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
- Down-The-Hole drills
- Rotary drills
- Rotary cutting
- Rotary crushing
- Rotary wearing
FOUR ACTIONS IN A DRILLING PROCESS…
- ENERGY TRANSFER – PERCUSSION / CRUSHING
- Top Hammer
- Down the Hole
- Does the real job = breaking the rock.
- Produced by rock drill or hammer’s impact energy and frequency.
- Typically between 1500 to 3600 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 5180 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%
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
- 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
- Hitting fractured and layered rock
- In soft rock
- 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
- 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
- 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
- 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 in fpm = Q *( 144 x 4 ) / Π( D^2 – d^2)
- 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 : 6000 ~ 9000 fpm
- For cuttings with average – low or dry density : 3000 ~ 6000 fpm
- Generally, optimum speed: 4000 – 7000 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