of feed pressure; feed force adjustments influence the rotation resistance, recommended constant rotation pressure 90-110 bar

 

 

 

 

 

 

 

 

 

 

 

Flushing

• Fuction is to:

  • Remove drill cutting from the hole bottom to the surface by air blowing or water flushing.

  • To cool a bit.

• Medium can be air, water, mist/foam.
• Bailing velocity It is derived by dividing the volume of air by the annular space available between the hole and drill-string.
• Has a direct effect on drilling efficiency.

Bailing velocity

• Bailing Velocity in fpm = Q *( 144 x 4 ) / Π( D^2 &#; d^2)
  • Where,

  • Q = Volume of air available in CFM &#; usually the FAD of compressor after adjustment for altitude

  • D = Diameter of hole

  • d = diameter of drill-rod

  • BV in fpm = feet per minute

• Recommended BV values

  • For cuttings with high or wet density : ~ fpm

  • For cuttings with average &#; low or dry density : ~ fpm

  • Generally, optimum speed: &#; fpm

Insufficient air: for Top Hammer <15 m/s, for DTH(Down The Hole)< 15m/s

• Low bit penetration rates
• Poor percussion dynamics
• Interupt drilling to clean holes
• Plugged bit flushing holes
• Stuck drill steel
• &#;Circulating&#; big chip wear

Too much air: for Top Hammer  >  30 m/s; for DTH  > 60 m/s 

• Excessive drill steel wear
• Erosion of hole collaring point
• Extra dust emissions
• Increased fuel consumption
• Correction factors

• High density rock

• Badly fractured rock (air lost in fractures &#; use water or foam to mud up hole walls)

• High altitude (low density air)

• Large chips

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