Hardrock_DTH_drilling

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HARDROCK DRILLSDTH Drilling training

• Background of the general DTH drilling statements

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DTH drilling statements

• Theory and technique
• The amount of energy in the stress wave depends
  on piston weight and piston velocity at impact.
• In DTH drilling, compressed air is used to make
  the piston move and strike
• In DTH drilling, the force on the bit at each
  strike is in the range of 50 100 tons and the
  striking frequency is in the range 1500 2400
  BPM
• Bit penetration into the rock is in the range of
  1 mm for each blow
• Please Read DTH(Down The Hole) Drilling
  Application
• (https//www.hardrockdrills.com/dth-drilling-
  application/)

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DTH drilling statements

• Energy and efficiency
• The harder the rock the more percussion force
  needed.
• Feed force, or weight on bit, must be correctly
  set to get maximum utilization of the shock wave
  energy in the system and avoid damage to tools
  and equipment
• Correct bit rotation speed is important for
  optimal rock cutting at each strike
• Flushing must deliver enough bailing velocity for
  efficient removal of rock cuttings
• The choke in the hammer can be varied to increase
  the flushing

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Converting energy

• Engine power P M2?n
• gt Transformed by the compressor
• Pneumatic power
• gt Transformed in the hammer
• Kinetic power P 1/2mv2 f
• gt Transformed to the bit.
• Strain power P Ac/E?(?2dt) f

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The amount of energy in the stress wave depends
on piston weight and piston velocity at impact.

• Right or wrong ??
• W 0.5mv2 kinetic energy of the piston
• ? Maximum which is available.
• What is the role of piston shape?
• Ø120 L 226 20 kg
• Ø80 L 510 20 kg
• Energy transfer, mechanical efficiency, stress
  level, rebound velocity.

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How is RH 550 doing?
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In DTH drilling, compressed air is used to make
the piston move

• Hydraulic DTH hammers are used in rare cases.
• Piston acceleration is caused by chamber
  pressures and working areas.
• Chamber pressures are defined by flow areas,
  timing points and chamber volumes.

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Example of air cycle modification
Impact Velocity m/s 8.6185 Impact Frequency
Hz 31.2012 Impact Power kW 9.1197 Air Flow
Out (NTP)m3/min 11.0143 Compressor power
(steps1) kW 94.5947 Total efficiency
9.6408 Stroke length mm 95
Impact Velocity m/s 11.053 Impact Frequency
Hz 27.9877 Impact Power kW 13.4548 Air Flow
Out (NTP) m3/min 12.7031 Compressor power
(steps1) kW 109.1581 Total efficiency
12.3259 Stroke length mm 105
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Pressure and Energy

• Energy by definition comes from force and stroke
  length
• W F s and F pA so W p A s
• Where
• W energy
• F force (which accelerates the piston)
• s stroke length of piston (how long distance
  force is acting)
• p pressure in working chamber
• A working area of piston
• gt Energy is directly ( linearly, power of 1)
  related to pressure

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Operating pressure vs energy
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Pressure and Velocity

• As a result of acceleration piston will have
  kinetic energy
• W 0.5 m v2
• Where W energy m piston mass and v piston
  velocity
• By combining these 2 equations we can write 0.5
  m v2 p A s
• Which can be written
• This tells us that pressure is related to 2nd
  power of velocity, and all other terms are
  constant.
• Or it can be written
• Which means velocity is related to square root of
  pressure, if you find it more practical as you
  normally set pressure and velocity is the
  result..

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Operating pressure vs velocity
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Pressure end frequency

• Frequency is ratio of velocity and stroke length
  (actually 2 times of stroke length, back and
  forth)
• gt As we know velocity is proportional to square
  root of pressure
• This means that also frequency is related to
  square root of pressure, when assuming constant
  stroke length.

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Operating pressure vs. frequency
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Pressure and power

• Power is product of energy and frequency
• P W f, where P power W energy f
  frequency
• When You multiply energy which is related to
  pressure linearly (power of 1) and frequency
  which is related to square root of pressure
  (power of 0.5), your impact power is related to
  the pressure by power of 1.5
• gt This is true only if we can allow impact
  velocity (energy) to increase, and stroke length
  remains the same.

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Operating Pressure vs. Impact Power
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Operation Pressure vs Practical Limitations

• Tricky part in current trend of increasing
  operating pressures (and hammer development in
  general) is that we can not allow impact velocity
  to increase
• With current Sandvik hammers impact velocity is
  already tuned to practical maximum.
• ? We can use only the frequency part of power
  increase.
• ?With increasing operating powers we can
  approximate that everything we gain in impact
  power is gained thru frequency increase.
• This means our practical impact power increase is
  proportional to square root of pressure.

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Theory vs. Practice
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Bit penetration into the rock is in the range of
1 mm for each blow

• 1500 2400 blows / min means 1.5 ..2.4m/min rate
  of penetration or does it?

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The harder the rock the more percussion force
needed.

• Human nature of driller keeps the power always in
  maximum.
• In soft rock formation overburden reduced power
  gives better overall result.
• Bit with bigger penetration resistance helps.

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The harder the rock the more percussion force
needed.

• Rock penetration resistance can be affected by
  bit selection.
• Please read Rock Drillability (https//www.hardroc
  kdrills.com/rock-drillability/) Rock Property
• (https//www.hardrockdrills.com/rock-properties-dr
  illability/)
• Button shape, size and angle.
• Mechanically it is indentation volume of buttons
  in contact defining penetration recistance.

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Feed force, or weight on bit, must be correctly
set to get maximum utilization of the stress wave
energy in the system and avoid damage to tools
and equipment

• Mechanical background for need of feed force is
  linear momentum and percussion frequency.
• DTH drilling is not as sensitive for feed force
  as top hammer.
• One definition of needed feed force
• Ffeed ?smvf (1ß)
• ?s System related constant (1.5 -gt 3)
• mv percussion mechanism linear momentum
• f percussion frequency
• (1ß) proportion of reflection

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What can you say about feed force here?
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Correct bit rotation speed is important for
optimal rock cutting at each strike
What is wrong with this one?
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Correct bit rotation speed is important for
optimal rock cutting at each strike
         
  Percussion frequency BPM 1900  
         
  Bit diameter mm 165  
         
  Indexing distance mm 9  
         
  Rotation speed RPM 33  
         
  Indexing distance      
  Hard abrasive rock mm 8 - 9  
  Normal rock mm 10 - 11  
  Soft rock mm 12 - 13  
         
Less than button diameter
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Flushing must deliver enough bailing velocity for
efficient removal of rock cuttings, choke in the
hammer can be varied to increase the flushing

• Normally uphole velocity is not any issue with
  modern compressors and hammers.
• Water together with cuttings can form restrictive
  blockage, so.
• Make sure that youve gotcuttings coming out of
  the hole while youre drilling.
• ?Otherwise..